ASNT NDT Handbook Volume 10 OVERVIEW

LEAK TESTING I 33

FIGURE3. Graphicaldecisiontree for step-by-stepselectionof leak testingmethods

HALOGEN HEATED ANODE HIGHER SENSITIVITY

DETECTOR PROBE INHERENT TRACER MASS SPECTROMETER 1 COMPARE THESE FACTORS IN
CHOICE OF LEAK TESTING METHOD
GAGE ~
l FOR EACH LISTING OF METHODS
IIN PLACE GAGE RESPONSE ON THIS CHART
LOWER EQUIPMENT COST
~
~

SONIC

PRESSURIZED MASS SPECTROMETER
SYSTEM INFRARED
HALOGEN HEATED ANODE
LEAK
LOCATION

EVACUATED SYSTEM INHERENT DETECTOR HIGH VOLTAGE DISCHARGE
GAGE RESPONSE

PRESSURE CHANGE

TRACER PROBE ~SONIC RADIOACTIVITY
MASS SPECTROMETER

-LEAK TEST A LOW SENSITIVITY HALOGEN HEATED ANODE
TEST RUN
AFTER HIGH BACK PRESSURIZING INFRARED MASS SPECTROMETER
SENSITIVITY TEST FLOW MEASUREMENT
INHERENT GAGE ----...-zl'.) zl'.)
~ .._ ~5; HHEAALTOEGDENANODE____5;
I
I .._ ~PRESSURE CHANGE u

EVACUATED

0

MULTIPLE MASS SPECTROMETER FLOW MEASUREMENT
SEALED
-INFRARED
LEAKAGE RATE SEALED WITH RADIOACTIVITY -RADIOACTIV-I-TY-------
MEASUREMENT TRACER HALOGEN MASS SPECTROMETER
HALOGEN HEATED ANODE
HEATED ANODE

AIR SEALED HIGH SENSITIVITY - INFRARED HALOGEN zl'.)
BACK PRESSURIZING
LEAK TO VACUUM PRESSURE MEASUREMENT --~ ------zl'.) HEATED ANODE ~
LEAK TO ATMOSPHERE LOW SENSITIVITY
INHERENT TRACER BUBBLE TESTING -INFRA-R-E-D----- u

FLOW MEASUREMENT I MASS SPECTROMETER ~
0
- -
BUBBLE
GAGE IN PLACE
i---
I
PRESSURE MEASUREMENT

FLOW MEASUREMENT

OPEN OR SINGLE INHERENT TRACER GAGE IN PLACE
SEALED UNITS
I I

34 I NONDESTRUCTIVETESTINGOVERVIEW

1. type and size of test object or system to be tested; FIGURE4. Tracergas probingtechniquesused
2. typical operating conditions of test object or system; for locatingleakswith sensitiveelectronicleak
3. environmental conditions during leak testing; detectioninstruments:(a) tracer probe
4. hazards associatedwith the probing medium and the technique;(b) detectorprobetechnique

pressure conditions involvedin testing; (aJ PROBE
5. leak testing instrumentation to be used and its
SOURCE VACUUM
response to the probing medium; and OF PUMP
6. the leakage rates that must be detected and the
TRACER GAS PROBE
accuracywith which measurements must be made.
(bJ
Gases and vapors are generallypreferred to liquid media
where high sensitivityto leakage must be attained; however, SOURCE
liquid probing media are used for leak testing in many spe-
cific applications. OF
TRACER GAS
Selection of Tracer Gas Technique
for Leak Location Only VACUUM
PUMP
As shown on the upper branch of the decision tree of
Fig. 3, tracer gas tests whose purpose is leak location only test application. However, each leak testing technique can
can be divided into a tracer probe technique and a detector have a different test sensitivity under different operating
probe technique (see Fig. 4). When choosing either tech- conditions. For example, a mass spectrometer leak detector
nique, it is important that leak location be attempted only is 10,000times more sensitive than a heated anode halogen
after the presence of a leak has been ascertained. The tracer vapor detection instrument when used for leak location in
probe technique is used when the test system is evacuated
and the tracer gas is applied from the outside of the pres- the tracer probe leak location test of an evacuated vessel.
sure boundary of the test system. The detector probe tech- However, if these two instruments are used for leak detec-
nique is selected when the test system is pressurized with tion on a pressurized test system, the halogen leak detector
gases including the tracer gas (if used) and the sniffing or is superior by approximatelythe same ratio. The reason for
sampling of the leaking gas is being done at atmospheric this apparent discrepancy becomes obvious upon close
pressure in the ambient air. This selection corresponds to examination of the operating characteristics of these two
the second decision point in the upper branch of the deci­ instruments. The mass spectrometer is designed for opera-
sion tree of Fig. 3. tion under vacuum conditions, whereas the halogen leak
detector is designed for operation in air at atmospheric
Factors InfluencingChoice pressure.
between DetectorProbe and
Tracer Probe Tests As another example, a helium mass spectrometer leak
detector may have a leakage sensitivity of 10-12 Pa-m3-s-1
One of the most difficult and important decisions is the (10-11 std cm3-s-1) during routine leak testing with dynamic
choice of which leak testing method should be used. A cor-
rect choice will optimize sensitivity, cost and reliability of leakage measurement techniques. On very small systems,
the leak testing procedure. Choice of an incorrect test this optimum sensitivitymay be increased to 10-15 Pa-m3-s-1
method makes leak testing less sensitive and less reliable, (10-14 std cm3-s-1) (a gain of l,OOOx) by using the static accu-
while adding to the difficultyof testing. One simplifiedway
to choose is to rank various leak testing methods by means mulation leakage measurement technique. However, the
of their leakage sensitivity. If this were sufficient, the test static leakage measurement technique is not the standard
engineer would only need to decide what degree of sensitiv- method of using the mass spectrometer leak detector.
ity is required and then to select the test method from
among those offering adequate sensitivity for the specific

LEAK TESTING I 35

Therefore, the last sensitivity stated above is subject to some Leak Location Technique with Tracer Probe Outside
question. It must be recognized that each method of leak an Evacuated System

detection or measurement is usually optimized for one par- When testing an evacuated system that has inleakage
ticular type of leak testing. Therefore, it can be a mistake to from the ambient atmosphere or from a tracer probe, the
first consideration in selection of a test method is whether
compare sensitivities of various leak testing methods under there is an inherent detector within the system. the inherent
detector might be a pressure gage of an electronic type or,
the same conditions, if each test is not designed to operate more desirably, a gage that is specifically responsive to the
partial pressure of a specific tracer gas. Vacuum systems
under these same conditions. often contain one or more types of vacuum pressure gages.
In Fig. 3, this point appears in the second main line from
Leak Location Technique with Detector Probe the top, for tracer probe testing of evacuated systems, and is
Operating at Atmospheric Pressure labeled inherent detector.

When testing a pressurized system that is leaking into If a vacuum pressure gage does not exist within the evac-
the atmosphere, the next decision point is whether or not uated system under test, other test methods must be exam-
the leaking fluid can be used as a tracer (this decision point ined individually to determine their limitations and
lies along the top branch of the tree of Fig. 3). For example, advantages for leak testing of this system. The tracer probe
most refrigeration and air conditioning systems are charged leak testing methods, in order of increasing leak sensitivity,
with a refrigerant gas (R-12 or R-22) which is a fluorocarbon time and cost, are ultrasonic, pressure change gage
to which the heated anode halogen vapor detector is specif- response, high voltage electrical discharge, heated anode
ically highly sensitive. When searching for leaks in operating halogen detector, infrared gas detector and mass spectrom-
systems of this type, the inherent tracer dictates the use of eter helium leak detector (highest in list). These methods
the halogen leak testing method. Because of potential envi- are listed vertically at the right-hand end of the second hor-
ronmental effects from fluorocarbons, some current sys- izontal branch in Fig. 3.
tems are being charged with R-134a refrigerant gas or sulfur
hexafluoride for use, respectively, with modified residual gas The methods shown in the upper half of Fig. 3 for leak
analyzer halogen leak detectors or electron capture halogen location are those in primary or most common usage. Other
leak detectors. methods, such as those employing radioactive tracer gases,
are not generally employed because of safety and other
If the pressurized test system contains ammonia gas, a operating problems associated with their use. However, if
chemical type of leak detector might prove to be optimum. none of the leak location methods described for detector
In certain cases where the mass spectrometer leak detector probe or tracer probe leak tests in the preceding discussion
is to be used, the presence of a specific gas (such as argon, is satisfactory for a specific application, more complicated
helium, or neon) within the system provides an excellent leak testing methods may be considered during selection of
inherent tracer. Alternative procedures involve pressurizing an appropriate leak testing method.
the test system with such a tracer gas, or a mixture of air
with tracer gas. Selection of Technique for Leakage
Measurement
Some other methods for leak location do not depend
upon the specific nature of the leaking gas; among these are The lower half of the decision tree diagram of Fig. 3 is a
the ultrasonic leak detector and bubble emission testing. In guide for step-by-step selection of optimum techniques for
some cases, the tracer gas might be suitable for use with leakage measurements. Leakage measurements can be
more than one testing method, e.g., helium could be used for divided into two different types based upon the nature of
bubble testing for large leaks, or for mass spectrometer test- the test objects whose leakage is to be measured. The first
ing for small leaks or quantitative leakage measurements. decision is based upon the accessibility of test surfaces on
the pressure boundaries of the test object. Test objects are
The detector probe leak testing methods, in order of classified by accessibility into two groups:
increasing leak sensitivity, time and costs, are ultrasonic,
bubble, chemical, pressure or flow gage response, infrared 1. open units, which are accessible on both sides of the
gas detector, mass spectrometer leak detector and halogen pressure boundary, for tracer probes or detector
vapor detector. These relative sensitivity ratings apply for probes;
detector probes searching with the detector inlet probe or
sniffer searching in air at atmospheric pressure. These alter-
native leak test methods are listed vertically at the right-
hand end of the top branch of the decision tree of Fig. 3.
The lowest cost, highest speed, simplest leak tests are at the
bottom of this list. The slower, more costly, higher sensitivity
test methods appear at the top of the list shown to the right
of the top branch of the decision tree of Fig. 3.

36 I NONDESTRUCTIVETESTING OVERVIEW

2. sealed units, which are accessible only on external FIGURE 5. Modes of leakage measurement
surfaces. used in dynamicleak testingtechniquesusing
vacuum pumping:(a) pressurized system mode
The second category usually consists of mass produced for leak testingof smaller components;
items such as transistors, relays, ordnance components and (b) pressurized envelopemode for leak testing
sealed instruments. In the lower portion of Fig. 3, this of largervolumesystems
choice is indicated first on the decision path for leakage
measurement. faJ
n7?"7~=777'?===~[o"E'.f-~
Practical Measurement of Leakage
Rates with Gaseous Tracers LEAK
DETECTOR
Principles of Leakage Measurement
SOURCE VACUUM
All leak detection with tracer gases involved their flow OF PUMP
from the high pressure side of a pressure boundary through
a presumed leak to the lower pressure side of the pressure TRACER GAS @::_o o I
boundary. When tracer gases are employed in leak testing, LEAK
instruments sensitive to tracer gas presence or concentra- fbJ DETECTOR
tion are used to detect outflow from the low pressure side of
the leak in the pressure boundary. Where leak tests involve SOURCE VACUUM
measurements of change in pressure or change in volume of OF PUMP
gas within a pressurized enclosure, the loss of internal gas
pressure or volume indicates that leakage has occurred TRACER GAS
through the pressure boundary (or temporary seals placed
on openings of the pressure boundary). When evacuated or Leakage Measurements of Open
low pressure .test systems or components are surrounded by Test Objects Accessible on Both
higher pressure media such as the earth's atmosphere, or a Sides
hood or test chamber containing gases at higher pressures,
leakage can be detected by loss of pressure in the external When test objects have pressure boundaries that are
chamber or by rise in pressure within the lower pressure accessible on both sides, the second decision in the selection
system under test. of a leakage measurement test method is whether the unit
can or should be evacuated during leak testing. Also, it
Classification of Techniques of Leakage would be important to determine if the leak test can be per-
Measurement with Tracer Gases formed with the tracer probe or detector probe at atmo-
spheric pressure. If one side of the pressure boundary can
Leakage rate measurement techniques involving the use be evacuated so that leakage occurs to vacuum and the leak
of tracer gases fall into two other classifications known as detector is placed in the vacuum system, more rapid leak
(1) static leak testing and (2) dynamic leak testing. In static testing methods can be used. In vacuum, the tracer gases
leak testing, the chamber into which trace gas leaks and can reach the detector quickly, particularly with dynamic
accumulates is sealed and is not subjected to pumping to tests in which the evacuated test volume is pumped rapidly
remove the accumulated gases. In dynamic leak testing, the
chamber into which tracer gas leaks is pumped continuously
or intermittently to draw the leaking tracer gas through the
leak detector instrumentation, as sketched in Fig. 5. The
leakage rate measurement procedure consists of first plac-
ing tracer gas within or around the whole system being
tested. A pressure differential across the system boundary is
established either by pressurizing the one side of the pres-
sure boundary with tracer gas or by evacuating the other
side. The concentration of tracer gas on the lower pressure
side of the pressure boundary is measured to determine
leakage rates.

LEAK TESTING I 37

and continuously. In this case, there is little possibility of Methods of Leakage Measurement in Evacuated
Systems with No Inherent Tracer
stratification of tracer gases. However, evacuation does not
If there is no inherent tracer or gage present within an
always produce the most sensitive and reliable leakage mea- evacuated test system, standard leak testing methods must
surements. If the test volume is extremely large, high pump- be considered. Methods for leak testing of evacuated sys-
ing speed is necessary to reduce response time. Such tems, in order of increasing leak sensitivity and cost of leak
auxiliary pumping will reduce the amount of tracer gas testing equipment, include gas flow measurement, pressure
reaching the leak detector. This, in tum, can reduce signal change measurement, heated anode halogen vapor leak
levels and leakage sensitivity. Other restraints may prevent detection and mass spectrometer helium leak detection.
evacuation of the test system to a sufficiently low vacuum These methods, listed vertically at the end of the
pressure. Conventional helium mass spectrometer leak next-to-bottom decision line in Fig. 3, should each be con-
detectors, for example, should be operated at vacuum pres- sidered individually and evaluated in terms of their advan-
sure of 0.1 Pa (10-3 torr) or lower. The structure of the tages and limitations. In most cases, all of the possible leak
equipment under test (particularlyif thin walls not intended testing methods should be considered. Selection depends
to withstand external pressure are involved) may prevent on pertinent factors. for example, a more sensitive leak test-
use of leakage rate measurement techniques in which the ing method might involve higher initial costs for equipment
leak detector must operate within a vacuum. In Fig. 3, the and test setups but, on the other hand, it might result in
lowest branch leading to the junction of the leak to vacuum great cost savings during testing programs, or provide
path and the leak to atmosphere path represents the point of greater reliability in leak testing results.
decision discussed in this paragraph.
Once the basic vacuum leak testing method has been
Selecting Specific Method for Leak Testing of selected, a second consideration involves selection between
static and dynamic test techniques. It is usually preferable to
Evacuated Test Units or Systems perform leak tests using a dynamic testing technique (tests
involving pumping of the vacuum system throughout the
As indicated along the next-to-bottom decision path at test period). However, static techniques of leakage rate
the center of Fig. 3, the first approach to selecting leak test measurement should also be considered. Static tests involv-
methods for units that can be evacuated is to determine ing rise or loss in pressure, or accumulation of tracer gases
whether or not there is an inherent tracer in the test system. over prolonged leak periods, are · slower than typical
For example, if in normal operation the system under test dynamic leak tests. However, higher sensitivity can be
contains one of the specific tracer gases such as helium or achieved in static tests if the volume under test is not exces-
halogenated hydrocarbons, a test method sensitive to that sive; this may be worth the extra effort.
specific tracer gas might be preferred. In this way, consider-
able savings in test time and cost can be realized if there is Selection of Test Methods for
no need to fill the system under test with a tracer gas. Systems Leaking to Atmospheric
Pressure
If no inherent tracer gas is available within the system
under test, the next decision step might be to determine if The choice of testing methods for test systems leaking to
there is a pressure or flow gage already present in the evac- atmospheric pressure should be made by following the same
uated system to be leak tested. If so, this gage might be used type of decision pattern as for leak testing of evacuated sys-
for leakage measurement in place of some additional type of tems. The decision path for this case appears at the bottom
leak detector. This internally available gage might be a sim- of Fig. 3. The leak testing methods applicable to testing of
ple vacuum dial, thermocouple or ionization gage or, in systems leaking to atmosphere, in order of increasing test
some fortunate cases, a mass spectrometer that is incorpo- sensitivity, are flow measurement, pressure measurement,
rated into the system as a part of its analytical instrumenta- immersion bubble testing, infrared gaseous leak testing,
tion or controls. Consideration need not be limited to those heated anode halogen leak testing, mass spectrometer
types of gages commonly used for leak testing. Any gas con- helium leak testing and leak testing using radioactive tracer
centration measuring equipment that happens to be avail- gases. A dynamic leak testing method should be used wher-
able may be used for leakage measurement. For example, it ever possible. After various dynamic leak test methods have
is possible to measure the pressure rise in a leaking vacuum been considered and those whose limitations are unaccept-
tube (evacuated electronic tube) by means of the plate cur- able have been rejected, a static leak testing method should
rent increase in a triode. This is not the intended function of
a triode electron tube but, beca~se its circuit is schemati-
cally similar to that of an ionization gage, it can be used for
leakage rate measurements during testing. This decision
point is that labeled gage in place in the two bottom decision
pathways shown in Fig. 3.

38 I NONDESTRUCTIVETESTINGOVERVIEW

also be considered. Although a static technique will increase 2. leak location methods using tracer gases with easily
leak testing time, it will also increase leak testing sensitivity. detectable physical or chemical properties (gases
with thermal conductivities or chemical properties
Purposes of Leak Testing to Locate differing from those of the pressurizing gas, gaseous
Individual Leaks halogen compounds and gases having characteristic
radiation absorption bands in the ultraviolet or
Leak testing for the purpose of locating individual leaks infrared spectral ranges); and
is required when it is necessary to detect, locate and evalu-
ate each leak; unacceptable leaks then can be repaired and 3. leak location methods involving the use of tracer
total leakage from a vessel or system brought within accept- gases with atomic or nuclear properties providing
able limits. Methods for detecting and locating individual easily detectable leak signals (helium and other inert
leaks are generally quantitative only in the sense that the gases having specific charge-to-mass properties that
lower limit of detectable leak size is determined by the sen- permit their sensitive detection by mass spectrome-
sitivity of the leak detecting indicators and test method ters and gaseous radioactive isotopes detectable with
used. Thus, only rather crude overall leakage rate informa- particle counters and radiation detectors).
tion could be approximated by adding the leakage rates
measured for the leaks that are detectable. Numerous dif- Tables 3 and 4 list some typical leak detection systems and
ferent leak detecting, locating and measuring techniques give their leakage sensitivities.
and devices are available. The selection of test equipment,
tracer gas and leak detection method is influenced by the Techniques for Locating Leaks with
following factors: Electronic 'Detector Instruments

1. size of the leaks to be detected and located; Figure 4 shows arrangements of two basic techniques for
2. nature and accuracy of leak test information required; locating leaks with electronic instruments that detect gas flow
3. size and accessibility of the system being tested; or presence of specific tracer gases: (1) the detector probe
4. system operating conditions that influence leakage; probe technique and (2) the tracer technique. With either, it
5. hazards associated with specific leak location is important that leak location pinpointing be attempted only
after the presence of a leak has been ascertained. When
methods; and choosing between the pressure test technique and the vac-
6. ambient conditions under which leak location tests uum test technique, both of the alternative techniques listed
above must be considered when the test object will withstand
are required to be carried out (wind or lack of air either pressure or vacuum. If a satisfactory choice of one
circulation and stratification effects can influence technique has been made, it is a good idea to compare it with
test sensitivity and personnel). a satisfactorychoice of the other technique, to see if reduced
cost or an easier test method might be possible.
Classification of Methods for
Locating and Evaluating Individual The detector probe leak location technique is used when
Leaks the system under test is pressurized and testing is done at
ambient atmospheric pressure. The tracer probe technique
Methods for location and evaluation of individual leaks is usually used when the system under test is evacuated and
can be categorized in various ways, including by types of the tracer gas comes from outside this system. The tracer
leak tracer utilized in the detection, location and possible probe technique is usually the most rapid test because the
measurements of individual leaks. A primary classification is tracer gas travels more rapidly in vacuum and so reaches the
that between the use of liquid tracers and the use of more leak detector in a shorter time. On the other hand, a higher
sensitive gaseous tracers. Leak location methods that pressure differential can be used with the detector probe.
depend upon tracer gas properties are listed below in gen-
eral categories, in order of increasing leak testing sensitivity Coordinating Overall Leakage
and complexity of test methods: Measurements with Leak Location
Tests
1. leak location methods that are independent of any
characteristic properties of the tracer gas (use of can- Leakage rate measurement techniques do not provide
dles, liquid and chemical penetrants, bubble testing information on the number and locations of individual leaks.
and sonic or ultrasonic leak tests, for example);

LEAKTESTING I 39

TABLE 3. Sensitivity limits of various techniques TABLE4. Relative ultimate leakage sensitivities
of leak location of leak testing methodsunder ideal conditions
with very high concentrationsof tracergasesa
MinimumDetectable
Method Leakage Rate Comments MinimumDetectable
Leakage Rate
Pa·m3·s-1 (std cm3·s-1 J
Pa·m3·s-1 (std cm3,s-1 J
Mass loss time limited pressu\rechange; generally Test Method
limited to sizableleaks;
good overall quantitative Liquid pressuredrop
measure; no information
on leak location; time- Gas pressuredrop

consuming Pressure rise

leak location only; fast; no Ultrasonicleak detector 10-2 110-11
clean up; can detect Volumetric displacementd 10-3 110-21
from distance; for fairly 10-3 110-21
Ultrasonics 0.05 10.5) large leaks only Gas discharge
Ammonia and phenolphthalein 1 0-3 to 10-4 f I 0-2 to I o-3)
simple to use; Ammonia and bromocresolpurple 1 0-3 to 1 0-4 f 1 0-2 to 1 0-31
1 0-3 to I 0-4 (I0-2 to 10­3J
Chemical s I 0-4 1::; I o-3) location only; may plug Ammonia and hydrochloric acid
penetrants small leaks; requires Ammonia and sulfur dioxide 1 0-3 to 10-4 I 1 0-2 to 1 o-3)
Halide torch 1 0-4 fl o-3)
clean up
for leak location; fluids Air bubbles in water I 0-4 to 10-5 f 1 0-3 to 1 0-4)

Bubbles 1 0-5 f 1 0-4) may plug smallleaks; Air and soap or detergent I 0-4 to I 0-5 f I 0-3 to 1 0-4)
requirescleanup Thermal conductivity
simple; compact; portable; Infrared 10-5 I 10-4)
6 x 1 o-5 to I6 x 1 0-4 to
inexpensive; sensitiveto 6 x 1 0-7 6 x 10-6)
various gases; operates
Thermal 1 0-6 f 1 o-5) in air Hydrogen-pirani 10­7 f 1 0-6)
conductivity operatesin air; sensitive I 0­1 to 1 o-s I I 0-6 to 10­11
Hot filament ionization gage I o-6 to 1 o-7 I I 0-5 to I 0-6)
I 1 0-12 claimedwith Mass spectrometersniffer test 1 0­1 to 1 0-9 f I o-6 to 1 o-sJ

sulfur hexafluoride); Halogen diode detector 5 x 1 0-7 15 x 1 o-s)
portable; requires
Halogen 10-10 f I o-9) cleanup; loses sensitivity Hydrogen bubbles in alcohol I o-s to 10-9 f 1 0-7 to I o-s)
with use; sensitiveto
ambient halide gases Paladiumbarrier detector fl o-9)
most accurate for vacuum Mass spectrometerenvelope test I 0-10
testing; expensive; 10-9 to 10-13 (10-10 to 10-12)
relativelycomplex; not Radioactiveisotopes

Mass I 0-13 I I o-12J as portable as halogen a. NUMBERS NOTTO BE USED AS GUIDES IN PRACTICALLEAKTESTING.
spectrometer detectors; much less b. DEPENDS ON VOLUME TESTEDAND PRESSURERANGE OF GAGE.
sensitivewhen used in c. DEPENDS ON VOLUME TESTED.
pressuretesting d. GAS TYPEFLOWMETERS.

The latter can only be determined by leak location test tech- sensitive leak location techniques and repaired if feasible,
niques. However, use ofthe leak location techniques alone until all detectable leak locations have been identified and
cannot give reliable assurance that no leaks exist, or that their leaks rectified. For final assurance that the test object
tests have revealed all leaks that exist. Without prior assur- or system meets leakage specification requirements, it may
ance that leaks do exist, leak location test techniques be necessary to repeat the overall leakage rate measurement
become arbitrary in application. to determine whether the total leakage rate falls within the
acceptable limits.
In practice, preliminary leakage testing is often done
first by less sensitive methods to permit detection, location Laser Based Leak Imaging 1
and rectification of gross leaks. Next, the operator can
determine if any additional leakage exists by an overall leak- The backscatter/absorption gas imaging (BAGI) tech-
age measurement of the entire test vessel, system, or com- nique is different from other laser based remote detection
ponent. Then each individual leak should be discovered by techniques in that it is designed for the sole purpose of
locating leaks or tracking gas clouds. It should not be con-
fused with other laser based gas detection techniques capa-
ble of measuring gas concentration. The BAGI technique is
a qualitative three dimensional vapor visualization scheme.

40 I NONDESTRUCTIVETESTING OVERVIEW

In its present state of development, the technique provides FIGURE6. Leak locationdemonstrationof the
no absolute gas concentration data but does quite effectively gas imagingtechnique
provide concentration distributions through its imaging
aspect. With this technique normally invisible gas leakage
becomes visible on a standard video display of the region of
interest. The image of the escaping gas allows the operator
to quickly identify the location of the leak. The technique

can detect tracer gas leakage of 2.7 x lo--5 Pa,m3,s-1
(2.7 x 1Q-4 std cm2,s-1) (50 g/yr equivalent) and displaying

the leakage in real time on a standard television monitor.
The principle of operation of the BAGI technique is the

production of a video image by backscattered laser light,
where the laser wavelength is strongly absorbed by the gas
of interest. When achieved, the result is that the normally
invisible gas becomes visible on a standard television moni-
tor. The technique has three basic constraints.

1. There must be a topographical background against FROM LASER IMAGING 5YjTEMS. REPRINTED WITH PERMISSION.
which the gas is imaged.
orthogonally positioned horizontal and verticaI'scan mirrors'.
2. The system must operate in an atmospheric
transmission window. The loaspetircableapmathisanindieicstesdcaninnteodthaecroinssstathnteantaeroguest field of
view area by
3. The gas of interest must absorb the laser light.
the same orthogonal mirrors. This ensures that the detector
To date, only infrared imaging systems have been consid-
ered because most hazardous gases are active absorbers in instantaneous field of view and the laser beam are in perfect
this spectral region. However, there is no reason why the
technique would not work in visible and ultraviolet atmo- synchronization and that the laser need irradiate only that
spheric transmission regions.
region of the target area viewed by the detector. This keeps
A simulated leak location demonstration at a cylinder the laser power requirements to a minimum and makes the
storage area is shown in Fig. 6. The prototype gas imaging system totally eye safe.
system shown here is a shoulder mount, multiwavelength
(tunable) unit with a 14 by 18 degree field of view and an · Training of Leak Testing Personnel2
effective range of about 30 m (100 ft). The shoulder mount
portion weighs about 10 kg (22 lb) and the cart weighs about Because of the many leak testing techniques and the
44 kg (97 lb). The system is water cooled and requires about multiple variations of each, leak testing could require more
900 W of the llO V AC power. The unit produces a real time training and knowledge than any of the other nondestruc-
standard black-and-white television picture on the cam- tive testing methods. Successful execution of many of these
corder style viewfinder for operator viewing or can be hard techniques by inspection personnel is highly dependent on
wired to a larger television monitor, as done in Fig. 6. For knowledge and skill. Nevertheless, there are fewer instruc-
this demonstration, a leak was simulated at the top of the tion and training materials available for leak testing than for
third gas cylinder from the right using a bromofluorocarbon other methods.
(CBrF3) refrigerant. The refrigerant is delivered to the top
of the cylinder by a plastic tube connected to a supply bottle SNT­TC­lA divides leak testing into four methods (see
sitting at the base of the cylinder. Table 5): bubble test (BT), pressure change test (PCT),
halogen diode leak test (HDLT) and mass spectrometer leak
The laser used in the gas imaging system is a tunable, test (MSLT). The recent revisions in the leak testing train-
5 W, C02 waveguide laser. Use of such a low laser power is ing outline of SNT­TC­lA expanded this list of four methods
possible because of the unique optical arrangement that to a total of 12 methods (see Table 5).
permits the laser beam and the instantaneous field of view
(IFOV) of an infrared detector to be scanned in synchro- The 34 variations in Table 5 more correctly reveal the
nization across the area of interest. The instantaneous field complex nature of leak testing and may also be the reason
of view produced by the small (0.05 x 0.05 mm [0.002 x why such a small percentage of ASNT membership is quali-
0.002 in.]), cooled infrared detector and a collimating lens is fied to Level III in the leak test method. At Level I, profi-
scanned in a rasterlike fashion across the target area by two ciency in one or two techniques is possible, but it would be

LEAKTESTINGI 41

TABLES. Leak testing methods and techniques TABLE6. Comparisonof leak rates

Methods Techniques Measuremen~ Approximate Approximate
Bubble Equivalentb
Bubble solution immersion; film solution std cm3·s-1 Equivalent
Ultrasonic/acoustic sonic/mechanical flow; sound
1·0-1 l std cm3/I Os steady stream
Voltage discharge generator J 0-2 1 std cm3/J 00 s I O-s-1
Pressure voltage spark; color change
hydrostatic; hydropneumatic; I 0-3 3 std cm3/h I -s-1
Ionization
Conductivity pneumatic J0-4 I std cm3/3 h O. J -s-1
photo ionization; flame ionization 10-s l std cm3/24 h __ c
Radiation absorption thermal conductivity; catalytic
Chemical based J0--6 l std cm3 /2 wk __ c
combustible __ c
Halogen detector infrared; ultraviolet; laser J o-7 3 std cm3/yr __ c
chemical penetrants; chemical gas __ c
Radioisotope f0-8 1 std cm3/3 yr __ d
Pressure change tracer
halide torch; electron capture; 10-9 1 std cm3/30 yr
Mass spectrometer
halogen diode 10-11 1 std cm3/3,000 yr
krypton-85
absolute reference; pressure rise; flow =a. 1 std cm3 6.1 x 10-2 std in.3•

measurement; pressure decay; b. ASSUMING BUBBLE OF 1 mm3 (6.1 x 10-5 in.3J VOLUME.
volumetric c. BUBBLES TOO INFREQUENT TO OBSERVEOR PARTIALLYDISSOLVED.
helium or argon, tracer probe d. SMALLESTDETECTABLELEAK BY MASS SPECTROSCOPY.
location; hooding total leakage;
detector probe location; sealed expected life of the product being tested. As a result, many
objects; residual gas analyzer tested objects with leaks that are 10 to 100 times smaller than

an acceptable level are rejected for repair or destruction. This

creates unnecessary cost and loss of profits. Some examples of

leaks that may affect certain products are as follows: chemical
process equipment, 10-2 to 10-1 Pa·m3-s-1 (10-1 to 1 std
cm3.s-1); torque converter, lo-4 to 10-5 Pa-m3·s-1 (lo-3 to

very difficult to meet the training and experience guidehnes lo-4 std cm3-s-1 ); beverage vcaacnuuemndr,ro10c-e0s~tosy1st0e-m7 ,Pa1-m~-+7 st!?
that are recommended by ASNT for more than two or three (lo----5 to l~-6 std cm3-s-1);
techniques. A brief listing for each technique may make you lQ-<S Pa-m3·s-1 (10-6 to 10-7 std cm ·s-1 ); mtegrated circuit
aware of your weaknesses. Variations of each technique may package, lo-<S to 10-9 Pa-m3.s-1 (10-7 to lQ-<S std cm3·s-1); pace-
require familiarity with different test equipment and tracers.
maker, 10-10 Pa·m3·s-1 (10-9 std cm3-s-1).
Many inspection people are also confused, when choos-
ing a technique, by the disadvantages and limitations in sen- Another reason training must be emphasized is that many
sitivity for each technique.
leak testing hazards may exist which cause injury to inspec-
Inspection personnel often have difficulty understanding . tion personnel, damage to test equipment, or damage to the
how extremely small some leaks are that they will try to find.
This also makes it difficult to realize that some leaks may be product being tested. The following examples illustrate
temporarily sealed by foreign material such as oil, grease, numerous hazards: flammable/toxic solvents for cleaning,
water, or even cleaning solvents. Improper handhng after
cleaning may temporarily prevent location of leaks that will flammable/toxic/explosive tracers, asphyxiation by vapors or
reappear at a later tim~t A comparison of leakage rates in
three different ways (Table 6) may help to visualize the size. tracer gases, access difficult on large objects, pneumatic and
hydrostatic pressure, radioactive tracer gases, compressed
When leak testing is performed with equipment capable
of locating and measuring leaks smaller than 10-9 Pa·m3·s-1 gas cylinders/regulators and strnctural stress.
(10-<S std cm3,s-1), tracer permeation through the test object
To summarize the need for leak testing methods train-
materials of constrnction may appear as a leak indication
several seconds to hours after application of the tracer. This ing, there are 11 reasons to expand this training: choice of
may require a knowledge of those materials that allow per- many techniques, sensitivity of various techniques, advan-
meation by the tracer being used.
tages and limitations of each technique, dependence of
Many Level II or III inspection personnel establish reject
specifications that are unreahstically small with respect to the techniques on testing skills and experience, leakage location
versus measurement, factors affecting measurement accu-

racy, employers' cutting cost by hiring entry level people

and minimizing training time, hazards to personnel and
products, few courses available that offer skills training, lim-

ited available training materials and the small number of

qualified Level III personnel.

42 I NONDESTRUCTIVE TESTING OVERVIEW

PART 2
SAFETY IN LEAK TESTING

General Safety Procedures for Test tracers. Many cleaning processes involve the use of liquid
Personnel solvents and vapors, some ofwhich presentpossible hazards
of flammability, toxicity, or asphyxiation. Liquid leak tracers
Test Personnel Dedication to Safety Procedures often have similar hazards, if vapors accumulate in working
areas. Ventilation must be provided to prevent hazardous
The range of applications of leak testing is s~ wide and vapor concentrations. Electrical systems must be enclosed
varied that no single set of safety rules for protection of per- or protected to prevent ignition of flammable vapors in air.
sonnel and property can be made to cover all cases. Leak Access to test surfaces, particularly on large structures,can
testing personnel must be made aware of job hazards and be be hazardous if scaffoldingis inadequate, lighting is insuffi-
receptive to proper training in order t~ protect. themselves cient, or bad housekeeping creates hazards such as oily work
and others working close by. On many jobs, testmg must be surfaces or obstructions in passageways.
performed at odd hours and under awkward conditions.
Night shift work, weekend work and work in unheated areas Psychological Factors and the
in winter and uncooled areas in summer are common occur- Safety Program
rences. Climbing through manholes, climbing ladders and
scaffolds, balancing on structural members, or other awk- The nature of leak testing work requires a competent
ward maneuvers may all be in a day's work. safety program. Much of the success of such a program
depends upon its acceptance by those to whom it is
In addition to technical abilities and training in test pro- directed. Never has there been a safety device or a safety
cedures, the competent technician must have other program that some human being could not disrupt or
attributes. He must be determined to do a safe job under impair. The human factors that operate at all levels in indus-
any circumstances. He must be willing to listen and to coop- try are perhaps the most potent factors for success or failure
erate with the many types of personnel encountered in the of a safety program. The president of a company, the safety
field, but he must not compromise the safety aspects of his director and the leak testing supervisor may either empha-
work for the convenience of himself, his crew, or someone size safety or subordinate it to production goals. Production,
else. maintenance and testing personnelare also importantcon-
tributors to safety and their full cooperation is vital. Individ-
Test personnel can acquire a proper attitude and point of ual differences affect personnel acceptance of a safety
view toward safety only through training coupled with expe- program. These differences must be recognized when moti-
rience. The training program should include first aid and vating work groups to use good safety practices at all times.
lifesaving techniques. In situations where irritating, toxic, or
corrosive dusts, gases, vapors or fluids are present, the test The safety program must be designed with an under-
technician should be given special training to make sure that standing of motivation of people. To want something is to
he is familiar with the properties of these substances and be motivated, but not to want something also requires
with the methods of controlling the hazards. Emergency motivation. To use a safety device to protect one's fingers
procedures must be learned and ~est per~onnel. must know from a saw is, perhaps, indicative of motivation for safe
where medical and hospital assistance is available at all practice. However, the desire to ignore a safety device that
hours. The leak testing technician should have more thor- interferes with production is also a matter of motivation.
ough training in accident preve?tion than. the regular plant Conflicting motivations should also be considered in any
or construction workers. For him, safety mvolves not a set attempt to understand human relations that influence the
pattern of activity, but a complex and constantly changing success of safety programs. Industry has recognized the
set of problems. effects that attitudes can have on production, plant morale
and plant safety. As a result, managementmay spend con-
Hazards in Leak Testing siderable effort to determine the attitudes of its workers.
Measuring, developing and changing attitudes constitute a
Precleaning of test surfaces is required for leak testi~g
where surface contamination might prevent entry of fluid

LEAK TESTING I 43

major problem for management and psychologists and are TABLE7. Combustible and toxicgases and vapors
of extreme importance to the safety program. detectableby area monitorsand alarm systems

Personnel Safety Training Requirements acetaldehyde dinitrobenzene methylbutylketone

There should always be concern with safety training of acetone dinitrotoluene methylcellosolve
personnel. The learning process starts at birth. Most early
safety training is through experience, as when a child may acetonitrile dipropyleneglycol methylchloride
have touched a stove and been burnt, played with a knife
and been cut, or fallen from a precarious treehouse and bro- acetylene methylether methylchloroform
ken a bone. However, personnel testing today's vessels that
hold gases, vapors and liquids at various temperatures and at tetrabromide epichlorhydrin methylcyclohexane
absolute pressures ranging from very high vacuum (nano-
pascals) to pressure levels of megapascals cannot afford to alcohol 2-ethoxyethanol methycl yclohexanol
learn safety by causing or experiencing disasters.
ally! alcohol ethylalcohol methylenechloride
Control of Hazards from Airborne
Toxic Liquids, Vapors and Particles c-allylglycidylether ethylamine methylethyl ketone

Toxic Gas and Vapor Sensors and Alarms ammonia ethylbenzene c-methylmercaptan

Detection and warning of the presence of toxic vapors or benzene ethylbromide naphtha
gases in a work area can be provided by various types of elec-
tronic instruments with detectors and alarm systems respon- benzoylchloride ethylbutylketone naphthalene
sive to many different airborne chemicals, fumes, smoke, or
particulate matter. For general protective service applica- benzoylperoxide ethylchloride naturalgas
tions, wall mounted, self-contained monitors can detect and
provide audible signals of the presence of various com- butane ethylether nitrobenzene
bustible gases, fumes and microscopicallysized airborne par- 2-butanone(MEK) ethylformate p-nitrochlorobenzene
ticulate contaminants. These are typicallyprovided with pilot
lights to indicate the presence of alternating current line 2-butoxyethanol ethylenediamine nitroethane
power and standby battery power. Flashing red lights are
actuated when abnormal concentrations of contaminants butylacetate ethyldichloride nitroglycerin
occur. The alarm sensitivity control can be adjusted to allow
compensation for normal ambient quiescent atmospheric butyl alcohol ethyleneoxide nitromethane
contamination levels. The sensor assembly of a typical gas
monitor and alarm system contains a heated semiconductor camphor formaldehyde nitrotoluene
element whose resistance to current flow varies as a function
of the type and quantity of gas molecules adsorbed on its sur- carbonmonoxide furfurylalcohol ozone
face. The heater effectively boils off adsorbed contaminants.
Sensor resistance is thus primarily a function of adsorbed gas carbontetrachloride fasoline pentane
molecules, whose number is related to their relative concen-
trations in the ambient air atmosphere. The sensor is chloroacetaldehyde ffycolmonoethyl 2-pentanone
designed for more than 50,000 exposures and can detect chlorobenzene ether perchloroethylene
50 µg,g-1 of many combustible and toxic gases and vapors,
including those listed in Table 7. c-chloroform heptane petroleum
1-chloro-I -nitropropanheexachloroethane distillate
Ventilation to Reduce Vapor Hazards in Solvent
Use Areas chloropicrin hexane phenyel ther

Many applications of leak testing in various industries chloroprene 2-hexanone propane
have, as a prerequisite to testing, some cleaning operation.
This operation often involves the use of volatile solvents cumene hexone propargylalcohol

cyclohexane hydrogen propyleneoxide
cyclohexanol hydrogenbromide propyne

cyclopentadiene c-hydrogenchloridesteam

DDT hydrogencyanide stibine

diacetonealcohol c-hydrogensulfide sulfurdioxide

diazomethane isoamyal lcohol tetrachloronaphthalene

diborane isobutylalcohol tetranitromethane

I , I dichloroethane isopropyal lcohol toluene

I . 2 dichloroethane ketone LLI trichloroethane

diethylamine L .P. gas I, 1,2 trichloroethane

diethylaminoethanol methane trichloroethylene

diisobutylketone methylacetylene trichloronaphthalene

dimethylamine methylal I ,2,3 trichloropropane

dimethylaniline methylalcohol trinitrotoluene

dimethylformamide methylamine turpentine

I, I methyln-amyl xylene

dimethyhl ydrazine ketone

FROM AMERICAN GAS AND CHEMICAL COMPANY. REPRINTED WITH PERMISSION.

which can contaminate the air within enclosures; therefore,
some consideration must be given to ventilating the working
areas with explosion proof equipment. Local exhaust sys-
tems have several inherent advantages as compared with
general ventilation for removal of atmospheric contami-
nants. They permit removal of hazardous vapors before they
spread throughout the work area, provide economy of air
flow and involve less heat loss. Operations where local
exhaust systems are impractical include situations where the

44 I NONDESTRUCTIVETESTING OVERVIEW

contami~ant is usually a solvent vapor. Local exhausts may Precautions in Handling and Use of Flammable
be unsuitable because there are a multitude of sources of Liquids
vapor, or the source may be extensive, or the amount of
ductwork to connect all the necessary hoods may be too In th~ h~ndling and use of flammable liquids, exposure
costly or impractical. of large liquid surfaces to air should be prevented. It is not
the liquids themselves that bum or explode but rather the
The basic purpose of volatile solvents used in industrial vapor-air mixture formed when liquids evaporate. There-
cleaning operations is to dissolve or loosen contamination fore, flammable liquids should be handled and stored in
such as grease, dirt and other impurities so as to facilitate closed containers. Low flash liquids in use should be cov-
their removal. The solvent may tend to evaporate into the ered or enclosed to avoid evaporation into the atmosphere.
atmosphere. This evaporation of volatile constituents leaves In both cases, the fluids should be enclosed wherever feasi-
behind some physically changed substance, which must be ?le. When the fluid is exposed to air for a specific operation,
removed from test surfaces. Thus, the use of solvents in it should again be covered or enclosed as soon as possible.
t~ese processes involves polluting the air with vapor. The
8:1m of the safety engineer is to keep this vapor concentra- .Th~ fl~sh point of a liquid is the lowest temperature at
tion as low as possible, certainly below the toxic limit. If which it gives off enough vapor to form flammable mixtures
lo~al exhaust systems are inadequate, such widely dis- with air and to produce a flame when a source of ignition is
t~bu:ed solvent vapors can sometimes be controlled by brought close to the surface. Other properties are factors in
dilutmg the general room atmosphere with outdoor air fast determining the hazards of flammable liquids, but the flash
enough to keep the concentration of toxic vapor in the air of point is the principal factor. The relative hazard increases as
the working space within safe limits. the flash point is lowered. The significance of this property
becomes more ap:earent when liquids of different flash
Control of Hazards of Flammable points are compared.
Liquids and Vapors
Control of Electricaland Lighting
Flammable Liquids and Vapors Hazards

Flammable liquids are usually subdivided into classes. As Hazards of Static Electricity with Flammable
defined by the National Fire Protection Association a Materials
flammable liquid is any liquid having a flash point below
60 °C ( 140 °F) and having a vapor pressure not exceeding Static electricity is an accumulation of motionless
275 kPa (40 lb-in.r'' absolute) at 38 °C (100 °F). charges generated by the contact and separation of dissimi-
lar materials. For example, static electricity is generated
Combustible liquids are those with flash points in the when a fluid flows through a pipe or from an orifice into a
~an~e of 60 ~o 93 °C ( 140 to 200 °F). Although they do not vessel and may set up high voltages. The principal hazards
1gmt~ as.easily as flammable liquids, they can ignite under created by static electricity are those of fire and explosion
certam circumstances and so must be handled with caution. caused by spark discharges occurring in the presence of
Th~ more common flammable and combustible liquids are flammable or explosive vapors, gases, or dust. A spark
vanous hydrocarbons, alcohols and their byproducts. They between two bodies occurs when there is no good electrical
are chemical combinations of hydrogen and carbon; the cond1:1ctive path between them. Hence, grounding and
combination may also contain oxygen, nitrogen, sulfur and bonding .of flam1:1~ble liquid containers is necessary to pre-
other elements. vent static electricity from causing a spark

Flammable liquids vaporize and form flammable mix- Bonding and Grounding to Prevent Sparks
tures when in open containers, when leaks or spills occur, or
when heated. The degree of danger is determined largely by A point of great danger from a static spark is the place
the following factors: where a flammable vapor may be present in the air, such as
at the outlet of a flammable liquid fill pipe or a delivery hose
1. the flash point of the liquid; ~ozzle. Static .spark ignition sources are prevented by bond-
2. the concentration of vapors in the air (whether the mg or grounding or both so they have the same static voltage
or potential.
vapor-air mixture is in the flammable range or not);
and The terms bonding and grounding often have been used
3. the possibility of a source of ignition at or above a interchangeably because of poor understanding of the dis-
temperature sufficient to cause the mixture to burst tinct functions indicated. Bonding is done to eliminate a
into flame.

LEAKTESTINGI 45

difference in potential between objects. The purpose of TABLE8. Selectionguide for personnel
grounding is to eliminate a difference in potential between protection indicators for toxic gasesand vapors
an object and ground. Bonding and grounding are effec- accumulating in leak testing areas
tively applied only to conductive bodies. The human body is
a conductive body which may differ in potential from Toxic Warning Color
ground or other bodies, so that it may also serve as a source Substance Concentration Change
of spark ignition. (parts per million)*

Although bonding will eliminate a difference in potential Ammonia 15 brown to white
between the objects that are bonded, it will not eliminate a Carbon monoxide 50 white to black
difference in potential between these objects and the earth Chlorine white to yellow
unless one of the objects possesses an adequate conductive Hydrazine 2 white to yellow
path to earth. Therefore, bonding will not eliminate the Hydrogen sulfide 5 white to brown
static charge, but will only equalize the potential between Nitrogen dioxide 5 white to yellow
the objects bonded. Ozone white to brown
I
Control of Electrical Power Supply Hazards 0.1

Electricity as a source of power is, in some ways, less * DATAAPPLYALSO TO AREA CONTAMINATION MONITORS.
hazardous than steam or other energy sources. However, FROM AMERICAN GAS AND CHEMICAL. REPRINTED WITH PERMISSION.
failure to take suitable precautions in its use creates condi-
tions that are certain to result in bodily harm or property in the test area within building enclosures), test personnel
damage or both. Although there have been recent advances (and others) cannot enter it without proper respiratory
in the control of electrical hazards, industry still has many equipment. In such cases, proper respiratory equipment
injuries and fatalities from preventable causes. Machine consists of a gas mask which contains its own oxygen supply.
tools can, with minimum expense and difficulty,be arranged
for maximum safety and efficiency. There are, however, cer- The oxygen required for breathing might be removed
tain hazards in the installation, maintenance and use of elec- from a test area or chamber accidentally. For example, if
tric wiring and equipment. Control of most of these hazards one of the halogenated hydrocarbons is used as a tracer gas,
is neither difficult nor expensive, but ignoring or neglecting it will stagnate and settle to the lowest area in the system. If
them may lead to serious accident. an operator is attempting to use a detector probe in this
area, the tracer that settles into low areas may eventually
Safety Precautions with Leak displace enough of the air to produce asphyxiation. In this
Testing Tracer Gases type of situation, it is necessary to provide adequate ventila-
tion. However, this ventilation must be performed carefully.
Tracer Gas Hazards in Leak Testing If the tracer gas is dispersed or blown away too rapidly from
the locations where it is escaping from the system under
Tracer gas safety aspects such as flammability, asphyxia- test, leak location by detector probe may become difficult or
impossible.
tion, or specific physiological effects as well as the possibility
of pressure vessel explosions must be considered. As long as For better understanding of the safety aspects, the fol-
lowing data are presented for several tracer gases that may
the nondestructive test engineer and the leak test technician be used. In addition, information is given on the availability
are aware of these considerations from the start, it is possi- of personnel protection indicators and area contamination
monitors which can provide warning indications of danger-
ble to leak test a vessel with minimum inconvenience or ous accumulations of toxic gases or vapors. See Table 8 for
color changes for various gases and vapors.

danger. ,

Hazard of Asphyxiation in Pools of Stratified Tracer Safety Precautions in Pressure and
Gases Vacuum Leak Testing

Most tracer gases are not toxic. However, if a question Safety Considerations in Leak Testing
exists about toxicity of any particular gas, a competent
authority should be consulted to assure personnel safety. When a pressure or a vacuum vessel is fabricated, some
None of the tracer gases such as helium, argon, neon or means of testing must be used to predict safe vessel perfor-
nitrogen will support human life. If a tracer gas replaces oxy- mance. It is sometimes necessary to exceed the designed
gen in a test vessel, hood, or enclosing chamber (or collects operating conditions during initial pressure testing. This

46 I NONDESTRUCTIVE TESTING OVERVIEW

requires many safety considerations to ensure proper pro- unless they should happen to collide with a safety shield or
tection of personnel. Safety aspects of pressure and vacuum glass pieces coming from the opposite direction. The hazard
leak testing operations are discussed in the remainder of
this section. (Hazards related to use of toxic or flammable of personnel injury by flying glass becomes particularly seri-
solvent vapors and tracer gases in leak testing are discussed
earlier in this section and should also be given careful con- ous when the capacity of the glass vessel exceeds about 30 L
sideration.) (1 ft3). For this reason, all evacuated bell jars should be

Hazard of Explosion or Implosion of Systems or enclosed in some type of safety shield.
Safety shields should be used on small thin wall vessels
Vessels Pressurized or Evacuated for Leak Testing
and glass bell jars under all vacuum conditions if an implo-
If a system to be leak tested is pressurized with tracer gas
or gas mixtures, rupture of its containment walls or pressure sion hazard exists. Because the pressure differential
boundaries could produce considerable damage. If the sys-
tem being pressurized is small, it might seem as if few pre- between atmospheric pressure (100 kPa) and a (crude) vac-
cautions would be necessary during pressurizing. However, uum pressure (10-1 kPa or 100 Pa) is essentially equal to
the damage from rupture of a gas filled volume results from
the total amount of gas it contains. Therefore, either a small atmospheric pressure (100 kPa), any additional increase in
system under high pressure, or a large volume system under
lower pressure, might be equally dangerous. The energy pressure differential is negligible as the contained absolute
stored in a pressurized gas volume is equal to the product of
its pressure and its volume. (The pressure in pascals or new- pressure is further lowered from 100 Pa to 1 Pa. The major
tons per square meter multiplied by the volume in cubic part of atmospheric pressure is thus exerted upon the bell
meters results in energy in joules, (Nvm) x m3 = Nori = J. By
comparison, 1 kg of gasoline contains approximately 44 MJ, jar or thin wall system when rough evacuation takes place.
enough to blow up a tank.) When pressurizing a system, a The increase in pressure difference resulting from further
pressure regulator fitted with a safety overpressure release pumping to obtain a high vacuum is very small. Thus, it is a
device should be installed, so that a pressure in excess of the
design pressure can never be applied to a vessel or system mistake not to use bell jar safety shields for any but the most
under leak test. moderate vacua. ~·

Pressurized vessels can fail by explosion due to the Safety Procedures and Problems of Code Pressure
energy stored in air or nonflammable gases used to pressur-
ize systems during leak testing. In systems that are evacu- Proof Testing of Systems before Leak Testing
ated during leak testing, implosion (violent collapse) failures
can result from external (atmospheric) pressures applied to Before leak testing, large systems may require proof test-
structures not designed for such loading. Where flammable ing to determine their capability to withstand leak test pres-
tracer gases are used in leak testing in the presence of air or surization. For example, the ASME Boiler and Pressure
oxygen, violent combustion or explosive chemical reactions Vessel Code specifies that all vessels should be hydrostatic
can occur. These hazards must be foreseen and carefully proof tested to 1.5 times the maximum allowable working
controlled to ensure safety during leak testing. pressure. The alternative to hydrostatic proof testing with
water is to perform a pneumatic proof test to 1.25 times the
Implosion is the collapse of a pressure boundary or the maximum allowable working pressure. The pneumatic proof
walls of a containment vessel or structure when evacuated test may be performed by pressurizing with gas to a high
and subjected to atmospheric or higher external pressures. pressure while all personnel are removed from the test area.
Many vessels and chambers are made for use under vacuum The disadvantage of the proof test made with gas or air pres-
to simulate high altitude or outerspace conditions where the sure is that if the system bursts during testing, considerable
maximum pressure differential that will ever be applied damage can result. Because water is relatively incompress-
across their boundaries is 100 kPa (1 atm) of external pres- ible under pressure (as compared with gases), the energy
sure. Systems fabricated of thin wall materials, glass, or foils released when a system bursts under water pressure is far
are not capable of withstanding high external or internal less than when the system bursts under an equal gas pres-
pressures. For example, although they are not internally sure. On the other hand, if hydrostatic testing is performed
pressurized, glass bell jars that are evacuated can become a before leak testing with gaseous tracers, small leaks will be-
dangerous source of flying glass as a result of implosions. come clogged with water. Therefore, if at all possible,
Pieces of flying glass, propelled by a pressure difference of hydrostatic testing should not be performed upon test ves-
approximately 100 kPa (1 atm), will travel great distances sels or systems where the allowable leakage rate is less than
10-7 Pa-mvs! (10-6std cm3·s-1).

Precautions in Selecting Sites for Leak Testing

Major factors determining size, shape and type of build-
ings and structures to be used for leak testing of compo-
nents need to be investigated. Catastrophes resulting in
large loss of life and heavy property damage often are due
to inadequate planning stage considerations. High hazard
leak testing operations should be located in small isolated

LEAK TESTING I 47

buildings of limited occupancy. Buildings can be designed Serious accidents may result from the misuse, abuse, or
so that internal explosions will produce minimum damage mishandling of compressed gas cylinders. Technicians
and minimum broken glass. Lower hazard operations can assigned to the handling of pressurized cylinders should be
justify large units. carefully trained and work only under competent supervi-
sion. Observance of the following rules will help control
Protecting Test Personnel during High Pressure hazards in the handling of compressed gas cylinders.
Testing
1. Accept only cylinders approved for use in interstate
Greater respect for high pressure testing has led to commerce for transportation of compressed gases,
increased emphasis on safety, with the result that overall
safety experience has been very good. This respect is well 2. Do not remove or change numbers or marks stamped
justified when one realizes that a valve stem operating at on cylinders,
200 MPa (3 x 104 lbr·in.-2) that fails and is blown out is pro-
pelled under conditions similar to those of a bullet fired from 3. Cylinders must never be moved unless the protective
a high powered rifle. The energy released from a completely cap is in place. Because of their shape, smooth sur-
liquid system should not be underestimated either. Liquid face and heavy weight, cylinders are dangerous to
compression, although small volumetrically by comparison carry by hand and some type of carrying device
with gas, is very much to be reckoned with in considering should be used when they must be moved without
potential forces to be handled upon pressure release. For the aid of a cart. Cylinders may be tilted and rolled on
example, a gasket 0.4 mm (1/64 in.) thick, blown between the bottom edge, but they should never be dragged,
split flanges under a pressure of more than 10 MPa (-1.4 x
103 lbr·in.-2), will release a thin sheet of water like a knife 4. Protect cylinders from cuts or abrasions,
edge which could cause injury or eye damage. 5. Do not lift a compressed gas cylinder with an electro-

Successful personnel protection during pressure testing magnet. Where cylinders must be handled by a crane
involves not only mechanical devices to guard against injury or derrick, when testing field erected vessels, carry
should failure occur, but thorough training of people, estab- them in a cradle or similar device and take extreme
lishment and enforcement of rigid safety rules and neces- care that they are not dropped. Do not use slings or
sary disciplinary action when justified. Without the proper chains,
attitude and respect for what is being handled, trouble is 6. Do not drop cylinders or let them strike each other
sure to occur. violently,
7. Do not use cylinders for rollers, supports, or any pur-
Safety Precautions with pose other than to contain gas,
Compressed Gas Cylinders 10. When empty cylinders are to be returned to the ven-
dor, mark them EMPTY or MT with chalk. Close the
Most of the gas used for leak testing is purchased in valves and replace the valve protection caps,
cylinders, which should be constructed and maintained in 11. Load cylinders are to be transported so as to allow as
accordance with regulations of the Interstate Commerce little movement as possible. Secure cylinders to pre-
Commission. The contents should be legibly marked on vent violent contact or upsetting.
each cylinder in large letters. 12. Always consider cylinders as full and handle them
with corresponding care. Accidents have resulted
when containers under partial pressure were thought
to be empty,
13. Use safety chains to secure cylinders during use to pre-
vent accidental falling is required practice (OSHA).

48 I NONDESTRUCTIVETESTING OVERVIEW

PART 3

HALOGEN TRACER GAS TECHNIQUES
AND LEAK DETECTORS

HalogenVaporTracer Gases and as R-134a tetrafluoroethane (C2H2F4), which contain much
Detectors fewer chlorofluorocarbons, are being manufactured as
replacements. Also available from refrigerant suppliers are
Leak testing with halogen vapor tracer gases uses leak refrigerant fittings and hose.
detectors that respond to most gaseous compounds that
contain halogens such as chlorine, fluorine, bromine, or PressureLeak Testing with
iodine. The elemental halogen gases are not commonly used Halogen (Sniffer} DetectorProbe
as tracers since they are toxic and typical halogen vapor
detectors do not respond sensitively to these elemental American Society for Testing and Materials E 427, Stan­
gases. Preferred halogen tracer gases are nontoxic chemical dard Practicefor Testingfor Leaks Using the Halogen Leak
compounds such as the common refrigerant gases and other Detector (Alkali­Ion Diode) covers procedures for testing
leak testing tracers. For example, Refrigerant-12 (com- and locating the sources of halogen tracer gas leaking at the
monly designated simply as R-12) is dichlorodifluo- rate of 1 x 10--8 Pa,m3-s-1 (1 x 10-7 std cm3,s-1). The test may
romethane. In addition to being a refrigerant, this gas is an be conducted on any device or component across which a
excellent halogen tracer gas since it is inert, nontoxic, liquid pressure differential of halogen tracer gas may be created
at moderate pressures and readily available in convenient and on which the effluent side of the area to be leak tested is
small and large containers. The only problem with this accessible for sniffing with the halogen leak detector. These
tracer is that it contains chlorofluorcarbons (CFCs) that may methods require halogen leak equipment with a full scale
harm the earth's ozone layer. This refrigerant is no longer readout of at least 3 x 10-10 Pa-m3-s-1 (3 x 10-9 std cm3-s-1)
made in the United States for this reason. on the most sensitive range, with a zero drift and sensitivity
drift not exceeding ± 15 percent of full scale during 1 min
If a closed component, pipe, vessel or system is pressur- on this range and of± 5 percent or less on other ranges.
ized with one of the halogen tracer gases, or with a mixture
of a halogen gas with air or nitrogen, a halogen vapor leak Five methods of halogen leak testing are described in
detector can be used to locate leaks and/or to measure the ASTM E 427:
rate of leakage. Three types of halogen leak sensors or
detectors used in halogen leak testing are ( 1) the halide 1. method A: direct sniffing with no significant halogen
torch, (2) the heated anode halogen detector and (3) the contamination in the atmosphere;
electron capture ( electronegative gas) detector.
2. method B: direct sniffing with significant halogen
The most popular halogen tracer gases for leak testing contamination in the atmosphere;
are the two refrigerant gases R-12 (dichlorodifluoro-
methane., CC12F 2) and R-22 (monochlorodifluoromethane, 3. method C: shroud test;
CHC1F2). These compounds may be available from local 4. method D: air curtain shroud test; and
refrigeration suppliers in containers varying in size from 5. method E: accumulation test.
small cans to full size pressurized liquid cylinders while pre-
viously manufactured supplies last. Newer refrigerants such Methods C, D and E are well adapted for automation of
valving and material handling.

LEAK TESTING I 49

PART4

REFERENCE LEAKS

Terminology Applicable to FIGURE 7. Categories of artificialphysical leaks
Reference, Calibrated or Standard commonlyspoken of as reference, calibration
Leaks or standard leaks

Physical leaks suitable for checking leak detector perfor- LEAKS
mance and leak test sensitivity are a vital component of
instrumentation for leak testing. The terms reference, cali- RESERVOIR NON RESERVOIR
brated and standard leaks have been used in the past to
identify these physical leaks. To many people, use of the PERMEATION CAPILLARY POROUS CAPILLARY POROUS
term calibration implies the existence of a universally PLUG PLUG
accepted standard such as those at the National Institute of
Standards and Technology (NIST). NIST did not issue ref- FIXED VARIABLE FIXED VARIABLE
erence standard leaks before 1981, although an effort was VALUE VALUE VALUE VALUE
initiated to develop leak standards, so that a solution in
some form may be forthcoming. Some leak testing specifica- FROM MARR-NASA. REPRINTEDWITH PERMISSION.
tions require that all calibrations be directly traceable to
standards maintained by NIST. Commercially available ref- are inspected. This ensures more uniform agreement of all
erence leaks can be traced to NIST. tests.

In some cases, accuracy in leakage measurement is not a Calibrated leaks may be divided into two distinct cate-
prime importance. Rather, most practical situations require gories: (1) reservoir leaks which contain their own tracer gas
that some particular leakage value not be exceeded. It need supply and (2) nonreservoir leaks to which tracer gas is
only be established that no leakage in the tested system is added during calibration. Figure 7 shows a classification of
greater than this allowable maximum leakage rate. This artificial physical leaks used for reference, calibration, or
practical approach to leakage specification requires some standard leaks.
arbitrary standard. However, if any doubt exists, one need
only reduce the leakage of this arbitrary standard physical The type and size range of the calibrated leak selected
reference leak by a sufficient safety factor to ensure that test should be comparable to the leakage rate and mode of flow
sensitivity meets the practical leakage requirement. in the system being leak tested.

Classification of Common Types of Modes of Gas Flow through Leaks·
Calibrated or Standard Physical
Leaks For each type of leak test, it is essential that the test
operator have a basic understanding of the types of flow that
Calibrated physical leaks are designed to deliver gas at a might occur within a leak. Different basic laws relate leak-
known rate. The most common use of such leaks is in the age rate to pressure difference across the leak, the range of
measurement of sensitivity of leak detectors. For large absolute pressure involved and the nature of the gaseous
evacuated systems, calibrated leaks are used to establish or fluid escaping through the leak. Three basic types of gas
confirm test sensitivity and response time for the test system flow that occur in leaks are known as:
or leak detector setup. Setup must be the same for calibra-
tion as it is during the test. Calibrated leaks are used also to 1. viscous flow, which typically occurs in leaks leaking at
measure the speed of vacuum pumps and to calibrate pres- atmospheric or higher pressure under pressure test-
sure gages. A standard physical leak makes feasible the ing conditions;
establishment of leakage rate requirements for specifica-
tions. It also provides a uniform reference standard for cali- 2. molecular flow, which usually occurs in leaks under
brating leak detectors at different locations where products vacuum testing conditions; and

3. transitional flow, which occurs under test conditions
intermediate between vacuum and pressures higher
than atmospheric pressure.

50 I NONDESTRUCTIVE TESTING OVERVIEW

PART 5

PRESSURE CHANGE TESTS FOR
MEASURING LEAKAGE RATES

Functions of Pressurizing Gases in Table 9 provides multiplying factors for converting pressure
Leak Testing values between other units and SI units. It includes conver-
sions between SI and the prior metric units such as kg-mm-2,
Atmospheric air and nitrogen are often used as pressur- millimeters of mercury (mm Hg) or torr, and micrometers of
izing fluids in leak testing and leakage measurements. Their mercury or millitorr. Also listed are conversions between SI
fluid pressure serves to create pressure differentials across and English units such as pounds per square inch (lbr·in.-2),
pressure barriers or walls. This pressure differential, in turn, inches of mercury (in. Hg) and atmospheres (atm).
causes the pressurizing gas to flow, by various mechanisms,
through leaks in the containment walls. Leaks are the physi- Compressibility of Gaseous and
cal holes or passageways that may exist in wall materials, Liquid Fluids
welds, or mechanical seals or joints. The fluid that flows
through the leak passageways constitutes leakage. The rate Gases are frequently regarded as compressible and liq-
of leakage, in turn is taken as a measure of the size of the uids as incompressible. Strictly speaking, all fluids are com-
leak. In general, the higher the differential pressure, the pressible to some extent. Although air is usually treated as a
greater the rate of leakage. With higher rates of leakage, the compressible fluid, there are some cases of flow in which
sensitivity of leak detection and leakage measurement is the pressure and density changes are so small that the air
typically increased. may be assumed to be incompressible. Examples include
the flow of air in ventilating systems and the flow of air
Closed systems with air or other gas pressures above around aircraft at low speeds. Liquids like oil and water may
atmospheric pressure (100 kPa) respond to leakage by pres- be considered as incompressible in many cases; in other
sure changes (within closed systems) or require inflow of gas cases, the compressibility of such liquids is important. For
to maintain constant pressure conditions. These pressure instance, common experience shows that sound waves travel
changes or rates of fluid flow can be used to determine (1) if through water and other liquids; such pressure waves
leaks are present or (2) the rates of leakage, when internal depend upon the compressibility or elasticity of the liquid.
volumes, fluid temperatures and other variables are known
or can be measured accurately. The physical properties and Pressure Change Tests for
characteristics of the pressurizing fluids must be known and Measuring Leakage Rates in
the effects of fluid reactions to various test conditions must Pressurized Systems
be calculated in order to make quantitative measurements
of leakage rates by these effects. Pressurizing gases should Operating Principles of Pressure Change Leakage
obey the Ideal Gas Laws. In some cases, the effects of water Rate Testing
vapor and other gaseous materials that do not obey the Gen-
eral Gas Laws must be determined and their effects sub- Leakage rate testing by measurement of pressure
tracted from the pressure measurements. changes in closed volumes requires that the system under
test be maintained at a pressure other than ambient atmo-
Conversion of Pressure spheric pressure. Pressure change leak tests can be made
Measurements to Systeme with either an evacuated or a pressurized test system. The
Internationale d'Unites (SI Units}
leakage rate Q is equal to the measured pressure change
The past few decades have seen many changes in the
units used to describe pressure levels. The Systeme Interna- 1"'..P multiplied by the test system's internal volume V and
tionale d'Unites (SI units) expresses pressure in pascals (Pa).

LEAKTESTING I 51

TABLE 9. Conversion factors for pressure values The pressure change leak testing procedure is used pri-
expressed in SI and in prior systems of units marily for leakage measurement in large systems. However,
with minor modifications, the pressure change technique
Convertfrom To Multiplyby can be used to measure leakage rates on test systems of any
size. This procedure is used only for measurement of leak-
pascal lbr·in.-2 1.4504 x I 0-4 age and is not well suited for location of individual leaks.
atmosphere (7 60 mm Hg) 9.8692 x 10-7 However, a leak may be localized to a closed off portion of a
torr (mm Hg) 7 .5006 x 10-3 system under test by pressure change test techniques.
millibar 1.0003 x 10-2
inches mercury 2.9530x J0-4 Sensitivity of Pressurized Mode Leakage Tests by
Pressure Change Techniques
lbr·in.-2 pascal 6.8948 x I 03
atmosphere (7 60 mm Hg) 6.8046 x 10-2 The sensitivity of leakage measurement during leak test-
torr (mm Hg) ing of pressurized systems with the pressure change tech-
millibar 5.1715x 101 nique is dependent upon the minimum detectable
inches mercury 6.8966 x 101 magnitude of pressure variation. Static pressure is measured
2.0360 at the start, at intervals and at the end of the leak testing
period. The sensitivity of this static leakage measurement is
atmosphere pascal l.01325xl05 largely dependent upon the time duration of the test and
lbrin.-2 l.4696x 101 the sensitivity and accuracy of the pressure measuring
7.60 x I 02 instruments. In the absence of uncontrolled temperature
torr (mm Hg) I .0135 x I 03 changes or severe outgassing effects, longer time intervals
millibar 2.9921 x 101 between initial and final measurements permit more sensi-
inches mercury tive measurements of pressure changes.

torr (mm Hg) pascal I .3332 x I 02 The accuracy of measurement of leakage rates in the
millibar lbr·in.-2 1.9337 x I 0-2 pressurized mode of pressure drop leak testing depends
atmosphere (760 mm Hg) I .31 58 x 10-3 upon how precisely the test volume Vis calculated and upon
millibar 1.3336 how . accurately the changes in pressure and temperature
inches mercury 3.9368 x I 02 can be measured. If the leakage rate is measured as a per-
centage of total enclosed fluid (mass) lost per unit of time,
pascal 9.9974 x I 01 then precision in calculating the enclosed volume may not
lbr·in.-2 I .4500 x I 0-2 be required. When using properly calibrated pressure mea-
atmosphere (7 60 mm Hg) 9.8666 x I 0-4 suring instruments in the pressurized mode, the accuracy of
torr (mm Hg) 7.4986 x I 0-1 leakage measurement by the pressure drop method can
inches mercury 2.9522 x I 0-2 often be traced to the National Institute of Standards and
Technology.
inches mercury pascal 3.3864 x I 03
lbr·in.-2 4.9116 x 10-1 Sources of Error in Pressurized Mode Leakage Tests
atmosphere (760 mm Hg) 3.3421 x I 0-2 by Pressure Change Techniques
torr (mm Hg) 2.54 x 10-1
millibar The test procedure for the pressurized mode of leakage
3.3873 x 101 measurement consists of filling the test system with gas and
observing any pressure decrease. The fundamental relation-
divided by the time interval M required for the change in ship is given in Eq. 1. Two large sources of error exists in this
system pressure to occur, as shown by Eq. 1: technique. The volume of the test system is difficult to cal-
culate for a large or complex system; however, it" could be
Q =. V­ ~p (Eq. l) measured by the additional leakage technique, which is also
M known as a verification test or a prooftest in practice. (This
Where: technique is not recommended for small volume systems
such as gasket interspaces because the measurement tech-
Q leakage rate in pascal cubic meter per nique may become a major source of errror.) An additional
known leak is added to the system under test. The system
second (Pa-m3-s-1 ); volume is then calculated from the effect of the additional
V enclosed system volume in cubic leakage upon the observed rate of pressure decrease. The
second source of error inherent in the pressure change
meters (m3);
M pressure change P1 - P2 during leak test in

Pa;and
~t time interval t2 ­ t1 during leak test in

seconds (s).

52 I NONDESTRUCTIVETESTINGOVERVIEW

technique exists when temperature variations during the have not yet been worked out for vacuum technology.The
test cycle tend to varythe pressure in the system. This error SI unit for pressure is the pascal (Pa), which is introduced
can be corrected by measuring system temperature during here as the unit of pressure in vacua. Many processes
the leak test. require medium levels of vacuum of the order of 0.1 to
1 Pa. However,for many applications such as high altitude
PressureChange Tests for simulationchambers, pressures much lower than 0.1 Pa are
Measuring Leakage in Evacuated required. Submultiplesof pascal, the millipascal(mPa) and
Systems micropascal (µPa), are used to describe pressures in this
range of hard vacua, to avoid use of negative exponents or
Introduction to Pressure Measurements in powers of ten. The previously used units of millimeters of
Evacuated Systems mercury (mm Hg) or of torr must be multiplied by 133 to
equal the pressure in pascals. The prior unit known as a
By popular usage, atmospheric pressure is taken as the micrometer of mercury pressure (µm Hg) is equal to pres-
upper limit of vacuum. Any pressure less than standard sure of 133 mPa. Because the pressure of the standard
atmospheric pressure (100 kPa) is some form of vacuum. atmosphere at sea level is 1.00 x 105 Pa or 101 kPa, it fol-
On Earth, vacuum pressure can be anything between abso- lows that perfect vacuum would have a (negative) gage
lute zero pressure and the barometer reading at the particu- pressure of(-) 101 kPa because the gage pressure in vac-
lar location and time. Earlier, the vacuum pressure was uum is referred to the standard atmospheric pressure at sea
measured in inches or millimeters of mercy below atmo- level.
spheric pressure. A vacuum of 720 to 740 mm Hg was con-
sidered to be a fairly goodvacuum. Now, using SI units, this The past few decades have seen many changes in the
same vacuum level would be expressed as an absolute pres- units used to describe pressure levelsin vacua. Early inves-
sure of 3 to 6 kPa (3 to 6 percent of normal sea level atmo- tigators described their vacuum pressure in terms of mil-
spheric pressure of 100kPa). limeters of mercury, where the atmospheric pressure at
standard conditionswas taken as 760 mm Hg. Hard vacuum
Meaning of Absolute Pressure and Gage Pressure pressures were later described in terms of micrometers of
in Vacuum Systems mercury (a micrometer is one millionth of a meter of mer-
cury).
As suggested earlier, the concept of a vacuum is related to
the pressure exerted by the earth's atmosphere. Atmospheric Effects of Weld Joint Design on Leak Testing of
pressure indicates the weight of a column of atmospheric air
per unit of sectional area measured at a particular altitude Evacuated Vessels
above sea level. With increasing altitude, pressure decreases
until, at some indefinitely great height above the earth's sur- For pressure vessels to be evacuated during leak testing
face (where only empty space exists), the pressure (and vesselsdesigned for vacuum operation), the weld joint
approaches absolute zero. An enclosure is said to be under design and preparation should avoid trapped volumes or
vacuum if its internal pressure is less than that of the sur- unwelded faying surface areas that will be exposed to the
rounding atmosphere. Because of atmospheric pressure vacuum side of the joint. Both form crevices that may hold
changes due to meteorological factors and altitude, the foreign matter that can outgas during evacuation, or may
numerical value assigned to gage pressure in vacuum is provide traps for tracer gases. Because cleaning of such
referred to atmospheric pressure under standard conditions crevicesis often impossible, joint design and weldingproce-
at sea level (an absolute pressure of 101 kPa). As vacuums dures must eliminate such traps. Welding should be per-
were improved, it became necessary to provide a scale of formed from the side of the joint that will be evacuated
absolute pressures (somewhat analogous to the scale of abso- whenever practical. The under bead often containsunavoid-
lute temperatures). The concept of a perfect vacuum corre- able microporosity too small to affect most strength and
sponds to the hypothetical state of zero absolute pressure. toughness properties of the welded structure. However, if
exposedto the vacuum, these voidscould act as trapped vol-
International System of Units (SI Units) for Vacuum umes. Leakage from this source can be avoided by at least
Pressures welding the cover (or seal) pass from the side of the pres-
sure boundary that will be evacuated. Figure 8a shows
The decision has been made to convert to International examples of preferred joint designs for systems that will be
System (SI) units, but the details of such preferred units exposed to high vacuum. Figure 8b showsundesirable joint
designs that provide dirt traps and create trapped volumes
(at the roots of butt welds made from two sides of the plate,
or fillet welds with unwelded areas between abutting
plates).

LEAK TESTING I 53

FIGURE 8. Examples of weld joint designs for welded vessels (preferred designs have no crevices or
volume traps open to evacuated side of pressure boundary; undesirable joints trap contamination and
tracer gases, which may outgas during evacuation or leak testing with sensitive mass spectrometer or
other vacuum leak detectors): (a) preferred designs of welded joints for evacuated vessels;

f bJ undesirable designs for welded joints in vessels to be evacuated

faJ

(bJ

LEGEND GAS TRAPS
0 = VACUUM SIDE
~ = CONTINUOUS WELD
G) = LOCATIONS OF PROBABLE
[::,,. = INTERMITTENT WELD

FROM MCDONNELL DOUGLASCOMPANY. REPRINTEDWITH PERMISSION.

54 I NONDESTRUCTIVETESTING OVERVIEW

PART6

LEAK TESTING OF VACUUM SYSTEMS

The Nature of Vacuum spectrometry, space simulation and leak detection. Many
other areas find application for vacuum equipment.
Definition of a Vacuum
Changes in Pressure Units Used for Vacuum
The word vacuum is derived from the Greek word Measurements
meaning empty. In practice, use is made of some type of
vessel (vacuum enclosure, chamber, or container) to contain The presently preferred SI unit for pressure is the pascal
a vacuum. When the enclosure is closed to the surrounding (Pa). The standard atmospheric pressure at sea level and
atmosphere and air or gas is removed by some pumping 0 °C is equal to 101.325 kPa. Earlier units used for pressure
means, a vacuum is obtained. Various degrees of vacuum in vacuum relate to atmospheric pressure indicated by the
can be obtained, depending on how much air is removed height (nearly 760 mm) of the mercury barometer column
from the enclosure. Common terms such as partial vacuum, at sea level and O °C. The unit known as the torr is defined
rough vacuum, high vacuum and ultrahigh vacuum are used as l/760th of the pressure of the mercury column. The torr
to describe degrees of vacuum. A vacuum is any pressure was named in honor of an Italian physicist, Evangelista Tori-
below the prevailing atmospheric pressure. Practically celli (1608-47) who was the inventor of the mercury barom-
speaking, a vacuum such that the containing vessel is empty, eter. The torr is, almost identical to the millimeter of
i.e., free of all matter (molecules), is never obtained. If this mercury (mm Hg), since there are 759.96 torr in a standard
were possible, the vacuum would be called a perfect or atmosphere. The difference between the two units amounts
absolute vacuum. to so little that torr and millimeter of mercury have been
used interchangeably in the past.
Applications of Vacuum Environments
Variation of Atmospheric Pressure with Altitude
Vacuum is used to reduce the interaction of gases or air
with solids and to provide control over electrons and ions by The mercury barometer is a device for measuring atmo-
reducing the probability of collision with molecules of air. spheric pressure. As the altitude increases, the pressure
Vacuum pumps are used by industry and laboratories to cre- decreases because fewer gas molecules press on any surface.
ate a vacuum environment for these operations. Most gases A knowledge of how the pressure changes with altitude is
react with solids to cause effects such as oxidation, which it very important in connection with various space studies.
may be necessary to avoid. In a vacuum environment, the Table 10 shows the relationship between pressure and alti-
necessary operation may be performed so that undesirable tude in the earth's atmosphere.
effects are either reduced or completely eliminated. For
example, unless most of the air is removed from an incan- Specifying Gas Flow Rates
descent light bulb, oxygen in its atmosphere will react with
the hot tungsten filament, causing it to bum out prema- The flow rate of liquids is expressed simply as volume
turely. An electron tube could not operate at atmospheric units per unit time, such as liters per second. When, however,
pressure. Electron flow would be impeded by collision with the flowrate of gases is considered, it is necessary to know not
air molecules due to the extremely small mean free path. In only the volume of a gas but its pressure and temperature as
addition, elements within the tube may react with the air. well. A cubic meter volume of gas at 100 kPa pressure and a
Other examples can be cited where vacuum is necessary to temperature of 20 °C (68 °F) will contain ten times as many
produce desired results that could be unattainable in any molecules as a cubic meter volume of gas at 10 kPa and 20 °C
other way. (68 °F). Only a complete statement of volume, displacement
rate, gas pressure and temperature can accurately describe
The use of vacuum is required in many industries and the total quantity of gas that flows per unit of time. In both
products. In addition to light bulbs and electron tubes, liquids and gases, it is mass flow that is of interest. For liquids
vacuum is employed in magnetrons, cathode ray tubes, of constant density, the mass rate of flow is directly propor-
semiconductor devices, solar cells, plating metals and plas- tional to volume flow rate. With gases, density varies both
tics, thin film deposition, lifting objects, plasma physics, with temperature and with pressure. Thus, for a given gas,
cryogenics, metallurgical processing, electron beam weld- volume displacement rate, pressure and temperature must be
ing, brazing, distillation, organic chemistry, packaging, mass known to define the mass flow rate.

LEAK TESTING I SS

TABLE 1 0. Change in atmospheric pressure with Typical units of pumping speed S would be cubic meter
altitude" per second (m3-s-1) or cubic feet per second (ft3,s-1). Consis-
tent with SI, m3-s-1 and its submultiples (such as dm3·s-1 =
Altitude Pressure L-s-1) are used in the Nondestructive Testing Handbook.
kilometers (miles)
pascal flb,·in.-2) Concepts of Throughput and Leakage Rate

Qb (O)b I .01325 x I 05 (14.70) In vacuum practice, the preferred description of the rate
8.99 x I 04 (13.04) of flow of gas is commonly called throughput. Throughput is
I (0.62) 7.95 x I 04 (11.53) the quantity of gas, or a measure of the total number of
2 (1.24) 5.40 x I 04 molecules at a specified temperature, passing an open sec-
5 (3.1 I) 2.65 x I 04 (7.83) tion of the vacuum system per unit time. Leakage rate is a
JO (6.22) 5.53 x 103 (3.84) similar measure of the total number of molecules at a speci-
20 (12.44) 79.8 fied temperature passing through a leak per unit time.
soc f3 I .09)c 3.2 x I 0-2 f0.80)
8.5 x 10-5 f I . I 57 x I 0-2) Q is the symbol commonly used for throughput of gas in
100 (62.18) 3 x I 0-7 (4.64 x J0-6) unit time. Throughput Q can be expressed by Eq. 3:
2QQd (I 24.36)ct 7.5 x I 0-9
I 1.23 x I Q-8)
500 (310.89)
1,000 (621 .79) (4.35 x 10-11)

fl.088x 10-12)

a. SOURCE:U.S. STANDARDATMOSPHERE 1976. NOAASIT 761562. Q = ­PtV­ (Pa ·m3 ·s-1)
b. INTERNATIONAL STANDARD.
c. JET LINER ALTITUDE.

d. LOW ORBIT.

(Eq. 3)

Concepts of Gas Quantity and Pumping Speed By combining Eq. 2 and 3, the product of pumping speed S
and gas pressure P can be equated to throughput by Eq. 4:
From the gas laws, it is known that the product PV of
pressure P and volume V is proportional to the number of (Eq. 4)
molecules in a sample of gas. In static systems, the PV prod-
uct is constant at a given temperature. This product PV is Equation 4 is the universal relationship on which vacuum
known as the quantity of gas. Common units of gas quantity pumping throughput calculations are based.
include:
Leak Testing of Vacuum Systems
1. pascal cubic meter (Pa-rn"), with Mass Spectrometer Leak
2. millimeter of mercury liters (um Hg·L), or torr liters Detector Techniques

(torr-L), Mass Spectrometer Leak Detectors on High
Vacuum Systems
3. micrometer of mercury liters (µm Hg·L);
4. atmospheric cubic centimeter (cm3 of volume at The mass spectrometer leak detector (MSLD) provides
almost ideal leak detection characteristics, including very
standard sea level atmospheric pressure or std cm3); high sensitivity and a basically rapid response. Well engi-
and neered and highly developed versions of the mass spectrom-
5. millibar liter (mbar-L). eter leak detector are commercially available. Two broad
categories are available, the helium mass spectrometer leak
The preferred SI unit of gas quantity is pascal cubic meter. detector and the residual gas analyzer. The helium mass
In steady flow, the same quantity of gas (number of spectrometer leak detector (usually referred to simply as a
helium leak detector) is adjusted to respond only to helium
molecules) that enters one end of a tube must leave at the gas (atomic mass = 4). The residual gas analyzer (RCA), on
other end, even though there may be different volumes of gas the other hand, can easily be adjusted to respond to any gas
entering and leaving per unit time. If the PV product is used in a fairly wide mass range. The residual gas analyzer is
as a measure of the amount of gas flowing through a tube, used primarilyto determine composition of gas in a vacuum
computation may be done with a minimum of complication.

The volumetric pumping speed S is the time rate of vol-
ume displacement, as given by Eq. 2: Volumetric pumping
speed,

sv (Eq. 2)

56 I NONDESTRUCTIVETESTING OVERVIEW

system. However, on small systems it can readily be used as a source of the spectrometer. By maintaining a known con-
leak detector with any gas in its mass range. Since the helium stant pumping speed in the unit and using a calibrated
leak detector is a special purpose instrument, it is more ver- helium leak, the proportionality constant can be deter-
satile, more convenient and usually more sensitive for leak mined. A single stage (ordinary) helium mass spectrometry
detection than the residual gas analyzer. Although several leak detector can detect partial pressures of about 1 nPa
types of mass spectrometer are used in these devices, the (10-11 torr). The minimum detectable leakage rate is
simple magnetic analyzer is most common by far.
defined to be that leakage rate that produces an output sig-
By choosing the suitable magnetic field strength and nal twice as large as any noise signal present in the detector.
acceleration voltage, the mass spectrometer can be tuned The minimum detectable leakage rate can be specified
to any mass of gaseous particle. Hence, any gas could be either for air or helium. The helium leakage rate is 2. 7 times
used as a tracer gas for leak detection. Helium has been the air leakage rate. For a single stage instrument, the mini-
chosen for the following reasons. It is present in the atmo- mum detectable leakage rate for air is about
sphere at a concentration of about 1 part in 200,000 7 x 10-11 Pa,m3,s-1 (7 x 10-10 std cm3,s-1). For a two stage
(5 µg,g-1). Thus, air leaks cause very little helium back-
ground in the detector. Helium is inert and readily avail- helium mass spectrometry leak detector commercially avail-
able in most countries. Because it is the lightest gas except able, the manufacturer claims that the minimum detectable
hydrogen, helium's diffusion and molecular flow rates are leakage rate for air is 5 x 10-13 Pa,m3,s-1 (5 x 10-12 std
the highest available with a nonhazardous gas. These prop- cm3,s-1 or 4 x 10-12 torr-Ls"). In order to achieve such high
erties are highly desirable in a tracer gas. sensitivities consistently, the helium leak detector must be
carefully maintained and used. It is good practice to check
Sensitivity of Helium Mass Spectrometer Leak the sensitivity with a calibrated standard leak before each
Detectors use (or at the byginning of each period of use) and to tune
the instrument for best sensitivity. This can be done quickly
The electrical output signal from a helium leak detector and easily and is a guarantee against the lost time and frus-
is proportional to the partial pressure of helium in the ion tration that can result from unknowingly using a poorly
operating detector.

LEAK TESTING I 57

PART 7

BUBBLE LEAK TESTING

Introductionto Bubble Techniques 3. The foam application technique is used for detection
of large leaks in which the applied liquid forms thick
Principles of Bubble Testing for Leaks suds· or foam. When large leaks are encountered, the
rapid escape of gas blows a hole through the foam
In leak testing by the bubble method, a gas pressure dif- blanket, revealing the leak location.
ferential is first established across a pressure boundary to be
tested. A test liquid is then placed in contact with the lower Inspection Liquids Used for Immersion Bubble
pressure side of the pressure boundary. (This sequence pre- Testing
vents the entry and clogging of leaks by the test liquid.) Gas
leakage through the pressure boundary can then be Typical bubble test liquids used in immersion leak tests
detected by observation of bubbles formed in the detection in industry include the following:
liquid at the exit points of leakage through the pressure
boundary. This method provides immediate indications of 1. water treated with a liquid wetting agent to reduce
the existence and location of large leaks ( 10-3 to surface tension and promote frequency of bubble
10-5 Pa·m3·s-1 ). Somewhat longer inspection time periods emissions (certain solid wetting agents are also very ef-
may be needed for detection of small leaks ( lQ-5 to fective in smallweight percentages, with water baths);
10---6 Pa-m+s+) whose bubble indications form slowly.
2. ethylene glycol (technical grade) undiluted;
In bubble tests, the probing medium is the gas that flows 3. mineral oil, with which degreasing of test specimens
through the leak due to the pressure differential. The test
indication is the formation of visible bubbles in the detec- following immersion leak tests may be necessary (if
tion liquid at the exit point of the leak. Rate of bubble for- mineral oil having a kinematic viscosity of 37.7 x 10---6
mation, size of bubbles formed and rate of growth in size of to 41.1 x 10---6 m2·s-1 (37.7 to 41.1 centistoke) at 25 °C
individual bubbles provide means for estimating the size of (77 °F) is used as the test liquid, it will meet material
leaks (the rate of gas flow through leaks). requirements of MIL-STD-202 Method 112A or its
successor; mineral oil is the most suitable test liquid for
Classification of Bubble Methods of Leak Testing by the vacuum technique of immersion bubble testing);
Use of Test Liquids 4. fluorocarbons of glycerine (fluorocarbons are not rec-
ommended for stainless steel or materials for nuclear
Bubble techniques for detecting or locating leaks can be applications; glycerine is a relativelypoor detection liq-
divided into three major classificationsrelated to the method uid with low sensitivityto bubble emissions); and
of using the test liquid. 5. silicone oil having kinematic viscosity of 20 x lQ---6
m2·s-1 (20 centistokes) at 25 °C (77 °F). This liquid
1. In the liquid immersion technique, the pressurized will meet the requirements of MIL-STD-202 Meth-
test object or system is submerged in the test liquid. od 112A or its successor for electronic components.
Bubbles then form at the exit point of gas leakage and However, silicone oil should not be used for leak test-
tend to rise toward the surface of the immersion bath. ing of parts to be subsequently painted or in welding
operations without special cleaning processes.
2. In the liquid film application technique, a thin layer of
test liquid is flowed over the low pressure surface of Bubble Testing by Liquid Film
the test object. An example of this solution film leak ApplicationTechnique
test is the well known soap bubble technique used by
plumbers to detect gas leaks. Films of detection liquid Technique of Liquid Film Application (Solution Film)
can be readily applied to many components and struc- Bubble Testing for Leaks
tures that cannot be conveniently immersed in a
d~tection liquid. For detection of small leaks, this liq- The liq~i~ film application technique of leak testing by
uid should form a thin, continuous, wetted film cover- bubble enussion can be used for any test specimen on which
ing all areas to be examined. a pressure differential can be created across the (wall) area

58 I NONDESTRUCTIVETESTING OVERVIEW

to be examined. An example of this technique is the applica- Ordinary household soap or detergents should not be used
tion ofleak test solutions to pressurized pipe line joints. This as substitutes for specified bubble testing solutions for criti-
test, also known as a solution film test, is most useful on pip- cal applications. The number of bubbles contained in the
ing systems, pressure vessels, tanks, spheres, compressors, solution during application should be minimized to reduce
pumps, or other large apparatus with which the immersion the problem of discriminating between leakage bubbles and
technique is impractical. Test liquid is applied to the low bubbles caused by the solution. In principle, a bubble will
pressure side of the test object area to be examined so that form only when there is leakage present. No liquid should
joints are completely covered with a film of bubble forming be used which is detrimental to the component being tested
liquid. The surface area is then examined for bubbles in the or other components in a system.
solution film.
Bubble Testing by the Vacuum Box
In no case should the test pressure exceed the specified Technique
maximum allowable working pressure for which the test
object has been designed unless analysis demonstrates that Vacuum box bubble leak testing provides for the detec-
higher pressures will not damage or endanger test personnel. tion of through-thickness discontinuities in welds and pres-
The area to be inspected should be positioned where possi- sure boundaries of systems containing air at atmospheric
ble so as to allow test liquid to lie on the surface without drip- pressure. It is used during construction to test pressure
ping off. Where necessary, it is allowable to position the test boundary welds of incomplete systems that cannot be pres-
surface so that inspection liquid flows off the test area, pro- surized. It is also used to test pressure boundary welds that
vided that a continuous film remains over the test area. are inaccessible for leak testing when the entire system is
All-position testing may be performed on large pressure ves- pressurized. It may also be used to create a pressure differ-
sels, weldments, tanks, spheres, compressors, pumps and ential for increasing the sensitivity of penetrant leak testing
other large apparatus. When one or more bubbles originate, techniques. Typical discontinuities detectable by this
grow, or release from a single point on the test object sur- method are through-thickness cracks, pores and lack of
face, this bubble formation should be interpreted as leakage. fusion. A bubble forming solution is applied to the surface
The point at which bubbles form should be interpreted as to be examined. A vacuum box with a viewing window large
the origin of leakage (the exit point of a physical leak). Usu- enough to view the complete area and to allow sufficient
ally, any component that does not show evidence of leakage light to enter the box for proper examination is placed over
is evaluated as acceptable. Leakage is cause for rejection of the test surface and then evacuated. A calibrated pressure
the test part except as specificallypermitted by the test spec- gage is placed in the vacuum box system to verify the
ifications. Where the leak is repairable in accordance with required pressure differential under test. The surface area
specifications, the component may be repaired and rein- visible through the vacuum box window is then viewed for
spected in accordance with the original leak testing accep- evidence of through thickness discontinuities by the forma-
tance procedures. After testing, any liquid or gas which is tion of bubbles on the surface. Through-thickness disconti-
detrimental to the test object should be thoroughly removed. nuities are indicated by the formation of a continuous chain
of bubbles in the film solution. Through-thickness indica-
Selection and Application of Bubble Forming tions are usually considered to be unacceptable and such
Solution Films welds should be repaired and retested. The formation of
single small bubbles may or may not be considered relevant,
The bubble forming solution used with the liquid appli- depending upon the type of test object and its intended
cation method of bubble emission leak testing should pro- applications.
duce a film that does not break away from the area to be
tested. The solution film should produce bubbles that do
not break rapidly due to air drying or low surface tension.

LEAK TESTING I 59

PARTS

HELIUM MASS SPECTROMETER LEAK
TESTING

Basic Techniques for Leak The basic techniques for helium leak detection are as
Detection with Helium Tracer Gas follows.

All techniques of leak detection using a mass spectrome- 1. In the helium detector probe method of Fig. 9, the
ter leak detector entail passage of a tracer gas through a pre- test object or system is pressurized internally with
sumed leak from one side to the other side of a pressure helium or a gas mixture containing helium. The mass
boundary and subsequent detection of tracer gas on the spectrometer leak detector is connected to the hose
lower pressure side. Figures 9 to 13 show some typical basic of a scanning probe that samples gas leaking from the
methods for leak testing with helium tracer gases. However, external surface into the surrounding atmosphere.
for each practical application, there is usually one helium This method is used to detect and determine leak
leak testing technique that gives optimum results. Factors to locations.
be considered when selecting helium leak test techniques
include: 2. In the helium tracer probe method of Fig. 10, the
mass spectrometer leak detector is connected to the
1. size, shape and location of the equipment to be internal volume of an evacuated test object (such as a
tested; vessel or piping system) while a helium spray tracer
probe is moved over the external surface to detect
2. choice between pressure, vacuum or both for and determine the specific locations of leaks.
testing;
FIGURE 1 0. Helium leak testingof evacuated
3. maximum leakage rate specified or that can be vessel or system with tracerprobe (for
tolerated; calibration,snifferprobe is moved past orifice
of standardleak at same speed and distance as
4. degree of automatic leak testing operation required; used in scanningsurface of system being leak
and tested)

5. number of parts or complexity of the system to be HELIUM TRACER PROBE
tested.
SYSTEM VALVE AUXILIARY
FIGURE 9. Helium leak testingof pressurized UNDER TEST PUMP
vessel or system with samplingprobe
(EVACUATED)
STANDARD
LEAK

SNIFFER OR
SAMPLING PROBE

tt

OPTIONAL VALVE lj

DIFFUSION OR I ~~ ~ ~
TURBOMOLECULAR l.
SYSTEM UNDER ...J
HELIUM PRESSURE
HELIUM PUMP OPTIONAL
LEAK ~ VALVE HELIUM
:::, LEAK
DETECTOR :::::;
DETECTOR
lJJ
I

FROM DU PONT INSTRUMENTS. REPRINTEDWITH PERMISSION. FROM DU PONT INSTRUMENTS. REPRINTEDWITH PERMISSION.

60 I NONDESTRUCTIVETESTING OVERVIEW

3. When vacuum leak testing by the hood method as FIGURE 12. Leak testingof sealed components
shown in Fig. 11, the mass spectrometer leak detec- internallypressurizedwith heliumtracer gas
tor is connected to the evacuated interior of the sys- and enclosedin a bell jar
tem under test. The test object or system is then
placed under a hood or within a chamber containing HELIUM LEAK
helium gas or an air helium mixture usually at atmo- DETECTOR
spheric pressure. This method can be used to deter-
mine the total leakage rate of the system. However, it FROM DU PONTINSTRUMENTS. REPRINTEDWITHPERMISSION.
cannot be used to determine the specific locations of
leaks. 6. In the dynamic method of leak testing of small sys-

4. In the belljar test method of Fig. 12, sealed compo- tems, the vacuum pumping system is throttled to
nents filled with helium or a gas mixture containing reduce the pumping speed so that greater helium
helium are placed in an evacuated testing chamber. tracer gas concentrations are attained in the mass
The mass spectrometer connected to this vacuum spectrometer detector. The dynamic method also
chamber detects helium leaking from any part of the permits detection of variation in leakage flow rates.
surfaces of the sealed test objects in the vacuum
chamber. This test does not permit location of leaks
on the test object surfaces.

5. In the accumulation method ofleak testing of Fig. 13,
leaking helium tracer gas is allowed to collect for a
period of time before being sampled by the leak
detector. This technique, also employed in parts per
million testing, can be adapted to several different
leak testing situations. The accumulation method
does not usually permit leak location. However, by
sealing off small surface areas and accumulating
tracer gas within the sealed volume, areas of leakage
can be localized.

FIGURE 1 1 . Hood methodof leak testingof
evacuated componentsinsertedinto hood or
envelopecontainingheliumatmosphere

SYSTEM UNDER FIGURE 13. Calibrationof microgramsper
TEST gram heliumaccumulationout-leakagetest:
arrangementfor detectingknown leakagefrom
(EVACUATED) heliumstandardleak with snifferprobe

HOOD CONTAINING HELIUM STANDARD LEAK
HELIUM AND AIR
MIXTURE I
~
SAMPLE PROBE
~
AUXILIARY HELIUM
::=:=:=:=:=:=:=:::::;--;::::::::itx~ MECHANICAL LEAK

PUMP DETECTOR

AUXILIARY OPTIONAL

DIFFUSION VALVE

OR AUXILIARY
ROUGHING
TURBOMOLECULAR HELIUM LEAK PUMP
DETECTOR
PUMP

ACCUMULATION
CHAMBER

FROM DU PONTINSTRUMENTS. REPRINTEDWITH PERMISSION.

LEAK TESTING I 61

PART9

ACOUSTIC LEAK TESTING

Principles of Acoustic Leak Testing same terminology may be applied to fluid leaks on or above
mechanical structures.

Principles of Acoustic Leak Detection Spectra/ Characteristics of Sounds Generated by
Fluid Leakage
Acoustic emission test techniques can be applied to
locate leaks in pressurized systems. Fluid leaks generate From observation of various spectra of sounds generated
sound waves when the fluid flow through the leak is accom- by fluid flow, both within boundaries and from orifices, it is
panied by turbulence, cavitation, or high-velocity flow. evident that vibration frequencies around 40 kHz are most
These sonic disturbances can be transmitted through the easily excited. Similar measurements of spectra of sounds
medium of the pressurizing fluid, through the containment generated by nature, both on land and beneath the water,
structure, or through the atmosphere surrounding the leak show spectra between 30 and 100 kHz. The acoustics gener-
location. Airborne vibrations can be detected at a distance ated by leaks may have a generating mechanism composed
from their source with directional microphones or acoustic of a form of a Culton whistle or Helmoltz resonator, in
probes. Leak detection and location from a distance through which case the sound may be confined to one or a few spec-
air or other fluids involves remote scanning of suspected tral lines. In other cases, the leak acoustics may be excep-
leak areas with a directional probe and coordinating direc- tionally broadbanded throughout the spectrum below
tion of the source of the characteristic hissing sound of a 100 kHz.
leak with the relative sound intensity. Certain precautions
must be observed if the sound source or leak location is to Factors Influencing Detectability of Leaks by
be reliably determined. These involve ( 1) avoidance of Acoustic Emission
sound path blocks or sound absorbing material that create
sound shadows between the leak and the acoustic sensor The ability to detect leaks acoustically depends upon the
and (2) recognition of possible sound reflectors such as flat, physical mechanisms of fluid flow in the leak, the sensitivity
hard surfaces which provide sound echoes from directions and selectivity of the detection instruments and to what
other than that of the original leak source. degree the leak is isolated from the detection sensor. A con-
duit may pass a liquid internally without an apparent leak-
Classification of Fluid Leaks in Terms of Their age of liquid, but a coupling may allow air, or any gas
Acoustic Emissions present, to penetrate into the fluid through a leak orifice in
the coupling. Such leaks are frequently described as viscos-
Leaks may be classified as internal or external to the ity dependent leaks. The high velocity, low pressure liquid
structure, but in either case they are undesired events. flow creates a condition that permits a low velocity, high
Leaks may be classified further as acoustically passive or pressure gas to be drawn into the leak orifice. Under these
active leaks. Active leaks emit sound generated by turbulent conditions, it is unlikely that acoustic sensors would detect
leakage flow. Passive leaks are those that emit no acoustic the location of the leak. However, under some conditions,
signals related to leakage flow. Passive leaks can sometimes ultrasonic detectors may detect the presence of gas entrap-
be detected with an artificial internal sound source trans- ment in the liquid from the resulting cavitation or turbu-
mitting signals through the leakage path to an external lence noise.
detector. However, leak detection depends upon the sensi-
tivity and selectivity of the vibration monitoring instru- Instrumentation for Ultrasonic
ments. For example, a leaking heart valve may be defined as Detection of Leaks
an internal leak within the human body. It would be identi-
fied as an acoustically active leak if the doctor is able to hear The components of the basic instrumentation used for
the leakage flow with his stethoscope. However, if the ultrasonic detection of leaks are analogous, in many ways, to
detection of this leak is limited to the use of tracer chemistry those of standard radio receivers used as direction finders.
and fluoroscopy and it cannot be detected with the stetho-
scope, the leak may be classified as acoustically passive. The

62 I NONDESTRUCTIVETESTING OVERVIEW

With the airborne signal ultrasonic detector, it is usually frequency generation and are usually enriched by many dif-
necessary to aim the directional sonic receiver toward the ferent modes of acoustic propagation within the walls of the
source of high frequencysound from the leak to obtain max- leaking structures. Within the metal walls, the ultrasonic
imum signal intensity. This operation is quite analogous to transmission modes typically include longitudinal waves,
that of turning a radio direction finder receiving antenna in shear (transverse) waves, Rayleigh waves, Lamb waves,
such a direction as to intercept the strongest signal from the interfacial waves and perhaps others. Multiple contact sens-
broadcast station or other signal source. ing transducers can be attached (or coupled by metallic
extension rods) to the metal walls to provide a leakage moni-
The airborne ultrasonic detector or contact probe toring system that may be temporarily emplaced or perma-
receives high frequency ultrasonic mechanical vibrations nently installed to detect the occurrence and location of
generated by leaks and converts these leak signals to high points of turbulent leakage. Automatic equipment, as well as
frequency electrical oscillations. These electrical signals are simple portable test equipment, can be used to analyze the
then amplified at their high ultrasonic frequencyrange of 35 leakage signals and locate the points where leakage occurs.
to 45 kHz, which results in an amplitude modulated signal
with a central or carrier frequency near 40 kHz. This stage Factors Influencing Feasibility of Acoustic Emission
corresponds to the radiofrequency amplifier stage of the Leakage Monitoring
conventional amplitude modulated radio receiver. The
amplifier high frequency input signal is then mixed with a For applications of acoustic emission leakage monitoring
frequency derived from an internal oscillator to provide a to large structures in industrial facilities, it is important to
difference frequency signal in the audiofrequency range. optimize the monitoring system to allow detection of small
This stage corresponds to the quite similar function of the
conventional radio receiver (omitting the intermediate fre- leaks that may be located at considerable distances, e.g.,
quency amplifier state sometimes used in radio receivers).
Finally, the audiofrequency signal for both radio receiver 100 m (330 ft) or more, from the acoustic emission sensors.
and ultrasonic leak detector is amplified and reproduced on This signal transmission efficiency factor becomes particu-
a loudspeaker or by headphones. This audible leak signal is larly important when acoustic emission monitoring is
interpreted by human listeners as the typical sounds of hiss- applied to buried pipeline. Holes must be dug at appropri-
ing leaks, vibrating objects, or voice signals, as the case may ate intervals to allow access for installation of contact sen-
be. In other words, the typical ultrasonic leak detector is sors or for installation of acoustic waveguides to transmit
only slightly different from the typical radio receiver, in that signals to the ground surface above the pipeline. The effec-
the original frequency range is ultrasonic (near 40 kHz) tiveness of acoustic emission leak detection and monitoring
rather than in the conventional amplitude modulated broad- systems is dependent upon the following factors:
cast frequency range (550 to 1,400 kHz).
1. the amplitude of the leakage signal;
Techniques of Leakage Monitoring 2. the amplitude of environmental acoustic background
with Multiple Acoustic Emission
Sensors noise;
3. the efficiency of transmission of the acoustic leakage
Acoustic Emission Leakage Monitoring of Large
Structures and Pipelines signal through the structure to surface mounted
acoustic emission sensors; and
A special application of multiple acoustic emission sen- 4. the efficiency of the signal processing instrument in
sors is monitoring of structures, large vessels and pipelines detecting, identifying and displaying the presence
for leaks that emit sound because of turbulent leakage and nature of leakage signals.
through leak holes and passageways. Acoustic emission sig-
nals are produced by a complex acoustic stresswave interfa- The first three factors listed above depend critically
cial coupling between the metal pressure containment and upon configuration of the structures under test, their oper-
the fluid escaping through the leaks. The leak signatures, ating conditions and environmental noise and vibration con-
i.e., the leakage signal amplitudes and frequency spectra, ditions, which are fixed and cannot be readily changed for
depend on the type of gas or liquid escaping through the purposes of utilizing acoustic emission leakage monitoring
leak opening, the rate of leakage, the differential pressure systems.
driving fluid through the leak, the leak (hole) size and many
other factors. The acoustic leak signals have broad bands of Selection of Frequency Response of Acoustic
Emission Monitors

The frequency response setting of the leakage monitor is
a critical variable whose value can be selected by the test
personnel for each specific leakage monitoring application.
This frequency parameter is quite important since the leak

LEAK TESTING I 63

signal amplitude, the environmental noise amplitude and [PWR] piping with pressure to 15.2 MPa [2,200 lbr·in.-2]),
the acoustic emission transmission efficiency from leak to leakage will involve two-phase flow. The characteristics of
sensing detectors do vary considerably as the test frequency the flow and the simultaneous generation of acoustic signals
is changed. The amplitudes of the leakage signals and their are extremely complex and difficult to model. Relatively sim-
frequency content (which is usually broad-band) are impor- ple two-phase flow models that appear at least qualitatively
tant considerations in selecting the monitor response fre- correct have been developed by Henry5 and by Collier.6 The
quency characteristics. Relatively strong leakage signals models predict that flow rate increases as the temperature of
have frequencies that vary from a few kilohertz to as high as the fluid decreases. An increase in flowwith decreasing tem-
800 kHz on some occasions. Environmental noise levels perature was, in fact, observed experimentally for an inter-
usually decrease rapidly at higher frequencies. The effi- granular stress-corrosion crack (IGSCC) leak.
ciency of acoustic transmission within the structures almost
always increases with decreasing frequency. Conversely, the Figure 14 shows an increase in the acoustic signal when
acoustic signal attenuation (weakening per unit of signal flow rate is increased at constant temperature. Here, the
travel distance) becomes greater as the signal frequency acoustic root-mean-square (rms) signal increases with flow,
increases. The monitor frequency response band should be as is expected.
selected to optimize the leakage signal and minimize the
structural and ambient noise signals - to improve the leak- Dependence of Acoustic Signal on Leak Rate
age signal-to-noise ratio and permit discrimination of leaks
of various sizes and distances from the sensor locations. For constant flow rates, the acoustic signal decreases
with fluid temperature. However, the effect is not severe
Acoustic Characteristics of Leaks3·4 near BWR operating temperatures.

andApsreasrseusrueltr-o7f.6theMhPiagh[ -1te,m10p0erlabtru·rine.-2(]-)2o8f2w°aCter[-i5n4b0oi°lFin]g) FIGURE 1 5. Leak flow rate versustransducer
signalin the 300 to 400 kHz bandwidth from
water reactor (BRW) piping (or pressurized water reactor leaking intergranularstresscorrosioncracking,
thermaland mechanicalfatigue cracks,valve
FIGURE 14. Field inducedintergranularstress and flange
corrosioncrack at 268 to 269 °c (514 to
516 °F), with transduceron waveguide 1 m 100
(40 in.) from leak: (a) acousticroot mean INTERGRANULAR
square signal;(b) water flow rate STRESS
CORROSION
(aJ 240 CRACK
E°......J 200 '--
160 '- _
v~oi ~.~s_ 120 ._'-- ...._

80'------'----...._ _
0

0.008. 0. I .____.___.._._..._._._U,,L_---'---'-.J....J...J..J..J...U_..l-..J'---'-''-L-LI.....______.___,.__L...L...Lu..u
0.007
0.006 0.063 0.632 6.32 63.2 632.0
0.005 (0.001) (0.01) (0.1) (I .OJ 110.0)
0.004 .__
_,____ ..;..._:....,._ ...._ o- WATER FLOW RATE
158 473
315 I 3 cubic decimeter per second
_ (gallons per minute)
TIME
(seconds) 630 LEGEND
o = FATIGUECRACK

• = 50 mm 12 in.) VALVE WITH STEM LEAK

o = FLANGELEAK

64 I NONDESTRUCTIVE TESTING OVERVIEW

Figure 15 presents the data on transducer signal ampli- the valve and flange have smaller signal amplitudes than
tude in the 300-400 kHz band for a transducer-waveguide · those due to intergranular stress corrosion cracking.
placed 1 m (3 ft) from the leak Fluid temperature was
274 °C (525 °F); the pressure was 7.4 MPa (1,070 lbr·in.-2). A similar result was found for fatig~e cracks relative to
Flow rates through the leak were measured by means of a intergranular stress-corrosion cracks. This result supports
turbine flow meter. The data were normalized so that the argument that, at least for water flow rates less than
results obtained from the various experiments (with the 63 cm3-s-1 (1 gal-min-1), intergranular stress-corrosion crack
same electronics and acoustic sensor) could be compared.
Acoustic signals from leaks with flow rates ranging from leaks may be distinguished from other leaks by comparing
0.13 to 538 cm3,s-1 (0.002 to 8.5 gal-min-1) have been ana- the ratios of acoustic signal intensity in low frequency ( 100
to 200 kHz) and high frequency (300 to 400 kHz) windows.
lyzed. Leaks from intergranular stress corrosion cracks, The actual ratio of signals will depend on the system and the
thermal and mechanical fatigue cracks, valves and flanges data must be corrected for frequency dependence of the
have been studied. The data presented in Fig. 15 compare attenuation. For the Argonne National Laboratory (ANL)
these leak sources. The general size of a leak can be esti- system, with the acoustic emission sensor on a waveguide,
mated from the rms signal in the 300 to 400 kHz range if the the ratio of rms (100 to 200 kHz)/rms (300 to 400 kHz) is '.5:2
distance to the leak is known. Further, for the same slow
rate, if the flow rate is less than 0.032 L-s-1 (0.5 gal-min-1), for an intergranular stress corrosion crack and ~2 for other
leak types with the signals corrected to a distance of 1 m
(3 ft) from the leak source.

LEAK TESTING I 65

PART 10

LEAK TESTING OF STORAGE TANKS

Detectionof External Leaksin devices. One aspect of the current Environmental Protec-
UndergroundStorage Tanks8 tion Agency (EPA) program is to develop easily performed
benchmark test procedures that can be used to evaluate the
Underground storage systems comprised of tank, piping performance of external petroleum hydrocarbon leak and
and associated components that contain petroleum hydro- release detectors. Performance test results for external
carbons (e.g., gasoline) or other hazardous materials repre- detectors in actual field use may differ from performance
sent a potential source of environmental contamination. measured by these benchmark tests.
Proper system design, installation, operation and mainte-
nance along with a leak detection program, can minimize Leak Testing of Aboveground
the detrimental effects of leaking underground storage Storage Tankswith Double Flat
tanks and piping. Devices capable of detecting petroleum Bottoms8
hydrocarbons lost from underground storage tank (UST)
systems can be used inside an underground storage tank sys- Leak Location Test Techniques
tem, i.e., in-tank, or external to an underground storage tank
system, i.e., out-of-tank. In-tank underground storage tank To comply with the Environmental Protection Agency
leak detectors generally detect losses with liquid level sen- (EPA) regulations and many state, county and local agency
sors. Out-of-tank underground storage tank detection sys- regulations, some aboveground storage tank (AST) owners
tems measure the presence of liquid phase or vapor phase have made design and leak testing (LT) requirement
hydrocarbons. Early detection of liquid phase or vapor changes in their aboveground storage tank flat bottom con-
phase petroleum hydrocarbons allows leaking underground struction specifications. Leak location techniques include:
storage tank systems or components to be removed from
service and repaired or replaced, thereby minimizing both 1. vacuum box bubble testing using soap solution,
environmental impairment, financial liability and economic commercial leak detector solution, linseed oil, or
loss of product. other suitable solution;

Historically, external petroleum hydrocarbon leak and 2. vacuum box liquid penetrant testing;
release detection devices have not been extensively used 3. vacuum box penetrant developer testing;
and, therefore, are used primarily in conjunction with new 4. ammonia tracer gas with ammonia sensitive paint;
underground storage tank installations. However, most 5. ammonia tracer gas with ammonia sensitive tape;
external leak and release detection systems can be 6. detector probe (sniffer) tracer testing using R-12 or
retrofitted at existing facilities. Because existing under-
ground storage tank facilities are potentially at greater risk R-22 halogen rich tracer with a halogen diode leak
of failure due to age, it is extremely important that leak and detector;
release detection devices be applied to these installations. 7. detector probe (sniffer) tracer testing using SF6 halo-
gen-rich tracer with an electron capture halogen leak
There are numerous commercially available external detector; and
leak and release detection devices designed exclusively for 8. detector probe (sniffer) tracer testing using helium
use with underground storage tank petroleum hydrocarbon with a helium mass spectrometer leak detector.
systems; however, there are no established industry wide
performance specifications or procedures for assessing the Disadvantages of Leak Location Test Methods
capabilities of these devices. The United States Environ-
mental Protection Agency (EPA) has been implementing an With the exception of the tracer gas tests, all of the listed
underground storage tank program and doing research on leak location tests have been used for many years to one
external and internal leak detection devices for under- degree or another on these structures. However, no leak
ground storage tanks. The Environmental Protection location test enables the test technician to determine the
Agency's Environmental Monitoring Systems Laboratory
has been participating in research on external leak detection

66 I NONDESTRUCTIVETESTING OVERVIEW

total leakage rate for a test system. Consequently, when a Double Bottom Designs
leak location test is completed, there cannot be total confi-
dence that all unacceptable leaks were detected. One practice to attempt to achieve quantitative bottom
leak testing results when constructing new or reconstructing
Quantitative (Volumetric) Test Techniques existing aboveground storage tanks with flat bottoms is to
specify a design that requires the installation of two bottoms.
Purchasers may specify aboveground storage tanks with
a double flat bottom design and quantitative leak test tech- The function of the inner bottom is to contain the stored
niques. Quantitative leak test techniques are intended to product with no unacceptable or objectionable leakage. A
assure purchasers that all unacceptable leaks have been function of the outer bottom would be to provide a closed
detected and repaired. These test techniques include: test system which could either be:
(1) pressure rise measurement; (2) pressure loss measure-
ment; and (3) constant pressure mass flow measurement. 1. pressurized with a tracer gas to a very low pressure
for a semi-quantitative detector probe (sniffer) test;
Applicable Design Standards
2. pressurized to a very low pressure for a quantitative
For many years the API 650, Standard for Welded Steel pressure loss measurement test;
Tanksfor Oil Storage has required either an air pressure test
or a 13.8 kPa gage (2 lbr·in.-2) pressure differential vacuum 3. partially evacuated for a quantitative pressure rise
box test. measurement test; and

Soap film, linseed oil, or other suitable leak detector 4. pressurized to a very low specific pressure and held at
solution is specified for leak testing all bottom lap or butt that pressure for the purpose of a mass inflow mea-
welds and the shell to bottom comer weld of this design of surement quantitative test.
aboveground storage tanks. Similarly, the API 620, Stan­
dard for Design and Construction of Large, Welded, Another functi~n of the outer bottom could be to use it
Low­Pressure Storage Tanks requires a 20.7 kPa gage as a catch basin to monitor for inservice leakage from the
(3 lbr·in.-2) vacuum box solution film test of all joints inner bottom. This function may be in addition to its use for
between flat bottom plates of aboveground storage tanks of a quantitative leak test or it may be its primary function.
this design.
Comparative Test Sensitivities of Leak Location
In May 1992 Appendix I on Underground Leak Detec­ Techniques
tion and Subgrade Protection was issued as an addendum to
API 650. It contains cross sections of typical arrangements Vacuum box bubble testing (VBBT) under field condi-
for leak detection at the tank perimeter on double flat bot- tions can produce an adequate test sensitivity of io-' to
tom or flexible membrane liner designs. It refers to API 10-4 Pa,m3-s-1 (10-2 to 10-3 std cm3,s-1) at a reasonable cost.
Recommended Practice 651 for guidelines on the use of With extra care 10-5 Pa,m3·s-1 (10-4 std cm3,s-1) range leak-
cathodic protection methods. It also refers to API Recom­ age size can be detected under field conditions, but to detect
mended Practice 652 on the use of linings to prevent inter- this smaller, less common leakage requires the expenditure
nal bottom corrosion. of additional time and money. This is not a highly technical
test method and thus requires a minimal amount of operator
The API 653, Standard for Tank Inspection, Repair, training. It also can be performed during construction of the
Alteration and Reconstruction was issued and was amended aboveground storage tank, saving time on the schedule
with Supplement 1. This standard, which covers tanks built because it does not require a closed test system that is to be
to API Standard 650 and its predecessor 12C, requires pressurized. For these reasons this is the test method that
either a vacuum box solution film test or a tracer gas test of has been most commonly used by contractors building
all flat bottom weld joints. It requires a vacuum box solution aboveground storage tanks. For that reason it is the test tech-
film test or a light diesel oil test of the shell-to-bottom cor- nique against which all others listed below are compared.
ner weld joint. No pressure differential is listed in this stan-
dard for the vacuum box test. Item C.2.3.i of the "Tank Vacuum box liquid penetrant testing (VBPT) of bottom
Out-of-Service Inspection Checklist" (Table C-2) in API lap or butt welds is performed by applying liquid penetrant
653 simply states "Vacuum test the bottom lap welds." to the test surface, removing the excess after the penetration
time has elapsed, applying the developer and then applying a
API 575, Standard on Inspection of Atmospheric and differential pressure with the vacuum box. This is a variation
Low Pressure Storage Tanks is to be issued as the guideline of vacuum box bubble testing that is normally only used in
for the Aboveground Storage Tank Inspector Certification situations where very small leakage is known to exist but has
Program. It will be based on the API 653 standard. escaped detection by the normal vacuum box test technique
or liquid penetrant test method used independently of each
other. Under field conditions the achievable sensitivity of

LEAK TESTING I 6 7

this test method is in the range of lo--4 to 10--5 Pa-m3-s-1 (10-3 mixture throughout the test system and the scanning speed
to 10-4 std cm3-s-1). However, compared to vacuum box bub- and the distance the sniffer is held from the test surface dur-
ing scanning (sniffing).
ble testing, it costs considerably more and requires more
background and experience to determine when the situation When performing a nonquantitative (semiquantitative at
warrants this approach. best) detector probe (sniffer) test of the flat bottom of an
aboveground storage tank, the amount of pressure that can
Vacuum box penetrant developer testing (VBDT) is a be applied (either single or double bottom) is limited to
special leak test technique that is normally only used for lap slightly higher than the weight of the bottom being pressur-
and butt welds in single bottoms. It is applied when leakage ized. This limitation is the ballooning of the bottom when
has been detected during the tank water (hydro) test and it the pressure exceeds the weight of the bottom.
is suspected that a normal vacuum box test would be inef-
fective because of the possibility of water laying against the As an example, 6.4 mm (0.25 in.) thick steel weighs
underside of the tank bottom in the area of the leak. If no about 0.49 kPa (10.2 lbr·ft-2). Thus, for a 6.4 mm (0.25 in.)
indications are found, then use the vacuum box to pull mois- thick steel bottom, the bottom will start to balloon when the
ture through the developer, indicating the leak area. For this pressure reaches 10.2/144 = 0.49 kPa (0.0708 lbr·in.-2). This
test method the developer is applied to the suspected area is the same as (0.0708) 27.7 in. H20-lbf1·in.-2 = 51 mm
or areas and allowed to dry. It is then visuallyinspected after (1.93 in.) H20 pressure. Allowing an additional 13 mm
a number of hours has elapsed (maybe overnight) for signs (0.5 in.) H20 pressure for some amount of bottom balloon-
of moisture bleed out into the developer indicating the area
of the leak. If no indications are found, the vacuum box is =ing, the maximum test pressure of 64 mm (2.5 in.) H20
used to pull moisture through the developer, indicating the
leak area. This test technique is normally used to detect equals 2.5/27.7 0.69 kPa (O.l lb-m."),
gross leakage but has the capability under production condi- The reduction in differential pressure from 101 kPa

tions of enabling an operator to detect leaka~e as smal! as (14.7 lbr·in.-2, or 1 atm) to only 0.69 kPa (O.l lbrin.-2) pres-
the I0-4 Pa-rrrl-s ! (IO- std cm3-s-1) range. This too reqmres sure reduces the attainable test sensitivity of viscous or tran-
sitional flow by an approximate factor of 220.
more experience in order to determine the best course of
action for the various situations that develop. Dilution of leakage tracer gas by surrounding air at a leak
further reduces test sensitivity by an additional factor of at
Ammonia sensitive paint or tape testing (AMPT or least ten. The test sensitivity attainable would be further
AMTT) with an ammonia gas mixture under the bottom can reduced by at least another factor of ten based on a tracer
result in a test sensitivity that enables the detection of leaks gas mixture of ten percent by volume. For outer bottoms,
with leakage as small as the I0-4 to 10-5 Pa-m3-s-1 (I0-3 to this mixture is achieved by flowing the tracer under the bot-
tom for a period of time or injecting it through coupling at
I0-4 std cm3-s-1) range. However, it is rarely used because of various point in the bottom. The shortcoming is that the
the hazards to human life that ammonia presents to those uniformity of the tracer gas mixture is not known. For inner
doing the testing. It also costs considerably more to perform bottoms, this mixture is obtained uniformly between the
than the vacuum box bubble test method. Likewise, leakage bottoms by evacuating the space between the bottoms to a
in the 10-4 std cm3-s-1 range is not very common and for that pressure of 0.10 (14.7 + 0.1) - 0.1 = 1.48 - 0.1 = 1.38
reason does not justify the additional cost to detect. It is also (9.6 kPa [l.4 lbr·in.-2] in round numbers) below atmosphere
more technical in nature and requires additional safety before backfilling and pressurizing with the tracer gas to
training and more experience. 64 mm (2.5 in.) H20 pressure.

Halogen diode detector probe (sniffer) testing (HDLT) If the space isn't evacuated to 9.6 kPa (1.4 lbr·in.-2)
with refrigerant R-12 or R-22 as the tracer gas was used but below atmosphere before pressurizing, then the uniformity
there is no marked increase in the pressure attainable under of the tracer gas would not be known and the mixture would
the bottom or in the test sensitivity over that which is attain- only be 0.10(100)/14.8 = 0.68 percent by volume and the
able by vacuum box testing. The reasons for the second item test sensitivity attainable would be reduced by a factor of
are given in the following paragraphs. 100/0.68 = 147 instead of a factor of ten. For this discussion,
a ten percent mixture is assumed.
If all test parameters, such as differential pressure, were
equal, any of the tracer gas leak location methods would be Thus, for this leak location test method performed under
capable of detecting smaller leakage than the other less the conditions described, the total reduction in test sensitiv-
technical test methods such as vacuum box testing, ammo- ity from the maximum realistic attainable test sensitivity
nia sensitive tape or paint, etc. However, such is not the case would be by a factor of approximately 220(10)(10) = 22,000
since the test parameters are not the same for each method. or 2.2 x 104•
Regardless of the tracer gas used, in addition to instrument
sensitivity, detector probe (sniffer) test sensitivity is depen- The maximum realistic test sensitivity attainable under
dent on the differential test pressure, the percentage by vol- field conditions for either a halogen diode or electron cap-
ume mixture of tracer gas, the uniformity of that tracer gas ture type leak detector probe (sniffer) test performed using
a 100 percent tracer gas mixture at a differential pressure of

68 I NONDESTRUCTIVETESTING OVERVIEW

about 100 kPa gage (15 lbr·in.-2) with a scanning speed of test sensitivity. Response time is faster than with a conven-
12.7 mm-s" (0.5 in-s ") and a sniffer probe to surface dis- tional sniffer probe, but the longer the probe the longer the
tance of 3 mm (0.125 in.) is on the order of 5 x response time. This allows the operator to place the helium
10-8 Pa-m3-s-1 (5 x 10-7 std cm3-s-1). mass spectrometer at one location near an entry hole and
scan welds of the entire bottom from that one location. The
Based on these values, the estimated test sensitivity for probe hose is so small as to be virtually weightless, whereas
this test method when performed on the bottom welds of an a halogen diode or electron capture instrument which
aboveground flat bottom ·storage tank would be about (5 x weighs several pounds must be carried with the probe.
10-7) . (2.2 x 104) = 1 x 10-2 std cm3-s-1.
A disadvantage is the added technical training and experi-
This is approximately the same test sensitivity as the ence needed to perform helium mass spectrometer detector
more economical vacuum box bubble test technique but probe leak testing versus that needed to perform halogen
costs considerably more to perform. It also requires much diode or electron capture detector probe leak testing.
more technical training and experience, particularly if those
performing or witnessing this method of testing are to Another disadvantage of the helium mass spectrometer
understand the actual test sensitivity that is being obtained. is the greater cost. Depending on the degree of sophistica-
tion of the model purchased, the cost of the helium mass
The test sensitivity for this test method can be increased spectrometer and associated equipment may cost anywhere
by increasing the percent of the tracer gas mixture and by from three to six times more than the best halogen diode or
attaching the detector probe (sniffer) to a pod or box placed electron capture instruments.
over a section of test area and waiting for tracer gas leakage
from potential leaks to accumulate. The test sensitivity Regardless of which leak location test technique is used
increase is greater for smaller boxes and/or longer accumu- to test the lap welds-in the bottoms, the welds outside the
lation times, but both of these factors rapidly increase test tank shell between the inner and outer bottoms on double
costs. Test sensitivities in the range of 10-5 to 10-6 Pa-m3-s-1 bottom designs must be leak location tested by some
(10-4to 10-5 std cm3-s-1) can be achieved, but at a consider- method if a quantitative leak test of this system is required
by the purchaser specification. The fastest and the most
able cost increase. economical test method is a bubble pressure test.

Electron capture detector probe (sniffer) testing (ECLT) Table 11 is a comparison of the aboveground storage
using sulfur hexafluoride (SF6) as the tracer gas has come tank flat bottom leak location test techniques discussed.
into greater usage because of the environmental impact of
hydrofluorocarbons and chlorofluorocarbons prevalent in Comparison of QuantitativeLeak
refrigerant R-12 and R-22. This method has approximately Testing Techniques
the same limitations for instrument sensitivity, test pressure
under flat bottoms, scanning speed and probe to surface dis- Pressure rise measurement is one of the quantitative leak
tance as the halogen diode detector probe test method dis- test techniques that has appeared quite frequently in many
cussed earlier. Thus, the estimated achievable test aboveground storage tank double bottom design specifica-
sensitivity is in the range of 10...J to 10-4 Pa-mvs! (l0-2 to tions. After required preliminary leak testing, the specifica-
10-3 std cm3-s-1) when detector probe (sniffer) leak testing tions normally require that the space between the double
bottoms be partially evacuated to some pressure below
the bottoms of aboveground storage tanks by this method. atmosphere and held at that pressure for a defined period of
Again, test sensitivities can be increased to the range of 10-5 time without any increase in the pressure (loss of vacuum).
to 10-6 Pa-m3-s-1 (10-4 or 10-5 std cm3-s-1) by the accumula- A typical requirement is to evacuate to a negative pressure
(vacuum) of 98 kPa (14.2 lb-in.:", also referred to as
tion technique, but at a considerable increase in cost. 735 mm Hg [29 in. Hg]) below atmosphere and hold for 8 h
Helium mass spectrometer detector probe (sniffer) test­ without any loss (degradation) of the vacuum. Another typi-
cal requirement is to evacuate to 68 kPa (9.8 lbr·in.-2, also
ing (MSx) using helium as the tracer gas. Again, as with the 508 mm Hg [20 in. Hg]) below atmosphere and hold for 24 h
halogen diode detector probe or the electron capture detec- without any loss of the vacuum.
tor probe, when using the helium mass spectrometer
(HMS) in the detector probe (sniffer) test mode, the attain- The basic pressure rise relationship is Q = !:l.PVIM.
able test sensitivity when testing an aboveground storage When Q = leakage rate in mass flow units of atm cc-s'" or
tank bottom is in the range of 10-3 to 10-4 Pa-m3-s-1 (10-2 to std cm''.s:', V = volume of the test system in cubic feet, !:l.P=
change in pressure in inches of mercury, M = change in
10-3 std cm3-s-1 ). Because a helium mass spectrometer is a time in hours, then Q = !:l.PV/38M, to give results in
high vacuum instrument, this detector probe (sniffer) pres- Pa-m3-s-1 instead of ft3-in. Hg-h-1.
sure test is the test technique for which it is least suited and
has the poorest sensitivity.

One advantage of using a helium mass spectrometer
with a pumped detector probe connected to a permeation
membrane accumulation chamber is the very long probe
hose, up to 60 m (200 ft), that may be used with no loss in

LEAK TESTING I 69

TABLE 11 . Comparisonof leak testingtechniquesfor abovegroundflat bottom storagetanks

Test Sensitivity Approximate Training

Test Technique (std cm3·s-1 J Relative Cost (hours) Equipment

Vacuum box bubble 10-2 to 1 o-3 J 2 vacuum box and solution
Vacuum box liquid penetrant J 0-3 to I Q-4 2 vacuum box, penetrant and
J.5
Vacuum box penetrant developer J o-3 unspecified developer
10-3 to J 0-4 0.5 unspecified
Ammonia sensitivepaint or tape 2 developer
8 to 12 vacuum box, ammonia paint or
Halogen diode or electron capture l 0-2 to J o-s 3 to 5
28 to 40 tape
Mass spectrometry l 0-2 to J o-s 4 to 6 tracer gas, leakdetector and

associated equipment
tracer gas, leak detector and

associatedequipment

As an example, if an aboveground storage tank were Pressure loss measurement is a quantitative leak test
30.5 m (100 ft) in diameter and the grating space between technique that should not be specified for a double bottom
the inner and outer bottoms were 13 mm (0.5 in.) and the system with the tank empty. This is because of the very small
grating occupied 20 percent of that space, the volume of positive pressure differential that a large flat membrane bot-
tom can tolerate before it balloons; i.e. 63 to 76 mm (2.5 to
=that space would be approximately V = (50)2 n(0.4)/12 3 in.) H20 pressure is maximum. Any slight variation in
temperature, and in turn the pressure, would make a sizable
7.4 m3 (262 ft3). change in the volume due to the movement of the inner bot-
If this volume were evacuated to a negative gage pres- tom. This would produce test results which would be impos-
sible to interpret.
sure (vacuum) of 98 kPa (14.2 Ib-m.") and held for 8 h
and the vacuum gauge reading had increased 2.5 mm Hg When this technique is specified, it should include the
(0.1 in. Hg) in that time, does this indicate: requirement that a head of water must be in the tank during
the test. If a tank contains 3.7 m (12 ft) of water, assuming
1. real leakage (Q) = (0.1)(262)/(38)(8) = 860 mPa-m3-s-1 an adequate foundation under the outer bottom, the space
(0.86 std cm3-s-1); between the bottoms could then be pressurized to 0.3 m

2. normal change in the system volume due to the =( 1 ft) more pressure than the head of water. This would
tendency of large flat membranes to change shape
with a very slight temperature change; or result in a pressure of 13(0.433) 38.3 kPa (or 5.6 lbr·in.-2)
without significant ballooning of the inner bottom. This
3. nothing significant because the pressure change is would also virtually eliminate temperature as a variable for
within the gage's listed accuracy of 0.33 percent of that space because the water would be a large heat sink that
full scale? This is approximately the listed accuracy would keep the temperature of that space stable. Plus, upon
for such gages. completion of the test, the drain or sampling pipes between
the bottoms could be checked for signs of moisture. If the·
When performing a quantitative pressure rise measure- hold test failed, an indication or lack of an indication of
ment test of the double bottom an aboveground storage moisture between bottoms would be indicative of the
tank, does a small pressure increase during such a test reveal source of the leakage.
real or apparent leakage? When this occurs, the test can be
repeated or continued for a longer time period in order to Mass flow measurement is a quantitative technique that
average down any errors. This may or may not produce a is not usually specified by purchasers but has several de-
more conclusive result, but in any event it will cost more cided advantages. First, it can be performed at a pressure
time and money in the process. either above or below atmospheric pressure, and second, it
can be performed in a minute or two, which eliminates
If the conclusion is that-this is real leakage, where is the temperature as a test variable. If conducting this test at a
leakage? Is it in the outer bottom, in the inner bottom, or in pressure above atmospheric, it is suggested that it be done
the perimeter welds between the bottoms outside the tank with water in the tank so that a pressure of a couple dozen
shell? To have any chance at reaching a conclusion, the only kilopascals (a few pounds per square inch) can be used
option is to retest the inner bottom and the perimeter welds rather than only several score millimeters mercury (a few
between the bottoms outside the tank shell. If leaks are inches mercury) of water pressure. A disadvantage of this
detected in one or both of these areas, the leaks can be test technique is that the mass flow meter must be pur-
repaired, the hold test rerun and the results will most likely chased for the specific pressure level and anticipated po-
be satisfactory. If no leaks are detected in either of these tential leakage rate range that might have to be measured.
areas, it would have to be assumed that the leak(s) were in
the outer bottom.

70 I NONDESTRUCTIVETESTINGOVERVIEW

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Tanks." Toppi Journal. Vol. 73, No. 12. Atlanta, GA: 37. Schlattmann, J. and J. Niewels. "A Systematic Devel-
Technical Association of the Pulp and Paper Indus- opment in Designing a Special Inspection Robot for
try (December 1990): p 105-109. Use in Private Sewer Lines." No Trenches in Town:

27. Miller, R.K."Tank-bottom Leak Detection in Proceedings of International Conference [Paris,
Aboveground Storage Tanks by Using Acoustic France]. J.P. Henry and M. Mermet, eds. Rotterdam,
Emission." Materials Evaluation. Vol. 48, No. 6. Netherlands: A.A. Balkema (1992): p 323-325.
Columbus, OH: American Society for Nondestruc- 38. Seliverstov, M.I. "Use of Sulfur Hexafluoride as the
tive Testing (June 1990): p 822-829.
Tracer Gas in Leak Detection." Soviet Journal of
28. Nickolaus, C.M. "Acoustic Emission Monitoring of Nondestructive Testing. Vol. 27, No. 8. New York, NY:
Aboveground Storage Tanks." Materials Evaluation. Plenum/Consultants Bureau (April 1992): p 599-604.
Vol. 46, No. 4. Columbus, OH: American Society for 39. Shakkottai, P. Apparatus for the Remote Detection of
Nondestructive Testing (March 1988):p 508-512. Sounds Caused by Leaks. United States Patent
No. 4,979,820 (December 1990).
29. Nordstrom, R. "Direct Tank Bottom Leak Monitor- 40. Shell Oil Company (Kruka, V.R. and R.W. Patterson).
ing with Acoustic Emission." Materials Evaluation. Subsea Pipeline Leak Detection. United States Patent
Vol. 48, No. 2. Columbus, OH: American Society for No. 4,996,879 (March 1991).
Nondestructive Testing (February 1990): p 251-254.
41. Spruin, WC. "Combination of Thermography and
30. Pastorello, J. "Study of Leak Detection Fluids." Pressure Tests to Combat Air Leakage Problems in
Materials Evaluation. Vol. 49, No. 8. Columbus, Building Enclosures." Thermosense IX: An Interna­
OH: American Society for Nondestructive Testing tional Conference on Thermal Infrared Sensing for
(August 1991): p 1,035-1,037. Diagnostics and Control [ Orlando, FL, May 1987].
SPIE Proceedings Vol. 780. Bellingham, WA: Interna-

tional Society for Optical Engineering ( Society of
Photo-Optical Instrumentation Engineers) (1987):
p 24-29.

LEAK TESTING I 7 3

42. Testrite, Incorporated. Method and Apparatus for 46. Tyson, J. "Optical Leak Testing: A New Method for
Detecting Leaks. United States Patent No. 4,625,545
(December 1986). Hermetic Seal Inspection." Nondestructive Charac­
terization for Advanced Technologies, Oakland,
43. Titov, LP., G.T. Lebedev, VA. Tyurin, Yu.M. Volkov, California. Columbus, OH: American Society for
N.I. Sevryukova and E.A. Ranneva. "A Pneumatic Nondestructive Testing (1991): p 182-186.

Method of Leak Testing with the Use of Leak Detec- 47. Tyson, J. "Real-time Optical Leak Testing of Micro-
tors Based on Aqueous Solutions of Surfactants."
electronic Hermetic Seals." Materials Evaluation.
Soviet Journal of Nondestructive Testing. Vol. 23, Vol. 49, No. 8. Columbus, OH: American Society
No. 9. New York, NY: Plenum/Consultants Bureau for Nondestructive Testing (August 1991):
(May 1988): p 607-613. p 970-972.
44. Tscheliesnig, P. and H. Theiretzbacher. "New Results
from the Detection of Micro-leakages in the Petro- 48. Weil, G.J. and R.J. Graf. "Infrared Thermograpy
chemical Industry." Proceedings of the 12th World
Conference on Non­Destructive Testing [Amsterdam, Based Pipeline Leak Detection Systems." Ther­
mosense XIII, Orlando, Florida. G.S. Baird, ed.
Netherlands, April 1989]. J. Boogaard and G.M. van Vol. 1,467. Bellingham, WA: International Society
for Optical Engineering (Society of Photo-Optical
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Science Publishers (1989):p 905-911.
45. Tscheliesnig, P., H. Molla Djafari, G. Krenn and H. 49. Worthington, WC. "Leak Testing - Part 2: Helium
Edinger. "An Acoustic Leak Detecting Pig." 6th
European Conference on Non Destructive Testing Leak Detection." InWte.Jr.naMtiocnGaolnAndagvalen,ceesd.inVNool.nd1e6­.
[Nice, France]. Vol. 1. Paris, France: Confederation structive Testing.
Francaise pour les Essais Non Destructifs
(COFREND) on behalf of the European Committee New York, NY: Gordon and Breach Science Pub-
for Nondestructive Testing (1994): p 563-568. lishers (1991): p 233-243.



3SECTION

LIQUID PENETRANT TESTING

Noel A. Tracy, Universal Technology Corporation, Dayton, Ohio

7 6 I NONDESTRUCTIVETESTING OVERVIEW

PART 1
DEFINITION AND PURPOSE OF LIQUID
PENETRANT TESTING

Liquid penetrant testing can be defined as a physical and discontinuity is expected to extend through the material. It
chemical nondestructive testing procedure designed to may be further enhanced by the application of a pressure
detect and expose surface connected discontinuities in non- differential.
porous engineering materials. The method relies on the
physical interaction between an appropriately formulated History
chemical liquid and the surface of a part. This interaction
causes the liquid to enter surface cavities and later to Liquid penetrant testing is one of the oldest of modern
emerge, visually indicating the location and approximate nondestructive testing methods. It was perhaps first used in
size and shape of the surface opening. The objective of liq- the railroad maintenance shops in the late 1800s. Parts to be
uid penetrant testing is to provide visual evidence of cracks, inspected would be immersed in used machine oil. After a
porosity, laps, seams and other surface discontinuities suitable immersion time, the parts were withdrawn from the
rapidly and economically with a high degree of reliability. oil and the excess surface oil wiped off with rags or wadding.
With proper technique, penetrant testing will detect a wide The part surfaces would then be coated with powdered
variety of discontinuities ranging in size from readily visible chalk or a mixture of chalk suspended in alcohol (whiting).
Oil trapped in discontinuities would bleed out, causing a
to microscopic. noticeable stain in the white chalk coating. This became
Liquid penetrant testing is popular because it is rela- known as the oil-and-whiting method.

tively easy to use and has a wide range of applications. The oil-and-whiting method was replaced by magnetic
Because it uses physical and chemical properties rather than particle inspection on steel and other ferrous parts in 1930.
electrical or thermal phenomena, it can be used in the field, However, industries using nonferromagnetic metals, espe-
far from power sources. Test equipment can be as simple as cially aircraft manufacturers, needed a more reliable and
a small, inexpensive kit of aerosol cans or as extensive as a sophisticated tool than used machine oil and chalk. In 1941,
large mechanized and automated installation. However, in fluorescent dye materials were added to a penetrating oil to
all cases, the success of liquid penetrant testing depends on make a fluorescent penetrant material. Nonfluorescent col-
cleanliness of test surfaces, on absence of contamination or ored penetrants, primarily red, were produced a little later.
surface conditions that can interfere with penetrant entry Since then, a large number of penetrant materials have
into and subsequent emergence from surface openings of evolved. Developments in the basic chemistry of penetrants
discontinuities, and on care by inspectors/operators to have led to surfactant- and water-base penetrants as well as
ensure proper processing techniques and observation of test improved oil-base formulations. New chemical additives
indications. Previous manufacturing processes also may and new dyes also have contributed to the production of
inhibit detection of some types of discontinuities by liquid penetrants having different levels of sensitivity. Materials
penetrants. For example, many seams and laps are forged for efficiently removing the excess surface penetrant and
shut by the hot rolling or piercing processes that create developers for enhancing the visualization of a discontinuity
these elongated discontinuities; in addition, local welding of indication also have evolved. The drivers behind continual
the metal or trapping of heat treat products within the developments probably are no different than for any other
opening can inhibit or prevent entry of the penetrant, mak- product: demand for improvement, inspection process eco-
ing penetrant testing ineffective. nomics and environmental concerns.

Penetrant testing is also used for leak testing. The same Basic Penetrant Testing Process
basic fundamentals apply but the penetrant removal step
may be omitted. The container is either filled with pene- Basic principles of the penetrant testing process have not
trant or the penetrant is applied to one side of the container changed from the oil-and-whiting days. These principles are
wall. The developer is applied to the opposite side, which is
inspected after allowing time for the penetrant to seep
through any leak points. This technique may be used on thin
parts where there is access to both surfaces and where the

LIQUID PENETRANTTESTING I 77

shown in Fig. 1. As the penetrant testing process evolved, 5. Visually examine surfaces for penetrant indications;
additional steps were added. Presently, the process consists interpret and evaluate the indications.
of six basic steps.
6. Postclean the part to remove process residues if they
l. Preclean and dry the test surfaces of the object to be will be detrimental to subsequent operations or the
inspected. Cleaning is a critical part of the penetrant part's intended function. (In some cases, a treatment
process and is emphasized because of its effect on the to prevent corrosion may be required.)
inspection results. Contaminants, soils or moisture,
either inside a discontinuity or on the part surface at Reasons for Selecting Liquid
the discontinuity opening, can reduce or completely Penetrant Testing
destroy the effectiveness of the inspection.
Some of the reasons for choosing penetrant testing are as
2. Apply liquid penetrant to the test surfaces and permit follows.
it to dwell on the part surface for a period of time to
allow it to enter and fill any discontinuities open to 1. Liquid penetrant testing can quickly examine all the
the surface. accessible surfaces of objects. Complex shapes can be
immersed or sprayed with penetrant to provide com-
3. Bemooe excess penetrant from the test surfaces. Care plete surface coverage.
must be exercised to prevent removal of penetrant
contained in discontinuities. 2. Liquid penetrant testing can detect ve:ry small surface
discontinuities. It is one of the most sensitive nonde-
4. Apply a developer, which aids in drawing any trapped structive testing methods for detecting surface discon-
penetrant from discontinuities and slightly spreading tinuities.
that penetrant on the test surface to improve the visi-
bility of indications. The developer also provides a 3. Liquid penetrant testing can be used on a wide variety
contrasting background on a part surface, especially of materials: ferrous and nonferrous metals and alloys;
for nonfluorescent indications. fired ceramics and cermets; powdered metal products;
glass; and some types of organic materials. Restrictions
FIGURE 1. Basic penetrantprocess:(a) apply on materials imposed by nature of the penetrant pro-
penetrant;(b) remove excess; (c) apply cess are covered in the discussion of limitations, below.
developer
4. Liquid penetrant testing can be accomplished with rel-
fa) PENETRANT atively inexpensive, nonsophisticated equipment. If
the area to be inspected is small, the inspection can be
(bJ accomplished with portable equipment.

(cJ INDICATION DEVELOPER 5. Liquid penetrant testing magnifies the apparent size of
discontinuities, making the indications more visible. In
addition, the discontinuity location, orientation and
approximate size and shape are indicated on the part,
making interpretation and evaluation possible. Typical
fluorescent penetrant indications are shown in Fig. 2.

6. Liquid penetrant testing is readily adapted to volume
processing, permitting 100 percent surface inspection.
Small parts may be placed in baskets for batch process-
ing. Specialized systems may be partially or fully auto-
mated to process many parts per hour.

7. The sensitivity of a penetrant testing process may be
adjusted through appropriate selection of penetrant,
removal method and type of developer. This allowsthe
penetrant process to be adapted to characteristics (e.g.,
composition, surface condition) of the part requiring
inspection and to be tailored to detect specified mini-
mum allowable discontinuities. Thus, inconsequential
discontinuities can be suppressed while larger discon-
tinuities of more concern are indicated. Figure 3 com-
pares indications of identical cracks produced by two
penetrant testing processes of different sensitivities.

78 I NONDESTRUCTIVE TESTING OVERVIEW

Disadvantages and Limitations of 4. Mechanical operations, such as shot peening, plastic
Liquid Penetrant Testing
media blasting (PMB), machining, honing, abrasive
1. Penetrant testing.depends upon the ability of pene-
trant to enter and fill discontinuities. Penetrant testing blasting, buffing, brushing, giinding or sanding will
will only reveal discontinuities open to the surface. smear or peen the surface of metals. This mechanical

2. Surfaces of objects to be inspected must be clean and working closes or reduces the surface opening of
free of organic or inorganic contaminants that will existing discontinuities. Mechanical working (smear-
prevent interaction of the penetrating media with a
surface. Organic surface coatings, such as paint, oil, ing or peening) also occurs during service when parts
grease or resin, are in this category. Any coating that
covers or blocks the discontinuity opening will pre- are in contact or rub together. Penetrant testing will
vent penetrant entry. Even when the coating does not not reliably detect discontinuities when it is per-
cover the opening, material at the edge of the open-
ing may affect entry or exit of penetrant and greatly formed after a mechanical operation or service use
reduce reliability of the inspection. Coatings in the
vicinity of a discontinuity will also retain penetrant, that smears or peens the surface. In some cases
causing backgrnund indications. Cleaning test sur- chemical removal (etching) of smeared metal may
faces is discussed in more detail below.
restore inspection reliability.
3. It is also essential that the inside surface of disconti-
nuities be free of materials such as corrosion, com- 5. Unless special procedures are used, penetrant test-
bustion products or other contaminants that would
restrict entry of penetrant. Because it is impossible to ing is impractical on porous materials, such as some
check inside discontinuities, one must trust that pro- types of anodized aluminum surfaces, other protec-
cesses selected to clean test surfaces will clean inside
surfaces of discontinuities also. tive coatings and porous nonmetallic parts. Pene-

FIGURE2. Fluorescentpenetrantindications of trant rapidly enters pores of the material and
fatigue cracksin an intake manifoldcase becomes trapped, This can result in an overall back-

ground fluorescence or color that could mask any
potential ,discontinuity indications. In addition,

removal of the penetrant may not be possible after
the inspection. ·

FIGURE 3. Cracked, brittle iron plated
couponshowing results of inspectionwith
two fluorescentpenetrant processesof
different sensitivities

FROM UNITED STATES AIR FORCE. FROM UNITED STATESAIR FORCE.

LIQUID PENETRANTTESTING I 79

6. Penetrants, emulsifiers and some types of developers Stationary Test Equipment
have very good wetting and detergent properties.
They can act as solvents for fats and oils. They also can The type of equipment most frequently used in fixed
clean ferrous materials so thoroughly that rust will installations consists of a series of modular work stations.
Each station accommodates a specific task. The number of
begin almost immediately if corrosion inhibitor is not stations in a processing line varies with the penetrant
applied. If allowed to remain in contact with human method used. Typical stations are as follows: dip tanks for
skin for extended periods, they may cause irritation. penetrant, emulsifier and developer; a number of drain or
dwell areas.] a wash area with lighting appropriate for the
Equipment Requirements type of penetrant used; a drying oven; and an inspection
booth. Thedrain or dwell stations can be roller top benches
Portable Equipment to hold the parts during the processing cycle. The usual
arrangement is to position a drain or dwell station following
Penetrant testing can be performed on installed parts each of the dip tanks, the wash station (if aqueous developer
(e.g., on aircraft or in power plants) or on parts too large to is not used) and the drying oven. Figure 4 illustrates a typi-
be brought to the inspection area. Penetrant materials are cal fluorescent penetrant testing system. Alternative sys-
available in aerosol spray cans and in small containers for tems have spray booths instead of tanks and overhead
brush or wipe applications. Portable kits are available for conveyers instead of roller top benches.
use in situations where electric power is not available.
Because the process required with portable kits is labor Small Parts Test Unit
intensive, portable penetrant applications are generally lim-
ited to localized area or spot inspections rather than entire There are inspection units designed for processing small
part surfaces. parts. The units are smaller than the stationary systems
described above, and some of the stations serve multiple
purposes. In use, the parts are loaded into wire baskets that

FIGURE 4. Typical fluorescent penetrant stationary inspection system

FROM UNITED STATESAIR FORCE.

80 I NONDESTRUCTIVETESTING OVERVIEW
FIGURE 5. Automated aerospace penetrant spray processing and inspectionsystem

are then batch processed through each of the stations. The Personnel Requirements
wash station may contain a water driven, rotary table with
spray jets to supplement the hand held spray wand. The apparent simplicity of penetrant testing is deceptive.
Very slight process variations during the performance of an
Automated Test Systems inspection can invalidate the inspection by counteracting the
formation of indications. It is essential that personnel per-
The penetrant testing process can be adapted for use forming penetrant testing be trained and experienced in the
with partially and fully automated processing equipment. penetrant process. All individuals who apply penetrant mate-
Semiautomation may consist of a conveyor system for mov- rials or examine components for penetrant indications
ing the parts through one or more of the processing steps; should be qualified. Qualification requires classroom and
penetrant, emulsifier or remover, rinse water and developer practical training, passing marks on examinations, and expe-
are manually applied. In fully automated systems, all of the rience. Typical qualification requirements are contained in
processing steps are mechanically performed with little or ANS1'ASNT CP-189-1991, Standard for Qualification and
no operator intervention. Automated equipment allows Certification of Nondestructive Testing Personnel; in EN
large numbers of parts to be processed with a minimum of 473, Qualification and Certification of NDT Personnel ­
personnel and time. Automated equipment also provides a General Principles; and in ISO 9712, Nondestructive Testing
more repeatable, though not necessarily more sensitive, ­ Qualification and Certification of Personnel. Another
testing process. widely used document intended as a guideline for employers
to establish their own written practice for the qualification
One type of automated equipment for inspection of large and certification of their nondestructive testing personnel is
structural aircraft components is shown in Fig. 5. Although ASNT Recommended Practice No. SNT­TC­lA, published by
the penetrant application, washing and drying are automatic, the American Society for Nondestructive Testing.
the ultraviolet light inspection and interpretation are manu-
ally performed by an inspector.

LIQUID PENETRANTTESTING I 81

PART 2

CLASSIFICATIONS OF PENETRANTS

Classification of Penetrants by Dye Classification of Penetrants by
Type Removal Method

Penetrants are generally classified by type according to A penetrant is further classified by the method used to
the dye contained in the penetrant. The penetrant testing remove it from the surface of a part after it has been on the
process relies on penetrant entering a discontinuity and sub- part a specified amount of dwell time. The penetrants are
sequently being drawn back out and made easily visible on formulated and manufactured for specific removal methods
the surface of a part. The amount of penetrant material designed to minimize removal of the penetrant that has
entrapped in discontinuities is usually very small. If the dis- seeped into a discontinuity. Each removal method has
continuity is to be detected, the very small amount of pene- advantages and disadvantages discussed below.
trant must be highly visible. In the oil-and-whiting days, it
was found that used or dirty oil was much more visible than Water Washable Penetrant
clean machine oil. Today chemists make penetrants visible
by dissolving dyes in a penetrating oil or other vehicle. Many penetrants contain an oil base insoluble in and
Based on the dye, penetrants are classified as one of the immiscible with water. This means that the excess penetrant
three typ,es described below. on a part cannot be removed with water. However, some
penetrants are carefully compounded mixtures of an oil base
Fluorescent Penetrant and an emulsifier and others have water or a surfactant as a
base rather than oil. Manufacturers provide these alterna-
Fluorescent penetrants contain fluorescent dye that tive formulations in ready-to-use penetrants, which may be
emits yellowish green light when exposed to ultraviolet or removed with water immediately after the penetrant dwell.
near ultraviolet light (with a wavelength of 320 to 400 nm). Depending upon requirements imposed by applicable pro-
This property is termed fluorescence. Very small quantities cess specifications, removal may be accomplished by direct-
of fluorescent penetrant will emit highly visible indications ing a controlled spray onto the part, by dipping and agitating
when exposed to ultraviolet light. the part in water or by wiping the part surface with a wet
lintfree cloth (after wiping first with a dry lintfree cloth).
Visible Penetrant
Postemulsifiable Penetrant
Visible dye or color contrast penetrants contain a dye vis-
ible under natural or white light. The visibility is further When used in the postemulsification process, penetrants
enhanced during the penetrant process by the application of can be formulated to optimize their penetrating and visibil-
a white developer. The white developer provides a high con- ity characteristics for higher sensitivity. Because postemulsi-
trast background for the colored penetrant when viewed fiable penetrants do not contain any emulsifying agent, they
under the appropriate light. Red dye is most common, are less likely to be removed from the discontinuity when
although some blue dye i~i!hlso used. the surface penetrant is being removed with water. Removal
from a surface is accomplished by applying an emulsifier in
Dual Mode (Both Visible and Fluorescent) a separate process step, normally by dipping the part into a
Penetrant tank of the emulsifier or spraying the emulsifier onto the
part. Depending on the type of emulsifier used, the emulsi-
Dual mode penetrants contain dyes that are both col- fier either converts the excess surface penetrant into a mix-
ored under white light and fluorescent under ultraviolet ture which forms an emulsion with the addition of water or
light. However, the intensities of the visible color (usually acts directly with the penetrant to form an emulsion subse-
red) and the fluorescent color (usually orange) are less than quently removed with water.
the colors produced by the single mode visible and fluores-
cent penetrants respectively.

82 I NONDESTRUCTIVE TESTING OVERVIEW

A postemulsifiable penetrant usually can be used with not take place in this mechanism of action. The surface
any emulsifier. However, qualifying/approving agencies may active agent in the remover helps displace penetrant from
choose to qualify a penetrant/emulsifier combination from the surface and prevents redeposition. Removal of excess
the same manufacturer. The manufacturer may offer the surface penetrant with hydrophilic emulsifiers in an immer-
same penetrant for use with different emulsifiers. A user sion or dip mode begins as the remover detaches the pene-
could use any penetrant/emulsifier combination that met trant from the surface. Mild agitation removes the displaced
the approval of a customer. penetrant from the part so that it cannot redeposit. When a
spray is used, the impinging droplets of water have the same
Solvent Removable Penetrant effect as agitation in a tank The hydrophilic emulsifier con-
tact time is directly related to its concentration. This applies
The term solvent removable actually applies only to the to both immersion and spray applications. The hydrophilic
removal process rather than the penetrant material since all emulsification process affords better control and, in addi-
penetrants can be removed with solvents. It is listed as a tion, allows for an effective and practical treatment and
type of penetrant because it is often used in that context. In recycling of the penetrant prerinse solution, thereby mini-
most applications the penetrants used in the solvent remov- mizing waste water pollution.
able process are the postemulsifiable type; however, water
washable penetrants can also be used. Solvent Removers

With this method excess penetrant is removed from a Solvent removers have traditionally been petroleum base
test surface by first wiping the surface with a clean, dry, lint- or chlorinated solvents. However, because the former is
free cloth or paper towel. After most of the surface pene- flammable and production of the latter was mandated to
trant has been removed, the remainder is removed with cease in December 1995, use of detergent cleaners or water
another clean cloth slightly moistened with the solvent. base solvents is increasing. Water itself can be used as a sol- '
Because the solvent removable method is very labor inten- vent for water washable penetrants. Often an emulsifier bas
sive, it is normally used when it is necessary to inspect a enough solvency to function also as a hand wipe remover.
localized area of a part or a part at its inservice site rather
than in an assembly line production environment. When Types of Developers
properly applied, the solvent removable method can be one
of the most sensitive penetrant testing methods available. Developer increases the brightness intensity of fluores-
cent penetrant indications and the contrast of visible-pene-
Types of Emulsifiers trant indications. Developer also provides a blotting action,
which serves to draw penetrant from within a discontinuity
Lipophilic emulsifiers are liquid blends that combine to the part surface until the thickness of the surface film of
with oil based penetrants to form a mixture that can be penetrant exuded from the discontinuity is increased to lev-
removed with a water spray. They are supplied in a ready-to- els above the threshold of visibility.Another developer func-
use form. Their action is based primarily on diffusion and tion is spreading the penetrant on the surface, enlarging the
solubility into an oil base penetrant. Parts are generally appearance of the indication.
dipped into tanks of lipophilic emulsifier, withdrawn and
placed at a drain station for a specified time. The diffusion Dry powder developers are applied to dry part surfaces
rate (emulsification time) will vary depending on the viscos- by air suspension, electrostatic spraying or part immersion.
ity of the emulsifier and the physical action of drainoff. The powder is light and fluffy and clings to the part surfaces
Therefore, it is important to control the emulsification time in a fine film. In most cases, dry powder developers should
to prevent emulsification of penetrant in the discontinuity. not be used with visible penetrant because they do not pro-
duce a satisfactory contrast coating on the surface of the
Hydrophilic emulsifiers, often referred to as removers, part. Electrostatic spraying is common in automated sys-
are composed of emulsifying agents dissolved in water and tems. For reasons of human safety, dry powder developers
are supplied in a concentrate form. They are diluted with should be handled with care. Like any other dust particle, it
water at concentrations of 5 to 30 percent and used as an can dry the skin and irritate the lining of breathing passages.
immersion dip with mild air or mechanical agitation, or as a
forced water spray rinse at dilution ratios up to 5 percent. Water soluble developers consist of a powder dissolved
Prerinse with a water spray normally preceding the applica- in water and applied by dipping a part in the solution, flow-
tion of hydrophilic emulsifiers reduces penetrant contami- ing the solution over a part or spraying the solution onto the ·
nation of the emulsifier. Hydrophilic means having an part. As the part is dried, a thin layer of the powder remains
affinity for water; lipophilic means having an affinity for oil. on the part. Stationary inspection equipment usually
includes a tank module for aqueous developer. Complete
Hydrophilic emulsifiers function through their detergent
and scraping or scrubbing (kinetic) action. Diffusion does