rates for other gases from helium leakage Varying the Pressure
rates determined by helium leak tests. Differential across a Leak
Computation of Because the rate of flow of a gas through a
Transitional Flow Rates leak will be a function of either the
molecular mass or the viscosity of the gas
Transitional flow, flow between the flowing, it is sometimes very important to
viscous and molecular regimes, is not know which type of leakage flow is
fully understood. A number of models occurring. This is especially true if a
have been proposed to estimate the flow leakage rate must be expressed in terms of
in this regime. The simplest and most one gas such as air, when leakage must be
used is the slip flow model. The flow is measured by detecting helium flow.
given by the following equation, Often, the capillary leak is used under
conditions that vary greatly from the
(3) πr4 conditions under which the leak was
Q2 = − Pa calibrated. The test gas, test pressure or
8nl both may be different from those used in
the calibration of the leak. If it is not
( )× λ possible to obtain a true calibration figure
P1 − P2 1 + 4 under the new test conditions, it becomes
r necessary to attempt an estimation. The
principles used in such estimations are
where r is tube radius (meter), l is tube presented next.
length (meter), P1 is pressure upstream Varying System Pressure to
(pascal), P2 is pressure downstream Identify Types of Flow in
(pascal), λ is mean free path and Pa = (P1 Leaks
+ P2)/2. If the tube diameter or effective
diameter is known, the flow for any gas If leakage, or flow, can be measured by
using a leak detector or a residual gas
can be calculated with the equation from analyzer, the type of flow can often be
identified by changing the pressure
the known pressures. causing the flow of gas. All techniques of
leak testing using a mass spectrometer
Also, the flow for gas can be estimated leak detector involve the passage of a
tracer gas through a presumed leak in a
from the known flow for another gas pressure barrier. This involves application
of tracer gas to the high pressure side of
under the same pressure conditions. the barrier and the subsequent detection
of the tracer gas on the lower pressure
Alternately, a crude estimate could be side. In general, there are three types of gas
flow: viscous, transition and molecular.
made using the formula for molecular The variables that control the type of gas
flow that occurs in leaks are (1) viscosity
flow. In general, estimation of flow based of the flowing gas or gas mixture (Pa·s),
(2) relative molecular mass Mr of the gas,
on the known flow of another gas in the (3) pressure difference causing the flow
(Pa), (4) absolute pressure in the system
transition range is not recommended. (Pa absolute) and (5) absolute temperature
of the flowing gas or gas mixture (K).
Effect of Absolute Gas Figure 10 shows the general relationship
Temperature on Molecular of flow type to gas pressure and radius of
Flow Leakage Rates tubular conductance.
The effect of absolute gas temperature on Conditions for Identification of
conductance when the leakage flow is Viscous Flow through Leaks
molecular should not be overlooked when
estimating leakage rates by use of When the pressure across a leak is
standard leaks. The conductance of both changed and the flow changes in
orifices and of tubes changes directly with proportion to the differences of the
the absolute gas temperature. Equation 4 squares of the absolute pressures, the
shows how the new flow rate Q2 at the leakage can be identified as viscous flow.
new absolute temperature T2 (K) compares Viscous flow occurs in high pressure
with the original flow rate Q1 at absolute systems, such as systems pressurized with
temperature 1, through a leak for which helium tracer gas and checked by the
both the leak path dimensions and the
pressure difference across the leak remain
constant, for the specific case of the
molecular flow rate at new absolute
temperature:
(4) Q2 = T2 Q1
T1
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helium detector probe method. Figure 11 flow of gases through leaks as a function
shows graphically how the viscous leakage of absolute gas pressures and diameter of
rate changes as internal pressure is varied leak passageways. In many cases, it is not
in test objects and systems leaking to the always practical to vary pressures on parts
atmosphere. Figure 12 shows similar under test to determine the types of leaks
graphical relationships for externally being detected. In instances where the
pressurized test objects with leakage to an leakage of a gas other than the tracer gas
internal volume that is highly evacuated. is of concern, it is best to assume the
worst possible condition, which may be
Conditions for Identification of either viscous or molecular flow.
Molecular Flow through Leaks
Calculating Effect of
If the flow of gas through a leak changes Pressure Changes with
in proportion with the difference between Viscous Flow through
the pressures acting across the leak, the Leaks
flow of gas is molecular. Molecular flow
usually occurs in vacuum testing Viscous flow occurs when the mean free
applications with helium spray
application of tracer gas and mass path of molecules of the gas is much
spectrometer leak detectors attached to
the internal volume of evacuated test smaller than the cross sectional
objects. Figure 13 shows graphically how
the molecular leakage rate varies linearly dimension of the physical leak. In this
with the pressure differential as external
pressure is varied on test objects and case, the leakage rate Q is proportional to
systems that are internally evacuated.
the differences in the squares of the
Conditions for Identification of
Transitional Flow through Leaks pressures on the opposite sides of the
If the flow changes in response to changes pressure barrier through which the leak
of pressure by some relation between
proportionality to differences in squares penetrates. If the viscous flow rate Q1 has
of pressures and proportionality to been determined for a difference between
difference in pressures, the leak involves
transitional flow. Figure 10 illustrates the pressure P1 and pressure P2 and then the
regimes for each of these three types of pressures are changed to new values P'1
and P'2, the new flow rate Q 2 can be
calculated by means of Eq. 5, for viscous
flow rate at a new pressure,
(5) Q2 = P ′12 − P ′22 Q1
P12 − P22
FIGURE 10. Graphical presentation of conditions for viscous, FIGURE 11. Graphical presentation of increase in viscous flow
leakage ratio when pressurizing with 100 percent tracer gas,
molecular and transitional flow of gases through leaks, in as a function of internal system pressure when leaking to
atmospheric air.
terms of absolute gas pressure at 25 °C (77 °F) and radius of
tubular conductance. Note that 1 Pa = 1.5 × 10–4 lbf·in.–2. 100 100 000
105 (4 × 103)
104 (4 × 102)
103 (4 × 101) Left
scale
102 (4 × 100)Radius of tube, mm (in.) Viscous 10 Viscous flow 10 000
101 (4 × 10–1) Leakage rate increase ratio 1000
1.0 Right
100 (4 × 10–2) scale
10–1 (4 × 10–3)
Transition
10–2 (4 × 10–4) Molecular
10–3 (4 × 10–5)
10–4 (4 × 10–6) Pa kPa MPa 100
mPa 100 1000 10 000 100 000
10–5 (4 × 10–7)
10–3 10–2 10–1 100 101 102 103 104 105 106 (15) (150) (1500) (15 000)
Pressure (Pa) Absolute pressure, kPa (lbf ·in.–2)
(outside of part at 100 kPa)
Calibrated Reference Leaks 89
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FIGURE 12. Graphical presentation of increase in viscous flow leakage rate ratio when
pressurizing a chamber with 100 percent tracer gas, as a function of external system pressure,
when leaking into internally evacuated test objects in the chamber.
Leakage rate increase ratio 1000 Left scale 1 000 000
800 Viscous flow 500 000
600
100 000
400 50 000
200
100
80
60
40
20
10 10 000
8 Right scale
6
5000
4
2
1 1000
0.10 0.30 1.00 3.00 10.0 30.0 100.0
(0.015) (0.045) (0.150) (0.45) (1.50) (4.50) (15.0)
Absolute pressure, MPa (lbf ·in.–2 × 103)
(inside of part at high vacuum)
FIGURE 13. Graphical presentation of increase in molecular flow leakage rate ratio with
molecular flow, as a function of external pressure of 100 percent tracer gas, when leaking into
internally evacuated test objects of systems (inside of test system or parts at high vacuum).
1000 Molecular flow
800
600
400
200
Leakage rate increase ratio 100
80
60
40
20
10
8
6
4
2
1 0.30 1.00 3.00 10.0 30.0 100.0
(0.045) (1.50) (4.50) (15.0)
0.10 (0.150) (0.45)
(0.015)
External pressure, MPa (lbf ·in.–2 × 103) absolute
(inside of part at high vacuum)
90 Leak Testing
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In Eq. 5, the pressures are all absolute test volumes, 100 percent helium at high
pressures in pascal or pound per square pressure may not be economical.)
inch (lbf·in.–2). The old and new flow rates
must be in the same units of leakage. In this example of pressuring up, the
Equation 5 for viscous flow through leaks new total viscous flow rate Q2 can be
estimated:
would apply for leak testing of systems at
Q2 = 42 − 12 2 × 10−5
higher than atmospheric pressure. It 12 − 02
applies to a helium detector probe test on = 30 × 10−5
an internally pressurized test system
leaking to the atmosphere.
Example Calculation of Capillary The actual helium leakage rate, because
Leakage Rate at Different
Pressures the final pressurized mixture is only
Assume that a capillary standard leak 25 percent helium, is only about
(flattened tube) has been calibrated for a 7 × 10–5 Pa·m3·s–1 (7 × 10–4 std cm3·s–1) of
nitrogen flow rate of 2 × 10–5 Pa·m3·s–1 helium. The result of pressuring up with
(2 × 10–4 std cm3·s–1) with atmospheric
pressure on the high side and zero air or nitrogen is an approximately linear
pressure (vacuum) on the low side. It is
desirable to predetermine the leakage rate increase in the helium flow rate through
if this same capillary leak is to be used
with twice atmospheric pressure on the the leak. This example calculation would
high side and atmospheric pressure on the
low side. (Note that the pressure be valid only for viscous leakage. (Note
differential between high and low sides of that 1 Pa·m3·s–1 = 10 std cm3·s–1.)
the leak is atmospheric pressure of
100 kPa (1 atm) in both old and new Limitations of Increasing Pressure
cases.) Because the leakage rate is so high, with Molecular or Transitional
it will be assumed that leakage occurs as Flow Leaks
viscous flow. By Eq. 5, the new flow rate
Q2 is calculated in Pa·m3·s–1: For molecular flow leaks, increasing
pressure with air would not result in an
Q2 = 22 − 12 2 × 10−5 increase in the helium flow rate. In the
12 − 02 transitional flow range, particularly when
dealing with gas mixture, the situation
= 6 × 10−5 (degree of enhancement of leak signals) is
extremely difficult to predict. In these
cases of unknown effects, it would be
useful to make a graphical plot of leak
signal amplitude as a function of total
pressure within the leaking vessel to aid
in determining the nature of a leak.
This new flow rate represents a threefold Correction for Aging of
increase when compared to the original Helium Membrane
flow rate obtained with internal Calibrated Leaks
atmospheric pressure leaking to vacuum.
As time passes, the internal helium
Example Calculation of Leakage pressure of glass permeation leaks is
Rate after Pressuring Up Helium depleted (see Fig. 14). This depletion
with Nitrogen results from the gas leaking from the
reservoir through the glass element and
Another situation often encountered in through any discontinuities in the
mass spectrometer leak testing involves a reservoir into the atmosphere. If there is
standard capillary leak used with a no appreciable leakage except through the
mixture of helium and nitrogen at high glass permeation membrane, then the
pressure. This case occurs most commonly depletion rate of the leak can be estimated
when the user attempts to increase from the original number of moles of
helium leak testing sensitivity by the helium in the reservoir and the original
technique of pressuring up. This leakage rate as follows. The leak rate N(t)
technique is used, for example, when a at a time t after the original calibration
large volume test object or system is N(to) can be determined from
tested with helium tracer gas and leaks are
detected with a detector probe. The vessel ( ) ( )(6) − N(t o ) ⋅ t
under test is originally filled with air at C(t o )
atmospheric pressure. The calibrated Nt = N to e
capillary leak is attached to the vessel and
absolute pressure is raised to a total of where the leakage rates are in mole per
200 kPa (2 atm) by injection of helium
tracer gas. Then compressed air or second, C(to) is the original number of
nitrogen is forced into the vessel, raising moles of gas in the reservoir and t is the
its absolute pressure even higher, for
example, to 400 kPa (4 atm). (For large elapsed time since the original calibration.
Calibrated Reference Leaks 91
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The amount that the leakage rate changes Frequently a linear approximation is
as a function of time depends on the used:
design of the calibrated leak and on usage
conditions and can vary from less than [ ]( )(8) Q = Qc 1 + a T − Tc
1 percent per year to more than
20 percent per year. where Qc is the leak rate at the calibration
temperature, Tc is the calibration
Correction for temperature, T is the temperature at test
Temperature of Helium conditions and a is a linear temperature
Membrane Standard Leaks coefficient, about 0.03 °C–1 (0.05 °F–1).
Using the linear temperature expression
The permeation rate of helium through will generally give adequate
glass is described by an exponential representation over small temperature
expression: variations, less than 5 °C (9 °F). For
temperatures differences of 30 °C (54 °F),
(7) Q = AT e − b errors as large as 75 percent can be made
T using this simplifying assumption. For the
lowest uncertainties the leak should be
where A (Pa·m3·s–1·K–1) and b (K) are calibrated close to the temperature at
which it will be used. A rough
constants and T is the absolute approximation to the linear correction is
given in Fig. 15.
temperature (K). Table 2 gives typical
values of the temperature coefficient b.3 Glass Capillary Leaks for Tracer
Gases Other than Helium
TABLE 2. Temperature coefficients (measured by the
National Institute of Standards and Technology) and For gases other than helium, such as
corresponding glass types for helium permeation leaks. argon, neon or hydrogen, permeability
rates in glass become small. The most
Temperature Probable common calibrating leak for these gases is
Coefficient (K) Glass Type a glass capillary leak (glass being chosen
for its ease of fabrication of small capillary
≤ 2500 borosilicate tubes and orifices). There are two areas in
2700 fused silica which these glass capillary leaks differ in
3000 Pyrex® 7740 characteristics from glass membrane leaks.
3600 Corning® 7052
1. Depletion of internal pressure due to
FIGURE 14. Decline in leakage rate as a function of depletionPercent loss in leakage rateaging of capillary glass leaks is a
rate. function of the length of time the
standard leak is in use, because the
100 rate of gas flow in a capillary leak is a
function of the total pressure drop
10 across the reference leak (not the
helium partial pressure difference that
1 controls the flow rates of helium
through glass membrane leaks). In a
capillary leak, there is no helium flow
unless the leak is being pumped on a
vacuum pump or leak detector system.
2. Glass capillary standard leaks exhibit a
negative temperature coefficient. This
means that the capillary tube or orifice
must decrease in diameter as
temperature rises. This diameter
reduction reduces the gas flow at a
faster rate than the internal pressure
rise increases the flow rate as
temperature rises.
0.1 1 10
0.1
Annual leak depletion rate for leak
(percent for year)
Legend
= 4 years = 6 months
= 2 years = 3 months
= 1 year
92 Leak Testing
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FIGURE 15. Temperature correction factor for a silica membrane standard helium leak used at
operating temperatures that differ from temperature during initial calibration. To correct
calibrated leakage rate for temperature, multiply by correction factor.
3.0
2.5
2.0
1.9 Fahrenheit
1.8
1.7
1.6
1.5
Correction factor
1.4
Celsius
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
–50 –40 –30 –20 –10 0 10 20 30 40 50
(–90) (–62) (–54) (–36) (–18) (18) (36) (54) (72) (90)
Temperature difference, °C (°F)
Calibrated Reference Leaks 93
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PART 4. Calibration of Standard Reference
Leaks
Commercial Sources for where Q is leakage rate (Pa·m3·s–1), P is
Calibrated Leaks pressure in the gas volume (pascal),
V2 – V1 is volume displaced during travel
Commercially available permeation leaks of the indicating fluid (cubic meter) and t
have been limited to helium in the past is time (second).
because glass elements were
predominantly used. It is now possible to Limitations of Isobaric Volume
obtain polymer permeation elements that Change Leak Calibration
function with other gases including
argon, sulfur hexafluoride and many The primary limitation of the isobaric
refrigerants. In addition many calibrated volume change technique is the size of
physical leaks are also commercially the capillary tube involved in the volume
available. The choice of gases in these measurement (see Fig. 16a). It is difficult
physical leaks, predominantly capillary to obtain a liquid that can be placed in a
type, are large and include most small capillary tube and that subsequently
noncorrosive, nontoxic industrial gases. can be forced out the other end. For this
reason, the practical limitation of the
Calibration Techniques for capillary tube technique of volume
Artificial Physical displacement measurement is in the range
Reference Leaks of 1 × 10–6 m3·s–1 (2 × 10–3 ft3·min–1). It
would theoretically be possible to use a
Calibrated leaks are available with eight slightly larger capillary and to take longer
decades of leakage values. Because of this periods of time between readings but
large range of leakage, calibration is errors might arise from permeation of gas
difficult. The five techniques of measuring either through the liquid slug or through
leakage rates are (1) isobaric volume
change, (2) pressure rise, (3) pressure FIGURE 16. Leak calibration by isobaric volume change:
drop across a known conductance, (a) with capillary tube; (b) with differential pressure gage.
(4) pressure measurement at constant
pumping speed and (5) comparison. (a)
Isobaric Volume Change Gas at atmospheric pressure
Calibration of Standard Leaks
Graduated capillary
A schematic diagram of the equipment
used in the isobaric volume change Slug of liquid
technique of leakage rate measurement is indicator
shown in Fig. 16. One side of the leak is
attached to a vacuum system; the other Vacuum
side is attached to a gas reservoir at
atmospheric pressure. To this reservoir is Leak
attached a capillary of known cross
section. A slug of indicating fluid is placed (b) Vacuum pump
in this capillary. As gas leaks from the
volume into the vacuum, the slug of fluid Differential
travels down this capillary to keep the
pressure in the reservoir constant. The pressure gage
leakage rate is determined by
measurement of the volume displaced by Piston P
the travel of the slug down the capillary:
Vacuum
( )(9) Q = P V2 − V1
t Leak
Gas at
atmospheric
pressure
Vacuum pump
94 Leak Testing
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the walls. An error might also be (10) Q = V d P
introduced by a change in barometric dt
pressure or a change of ambient
temperature. This becomes particularly where Q is leakage rate (Pa·m3·s–1), V is
critical in the calibration of small leaks, volume of evacuated chamber (m3), P is
because a slight temperature change pressure in chamber (Pa absolute), t is
might produce a volume change greater time (s) and dP/dt is time rate of pressure
than that due to efflux of gas. change (Pa·s–1).
Selection of Liquid for Capillary Limitations of Leak Calibration by
Slug That Indicates Volume Pressure Rise
Change
The major difficulties with the pressure
It is desirable that the indicating fluid not rise calibration technique occur in
be permeable to the gas being calibrated. measurement of pressure. The pressure in
For this reason, mercury is the preferred an evacuated system usually does not stay
indicating fluid. Because of its high constant, but gradually increases due to
surface tension, mercury cannot be placed outgassing of the walls of the chamber.
in a small capillary. This drastically limits The pressure rise due to this desorption
the size of the leak that may be calibrated must be taken into account in
with mercury. For these measurements it calculations. The type of pressure
is desirable to use a liquid with a low instrumentation to be used for the
vapor pressure so that the leak is not measurement depends on the range of
contaminated by the calibration fluid. pressures that are expected to be
Unfortunately, most liquids of low vapor measured. Table 3 lists some gages that
pressure are also of high viscosity and may be used and their ranges.
make it difficult to obtain an accurate
measurement of the flow of liquid The effect of desorption on the
displaced in the capillary. These fluids also uncertainty of the measurement will
tend to form bubbles at the end of the depend on the ratio of the apparent
capillary. The added pressure necessary to leakage because of gas desorption to that
remove the bubble of liquid from the end of the leakage to be measured. It should
of the capillary prevents the experiment be recognized that the rate of desorption
from being isobaric. is usually not constant and will in general
be a function of temperature.
Piston and Differential Pressure
Gage in Isobaric Volume Change TABLE 3. Gages for pressure rise leak calibration.
Tests
Gage _______P_r_es_su_re_R_a_n_ge__________
Another technique for measuring volume
displacement is with a piston to replace Pa (lbf·in.–2)
the effluent gas. In this technique, a
differential pressure gage is used to Mass spectrometer < 10–3 (< 1.5 × 10–7)
measure the pressure in the gas volume (1.5 × 10–8 to 1.5 × 10–14)
and the piston is manually pushed into Molecular drag 10–4 to 10–10 (1.5 × 10–5 to 1.5 × 101)
the volume at such a rate as to keep the
pressure constant. The pressure gage need Capacitance diaphragm 10–1 to 105
not be calibrated because the readings are
made only when the differential pressure FIGURE 17. Leak calibration by pressure rise technique.
gage is at zero indication. This technique
can readily measure leakage as low as Gas at Pressure
10–9 Pa·m3·s–1 (10–8 std cm3·s–1). atmospheric gage
Pressure Rise Calibration pressure P
of Standard Leaks
Leak
The second technique of calibrating leaks
is by means of the pressure rise technique. Vacuum
A leak and its gas supply are attached to
an evacuated chamber of known volume Vacuum pump
in the arrangement sketched in Fig. 17.
The leaking gas is allowed to accumulate
in this volume and the pressure rise is
measured at various intervals. The leakage
may then be computed by Eq. 10:
Calibrated Reference Leaks 95
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Practical Example of Leak leak. Again, the pressure rise of the
Calibration by Pressure Rise evacuated system would be measured, this
time with the valve open so that air enters
As noted just previously, the pressure rise through the standard leak to be
technique for calibrating a standard calibrated. The rate of pressure rise is
reference leak in a laboratory depends on higher with the leak in place and the
the outgassing surface area as well as the curve from the second test with the leak
volume of the system to be evacuated. admitting air to the evacuated chamber
The upper size limit for large leaks to be would be higher than the initial curve, as
measured by this technique would be indicated by the higher curve of Fig. 18.
governed mainly by the largest size of test The vertical difference between the two
volume that could be realistically placed curves (with and without the leak opened
within a laboratory. Probably leaks as to the evacuated chamber) indicates the
large as those with 1 Pa·m3·s–1 theoretical rate of rise or pressure due to
(10 std cm3·s–1) leakage rates would be the leak.
near the upper limit.
With this number, together with the
The size limit for small leaks measured values for system volume and test time,
by the pressure rise calibration technique the rate of leakage through the standard
would be governed by the accuracy of leak under calibration test can be
measurement of the volume of the calculated. Figure 19 shows the relation of
evacuated test system and the accuracy pressure difference to the elapsed test time
with which the pressure change could be and approaches a linear (straight line)
measured. These may place the lower relationship. The leakage rate is computed
limit of leak size in the range of leakage in SI units from the relation
rates from 10–5 to 10–6 Pa·m3·s–1 (10–4 to
10–5 std cm3·s–1). (11) Q = V ∆ P
t
In a practical industrial laboratory,
calibration would be performed by where V is volume (cubic meter), ∆P is
measuring the rate of pressure rise of a pressure difference (pascal) and t is
well conditioned evacuated system volume elapsed test time (second). For example,
when closed off from external sources of for the case shown in Fig. 19, the
gases. The result may be a curve similar to calculation is as follows:
the lower curve shown in Fig. 18.
Q = 0.382 × 0.0169
Following that test, the reference 700
physical standard leak would be attached
to the same test volume, which would be = 9.2 × 10−6 Pa ⋅ m3 ⋅ s−1
evacuated to the same vacuum level as in
the first test, with the valve closed = 9.2 × 10−5 std cm3 ⋅ s−1
between the chamber and the standard
FIGURE 18. Pressure rise as a function of time elapsed after FIGURE 19. Pressure difference resulting from leakage
evacuating test chamber, during calibration tests of physical through standard leak.
reference leak.
20.0 (150) Air
Pressure rise with leak attached 18.7 (140)
17.3 (130 100 200 300 400 500 600 700
13.33 (100) 16.0 (120) Pressure rise elapsed time (s)
Pressure, mPa (µtorr) 14.7 (110)
Pressure, mPa (µtorr) 13.3 (100)
Pressure rise without leak 12.0 (90)
10.7 (80)
1.33 (10) 9.3 (70)
8.0 (60)
0 100 200 300 400 500 600 700 6.7 (50)
Pressure rise elapsed time (s) 5.3 (40)
4.0 (30)
2.7 (20)
1.3 (10)
00
0
96 Leak Testing
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Calibration of Standard Calibration of Standard
Leaks by Pressure Drop Leaks by Pressure
across a Known Measurement at Constant
Conductance Pump Speed
A third technique of measuring leakage A fourth technique of calibrating the flow
rates is by measuring the pressure drop of a leak is by measuring the pressure it
across a known conductance C. This produces in a vacuum system that is
technique is illustrated in Fig. 20. The pumped at a known speed (see Fig. 21).
pressure drop (P1 – P2) across a known This is the limiting case for Eq. 12, when
conductance is proportional to the flow P2 is zero. The equation then being used
rate Q, as indicated by Eq. 12: takes the form of Eq. 13:
( )(12) Q = C P1 − P2 (13) Q = S P
With molecular flow the conductance where S is the pumping speed (m3·s–1) of
C may be designed from theoretical the system (usually governed by an
grounds and such a conductance can be orifice) and P is the ultimate pressure
accurately constructed. (pascal) attained within the vacuum
chamber while being pumped. The system
Limitations of Pressure Drop Leak is usually constructed so that the
Calibration Technique pumping speed is controlled by molecular
kinetics considerations and can be
The major difficulty with the pressure rigorously calculated from theoretical
drop calibration technique is in obtaining grounds. The disadvantage of the
accurate pressure measurements. pumping speed technique is, again, that
Ionization gages have been used for the the pressure of the system must be
pressure measurement in evacuated accurately measured. If the leakage Q and
systems, but their readings are often the pumping speed S are known, P can be
questionable. Because their sensitivities derived using the above equation. This
are more often in doubt, pressure drop type of system has also been used to
leakage tests are also used to calibrate calibrate pressure gages.
ionization gages.
Mass Spectrometer As Pressure
An alternative to using an ionization Gage in Leak Calibrations
gage is to use a molecular drag gage,
sometimes referred to as a spinning rotor If a mass spectrometer is used as the
gage. This instrument is stable with time pressure gage, some accuracy is gained
and can achieve accuracies of ±10 percent because the error due to outgassing is
even if uncalibrated over the pressure minimized. The pumping speed system
range of 10–4 Pa to 10–1 Pa (1.5 × 10–8 to has essentially the same flow pattern as
1.5 × 10–5 lbf·in.–2). the mass spectrometer leak detector. In a
mass spectrometer leak detector, Eq. 13
takes the form of Eq. 14:
FIGURE 20. Leak calibration by pressure drop across a known FIGURE 21. Leak calibration by pressure measurement at
conductance. constant pumping speed.
Gas at Pressure Gas at Pressure
atmospheric gages atmospheric gage
pressure P1 P2 pressure P
Leak
Leak
Limiting
conductance
Conductance
Vacuum pump Vacuum pump
Calibrated Reference Leaks 97
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(14) Q = S K a = C1 S changes and that the pumping speed of
the system is stable over the testing
where Q is leakage rate (Pa·m3·s–1); S is period. To minimize uncertainties due to
pumping speed, a constant (m3·s–1); K is nonlinearities in the mass spectrometer
conversion factor for pressure from the calibrated leak should be closely
collector current reading (Pa·mA–1); a is matched to the unknown leak. For the
collector current reading in mass most accurate results, it is usually
spectrometer (mA); and proportionality necessary to have the two leaks register
constant C1 equals Ka (Pa). In most cases, signals in the same decade of the
a and K are not known, but a measuring instrument.
proportionality constant C1, the product
of these two numbers, is used. Providing Matching Standard
that the response of the leak detector is Leakage Rate to
linear, the mass spectrometer can be used Permissible Leakage Rates
to calibrate leaks by comparison to
standards calibrated by other techniques. The standard or reference leakage rate
used in leak testing should be of the
Comparison Calibration approximate value of the permissible
leakage rate of the test object. This must
A fifth type of calibration is by be so if the response of the detector to
comparison with a calibrated leak whose leakage is not linear. The smaller the
measurement is traceable to the National standard leakage rate, the greater the
Institute of Standards and Technology. difficulties associated with it. If the
This technique can be used over a wide standard leak is substantially different
range of leakage rates, 10–11 to from the permissible leakage (a
10–3 Pa·m3·s–1 (10–10 to 10–2 std cm3·s–1) contingency that may result from the
and with a wide range of gases. With a difficulty of making small standard leaks),
mass spectrometer type leak detector the response of the detector to different
(helium only) a calibrated leak may be leakage rates becomes important.
compared to a leak whose leakage rate is
to be determined. The leakage rate is Calibration of Standard
calculated with the following expression: Leaks with Different Gases
(15) Q unk = Q std H unk Basic leakage rate measurements are
H std necessary for the calibration of primary
standard leaks used in connection with
where Qunk is the leakage rate of the tracer gas leakage rate measurement
unknown leak and Qstd is the leakage rate systems. Fortunately, controlled
of the calibrated leak, H is the mass laboratory conditions are practical for
such calibrations and time is not an
spectrometer signal corresponding to the essential factor. Figure 22 shows
schematically two basic systems, constant
two measured leakage rates. This equation
assumes that the mass spectrometer gives
a linear response to partial pressure
FIGURE 22. Standard leak calibration, dQ/dt: (a) pressure change calibration system;
(b) volume change calibration system.
(a) Pressure gage
(McLeod)
}Gas supply or vacuum V { Vacuum system
Leak
(b) Pressure gage
}Gas supply or vacuum { Vacuum system
Leak
V
98 Leak Testing
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volume and constant pressure. The Closing
leakage rate Q for the pressure change
calibration system of Fig. 22a is given by Calibrated leaks have a vital role in leak
Eq. 16: testing programs. Attention to the proper
calibration techniques can enhance the
(16) Q = d (PV ) = V dP operator’s understanding of the test
dt procedure and hence improve the
dt reliability of the leak testing being
performed.
The leakage rate Q for the volume
change calibration system of Fig. 22b is
given by Eq. 17:
(17) Q = d (PV ) = P dV
dt dt
It may be noted that no reservoir is
shown for the leak in Fig. 22. Elimination
of a fixed upstream leak reservoir has two
important advantages. First, using a
vacuum upstream permits an evaluation
of outgassing and other extraneous
sources of gas arising in the calibration
system. Second, the same leak element
can be calibrated for many leakage rate
values for various gases simply by varying
the upstream gas and pressure.
For leaks in the 10–9 Pa·m3·s–1
(10–8 std cm3·s–1) range, accumulation
times as long as a week have been used
for increasing measured pressure change
in the constant volume manifold V of the
V(dP/dt) calibration system. Conversely,
times of the order of 100 s have been used
for increasing the pressure in the known
volume V from an insignificant pressure
to an arbitrary pressure of 0.4 Pa
(5.8 × 10–5 lbf·in.–2) in the P(dV/dt) system
for leaks in the 10–5 Pa·m3·s–1 range.
The procedure for this second
technique is as follows. The known
volume V is evacuated to a negligible
pressure, for instance less than 10 mPa
(1.5 × 10–6 lbf·in.–2), and then valved off.
The valve to the vacuum system is then
closed; thereafter, gas from the leak is
admitted to the manifold. At the instant
the pressure in the manifold attains a
preselected value P, a timer is started.
Opening the valve to V will lower the
manifold pressure temporarily, but the
pressure will again increase steadily
because of the continued inflow of gas
from the leak. When the pressure again
climbs to the value P, the time is stopped.
The only difference between the
conditions when starting and stopping
the time is that the pressure in V
increased from essentially O to P.
Calibrated Reference Leaks 99
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References
1. Nondestructive Testing Handbook,
second edition: Vol. 1, Leak Testing.
Columbus, OH: American Society for
Nondestructive Testing (1982).
2. Ehrlich, C.D. and J.A. Basford.
“Recommended Practices for the
Calibration and Use of Leaks.” Journal
of Vacuum Science and Technology A —
Vacuum, Surfaces, and Finishes. Vol. 10,
No. 1. New York, NY: American
Institute of Physics, American Vacuum
Society (Jan.-Feb. 1992): p 1-17.
3. Abbott, P.J. and S.A. Tison.
“Commercial Helium Permeation Leak
Standards: Their Properties and
Reliability.” Journal of the Vacuum
Society of America A — Vacuum,
Surfaces, and Finishes. New York, NY:
American Institute of Physics,
American Vacuum Society (May-June
1996): p 1242-1246.
100 Leak Testing
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4
CHAPTER
Safety Aspects of Leak
Testing
Gerald L. Anderson, American Gas and Chemical
Company, Northvale, New Jersey
Robert W. Loveless, Nutley, New Jersey
Charles N. Sherlock, Willis, Texas
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PART 1. General Safety Procedures for Test
Personnel
Test Personnel Dedication The United States Department of
to Safety Procedures Transportation is responsible for the rules
governing training requirements for
The range of applications of leak testing is handlers of hazardous materials
so wide and varied that no single set of (HAZMAT). The Code of Federal
safety rules for protection of personnel Regulations1 states the requirement that
and property can be made to cover all hazardous materials handlers receive
cases. Leak testing personnel must be training at least every two years by
made aware of job hazards and be someone licensed to provide such
receptive to proper training to protect training.
themselves and others working close by.
On many jobs, testing must be performed Hazards in Leak Testing
at odd hours and under awkward
conditions. Nightshift work, weekend Precleaning of test surfaces is required for
work and work in unheated areas in leak testing where surface contamination
winter and uncooled areas in summer are might prevent entry of fluid tracers. Many
common. Climbing through manholes, cleaning processes involve liquid solvents
climbing ladders and scaffolds, balancing and vapors, some of which present
on structural members or other awkward possible hazards of flammability, toxicity
maneuvers may all be in a day’s work. or asphyxiation. Liquid leak tracers often
have similar hazards, if vapors accumulate
In addition to technical abilities and in working areas. Ventilation must be
training in test procedures, competent provided to prevent hazardous vapor
technicians must have other attributes. concentrations. Electrical systems must be
They must be determined to do a safe job properly grounded and enclosed or
under any circumstances. They must be protected to prevent ignition of
willing to listen and to cooperate with the flammable vapors in air. Access to test
many types of personnel encountered in surfaces, particularly on large structures,
the field, but they must not compromise can be hazardous if scaffolding is
the safety aspects of their work for the inadequate, lighting is insufficient or bad
convenience of themselves, their crew or housekeeping creates hazards such as oily
someone else. work surfaces or obstructions in
passageways.
Need for Safety Training of Test
Personnel Special Safety
Considerations in Testing
Test personnel can acquire a proper Systems under Pressure
attitude and point of view toward safety
only through training coupled with When a pressure or a vacuum vessel is
experience. The training program should fabricated, some means of testing this
include first aid and lifesaving techniques. vessel must be used to predict safe
In situations where irritating, toxic or performance. It is sometimes necessary to
corrosive dusts, gases, vapors or fluids are exceed the designed operating conditions
present, test technicians should be given during initial pressure testing. This
special training to make sure that they are pressurization requires many safety
familiar with the properties of these considerations to ensure proper protection
substances and with the methods of of personnel. Greater respect for high
controlling the hazards. Emergency pressure has led to increased safety
procedures must be learned and test emphasis, with the result that overall
personnel must know where medical and safety experience has been good.
hospital assistance is available at all hours.
Leak testing technicians should have
more thorough training in accident
prevention than the regular plant or
construction workers. For leak testing
personnel, safety involves not a set
pattern of activity but a complex and
constantly changing set of problems.
102 Leak Testing
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Psychological Factors and disasters. In today’s industry it is the
the Safety Program responsibility of the employer to provide
employees adequate training on safety
The nature of leak testing work dictates practices for for their job responsibilities.
that a competent safety program be used.
Much of the success of such a program
depends on its acceptance by those to
whom it is directed. Never has there been
a safety device or a safety program that
some human being could not disrupt or
impair. The human factors that operate at
all levels in industry are perhaps the most
potent factors for success or failure of a
safety program. The president of a
company, the safety director and the leak
testing supervisor may either emphasize
safety or subordinate it to production
goals. Production, maintenance and
testing personnel are also important
contributors to safety and their full
cooperation is vital. Individual differences
affect personnel acceptance of a safety
program. These differences must be
recognized when motivating work groups
to use good safety practices at all times.
The safety program must be designed
with an understanding of motivation of
people. To want something is to be
motivated, but not to want something
also requires motivation. To use a safety
device to protect one’s fingers from a saw
shows motivation for safe practice.
However, the desire to ignore a safety
device that interferes with production is
caused by still other motives. Conflicting
motivations should also be considered in
any attempt to understand human
relations that influence the success of
safety programs. Industry has recognized
the effects that attitudes can have on
production, plant morale and plant safety.
As a result, management should spend
considerable effort to determine the
attitudes of its workers. Measuring,
developing and changing attitudes
constitute a major problem for personnel
and psychologists and are of extreme
importance to the safety program.
Personnel Safety Training
Requirements
There should always be concern with
safety training of personnel. The learning
process starts at birth. Most early safety
training is through experience, as when a
child may have touched a stove and been
burnt, played with a knife and been cut or
fallen from a precarious treehouse and
broken a bone. However, personnel
testing today’s vessels that hold gases,
vapors and liquids at various temperatures
and at pressures ranging from high
vacuums (in nanopascal) to high pressures
(in megapascal) cannot afford to learn
safety by causing or experiencing
Safety Aspects of Leak Testing 103
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PART 2. Control of Hazards from Airborne Toxic
Liquids, Vapors and Particles
Toxic Gas and Vapor Ventilation to Reduce
Sensors and Alarms Vapor Hazards in Solvent
Use Areas
Detection and warning of the presence of
toxic vapors or gases in a work area can be Many applications of leak testing in
provided by various types of electronic various industries have, as a prerequisite
instruments with detectors and alarm to testing, some cleaning operation. This
systems responsive to many different operation often uses volatile solvents that
airborne chemicals, fumes, smoke or can contaminate the air within
particulate matter. For general protective enclosures; therefore, some consideration
service applications, wall mounted, must be given to ventilating the working
self-contained monitors can detect and areas with explosion-proof equipment.
provide audible signals of the presence of Local exhaust systems have several
various combustible gases, fumes and inherent advantages over general
microscopically sized airborne particulate ventilation for removal of atmospheric
contaminants. These are typically contaminants. They permit removal of
provided with pilot lights to indicate the hazardous vapors before they spread
presence of alternating current line power throughout the work area, they provide
and standby battery power. Flashing red economy of air flow and they involve less
lights actuated when abnormal heat loss. Local exhaust systems are
concentrations of contaminants occur. impractical where the contaminant is
The alarm sensitivity control can be usually a solvent vapor. Local exhausts
adjusted to allow compensation for the may be unsuitable because there are a
normal ambient quiescent atmospheric multitude of sources of vapor, or the
contamination levels. source may be extensive, or the amount
of ductwork to connect all the necessary
The sensor assembly of a typical gas hoods may be too costly or impractical.
monitor and alarm system contains a
heated semiconductor element whose The basic purpose of volatile solvents
resistance to current flow varies as a used in industrial cleaning operations is
function of the type and quantity of gas to dissolve or loosen contamination such
molecules adsorbed on its surface. The as grease, dirt and other impurities and so
heater effectively boils off adsorbed facilitate their removal. The solvent may
contaminants. The sensor resistance is tend to evaporate into the atmosphere.
thus primarily a function of the adsorbed This evaporation of volatile constituents
gas molecules, whose number is related to leaves behind some physically changed
their relative concentrations in the substance that must be removed from test
ambient air atmosphere. The sensor is surfaces. Thus, the use of solvents in these
designed for more than 50 000 exposures processes involves polluting the air with
and can detect 50 µL·L–1 of many vapor. The aim of the safety engineer is to
combustible and toxic gases and vapors, keep this vapor concentration as low as
including those listed in Table 1. possible, certainly below the toxic limit. If
local exhaust systems are inadequate, such
Selecting Leak Testing Sites with widely distributed solvent vapors can
Adequate Ventilation sometimes be controlled by diluting the
general room atmosphere with outdoor
When possible, testing of structures such air fast enough to keep the concentration
as pressure vessels should be performed in of toxic vapor in the air of the working
a well ventilated area isolated far from space within safe limits.
other processes such as welding or
grinding. A room is desirable with a high Ventilation Rate
roof, adequately ventilated at its apex and Calculations for Safe Use
with enough low level inlets. Conversely, of Vaporizing Solvents
a small room with a low roof and a
minimum of opening for ventilation The rate of solvent evaporation can easily
should not be used for testing with be ascertained, as can the chemical nature
potentially dangerous tracer gases such as
hydrogen.
104 Leak Testing
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of the solvent. It is known that the weight toxicity, order and experience. Equation 1
of a given volume of vapor that converts to Eq. 2 in English units:
evaporates from a liquid is proportional to
its molecular weight. It is possible, then, 4 × 108 W
to calculate how much air must be mixed M
with a solvent vapor to hold the ( ) ( )(2)
concentration down to safe limits. VR = VDC
Table 2, from which general ventilation
can be calculated, is based on the formula where VR is rate of ventilation (ft3·min–1);
of Eq. 1 (in SI units): W is rate of solvent evaporation
(lbm·min–1); M is molecular weight of
2.4 × 107 W solvent (unified atomic mass unit); and
M
( ) ( )(1) VDC is ventilation design concentration
VR = VDC (from Table 2).2
where VR is rate of ventilation (m3·min–1); Neither the maximum allowable
W is rate of solvent evaporation
(kgm·min–1); M is molecular weight of concentration (MAC) nor the threshold
solvent (unified atomic mass unit); and
limit value (TLV) should be used for
VDC is ventilation design concentration
calculating the ventilation design
(from Table 2).
concentration. The degree of vapor
Equation 1 does not give the
dilution in the working space is bound to
maximum acceptable concentration for
be uneven. In addition, the
the compound. Instead, it is the
concentrations must always be
ventilation design concentration that has
maintained below the MAC or TLV to
incorporated in it a safety factor based on
provide a factor of safety. In turn, this
factor of safety depends on whether the
solvent vapor is to be controlled because
TABLE 1. Combustible and toxic gases and vapors detectable by area monitors and
alarm systems.
Acetaldehyde 1,1 dichloroethane 2-hexanone Nitroglycerin
Acetone 1,2 dichloroethane Hexone Nitromethane
Acetonitrile Diethylamine Hydrogen Nitrotoluene
Acetylene tetrabromide Diethylamino ethanol Hydrogen bromide Ozone
Alcohol Diisobutyl ketone c-hydrogen chloride Pentane
Allyl alcohol Dimethylamine Hydrogen cyanide 2-pentanone
c-allylglycidylether Dimethylaniline c-hydrogen sulfide Perchloroethylene
Ammonia Dimethylformamide Isoamyl alcohol Petroleum distillate
Benzene 1,1 dimethylhydrazine Isobutyl alcohol Phenylether
Benzoyl chloride Dinitrobenzene Isopropyl alcohol Propane
Benzoyl peroxide Dinitrotoluene Ketone Propargyl alcohol
Butane Dipropylene glycol Liquid propane gas Propylene oxide
2-butanone (MEK) Methane Propyne
2-butoxyethanol methyl ether Methyl acetylene Refrigerant-11, -134a etc.
Butyl acetate Epichlorhydrin Methylal Steam
Butyl alcohol 2-ethoxyethanol Methyl alcohol Stibine
Camphor Ethyl alcohol Methylamine Sulfur dioxide
Carbon monoxide Ethylamine Methyl n-amyl ketone Sulfur hexafluoride
Carbon tetrachloride Ethyl benzene Methyl butyl ketone Tetrachloronaphthalene
Chloroacetaldehyde Ethyl bromide Methyl cellosolve Tetranitromethane
Chlorobenzene Ethyl butyl ketone Methyl chloride Toluene
c-chloroform Ethyl chloride Methyl chloroform 1,1,1 trichloroethane
1-chloro-1-nitropropane Ethyl ether Methylcyclohexane 1,1,2 trichloroethane
Chloropicrin Ethyl formate Methylcyclohexanol Trichloroethylene
Chloroprene Ethylenediamine Methylene chloride Trichloronaphthalene
Cumene Ethyl dichloride Methyl ethyl ketone 1,2,3 trichloropropane
Cyclohexane Ethylene oxide c-methyl mercaptan Trinitrotoluene
Cyclohexanol Formaldehyde Naphtha Turpentine
Cyclopentadiene Furfuryl alcohol Naphthalene Xylene
DDT Gasoline Natural gas
Diacetone alcohol Glycol monoethyl ether Nitrobenzene
Diazomethane Heptane p-nitrochlorobenzene
Diborane Hexachloroethane Nitroethane
Hexane
Safety Aspects of Leak Testing 105
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of its inherent toxicity or its disagreeable directly involves the work space volume.
odor. The rule of thumb based on room air
changes per minute, thus in widespread
Example of Ventilation Rate use over many years, has been used
improperly more often than properly. This
Calculation is especially true when there are
unwanted contaminants being released
For example, suppose that 3 L of methyl within the space.
ethyl ketone were evaporated per hour.
One liter of methyl ethyl ketone requires Example of Evaluation of Health
a ventilation quantity of 1800 m3 of air;
3 L would then require 3 times 1800 Hazard from Dilution Rate Table
equals 5400 m3 of air. If this is needed per
hour, the ventilation rate per minute The following is an example in which the
would be 5400 divided by 60 equals degree of health hazard resulting from a
90 m3·min–1. (Users of the English system solvent exposure is to be evaluated using
should use a conversion of 35 ft3·m–3 and data from Table 2. Trichloroethylene is
2 pt·L–1, or 17 ft3·pt–1 for each m3·L–1.) being used in an enclosed 6 × 6 × 3 m
work space. In an 8 h day, 20 L of the
It is important to note that this solvent are lost through evaporation.
example assumes there is perfect mixing There are two air changes per hour. Is
of the clean air with the solvent vapor, there a potential health hazard?
but in practice this does not occur. The
ventilation rate calculated is therefore a Solution in metric units. The work
minimum. It should be increased space volume is 6 × 6 × 3 = 108 m3.
depending on other factors involved, such Ventilation rate at two changes per hour
as type and location of air diffusers, provides 2 × 108 = 216 m3·h–1. The rate of
location of people in the working space solvent evaporation is 20/8 = 2.5 L·h–1.
and relative toxicity of the vapor.
The dilution rate or ventilation ratio is
The volume of the space in which the 216 divided by 2.5 = 86 m3·L–1. The
work is done does not enter the proper ventilation ratio (from Table 2)
calculation for ventilation design should be 2700 m3·L–1. Therefore, the
concentration. This is a variance from the ventilation rate is totally inadequate and a
common practice of specifying ventilation health hazard is indicated. At least 2700
requirements in terms of number of air divided by 86 is 31 times as much
changes per minute, which of course ventilation is required for the safe
TABLE 2. Dilution rates for common industrial solvents recommended for use in ventilation design (SI units), after
Hemeon.2
Solvent Molecular Ventilation Ratio or Dilution Rated Possible
Weighta Densityb VDCc (Quantity of Air per Unit, Solvent)
(M) (kg·m–3) (µL·L–1) Complaints
(m3·kg–1) (m3·L–1) (ft3·lb–1) (ft3·pt–1) If Twice VDCc
Exceeded
Acetone 58 790 150 2800 2200 46 000 38 000 Disagreeable
Benzene 78 880 ——e ——e ——e ——e ——e ——e
Carbon tetrachloride 154 1580 ——e ——e ——e ——e ——e ——e
Ether 74 720 4300 3000 Disagreeable
Ethyl alcohol 46 790 75 2100 1600 72 000 54 000 Disagreeable
Isopropyl alcohol 60 790 250 2700 2100 34 000 28 000 Disagreeable
Methanol 32 800 150 7500 6000 45 000 37 000 Toxic
Methyl-ethyl ketone 72 810 100 2200 1800 125 000 103 000 Disagreeable
Pentachloroethane 202 1670 150 ——e ——e 37 000 37 000 ——e
PMV naphtha 110 750 ——e 1100 Disagreeable, toxic
Stoddard solvent 130 800 200 300 ——e ——e Disagreeable
Tetrachloroethane 168 1580 500 370 300 18 000 14 000 Toxic
Tetrachloroethylene 166 1620 29 900 45 000 Disagreeable, toxic
Toluene (toluol) 92 870 5 2300 6000 5000 Toxic
Trichloroethane 133 1440 100 1400 2300 480 000 790 000 Disagreeable, toxic
Trichloroethylene 131 1460 100 2600 2600 Disagreeable, toxic
Xylene (xylol) 106 880 100 1800 2700 24 000 40 000 Disagreeable
100 1800 2700 44 000 39 000
3000 30 000 45 000
75 30 000 45 000
50 000 46 000
a. Atomic mass units.
b. Same as g·L–1 or mg·cm–3.
c. Ventilation design concentration, not to be identified with values of maximum acceptable concentration or threshold values employed in appraising
conditions since all VDCs include a factor of safety.
d. Ventilation ratio (or dilution rate) is the ratio of the volume of air (m3 or ft3) to the volume or weight of solvent evaporated.
e. Dilution system is not recommended in this case.
106 Leak Testing
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operation of this facility. Note that this Because toxicity is not a definite
type of calculation is valid only if the air physical constant but rather the degree to
contaminant is uniformly distributed at a which a substance will affect living cells
relatively low concentration. Where the under certain conditions, it can be
air contaminant is localized in high measured only after recognizable changes
concentrations, more complex means of have occurred following absorption. Some
evaluating the hazard must be used. changes such as impaired judgment or
delayed reaction time may be produced at
For users of the English system, the levels too low to cause actual cell damage.
preceding example could be stated as Then too, toxicity depends on the dose,
follows. Trichloroethylene is used in a rate, means and site of absorption. Other
room of 20 × 20 × 10 ft. In an 8 h day, pertinent factors include the ambient
5 gal are evaporated; there are two air temperature and the working conditions,
changes per hour. The solution in English as well as the general state of health,
units is, for room volume, 20 × 20 × 10 = individual differences, tolerance and diet
4000 ft3; for ventilation rate, 2 × 4000 = of individual personnel.
8000 ft3·h–1.
Estimating Toxicity Values
Rate of solvent evaporation = 5 divided and Lethal Doses of Toxic
by 8 equals, in United States units, Materials
0.6 gal·h–1 or 5 pt·h–1.
The first attempts at estimating the
Ventilation ratio is 8000 divided by 5 is toxicity of a substance are usually made
1600 ft3·pt–1. Ventilation ratio according on the basis of animal experiments. Data
to Table 2 is 2700 m3·L–1 or 17 times 2700 from these experiments are expressed as
is 46 000 ft3·pt–1. At least 46 000 divided lethal doses (LD) in milligrams of
by 1600 = 29 times more ventilation is substance per kilogram of body weight of
required. the test animal. The commonly used
expressions are the following: MLD,
Evaluation of Toxicology minimum lethal dose, the smallest dose
and Health Hazards of that kills one of a group of test animals;
Materials LD50, lethal dose for 50 percent, the dose
that kills one half of a group of test
The toxicity of a material is not animals (usually ten or more); LD100,
synonymous with its health hazard. lethal dose for 100 percent, the dose that
Toxicity is the capacity of a material to kills all of a group of test animals (usually
produce injury or harm. Hazard is the ten or more). These doses may also be
possibility that a material will cause injury expressed as lethal concentrations (LC) for
when a specific quantity is used under airborne toxic substances.
specific conditions. The key elements to
be considered in evaluating a health Substances can then be rated according
hazard are the following. to their relative toxicity as shown in
animal experiments (Tables 3 and 4).4-6
1. How much of the material is needed The probable lethal dose for humans is
to produce injury? often estimated from animal tests. These
ratings are based on the results of short
2. What is the probability that the term exposures only. It is possible in
material will be absorbed by the body
to produce injury?
3. What protective equipment is in use?
TABLE 3. Combined tabulation of toxicity classes, after Roehrs and Center.3
Commonly LD50 Single 4 h Vapor Exposure LD50 Skin Exposure Probable Lethal
Used Term Oral Dose for Ratsa for Rabbits _____D_o_s_e__f_o_r_H__u_m__a_n_s_____
Causing 2 to 4 Deaths in
(g·kg–1) (g·kg–1) SI (English)
Six-Rat Group
(µL·L–1)
Extremely toxic ≤0.001 >10 ≤0.005 50 mg (taste) (1 grain [taste])
Highly toxic 0.001 to 0.05 10 to 100 0.005 to 0.043
Moderately toxic 0.05 to 0.5 0.044 to 0.340 4 cm3 (1 tsp)
Slightly toxic 0.5 to 5.0 100 to 1000 0.35 to 2.81
Relatively nontoxic 5.0 to 15.0 1000 to 10 000 2.82 to 22.6 30 cm3 (1 oz)
Practically nontoxic 10 000 to 100 000
>15.0 >100 000 >22.6 0.5 L (1 pt)
1 L (1 qt)
>1 L (>1 qt)
a. Grams of dose per kilogram of rat.
b. Parts of vapor in million parts of air.
Safety Aspects of Leak Testing 107
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actual, long term chronic exposure for a hazards, e.g., those with carcinogenic,
substance to prove highly toxic, even mutogenic, teratogenic or other
though short term exposure tests reproductive effects, although they may
indicated a low order of toxicity. However, review other effects as needed.
animal experiment data are difficult to
interpret and apply to human exposures. The permissible exposure levels of
Such data are valuable only as guides to hazardous substances that have been
be used in estimating the gross toxicity of adopted by OSHA to provide a safe,
a substance and as leads for further healthful work environment for all
investigations. persons are cited as Occupational
Standards (OSHA). These are given in an
NIOSH Evaluations of Exposure to annually updated NIOSH Registry of Toxic
Effects of Chemical Substances. NIOSH
Toxic Substances Criteria Documents contain
environmental and medical
In the United States, the Department of recommendations related to specific
Health, Education and Welfare (HEW), the substances and processes.
Occupational Safety and Health
Administration (OSHA) and the National Management and test personnel can
Institute for Occupational Safety and use NIOSH published resources to
Health (NIOSH) conduct critical reviews determine probabilities of hazards with
of occupational hazards, prepare criteria new test materials, interacting
documents, recommend standards of combinations of chemical materials and
exposure and list toxic effects of chemical environmental hazards. In all cases of
materials. Under no circumstances can the doubt, however, reference to experts in
toxic dose values presented for chemical the field for consultation and guidance is
substances be considered as being recommended.
definitive values for describing safe versus
toxic doses for human exposure. Limitations of Safety
Concentrations of chemical substances in Warnings
the work environment that may be safely
tolerated can be determined only by a This volume is limited to leak testing and
critical evaluation of all available endeavors to provide comprehensive and
pertinent data by experienced useful information and data on test
investigators. techniques and applications. It is not
possible, within its scope, to advise users
NIOSH special occupational hazard of all potential hazards and toxic or
reviews analyze and document, from a dangerous substances. In this book, only
health standpoint, the problems partial information and warnings can be
associated with a given industrial included, so workers and test personnel or
chemical, process or physical agent and management should look up more
recommend the implementation of complete data in publications from
engineering controls and work practices NIOSH and other sources for complete
to relieve these problems. The evaluations information. Qualified assistance should
pertain primarily to special alleged
TABLE 4. Guidelines for evaluating acutea dosages differentiating relatively toxic from nontoxic substances taking into
consideration the route of administration to experimental animals and the dose causing deathb. After Hine and
Jacobson5 and NIOSH 78-104A.6
Rectal 24 h Subcutaneous
Intraduodenum Inhalation Intraperitoneal Intradermal Other Unspecified
Intracervix Maximum Skin Intrapleural Implant Parenteralc Parenteral Unreported
Species (mg·kg–1) (g·kg–1) (g·kg–1) (g·kg–1) (g·kg–1) (g·kg–1) (g·kg–1) (g·kg–1)
Frog, gerbil, hamster 2.5 1.0 1.4 1.0 5.0 0.75 1.0 2.5
Mouse, rat, squirrel 5.0c 10.0c
Bird, chicken, duck, guinea, 10.0 2.0 2.8 2.0 20.0 1.5 2.0 5.0
pig, pigeon, quail, rabbit, turkey 10.0 4.0 2.8c 4.0 20.0 3.0 4.0 10.0
Cat, cattle, dog, goat, horse,
4.0 5.6 4.0 3.0 4.0 10.0
monkey, pig, sheep
a. Applies to those substances for which acute or short-term toxicity characterizes the response, e.g, fast-acting substances, irritants, narcosis-producing
substances, and most drugs. Does not apply to substances whose characteristic response results from prolonged exposures, e.g., silica, lead, benzene,
carbon disulfide, carcinogens. Concentrations more appropriately characterizing the toxicity of long- or slow-acting substances are derived from nonacute
toxicity studies.
b. Calculated from experimental data (Stokinger).
c. Intravenous, intramuscular, ocular, intracerebral, intratracheal, intraplacental, intravaginal, intrarenal.
108 Leak Testing
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be sought from experts in safety, legal The most common halogenated
requirements, governmental regulations, hydrocarbons, arranged in increasing
safety engineering, health and medical order of ability to produce narcosis, are
practice, wherever the possibility of vinyl chloride, methyl chloride, ethyl
hazards may exist. Special reference chloride, ethylene dichloride, ethyl
should be made by leak testing personnel bromide, carbon tetrachloride,
and supervision to applicable plant safety dichloromethane (also called methylene
rules; to procedures used in case of chloride), methyl chloroform (also called
accidents; to local, municipal, county, 1,1-trichloroethane and
state and national laws and regulations; 1,1,2-trichloroethane), trichloroethylene,
and to qualified safety and health methyl bromide, tetrachloroethylene (also
agencies, organizations and experts for called perchloroethylene),
advice on health and safety. The warnings pentachloroethane and tetrachloroethane
and precautions given in this book are (see a dictionary of commercial
based on experience in industry during chemicals). Tetrachloroethane is about 40
application of leak tests. They do not times as strong a narcotic as vinyl
foresee the possibilities and nature of chloride. An acute exposure to the more
potential future accidents, nor do they narcotic of these compounds may result
include the constantly changing in unconsciousness for a surprisingly long
identifications of toxic or hazardous period, with eventual recovery.
substances included in publications of Unconsciousness for eight weeks has been
governmental and other health and safety reported in a case of methyl bromide
agencies and organizations. poisoning. It is to be noted that the
preceding listing is not in the same order
Precautions with Specific as the chronic toxicity of these
Fluids halogenated hydrocarbons. Chronic
toxicity due to low rates of exposure over
Acetone and Other Ketones long periods of time has been the more
common problem in industry.
Acetone and other ketones are typical
solvents and metal cleaning compounds Tetrachloroethane, the most toxic of
used widely in industry. Acetone the common chlorinated hydrocarbons,
(dimethyl ketone) is a very flammable has no particular warning signs or
liquid that should be handled and stored symptoms. It can produce extremely
with precautions against fire and severe poisoning from continuous
explosion. In spite of the large quantities exposure to fairly low concentrations.
of acetone used in industry and its high Tetrachloroethane is a very dangerous
volatility, there are no known compound because inhalation of it at a
documented reports of serious industrial concentration barely perceptible by odor
poisoning. Experimental work has shown can lead to extensive injury. Carbon
that acetone is a narcotic. Overexposure tetrachloride, methyl chloride,
will lead to moderate irritation of the dichloroethylene and trichloroethylene
eyes, nose and throat and to headache, show decreasing chronic toxicity in
stupor and a general feeling of oppression. approximately that order. Introduction of
The absorbed acetone is eliminated slowly a bromine or iodine atom into one of the
and the symptoms are persistent. Contact halogenated hydrocarbons generally
with skin and eyes should be avoided by increases the toxicity as compared to that
the use of protective clothing. In areas of of the corresponding chlorine compound.
vapor concentration, approved respiratory In contrast, introduction of a fluorine
protective equipment should be used. atom generally reduces the toxicity as
compared to that of the corresponding
Precautions with Halogenated chlorine compound.
Hydrocarbons
The methyl compounds, particularly
Halogenated hydrocarbons are typically methyl chloride and methyl bromide, are
colorless volatile liquids with excellent in a special class because of their delayed
organic solvent properties and are widely action. Minor symptoms may appear
used. Hydrocarbons having only one or during an acute exposure to these
two halogens are usually flammable and compounds; severe symptoms may appear
less toxic than similar hydrocarbons with after a delay of several hours to several
complete halogen substitution. Thermal days.
decomposition of halogenated
hydrocarbon vapors occurs and poisonous Precautions with Carbon
gases may be formed when they come Tetrachloride
into contact with a heat source, such as a
red hot surface, flame or electric arc. Carbon tetrachloride (CCl4) is a
halogenated hydrocarbon liquid that is
colorless, nonflammable and has a
characteristic odor. Synonyms for carbon
tetrachloride include tetrachloromethane
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and perchloromethane. Carbon person may process any fully halogenated
tetrachloride is used as a solvent, chlorofluoroalkane into any aerosol
degreaser and chemical constituent and propelled article. . . .”
can act to remove the natural liquid cover
of human skin. With repeated contact Prevention of Personnel Exposures
with the skin, it can lead to a dry, scaly, to Halogenated Hydrocarbons
fissured skin condition known as
dermatitis. Chronic poisoning including Because exposure of testing personnel to
liver damage comes from long, continued halogenated hydrocarbons is almost
absorption of fairly small amounts of invariably by inhalation, the most
carbon tetrachloride over a long period. valuable measures to prevent poisoning
Barrier creams, gloves, protective clothing are enclosure and ventilation at the point
and masks should be used as appropriate where vapor is released. However, several
where exposure occurs. The major of the chlorinated hydrocarbons are
problem in prevention of injuries from apparently much more toxic by skin
carbon tetrachloride is that of prevention contact than has been believed. Skin
of inhalation of carbon tetrachloride contact should therefore be avoided
solvent vapor. Oxidative decomposition because of the probability that where
by flame causes it to form phosgene (a there is skin contact there will also be a
poisonous gas) and hydrogen chloride, severe inhalation exposure.
also a poisonous gas. Carbon tetrachloride
is now prohibited in many instances. Precautions with Aromatic
Hydrocarbons
Precautions with Fluorocarbon
and Refrigerant Gases Aromatic hydrocarbons are widely used as
solvents and chemical intermediates. The
Fluorocarbons are hydrocarbons basic aromatic nucleus is benzene, C6H6.
containing fluorine; they may contain Because of its health hazards, benzene has
other halogens in addition to fluorine. been replaced as a commercial solvent by
Generally these compounds are colorless toluene and other less toxic compounds.
nonflammable gases. Decomposition of Typically, the vapor of aromatic
chlorine-containing fluoromethanes, hydrocarbons causes central nervous
caused by contact with an open flame or system depression and other effects. Vapor
hot metal, produces hydrogen chloride, is absorbed through the lungs and the
hydrogen fluoride, phosgene, carbon liquid may be absorbed through the skin.
dioxide and chlorine. Repeated and prolonged skin contact may
cause defatting of the skin, which leads to
The fluorocarbons are used primarily as dermatitis. Chronic benzene poisoning
refrigerants, leak testing tracer gases and can be fatal.
fire extinguishers and in degreasing of
electronic equipment. They have found Precautions with Methyl Alcohol
wide use due to their relatively low
toxicity and nonflammability. Trademarks Methyl alcohol (CH3OH) is a colorless,
including Freon®, Genetron® and Isotron® volatile liquid with a mild odor. It is used
have been used for a number of in synthesis of many chemicals and as an
fluorocarbons used in refrigeration. The industrial solvent. Contact of methyl
fluorocarbon compounds may produce alcohol with the skin can produce mild
mild irritation in the upper respiratory defatting and a mild dermatitis that can
tract, perhaps caused by their be avoided by use of barrier creams and
decomposition products. Dermatitis protective clothing. Methyl alcohol is
occurs only rarely from contact with these virtually nonirritating to the eyes or upper
materials. respiratory tract at concentrations in air
below 2000 µL·L–1; it is difficult to detect
In the United States, the by odor at less than this level.
Environmental Protection Agency took
action to essentially ban the Methanol (methyl alcohol) poisoning
chlorofluorocarbons in aerosol spray cans is usually produced by swallowing the
that release the chemicals to the liquid or inhaling high concentrations of
atmosphere with each use of the can. The vapor in an enclosed area. The signs of
law itself is written in two parts, which poisoning include headache, nausea,
are integrated. The first part is vomiting, violent abdominal pains,
administered by the Food and Drug aimless and erratic movements, dilated
Administration. The second part, which pupils, sometimes delirium and such eye
covers penetrants, is administered under symptoms as pain, tenderness on pressure
the Toxic Substances Control Act. The and, occasionally, blindness. Direct action
exact wording appears in Parts 712 and of the liquid or the vapor on the skin and
762 of this act and in the Federal Register mucous membranes may produce an
of March 17, 1978. The important irritation and inflammation.
wording appears in paragraph 762.12(a),
as follows: “After December 15, 1978 no One of the peculiarities of methanol
poisoning is its exceptionally severe action
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on the optic nerve. About one half of all Precautions with Stoddard Solvent
the serious cases of methanol poisoning
result in some impairment of vision. This Stoddard solvent is a registered
loss is usually permanent and may vary commercial standard of the U.S.
from dimness or blind spots scattered Department of Commerce for a dry
through the visual field to total blindness. cleaning solvent. Its specifications are that
it has a flash point of 37.8 to 43.3 °C
Precautions with Glycols and (100 to 110 °F), evaporates without
Glycol Derivatives residue and consists of aliphatic, saturated
materials and, in some formulations, 15
Glycols are dihydric alcohols, which are to 20 percent aromatics. The fire hazard is
colorless, odorless liquids. Glycols are about that of kerosene. It is available
soluble in water and in alcohol, have high under a number of trade names.
boiling points, have low freezing points
and are used as solvents and antifreeze. Precautions with Toluene
These compounds have relatively low
toxicity and the major hazard appears Toluene is seldom a source of acute
when the liquids are heated during poisoning, although its inherent acute
processing. toxicity is somewhat higher than that of
benzene. It is a flammable, colorless liquid
Precautions with Ethylene Glycol of rather strong aromatic odor that serves
Ethers somewhat as a warning of high
concentration. At concentrations of 500
Ethylene glycol ethers are only mildly to 1000 µL·L–1, toluene is strongly
irritating to the skin. Vapors may cause irritating to the eyes and respiratory
conjunctivitis and irritation of the upper system. In higher concentration, it is a
respiratory tract. Temporary corneal narcotic and the signs of acute poisoning
clouding may also result and may last are headaches, drunkenness, nausea,
several hours. Acetate derivatives cause vomiting and ultimately unconsciousness.
greater eye irritation than the parent Toluene does not appear to produce the
compounds. The butyl and methyl ethers severe and often fatal depression of the
may penetrate the skin readily. Symptoms blood forming organs seen in chronic
from repeated overexposure to glycol benzene poisoning. In case of acute
ether vapors are fatigue and lethargy, exposure to toluene, the person should be
headache and tremor. Glasses and taken to fresh air as soon as possible.
protective clothing can be used to prevent Oxygen should be given and, if breathing
skin absorption. Respiratory protection has stopped, artificial respiration should
maybe needed if ventilation is poor or be administered immediately. A physician
glycol compounds are heated or atomized. should be called at once.
Precautions with Petroleum Precautions with
Derivatives Trichloroethylene
Naphtha is a rather indefinite term for Trichloroethylene is a halogenated
any one of a number of solvent mixtures hydrocarbon used primarily as a
derived from petroleum. One should degreasing compound. It has no flash
define it more carefully before attempting point as such, but at elevated
to assess the hazard. The naphthas are temperatures and with a high energy
irritating to the skin, conjunctiva and ignition source, such as a welding arc, its
mucous membranes of the upper vapors can and will explode. Toxic
respiratory tract. Skin chapping and decomposition products, mainly
photosensitivity may develop after hydrogen chloride with some phosgene,
repeated contact with liquid naphtha. If both highly poisonous gases, may also be
confined by clothing against the skin, the formed under these conditions. Phosgene
naphthas may cause skin burn. Workers may be formed inside a cigarette when
should use barrier creams, protective smoking in an area where
clothing, gloves and masks where trichloroethylene vapors are present.
exposure to naphtha vapor is likely.
Sufficient quantities of naphtha cause Trichloroethylene may have a
central nervous system depression. depressant action or, as with other
Symptoms include inebriation, followed chlorinated hydrocarbons, cause
by headache and nausea. In severe cases, alteration of the heart rhythm, or lead to
dizziness, convulsions and addiction. Although some absorption may
unconsciousness may result. If benzene is occur through the skin, trichloroethylene
present, coal tar naphthas may produce has mainly a defatting and dermatitis-
leukemia. producing skin effect.
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Precautions with Xylene two toxological effects from this inert gas
are asphyxiation and radiation exposure.
Xylene, C6H4(CH3)2, is a mixture of
isomers and may contain numerous other To satisfy federal and state licensing
solvent compounds. It is used as a solvent requirements in the United States, the
and is specified in some tests to detect the pressurization systems are provided with a
water content of penetrant materials. room enclosure; an exhaust system for
Xylene vapor may cause irritation to the typically 3 to 5 min room air exchange; a
eyes, nose and throat. Repeated or series of interlocking safety circuits for the
prolonged skin contact may cause drying proper exhaust air flow; and the detection
and defatting of the skin, which may lead of any radioactive gas in the room or
to dermatitis. Liquid xylene is irritating to exhaust.
the eyes and mucous membrane.
Aspiration of a few milliliters may cause The regulatory agencies monitor and
severe effects. Repeated exposure of the enforce these requirements as well as
eyes to high concentrations of xylene continuous monitoring film badges to
vapor may cause irreversible eye damage. document worker and room exposure
levels. A total dump of a typical
When xylene vapor concentrations krypton-85 leak testing system would
exceed allowable standards, full face require immediate operator evacuation of
masks with organic vapor cannisters or air the machine enclosure, which would
supplied respirators should be furnished. typically result in nondetectable radiation
Impervious protective clothing and gloves exposure as measured on state-of-the-art
should be worn by personnel exposed to film badges.
liquid xylene. Xylene wet clothing should
be changed quickly. Goggles or safety Precautions with Dry
glasses are advised. Barrier creams may be Powder Developers
useful.
Dry powder developers as used in some
Hazards of Oxygen Deficient liquid leak tracers are subject to dusting
Atmospheres and other behavior characteristics of dry
powder materials. Safety procedures such
Oxygen deficiency designates an as the following should be observed.
atmosphere having less than the
percentage of oxygen found in normal air. 1. Avoid continued excessive inhalation.
Normal air contains about 21 percent 2. Use a well fitting dust mask and
oxygen at atmospheric pressure. When
the oxygen concentration in air is reduced adequate ventilation.
to approximately 16 percent, many 3. Wear eye protection when filling or
individuals become dizzy, experience a
buzzing in the ears and have a rapid emptying a hopper.
heartbeat. 4. Any dry powder material can build
In addition to tests for toxicity, the static electricity charges when
oxygen content of the atmosphere of a subjected to the friction of mixing,
vessel or similarly confined space sliding or conveying. Proper
suspected of being oxygen deficient precautions such as adequate electrical
should be determined by preentry and grounding of equipment and not
subsequent tests made with instruments having flammable liquids in the area
approved for the purpose by the United should be taken.
States Bureau of Mines. No one should
enter or remain in a vessel or enclosed For further information, refer to
space that tests show has less than NFPA 77-1993, Recommended Practice on
16 percent oxygen in its atmosphere at Static Electricity.7
any time unless wearing approved
respiratory protective equipment such as a
fresh air hose mask or self-contained or
self-generating breathing apparatus.
Various types of self-contained
compressed air breathing apparatus,
approved by the U.S. Bureau of Mines,
have proved satisfactory in oxygen
deficient atmospheres. They are especially
useful where it is difficult to run an air
supply hose line.
Precautions with Krypton-85 Gas
Krypton-85 gas is used in leak test
pressurization systems in concentrations
near 0.01 percent in nitrogen or air. The
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PART 3. Flammable Liquids and Vapors
Definition of Terms atmosphere. In both cases, the fluids
Characterizing Flammable should be enclosed wherever feasible.
Liquids and Vapors When the fluid is exposed to air for a
specific operation, it should again be
Flammable liquids are usually subdivided covered or enclosed as soon as possible.
into classes. As defined by the National
Fire Protection Association, a flammable Flash Point of a Flammable Liquid
liquid is any liquid having a flash point
below 60 °C (140 °F) and having a vapor The flash point of a liquid is the lowest
pressure not exceeding 275 kPa absolute temperature at which it gives off enough
(40 lbf·in.–2) at 37.8 °C (100 °F). vapor to form flammable mixtures with
air and to produce a flame when a source
Combustible liquids are those with of ignition is brought close to the surface.
flash points in the range of 60 to 93 °C Other properties are factors in
(140 to 200 °F). Although they do not determining the hazards of flammable
ignite as easily as flammable liquids, they liquids, but the flash point is the principal
can ignite under certain circumstances factor. The relative hazard increases as the
and so must be handled with caution. The flash point is lowered. The significance of
more common flammable and this property becomes more apparent
combustible liquids are various when liquids of different flash points are
hydrocarbons, alcohols and their compared.
byproducts. They are chemical
combinations of hydrogen and carbon; Examples of Flash Points of common
the combination may also contain Liquid Fuels
oxygen, nitrogen, sulfur and other
elements. Kerosene and number 1 fuel oil have flash
points of about 43 to 74 °C (110 to
Factors Influencing Hazards of 165 °F) but ASTM D 396, Specification for
Flammable Liquids Fuel Oils,8 will permit a flash point as low
as 38 °C (100 °F) for number 1 fuel oil. At
Flammable liquids vaporize and form ordinary room temperatures of 22 °C
flammable mixtures when they are in (72 °F), these oils do not give off
open containers, when leaks or spills dangerous quantities of vapor. On the
occur or when the flammable liquids are other hand, gasoline gives off vapor at a
heated. The degree of danger depends on rate sufficient to form a flammable
the following: (1) the flash point of the mixture with air at temperature as low as
liquid, (2) the concentration of vapors in –45 °C (–50 °F).
the air (whether the mixture of vapor and
air is in the flammable range) and (3) the Any flammable liquid, when heated to
possibility of an ignition source at or a temperature above its flash point, can
above a temperature sufficient to cause produce vapors in sufficient quantity to
the mixture to burst into flame. produce an explosive mixture in the air.
For example, when heated, heavy fuel oil
Precautions for Flammable may produce flammable vapors just as
Liquids readily as gasoline does at –20 °C (–4 °F).
In the handling and use of flammable Autoignition Temperature
liquids, exposure of large liquid surfaces
to air should be prevented. It is not the Autoignition temperature is the lowest
liquids themselves that burn or explode, temperature at which a flammable gas or
but rather the vapor-and-air mixture vapor-and-air mixture will ignite under
formed when liquids evaporate. Therefore, defined conditions without an external
flammable liquids should be handled and source of ignition. Flammable vapors and
stored in closed containers. Low flash gases in oxygen will spontaneously ignite
liquids in use should be covered or at a lower temperature than in air and
enclosed to avoid evaporation into the their autoignition temperature may be
influenced by the presence of catalytic
substances.
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Flammability Limits of Vapor Volatility is the tendency or ability of a
liquid to vaporize. Such liquids as alcohol
Concentrations and gasoline, because of their well known
tendency to evaporate rapidly, are called
Flammable liquids have a minimum volatile liquids.
concentration of vapor in air below which
propagation of flame does not occur on Boiling Points of
contact with a source of ignition. There is Flammable Liquids
also a maximum proportion of vapor or
gas in air above which propagation of The boiling point of a liquid is that
flame does not occur. temperature at which the vapor pressure
of the liquid equals the atmospheric
The extremes of vapor or gas pressure. Increasing the liquid
concentration with air which, if ignited, temperature causes vapor to be given off
will just propagate flame, are known as more readily. Liquids with low boiling
the lower and upper flammable limits. points generally volatilize more readily
These are usually expressed in terms of than those with higher boiling points.
percentage by volume or weight of gas or However, there is not consistent
vapor in air. These limits are also relationship between boiling point and
commonly referred to as, respectively, the evaporation rate.
lower and upper explosive limits.
Definitions for Vapor Volume and
A mixture with less than about 1.0 Evaporation Rate
percent by weight of gasoline vapor is too
lean and propagation of flame will not Vapor volume is the number of liters of
occur on contact with a source of solvent vapor formed by evaporation of
ignition. Similarly, if there is more than 1.0 L of liquid at standard temperature
about 8 percent of gasoline vapor, the (20 °C). In English units, the vapor
mixture will be too rich. Other gases such volume is the number of cubic feet of
as hydrogen, acetylene and ethylene have solvent vapor formed by the evaporation
a wider range of flammable limits. of 1 gal (imperial or United States gallon),
of a liquid at 68 °F. One can always find
Flammability Ranges (Explosive vapor volume by using the mole (an
amount of gas or liquid whose weight in
Range) grams equals its molecular weight.) This
number of grams, equal to the molecular
Flammable range is the difference weight of the substance (at 0 °C and
between the lower and upper flammable 101.3 kPa), evaporates to 22.4 L at
limits, expressed in terms of percentage by standard temperature and pressure.
volume of vapor or gas in air. It is also
often referred to as the explosive range. Evaporation rate is the ratio of time
For example, the limits of the flammable required to evaporate a measured volume
range of gasoline are generally taken as of liquid to the time required to evaporate
1.4 to 7.6 percent, which is relatively the same volume of a reference liquid
narrow. Thus, a mixture of 1.4 percent under ideal test conditions. The higher
gasoline vapor and 98.6 percent air is the ratio, the slower the evaporation rate.
flammable, as are all the intermediate
mixtures up to and including 7.6 percent
gasoline vapor and 92.4 percent air. The
range is the difference between these
limits, or 6.2 percent.
Effects of Diffusion Rate, Containers for Flammable
Vapor Pressure and Liquids
Volatility
Portable containers should be provided
Rate of diffusion is the tendency of one gas with flame arresters installed in the vent
or vapor to disperse into or mix with or opening. If a number of different
another gas or vapor. This rate depends flammable liquids are handled, safety cans
on the density of the vapor or gas as should have distinct stripes, or
compared with that of air. Whether a identification lettering should be placed
vapor or gas is lighter or heavier than air on them so as to reserve certain cans for
determines to a large extent the means of their respective liquids and to help reduce
solving ventilation problems. the chance of the liquids being mixed.
Safety can caps should be regularly
Vapor pressure is the partial pressure (in inspected for proper operation and
kilopascal or in lbf·in.–2) exerted by the sealing.
vapor of a volatile liquid, when in
equilibrium with the surface of the liquid,
as determined by standard ASTM D 323,
Test Method for Vapor Pressure of Petroleum
Products (Reid Method).9
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Restriction of Smoking and
Lighters in Flammable
Material Areas
Smoking and carrying of lighters, strike-
anywhere matches and other spark
producing devices should be prohibited in
buildings or areas where flammable
liquids are stored, handled or used. The
extent of the restricted area will depend
on the type of products handled, the
design of the building design, local
conditions and compliance with local,
state and federal regulations for
flammable material areas.
Safety Aspects of Leak Testing 115
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PART 4. Electrical and Lighting Hazards
Hazards of Static Electricity A discharge of static electricity is a
with Flammable Materials possible cause of ignition, so all metal
parts likely to become charged should be
Static electricity is an accumulation of grounded. When testing with gases such
motionless charges generated by the as hydrogen, it would also be sensible for
contact and separation of dissimilar personnel to avoid wearing clothing that
materials. For example, static electricity is might produce static charges and for them
generated when a fluid flows through a to wear shoes with conducting soles.
pipe or from an orifice into a vessel and Another precaution is the use of reduced
may set up high voltages. The principal sparking or nonsparking tools.
hazards created by static electricity are
those of fire and explosion caused by Bonding and Grounding to
spark discharges occurring in the presence Prevent Electric Sparks
of flammable or explosive vapors, gases or
dust. A spark between two bodies occurs A point of great danger from a static spark
when there is no good electrical is the place where a flammable vapor may
conductive path between them. Hence, be present in the air, such as at the outlet
grounding and bonding of flammable of a flammable liquid fill pipe or a
liquid containers is necessary to prevent delivery hose nozzle. Static spark ignition
static electricity from causing a spark. sources are prevented by bonding or
grounding or both so they have the same
Avoidance of Sources of static voltage or potential.
Ignition of Flammable
Gases and Vapors The terms bonding and grounding
often have been used interchangeably
When using potentially explosive gases, because of poor understanding of the
the test area should be free from obvious distinct functions indicated. Bonding is
sources of ignition. Smoking should be done to eliminate a difference in potential
prohibited and signs should be posted to between objects. The purpose of
warn of the hazards. Electrical equipment grounding is to eliminate a difference in
may also present a problem. If there is a potential between an object and ground.
possibility that, in the event of leakage, Bonding and grounding are effectively
such equipment will be in an explosive applied only to conductive bodies. The
environment, then either the equipment human body is a conductive body that
should be repositioned outside the danger may differ in potential from ground or
area or specifically chosen equipment other bodies, so that it may also serve as a
should be used. source of spark ignition.
Although hydrogen presents the most Although bonding will eliminate a
severe risk, the above precautions are also difference in potential between the
relevant if other flammable tracer gases objects that are bonded, it will not
are used. When large components are eliminate a difference in potential
tested, or when large volumes of between these objects and the earth
hydrogen are used, it may be advisable to unless one of the objects possesses an
provide monitors that give a continuous adequate conductive path to earth.
indication of the hydrogen and air Therefore, bonding will not eliminate the
content in the test area. Intrinsically safe static charge but will only equalize the
detectors are available. Gas monitoring potential between the objects bonded.
may also be advisable when high vacuum
vessels are being chemically cleaned Electrical Power Hazards
before evacuation. Cleaning techniques
often include washing with benzene, Electricity as a source of power is, in some
acetone or alcohol. The interior of the ways, less hazardous than steam or other
vessel as well as the environment may energy sources. However, failure to take
contain an explosive mixture. Extreme suitable precautions in its use creates
precautions should be taken when using conditions that are certain to result in
these materials. bodily harm or property damage or both.
Although there have been advances in the
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control of electrical hazards, industry still Effects of Electric Current
has many injuries and fatalities from on Human Body
preventable causes. Machine tools can,
with minimum expense and difficulty, be Death or injury by electric shock may
arranged for maximum safety and result from the following effects of current
efficiency. There are, however, certain on the body.
hazards in the installation, maintenance
and use of electric wiring and equipment. 1. Electric current may cause contraction
Control of most of these hazards is of the chest muscles, which may
neither difficult nor expensive, but interfere with breathing to such an
ignoring or neglecting them may lead to extent that death will result from
serious accident. asphyxiation when the exposure is
prolonged.
Electrical Injury and Fatal Levels of
Body Current 2. Electric current may cause temporary
paralysis of the nerve center, which
Current flow is the factor that causes may result in failure of respiration, a
injury in electric shock. The severity of condition that often continues until
electric shock injury is determined by the long after the victim is freed from the
amount of current flow through the circuit.
victim. Experimental and field data from
authoritative sources indicate that, in 3. Electric current may interfere with
general, an alternating current of 0.1 A at normal rhythm of the heart, causing
commercial frequency (60 Hz) may be ventricular fibrillation. In this
fatal if it passes through the vital organs. condition, the fibers of the heart
Similarly, it is estimated that a current muscles, instead of contracting in a
value of 0.02 A is the limit at which an coordinated manner, contract
individual can still release himself from separately and at different times.
an object held by the hand. Such current Blood circulation ceases and death
flow may readily result from body contact ensues, because apparently the heart
with low voltage sources of ordinary cannot spontaneously recover from
lighting or power circuits. this condition. It has been estimated
that 0.1 A flowing through the body
Limiting Current Flow to cavity (chest) is sufficient to cause
Human Body ventricular fibrillation.
Because current flow depends on voltage 4. Electric current may suspend heart
and resistance, these factors are action by muscular contraction (on
important. Other factors affecting the contact with heavy current). In this
amount of injury are the parts of the body case, the heart may resume its normal
involved, the duration of current flow rhythm when the victim is freed from
through the victim and the frequency the circuit.
with alternating current. Resistance to
current flow is mainly to be found in the 5. Electric current may cause
skin surface. Callous or dry skin has a hemorrhages; destruction of nerves,
fairly high resistance, but a sharp decrease muscles or other tissues; or extensive
in resistance takes place when the skin is skin burn from heat due to heavy
moist. Once the skin resistance is broken current or electric arcs.
down, the current flows readily through
the blood and body tissues. Grounding In general, the longer the current flows
conditions often determine resistance to through the body, the more serious may
current flow from the human body to be the result. Considerable current is
earth or grounded structures. likely to flow from high voltage sources
and in general only very short exposure
Whatever protection is offered by skin can be tolerated if the victim is to be
resistance decreases rapidly with increase revived.
in voltage. High voltage alternating
current at 60 Hz causes violent muscular Injuries from electric shock are less
contraction, often so severe that the severe when the current does not pass
victim is thrown clear of the circuit. through or near nerve centers and vital
Although low voltage also results in organs. In most electric accidents in
muscular contraction, the effect is not so industry, the current flows from hands to
violent. The fact, however, that low feet. Because such a path involves both
voltage often prevents the victim from the heart and the lungs, results are usually
freeing himself from the circuit makes serious.
exposure to it dangerous.
Treatment of Victims of
Electric Shock
Statistics indicate that only a small
percentage of those who recover from
electric shock show permanent disability.
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In many cases, the victim may be saved Explosion-Proof Electrical
by prompt application of Fittings
cardiopulmonary resuscitation because a
common result in electrical accidents is When using potentially explosive gases,
failure of that part of the nervous system the test area should be free from obvious
that controls breathing. Therefore, it is sources of ignition. Smoking should be
essential that persons working with prohibited and signs should be posted to
electrical power equipment be instructed warn passersby of the hazards. Electrical
in the modern technique of equipment may also present a problem. If
mouth-to-mouth or mouth-to-nose there is a possibility that, in the event of
resuscitation and cardiopulmonary leakage, such equipment will be in an
resuscitation as developed by the explosive environment, then either the
American Heart Association. Immediate equipment should be repositioned outside
treatment should be applied to victims of the danger area, or else specifically
electric shock and should be continued chosen, safe, explosion-proof equipment
until they revive or until death is should be used.
diagnosed by a physician or until rigor
mortis sets in. Standard electrical fittings, considered
safe for ordinary application, are
Hazards of Electric Arcs obviously unfit for installation in
locations where flammable gases and
Another type of injury is burns from vapors or other easily ignitable flammable
electric arc flashes or from human contact materials are present. Sparks and electric
with energized electric power equipment. arcs originating within electrical switches
Such burns are usually deep and slow to and fittings have been the igniting
heal and may involve large areas of the medium in costly fires and explosions.
body. Even welding arcs are also sources
of arc flashes. Hot weld metal, welding Selection of Electrical Fittings for
slag and electrode stub ends can produce
severe burns if touched. Side shielded Hazardous Locations
safety glasses, glasses that do not transmit
ultraviolet radiation and proper use of Before fittings are selected for a hazardous
welding helmets all help avoid welding location, it is necessary to determine the
arc flash injuries to the eye. exact nature of the flammable materials
present. For instance, an electrical fitting,
Hazards of Electrical Extension Cords found by test to be safe for installation in
an atmosphere of combustible dust, may
Extension cords should be of a type listed be unsafe for operation in an atmosphere
by the Underwriter’s Laboratories and containing flammable vapors or gases.
should be labeled to show that they meet
all requirements of the National Electrical It is impossible to prevent highly
Code.10 They should be inspected flammable gases from entering the
regularly. Kinking or excessive bending of interior of either an explosion resistant or
the cord should be avoided to prevent the an ordinary wiring system. They will
wire strands from breaking. Broken eventually enter the entire line through
strands may pierce the insulated covering the joints and through the breathing of
and become a shock or short circuit the conduit system caused by temperature
hazard. Old insulation on extension cords changes. Furthermore, gaseous vapors will
often becomes brittle and creates a shock fill every crevice whenever covers are
or short circuit hazard. Ordinary twisted removed. For these reasons, it is
lamp cord should never be used for impossible to provide an entirely vapor-
extension cords or lamps in vessels or on proof switch unit or to regulate
damp or metallic floors and should never temperatures or keep the air free from
be used where it will be exposed to flammable gases inside the electrical
mechanical wear. fittings.
Cord for use with portable power tools To protect that area classified as a
and equipment is made in several grades, hazardous location, it is necessary to have
each of which is designed for a specific positive confinement of the arc, heat and
type of service. Rubber sheath cord should explosion within the internal limits of
be used with portable electric tools and explosion-proof fittings. These fittings are
with extension lamps in vessels or other constructed to completely imprison the
grounded enclosures. Special types of dangerous arcing, intense heat and
synthetic rubber or plastic covering subsequent explosion so that the gas
should be considered when the cord is to laden air outside does not become ignited.
be used in areas where it may come in
contact with oils or solvents. Double Protective Enclosures for Electrical
insulated electrical tools should be
selected for maximum safety. Apparatus
A useful substitute for explosion-proof
equipment is to enclose nonexplosion-
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proof apparatus in metal boxes and pass a sufficient light for general safety and for
stream of nitrogen or even air into the ordinary visual needs.
box to maintain it slightly above
atmospheric pressure. However, it should Light intensity on a surface varies
be kept in mind that all equipment can be inversely with the square of the distance
hazardous and should only be used with between the surface and a small area
due regard to the hazards involved. Put source of light. A source 3 m (about 10 ft)
only that equipment in the test area that above a surface would give four times
must be there. Where possible, use air more light to the work area than would
operated equipment instead of electrical the same source 6 m (about 20 ft) high.
equipment. Where visual needs are more critical,
additional lighting can thus be provided
It is necessary that electrical equipment by fixtures placed fairly close to the area
be explosion-proof throughout the entire needing more light.
building and not solely in the test area, in
cases where explosive vapors may travel Precautions with
to other parts of the building should a Ultraviolet Sources Used
leak occur. Under some circumstances, the for Inspection with
test area can be sealed to prevent escape Fluorescent Leak Tracers
of vapors to other areas.
Fluorescent penetrants and leak tracers
Lighting as a Factor in require intense illumination with
Industrial Safety ultraviolet and near ultraviolet radiation
sources to make test indications visible.
The proportion of industrial accidents Properly enclosed, shielded and filtered
attributable to poor lighting has been ultraviolet radiation sources used for
estimated to be from 15 to 25 percent. inspection emit radiation in the 320 to
Good lighting contributes greatly to 400 nm wavelength range, well above the
safety, as well as increasing efficiency and more hazardous shorter wavelength
morale. Daylight is an ideal type of ranges of hard ultraviolet radiation.
illumination. For the most effective use of Failure to use proper filters and lamp
daylight, a definite relationship of floor to enclosures could permit such hard
window must be maintained. Sudden ultraviolet radiation from mercury vapor
transitions from brightly lighted to dim arc lamps, welding arcs and fluorescent
areas and vice versa are dangerous; the tubular ultraviolet lamps to escape. The
result is momentary blindness due to the following discussion lists hazards and
lag in eye accommodation. Gradations of precautions for control of ultraviolet
light between areas of different intensities radiation and notes its physiological
will remedy this difficulty. effects.
Artificial Lighting Effects of Hard Ultraviolet
Radiation
Artificial lighting has become so accepted
as an element of modern life that its Hard (short wavelength) ultraviolet
original supplementary character has been radiation has long been known to
largely forgotten. Artificial lighting has produce physical, chemical and
become the major source of illumination physiological effects, so some evaluation
because natural light is undependable, of these effects and the degree of hazard
especially in the winter when work involved in ultraviolet radiation is in
schedules do not coincide with daylight. order.
For continuous shift operation, artificial
light is essential. For other types of Physically, ultraviolet radiation is the
operation, it must be relied on from 20 to portion of the electromagnetic spectrum
50 percent of the total working hours, with wavelengths between those of visible
excluding overtime work or night work. light and X-rays. Therefore, as might be
expected, the long wave portions behave
General Lighting very much like visible light and the short
wave portions have some of the properties
General lighting is the base or minimum of X-rays. The middle ranges have
amount of light required. It has been properties of their own that are not
defined as uniform distribution of light to common to other portions of the
produce equivalent seeing conditions spectrum.
throughout an interior. Localized general
lighting sources usually are arranged 3 m
(about 10 ft) or more above the work to
prevent too great a contrast in brightness
between the more highly lighted work
area and the adjacent areas and to provide
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Filters for Transmission or bread down or otherwise alter many
Absorption of Ultraviolet molecules even without the presence of
Radiation oxygen, so oxidation is not the only
chemical action it produces. Ultraviolet
Ultraviolet radiation can be transmitted, radiation, being at the long wave end of
absorbed and refracted or bent just like the range, is probably only slightly more
visible light, although usually by chemically active than visible light.
substances other than those normally However, as with visible light, long
used for visible light. For instance, exposure to high intensity ultraviolet
ordinary window glass transmits quite radiation can be expected to have its
well in the longer wavelengths, but effect.
becomes opaque to wavelengths shorter
than 310 nm. Thus, it will transmit Chemical Reactions Caused
ultraviolet radiation but absorb the by Ozone
shorter, more harmful wavelengths.
Therefore, ordinary glass is a good Hard (short wave) ultraviolet radiation
protective shield against hard ultraviolet also produces ozone, which itself is a
radiation. strong oxidant. Ozone (O3) is a bluish gas
with a characteristic pungent odor and is
A number of suitable filters and glass found naturally in the atmosphere as a
types will remove all ultraviolet radiation result of the action of solar radiation and
while permitting visible light to pass. In lightning in electrical storms. It is also
cases where short wave ultraviolet formed in corona discharges around high
radiation must be transmitted, special voltage conductors and is generated by
glasses are available. Some glass will X-ray and ultraviolet radiation, electric
transmit wave lengths as short as 280 nm arcs (including welding arcs), mercury
and other glass to 230 nm. Below this vapor lamps and linear accelerators.
point, quartz, particularly in the
crystalline form, must be used.
Reflection of Ultraviolet Physiological Effects of
Radiation Hard Ultraviolet Radiation
Ultraviolet can also be reflected, but often Physiologically, ultraviolet radiation can
by materials different from those used to produce a variety of effects, depending
reflect visible light. Most white metals strongly on the wavelength. Short wave
reflect ultraviolet radiation although not ultraviolet radiation, as previously stated,
as strongly as they reflect visible light. produces ozone. Ozone is a very toxic
Silver is an exception, reflecting to about compound that may cause death due to
360 nm, with absorbing shorter lung congestion and edema. Its maximum
wavelengths. Aluminum and polished allowable concentration is 0.1 part per
iron are good ultraviolet reflectors. Some million (0.2 mg·m–3). Ozone is produced
white pigments such as magnesium oxide, essentially at wavelengths below 260 nm.
aluminum oxide and calcium carbonate The properly filtered mercury arc
are good reflectors, whereas others such as ultraviolet radiation sources used with
titanium dioxide and zinc oxide are poor fluorescent leak tracers do not produce
ultraviolet reflectors. Dark visible colors, ozone.
particularly greens, browns and reds, are
usually poor reflectors and good absorbers Ultraviolet radiation also has a
of ultraviolet. These factors should be kept germicidal effect and is used for
in mind when designing ultraviolet sterilization. This effect reaches a
radiation inspection booths.11 maximum at 260 nm and falls off rapidly
to nearly zero at 320 nm. The action is
Chemical Reactions Excited effective on almost all bacteria as well as
by Hard Ultraviolet some fungi and molds. Thus, ultraviolet
Radiation radiation is a very useful tool for
disinfecting surfaces as well as room air
Ultraviolet radiation is also chemically while it passes through enclosed
active and accelerates many reactions. Of ventilating systems. Sterilizing lamps
particular importance are oxidation and should not be placed where human eyes
molecular breakdown. Oxidation is a or skin can be exposed to their radiation.
primary cause for the breakdown of paint
vehicles and the fading of dyes and other Skin Inflammation Caused by
colorants. Powerful ultraviolet will also
Ultraviolet Radiation
Another well known effect of ultraviolet
radiation is the production of erythema or
skin inflammation, commonly known as
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Relative effectivenesssunburn. This effect is produced strongly Eye Irritation Caused by Ultraviolet
by certain wavelengths and not at all by Radiation
others, as shown in Fig. 1. Thus, the short
germicidal wavelengths produce Eye irritation is one further physiological
considerable inflammation, whereas effect due to ultraviolet radiation. There
certain middle wavelengths are relatively are two types of irritation. The first is a
ineffective. For those using ultraviolet bluish haze noted when the eyes are
radiation for inspection, the important exposed to ultraviolet, particularly of the
fact is that there is essentially no longer wavelengths. This is irritating,
erythemal effect above 320 nm. Because causing headaches and, in extreme cases,
the light used for inspection is essentially nausea but is otherwise not harmful. It is
365 nm, inspectors do not become caused by fluorescence of certain portions
sunburned from their work with properly of the eye when exposed to ultraviolet
filtered and shielded black lights. radiation.
One of the serious concerns about The second type of irritation is
possible effects of any radiation is its photokeratitis followed by conjunctivitis.
tendency to produce cancer. The United This is essentially snow blindness. It
States government in its role as a includes a feeling of sand in the eyes,
consumer protection agency has allergy to light, tear formation and finally
conducted studies on carcinogenic and blindness. These symptoms usually begin
other health hazards of ultraviolet 6 to 12 h after exposure and last from 6 to
radiation. This work is summarized in the 24 h, with all symptoms disappearing in
document Criteria for a Recommended 48 h. There is not cumulative effect but,
Standard for Occupational Exposure to on the other hand, no tolerance is
Ultraviolet Radiation,12 which concludes developed from repeated exposure as is
that cancer can be produced by long the case with sunburn. This effect is
exposure to sunlight rich in midrange caused only by the wavelengths shorter
ultraviolet. However, the cancer than 310 nm and so should be no
producing effect is directly proportional problem in inspection operations as long
to the erythemal or sunburn effect. as the light is passed through the normal
Therefore, the ultraviolet radiation used filters.
for inspection purposes is not a probable
cause of cancer. Protective Glasses to Shield Eyes from
Ultraviolet Radiation
FIGURE 1. Standard curve for erythemal effectiveness of
various wavelengths of hard ultraviolet radiation. Note that Should eye irritation be a problem, yellow
radiation used in inspection with fluorescent leak tracers lies tinted eye glasses will offer complete
in the range of 360 nm and above and does not have protection, particularly if equipped with
significant hazardous effects. side shields. Such glasses are sold as
shooter’s glasses and can be provided by
1.0 local oculists.
0.9 Recommended Limits for
Personnel Exposure to
0.8 Ultraviolet Radiation
0.7 In Criteria for a Recommended Standard for
Occupational Exposure to Ultraviolet
0.6 Radiation,12 recommended limits for
personnel exposure to ultraviolet
0.5 radiation in the 314 to 400 nm range
were listed as 1.0 mW·cm–2 for exposures
0.4 exceeding 1000 s and 100 mW·cm–2 for
exposures under 1000 s (about 16 min).
0.3
The OSHA Environmental Standard
0.2 was 10 mW·cm–2 over any 1 h period.
0.1 Recommended Limits for
Exposure to Krypton-85
250 260 270 280 290 300 310 Gas
Wavelength (nm) Although regulations in some of the
United States vary in specific exposure
limits, the Code of Federal Regulations,
Title 10, Part 20,1 sets standards for the
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allowable cumulative annual exposure:
1 mSv (100 mrem) for the general public,
50 mSv (5 rem) for the whole body,
150 mSv (15 rem) for the lens of the eye,
0.5 Sv (50 rem) for the skin and
extremities and 5 mSv (500 mrem) for an
embryo or fetus.
The prevailing industrial philosophy is
that any unnecessary exposure should be
prevented. The gamma radiation from
krypton-85 has a 514 keV energy and
represents 0.46 percent of the emission
from krypton-85 gas. This is considered to
be a very week photon with potential for
very little tissue damage. The beta particle
emitted by krypton-85 is quite weak and,
when the gas is leaked into a hermetic
device, the beta particles rarely can
penetrate the walls of the device. In actual
leak testing, the krypton-85 gas that has
leaked into the device is measured by
detecting the total radiation seen through
the walls of the device using highly
sensitive scintillation detectors.
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PART 5. Safety Precautions with Leak Testing
Tracer Gases
Tracer Gas Hazards in Leak Personnel Protection
Testing Badges to Warn of
Excessive Exposure to
Tracer gas safety aspects such as Toxic Gases
flammability, asphyxiation or specific
physiological effects as well as the Personnel protection indicators (PPIs) are
possibility of pressure vessel explosions plastic badges with pocket clips that have
must be considered. So long as the sensors that react chemically with
nondestructive test engineer and the leak concentrations of various gases or vapors
test technician are aware of these used as tracers in leak testing. They
considerations from the start, it is possible provide forewarning of excessive exposure
to leak test a vessel with minimum to the toxic substances by means of color
inconvenience or danger. changes, as listed in Table 5. These
personnel protection indicators are
Most tracer gases are not toxic. sensitive to the accumulated personal
However, if a question exists about the exposure of the badge wearer to the
toxicity of any particular gas, a competent concentration of gas in the leak testing
authority should be consulted. Many area. The Occupational Safety and Health
tracer gases will not support human life. If Administration of the United States
such tracer gases replace oxygen in a defines the critical exposure period to be
vessel, this vessel cannot be entered an 8 h shift. A color change of the
without proper ventilation. In this case, protective badge at any time during an
proper ventilation consists of a gas mask 8 h shift indicates that the badge wearer
that contains its own air-oxygen gas has received his or her maximum safe
supply. exposure. Table 5 lists the concentrations
of toxic gas or vapor in air, which are
The oxygen required for breathing may designated as the critical accumulations.
be accidentally removed from an area. For Also listed in Table 5 are the color
example, if one of the halogenated changes that occur on exposure of
hydrocarbons is used as a tracer gas, it personnel protection badges to the
may stagnate and settle to the lowest area. specific tracer gases for which they are
If a technician is attempting to use a sensitive.
detector probe in this low area, the tracer
gas that settles may eventually displace Although the personnel protection
enough of the air to produce indicator badges are normally worn on
asphyxiation. To avoid this condition,
adequate ventilation must be provided. TABLE 5. Selection guide for personnel
However, this ventilation must be protection indicators for toxic gases and
performed carefully. If the tracer gas is vapors accumulating in leak testing
removed too rapidly from the place where areas. Data apply to both personnel
it is escaping from the vessel, leakage protection and area contamination
location may be difficult. monitors.
To aid in a better understanding of the Warning
safety aspects, the following data are
presented below for several tracer gases Toxic Concentration Color
that may be used. In addition, Substance
information is given on the availability of (µL·L–1) Change
personnel protection indicators and area
contamination monitors that can provide Ammonia 15 Brown to white
warning indications of dangerous Carbon monoxide 50 White to Black
accumulations of toxic gases or vapors. Chlorine White to yellow
Hydrazine 2 White to yellow
Hydrogen sulfide 5 White to brown
Nitrogen dioxide 5 White to yellow
Ozone 1 White to brown
0.1
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breast pockets so supervisory personnel excessive accumulations of the specific
and coworkers can easily see the status of toxic gas are present.
the indicator, a person working alone can
monitor his or her own status more easily Each of the contamination monitors
by clipping the badge to his or her belt. listed in Table 6 indicates the accumulated
Replacement color change buttons are exposure to the specific gas to which the
available to be inserted into these badges work area has been exposed during the
because the color changes occurring on 8 h measurement period set by the
exposure are permanent. Continuous use Occupational Safety and Health
of suitable personnel protection indicators Administration. A color change at any
would be appropriate during leak testing time during this 8 h interval indicates
operations. In addition, such leak testing that anyone in the area is being exposed
areas can be monitored by area to a toxic gas concentration in excess of
contamination monitors, as described the safe maximum.
next.
Portable Electronic
Contamination Monitoring Instrument for Locating
of Excessive Accumulations Small Combustible or Toxic
of Toxic Gases Gas Leaks
Area contamination monitors (ACMs) for Figure 2 shows a portable, hand held
atmospheric accumulations of gases and electronic sensing instrument with
vapors such as ammonia, chlorine, pointing indicator; the instrument is used
hydrazine, hydrogen sulfide, nitrogen both for personnel protection and as a
dioxide or ozone are self-adhesive filter tracer gas detector in leak testing. It
papers that chemically react to detects all combustible and many
concentrations of various gases or vapors. noncombustible toxic gases and vapors,
Indicating by means of color changes including the following: acetone, alcohol,
listed in Table 6, these area monitoring ammonia, benzene, butane, carbon
indicators are normally mounted on walls monoxide, carbon tetrachloride, ethane,
or bulkheads that are easily seen by ethylene oxide, gasoline, hydrogen,
supervisory personnel and by leak testing
workers. Ideally, the monitors should be FIGURE 2. Portable personnel protection monitor and
placed opposite an entrance door with a detector for leak testing of certain combustible or toxic
window (within buildings) or at locations gases.
where they are visible prior to entry in
open areas, so that personnel can see their
indications and do not enter any
contaminated areas unnecessarily.
By contrast, the area contamination
monitors for carbon monoxide is a
triangular wall mounting plaque. The
propane monitor is a vial of crystals. Both
of these monitoring indicators change
color, as indicated in Table 6, when
TABLE 6. Selection guide for area
contamination monitors for toxic gases
and vapors accumulating in leak testing
areas.
Toxic Critical Color
Substance Change
Concentration
(µL·L–1)
Ammonia 15 Brown to white
Carbon monoxide 50 White to Black
Chlorine White to yellow
Hydrazine 2 White to yellow
Hydrogen sulfide 5 White to brown
Nitrogen dioxide 5 White to yellow
Ozone 1 White to brown
Propane 0.1 Purple to yellow
0.001
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turpentine, hydrogen sulfide, liquid liquid ammonia from the skin surface can
propane gas, methane, methyl ethyl cause frostbite. Anyone working with
ketone, naphtha, natural gas, propane, liquid ammonia must wear rubber gloves,
steam, sulfur dioxide, toluene, chemical protection clothing and goggles
trichloroethylene and xylene. and a rubber or plastic apron.
This instrument does not detect carbon Hazards of Explosion or Ignition with
dioxide. It is a low cost, simple leak tracer Ammonia
designed to locate small leaks. A flexible
1 m (3 ft) extension hose can be used to Ammonia cylinders should never be
sniff leaks in less accessible locations directly heated by steam, direct electric
behind pipes or around complex pipe coils or flames. Uncontrolled heating of a
connections. Its use is often more cylinder can cause the liquid to expand to
convenient than using bubble tests and a point where dangerous pressures will be
the small battery operated hand held developed. Heating is done in a
detection and indicating instrument is thermostatically controlled water or oil
often more feasible than larger electronic bath. In no case should the temperature
instruments requiring connections to be allowed to exceed 50 °C (120 °F).
alternating current power outlets. Its
sensor is reported to detect 50 µL·L–1 of Ammonia represents a possible
gas or vapor contaminant in atmospheric flammability hazard. A mixture of air and
air and is designed for over 50 000 ammonia containing from 15 to
exposures to gases. 28 percent ammonia by volume will
ignite when sparked or exposed to
Precautions with Ammonia temperatures exceeding 50 °C (120 °F).
Gas Therefore, flames and sparks should not
be allowed in the area where ammonia is
Ammonia (NH3) is used as a tracer gas for being used.
many chemical indicator leak tests. At
room temperature and atmospheric As another noteworthy consideration,
pressure, ammonia is a colorless, alkaline ammonia can combine with mercury to
gas having a pungent odor, which form explosive compounds. Therefore,
provides ample warning of its presence. instruments containing mercury (such as
Ammonia gas is irritating to the eyes and manometers) should not be used where
to moist skin. However, concentrations of they will be exposed to ammonia.
ammonia gas in air in the concentration
range below 50 µL·L–1, although not Precautions with Argon
harmful, are a considerable nuisance, so Gas
that people tend to avoid them. It is
therefore unlikely that an individual On some occasions, argon (Ar) is used as a
would unknowingly become overexposed leak tracer gas. It is the most abundant
to ammonia gas. member of the rare gas family, which
consists of helium, neon, argon, krypton
Physiological Effects of Ammonia Gas and xenon. All of these gases are
monatomic and are characterized by their
Table 7 lists the physiological effects of extreme chemical inactivity. Argon, a
various concentrations of ammonia. The
corrosive action of high concentrations TABLE 7. Physiological effects of various concentrations of
(above 700 µL·L–1) can cause extensive ammonia gas (NH3).
injuries to the eyes, including severe
irritation, hemorrhaging and swollen lids. Atmospheric Physiological Effects
If not treated immediately, partial or total
loss of sight may result. The mucous Concentration
lining of the mouth, throat, nose and (µL·L–1)
lungs is particularly sensitive to ammonia
attack. 20 First perceptible odor.
40
Precautions with 100 A few individuals may suffer slight eye
Anhydrous Liquid 400 irritation.
Ammonia 700
1700 Noticeable irritation of eyes and nasal passages
Contact with anhydrous liquid ammonia 5000 after few minutes’ exposure.
is intensely irritating to the mucous
membranes, eyes and skin. Contact with Severe irritation of the throat, nasal passage
the skin will produce severe burns and the and upper respiratory tract.
freezing effect due to rapid evaporation of
Severe eye irritation. No permanent effect if
the exposure is limited to less than 0.5 h.
Serious coughing, bronchial spasms; less than
0.5 h of exposure may be fatal.
Serious edema, strangulation, asphyxia, fatal
almost immediately.
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colorless, odorless and tasteless gas, is Characteristics of
nontoxic. However, argon can act as a Refrigerant-12 Gas
simple asphyxiant by displacing the
amount of air necessary to support life. The halogen tracer gas
dichlorodifluoromethane (CCl2F2) was
Precautions with Carbon widely used in the 1980s. This was the
Dioxide Gas refrigerant-12 gas used in air conditioners.
It is a colorless, nonflammable gas at
Carbon dioxide (CO2) is a nonflammable, normal temperatures and pressures. In
colorless, odorless and slightly acid gas concentrations of less than 20 percent (by
which is about one and one half times as volume), refrigerant-12 is odorless. At
dense (heavy) as air. The normal high concentrations, its odor is mild and
concentration of carbon dioxide in the somewhat ethereal and similar to that of
atmosphere is 0.03 percent, or 300 µL·L–1. carbon tetrachloride. Refrigerant-12 is
Gaseous carbon dioxide is not a readily liquefied and is usually supplied in
chemically active compound as such and steel cylinders as a liquefied gas under its
high temperatures are generally required own vapor pressure of about 480 kPa
to promote its chemical reactions. (70 lbf·in.–2 gage) at 21 °C (70 °F).
However, aqueous solutions of carbon Refrigerant-12 gas has also been known by
dioxide are acidic and many reactions several trade names, including Freon® 12.
occur readily. Its extensive use as a propellant for spray
cans has been discontinued in the United
When it replaces breathable air, carbon States. Its manufacture in and its
dioxide acts as a simple asphyxiant. importation into the United States have
Because it is heavier than air and does not been banned. However, if this gas is
diffuse readily, pure carbon dioxide may sprayed on very hot metallic surfaces or in
collect in confined, unventilated areas or the presence of flames, it can dissociate to
in lower regions of large vessels. Gaseous form deadly toxic gases such as phosgene.
carbon dioxide is the regulator of the
breathing function. An increase in the Refrigerant-12 gas is practically
amount of carbon dioxide inhaled will nontoxic. It shows no toxic effects in
cause an increased rate of breathing. The guinea pigs in concentrations up to at
body, while exercising, will burn more least 20 percent by volume for 2 h
oxygen and the product of this exposures. In higher concentrations,
combustion will be higher concentrations refrigerant-12 may produce some
of carbon dioxide. These higher physiological action, caused primarily by
oxygen deficiency. The generally accepted
TABLE 8. Physiological effects of carbon maximum allowable refrigerant-12
dioxide gas in air. concentration for an 8 h daily exposure of
personnel is 1000 µL·L–1.
Carbon Dioxide
Precautions with Helium
Gas in Air Increased Gas
(mL·L–1) Lung Ventilation Helium (He) is widely used as a tracer gas
in leak testing with the mass spectrometer
1 to 10 Slight and unnoticeable leak detector. It is the lightest member of
20 50 percent the rare gas family and is a chemically
30 100 percent inert, colorless, odorless and tasteless gas.
50 300 percent (breathing Helium is not toxic but can act as an
asphyxiant by displacing the air necessary
becomes laborious) to support life. Because of its low density,
helium tends to rise to the top regions of
concentrations of carbon dioxide produce closed vessels or enclosures, where it
higher rates of breathing listed (Table 8). could lead to asphyxiation of workers at
these elevations.
Concentrations of 10 percent
(100 000 µL·L–1) of carbon dioxide in Characteristics of
breathing air can produce Hydrogen Chloride Gas
unconsciousness; concentrations of 10 to
25 percent may cause death with To some degree, hydrogen chloride (HCl)
exposures of several hours. A has also been used as a tracer gas.
concentration of 5 percent may produce Anhydrous hydrogen chloride is a
shortness of breath and headache. colorless, pungent, nonflammable,
Continuous exposure to 1.5 percent
carbon dioxide may cause changes in
some physiological processes.
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corrosive gas with a suffocating odor. It is or tantalum). When hydrogen chloride is
heavier than air, soluble in water and used at higher pressures, it is necessary to
fumes strongly in moist air. The aqueous use extra-heavy steel pipe throughout. No
solution is known as hydrochloric acid (or galvanized pipe or bronze valves should
muriatic acid) and may contain as much be used.
as 38 percent hydrogen chloride.
Hydrogen chloride is supplied in cylinders Precautions with Hydrogen
in the form of a gas over a liquid. The Gas
cylinder pressure is about 4.2 MPa
(610 lbf·in.–2 gage) at 21 °C (70 °F). As Hydrogen (H2) is colorless and odorless
long as liquid is present in the cylinder, and is the lightest gas known. It is
this pressure remains fairly constant. nontoxic but can act as an asphyxiant by
When the liquid phase is exhausted, displacing the necessary amount of air
cylinder pressure drops rapidly. required to support life. Because hydrogen
is much lighter than air, it tends to collect
Physiological Effects of Hydrogen near the top of closed vessels. Hydrogen,
in combination with air or oxygen, can
Chloride Gas explode with great violence. Hydrogen
gas, although relatively inactive at
Hydrogen chloride is a highly toxic gas ambient temperatures, reacts with almost
that severely irritates the upper respiratory all the other elements at high
tract and is corrosive to the eyes, skin and temperatures and is considered to be a
mucous membranes. It may produce very dangerous tracer gas. For this reason,
dermatitis on repeated exposures. Eye hydrogen should be avoided if at all
contact may result in reduced vision or possible.
blindness. Ingestion may be fatal.
Hydrogen chloride concentrations of 0.13 When large vessels are tested or when
to 0.2 percent (1300 to 2000 µL·L–1) in air large volumes of hydrogen are used, it
are lethal for human beings in exposure may be advisable to provide monitoring
lasting only a few minutes. The maximum equipment that gives a continuous
hydrogen chloride concentration that can indication of the hydrogen and air
be tolerated for exposures of 60 min is in content in the test area. Intrinsically safe
the range of 0.005 to 0.01 percent (50 to detectors are available. This precaution
100 µL·L–1). However, the unpleasant may also be advisable when high vacuum
effects of hydrogen chloride provide vessels are in the process of being
adequate warning, leading to prompt chemically cleaned before evacuation
voluntary withdrawal of personnel from because the vessel interior as well as the
hydrogen chloride contaminated surrounding environment may contain an
atmospheres. explosive mixture.
Precautions with Hydrogen Chloride Precautions with
Radioactive Krypton-85
Gas Gas
Personnel who handle hydrogen chloride Krypton gas is completely chemically
gas must wear protective clothing such as inert and, thereby, forms no chemical
rubber or plastic aprons, rubber gloves combinations with any material used in
and suitable gas tight safety goggles. the tested components. Radioactive
Appropriate gas masks with cannisters or krypton-85 tracer gas, due to its chemical
supplied air respirators should be provided inertness, does not participate in any
when hydrogen chloride vapor metabolic processes in the body if inhaled
concentrations are excessive. Woolen or ingested in any way. If accidentally
outside clothing or other acid-resisting inhaled for a short time, normal breathing
fabrics are also recommended for of noncontaminated air will rapidly
personnel handling hydrogen chloride. remove the radioactive krypton gas from
Personal hygiene and showering after the lungs and body tissue into which it
each work shift should be encouraged. might be diffused. With an adequate
When hydrogen chloride is supplied from ventilating system, proper gamma ray
cylinders, users should always shut off shielding of storage tanks and reasonable
their hydrogen chloride lines from the use care, krypton-85 tracer gas can be handled
end, closing valves successively backward with negligible risk to the operators.
to the cylinder.
Krypton-85 has a radioactive half life
Dry gaseous hydrogen chloride is or 10.76 yr. Over 99 percent of the
essentially inert to metals and does not disintegrations give no gamma rays but
attack the commonly used structural emit beta particles with a maximum
metals under normal conditions of use energy of 0.67 MeV. Only 0.7 percent of
(room temperature and atmospheric
pressure). In the presence of moisture,
however, hydrogen chloride will corrode
most metals (other than silver, platinum
Safety Aspects of Leak Testing 127
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the disintegrations yield 0.514 MeV mild to marked euphoria. These responses
gamma rays. The primary usefulness of are similar to those associated with
krypton-85 for leak testing depends on alcoholic intoxication.
this small proportion of gamma emitting
disintegrations, reinforced in some Precautions with Nitrous
applications by the emission of low Oxide
energy bremsstrahlung or very soft X-rays.
Nitrous oxide (N2O) is used as a tracer gas
Many industrial hand held portable in the performance of some leak tests,
survey meters are used to detect the such as those using the infrared leak test
presence of trace quantities of krypton-85 method. Nitrous oxide is a colorless,
gas. The high percentage of beta particle nonflammable gas with a slightly sweetish
emission allows for detection of taste and odor. It is nontoxic and
nanocuries of krypton-85 gas in the air. nonirritating and must not be confused
Additionally, all equipment approved to with other nitrogen oxides that can be
handle krypton-85 gas is required to have harmful. Nitrous oxide is a rather weak
air monitors in continuous operation to anesthetic and must be inhaled in high
detect any airborne krypton-85 gas and concentrations, mixed with air or oxygen,
initiate an alarm. when regularly used as an anesthetic in
medicine and dentistry. Medical and
Precautions with Methane dental personnel who repeatedly inhale
Gas this gas over a long period of time are
known to suffer nerve damage. When
Methane is sometimes used as leak testing inhaled without oxygen, nitrous oxide is a
tracer gas. Natural gas consists primarily simple asphyxiant. Inhalation of small
(85 percent) of methane. Methane gas amounts of nitrous oxide often produces a
(CH4) in its pure state is flammable, type of hysteria, which accounts for its
colorless, odorless and tasteless and is not common name of laughing gas.
considered toxic. It can act as a simple
asphyxiant where, present in high It is to be recognized that most other
concentrations, it displaces the oxygen nitrogen oxides can be harmful.
necessary to sustain life. As an example, California’s Occupational Safety and
coal miners frequently breath air Health Administration standard for all
containing 9 percent methane and do not nitrogen oxides combined is a
appear to suffer. When concentration concentration of 5 µL·L–1 ceiling for an
increases above this point, pressure on the 8 h occupational standard. The California
forehead and eyes is noticed. However, ambient air standard for nitrogen-oxygen
this pressure disappears again on pollutants is 0.25 µL·L–1 for 1 h. The
breathing fresh air. Methane in mixtures Federal standards in 1978 for nitrogen
with air or oxygen burns rapidly. Ignition oxides, determined as time weighted
leads to explosions similar to many coal averages (TWAs), are 25 µL·L–1 or
mine explosions. Incomplete combustion 30 mg·m–3 for nitric oxide (NO), 5 µL·L–1
of methane gas may produce carbon or 9 mg·m–3 for nitrogen dioxide (NO2)
monoxide, a toxic gas. and 2 µL·L–1 or 5 mg·m–3 for nitric acid
(HNO3).
Precautions with Nitrogen
Gas Precautions with Oxygen
Nitrogen (N2) is not often used as a tracer Even though oxygen (O2) is not often
gas but may be used to backfill vacuum used as a tracer gas, there should be full
vessels or may be mixed with a tracer gas awareness of its potential hazards. Oxygen
and introduced into a vessel before a is a colorless, odorless, tasteless gas and its
pressure leak test. Nitrogen gas comprises outstanding properties include its ability
about 79 percent by volume of the air. It to sustain animal life and to support
will not burn and will not support combustion. Inhalation of 100 percent
combustion. It is nontoxic; however, oxygen at atmospheric pressure (100 kPa
nitrogen can act as an asphyxiant by or 1 atm) will irritate the throat although
displacing the amount of air necessary to symptoms of oxygen poisoning do not
sustain life. This gas is extremely inert, occur if the exposure is relatively short.
except when heated to very high Long periods of exposure to higher
temperatures where it combines with oxygen pressures can adversely affect
metals to form nitrides. At pressures of neuromuscular coordination and the
400 kPa (4 atm) or higher, the gaseous power of attention. Inhalation of oxygen
nitrogen in normal air induces a narcotic when its partial pressure exceeds 200 kPa
action evidenced by decreased ability to (2 atm) may result in the signs and
work, mood changes and frequently a symptoms of oxygen poisoning. These
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include tingling of fingers and toes, make it unlikely that any person would be
acoustic hallucination, confusion, muscle able to remain in such a contaminated
twitching (especially about the face) and atmosphere unless he or she were
nausea. The final result of such exposure unconscious or trapped. The adverse
may be convulsion, which ceases as soon effects of sulfur dioxide are heightened by
as exposure to high partial pressures of the presence of dust, dirt, soot or other
oxygen is terminated. Note that carbon particulates in the air. If particulates are
dioxide enhances the toxicity of oxygen high in concentration in the air, even a
and the narcotic effect of nitrogen. little sulfur dioxide can cause illness.
Precautions against Oxygen Fires and Chronic exposure to sulfur dioxide
may result in fatigue, altered sense of
Explosions smell and chronic bronchitis symptoms.
Short acute exposure to sulfur dioxide gas
Pressurized oxygen reacts violently with has severe effects. A concentration of 8 to
oil, grease, fuel gases or metallic particles, 12 µL·L–1 causes throat irritation,
often producing flames or violent coughing, constriction of the chest, tears
explosions. The cylinders in which and smarting of the eyes; a concentration
gaseous oxygen is supplied are often of 150 µL·L–1 causes extreme irritation and
pressurized to 14 or 15 MPa can be tolerated for only a few minutes;
(2.2 × 103 lbf·in.–2 gage). Thus, oil, grease and a concentration of 500 µL·L–1 causes a
or readily combustible materials should sense of suffocation because it is so
never be allowed to come into contact acutely irritating. Acute overexposure to
with interiors of oxygen cylinders, valve, sulfur dioxide may result in death from
pressure regulators and fittings. These asphyxiation.
components should never be lubricated
with oil, grease or other combustible Precautions with Sulfur Dioxide
substances containing hydrocarbons.
Sulfur dioxide should be handled only in
Oxygen gages, regulators and fittings a well ventilated area, preferably using a
should never be used for compressed air hood with forced ventilation. Personnel
(which may contain lubricants from air handling sulfur dioxide should wear
pumps). Similarly, gages regulators and chemical safety goggles or plastic face
fittings used with air or other gases should shields (or both), approved safety shoes
never be used on oxygen systems, for fear and rubber gloves. Additional gas masks,
of violent explosions. It is also advisable airline gas masks and self-contained
never to use manifolds for pressurized breathing apparati should be at hand for
oxygen systems unless these are designed emergencies. Instant acting safety showers
and constructed with the advice and should be available in convenient
control of a qualified engineer. Manifolds locations.
must comply with applicable regulations
and safety procedures. Cylinders of Where sulfur dioxide gas is excessive,
oxygen should not be stored near the worker should be supplied with a full
cylinders of acetylene or fuel gases. face piece cartridge, canister respirator or
supplied air respirator. Goggles, protective
Characteristics of Sulfur clothing and gloves should be worn if
Dioxide splashes of liquid are likely. In areas of
splash or spill, impervious clothing
Sulfur dioxide (SO2), through extremely should be supplied. If work clothes are
undesirable, is sometimes used in the leak wetted by sulfur dioxide, they should be
testing of welded pressure vessels. It is a removed promptly and the skin area
highly irritating, nonflammable, colorless washed thoroughly.
gas at room temperature and atmospheric
pressure. Liquid sulfur dioxide may cause
skin and eye burns on contact with these
tissues as a result of the freezing effect of
sulfur dioxide liquid on the skin or eyes.
Sulfur dioxide is also a highly irritating
gas in the vapor form, but is readily
detectable in concentrations of 1 to
3 µL·L–1 and so provides ample warning of
its presence.
Slight tolerance, at least up to the odor
threshold and general acclimatization are
common. Sensitization in a few
individuals, particularly young adults,
may develop following repeated exposure.
In higher concentrations, the severely
irritating effects of gaseous sulfur dioxide
Safety Aspects of Leak Testing 129
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PART 6. Safety Precautions with Compressed
Gas Cylinders
Handling and Use of possible. Secure cylinders to prevent
Compressed Gas Cylinders violent contact or upsetting.
12. Always consider cylinders as full and
Most of the gas used for leak testing is handle them with corresponding care.
purchased in cylinders, which should be Accidents have resulted when
constructed and maintained in containers under partial pressure were
accordance with regulations of the thought to be empty.
Interstate Commerce Commission. The 13. Use of safety chains to secure cylinders
contents should be legibly marked on during use to prevent accidental
each cylinder in large letters. falling is required practice by the
Occupational Safety and Health
Serious accidents may result from the Administration.
misuse, abuse or mishandling of
compressed gas cylinders. Technicians Precautions for Storage of
assigned to the handling of pressurized Compressed Gas Cylinders
cylinders should be carefully trained and
work only under competent supervision. Store compressed gas cylinders with
Observance of the following rules will protective caps properly installed in safe,
help control hazards in the handling of dry and well ventilated places prepared
compressed gas cylinders. and reserved for this specific purpose.
Cylinders should be stored on a level,
1. Accept only cylinders approved for use fireproof floor and should be chained in
in interstate commerce for place or provided with barriers to prevent
transportation of compressed gases. them from falling over. Flammable
substances such as oil and volatile liquids
2. Do not remove or change numbers or should not be stored in the same area as
marks stamped on cylinders. pressurized gas cylinders. Cylinders
should not be stored near arc welding
3. Never move cylinders unless the areas, elevators, gangways, stair wells or
protective cap is in place. Because of other places where they could be knocked
their shape, smooth surface and heavy over, arc gouged or damaged. Cylinders
weight, cylinders are dangerous to are not designed for temperatures in
carry by hand and some type of excess of 55 °C (130 °F). Accordingly, they
carrying device should be used when should not be stored near sources of heat
they must be moved without the aid such as radiators or furnaces, nor near
of a cart. Cylinders may be tilted and highly flammable substances like gasoline.
rolled on the bottom edge, but they
should never be dragged. Cylinder storage should be planned so
that cylinders will be used in the order in
4. Protect cylinders from cuts or which they are received from the supplier.
abrasions. Empty and full cylinders should be stored
separately, with empty cylinders being
5. Do not lift a compressed gas cylinder plainly identified as such to avoid
with an electromagnet. Where confusion. Group together cylinders that
cylinders must be handled by a crane have held the same contents.
or derrick when testing field erected
vessels, carry them in a cradle or Precautions in Indoor
similar device. Take extreme care that Storage of Oxygen and
they are not dropped. Do not use Fuel Gas Cylinders
slings or chains.
Cylinders of oxygen must not be stored
6. Do not drop cylinders or let them indoors close to cylinders containing
strike each other violently. flammable gases. Unless they are stored
apart, oxygen cylinders and flammable
7. Do not use cylinders for rollers, gas cylinders must be separated by a fire
supports or any purpose other than to resistive partition. A direct flame or
contain gas.
10. When empty cylinders are to be
returned to the vendor, mark them
EMPTY or MT with chalk. Close the
valves and replace the valve protection
caps.
11. Load cylinders to be transported so as
to allow as little movement as
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electric arc should never be permitted to cause gas under high pressure to
contact any part of a compressed gas escape.
cylinder. 3. Make sure the threads on a regulator
or union correspond to those on the
Acetylene and liquefied fuel gas cylinder valve outlet. Do not force
cylinders should be stored with the valve connections that do not fit.
end up. The total capacity of acetylene 4. Open cylinder valves slowly. A
cylinders stored inside a building should cylinder not provided with a
be limited to 60 m3 (2000 ft3) of gas, handwheel valve should be opened
exclusive of cylinders in use or connected with a spindle key, a special wrench or
for use. Quantities exceeding this total other tool provided or approved by
must be stored in a special room, located the gas supplier.
in a separate building or outdoors and 5. Do not use a cylinder of compressed
built in accordance with the specifications gas without a pressure reducing
of NFPA 51, Standard for the Design and regulator attached to the cylinder
Installation of Oxygen-Fuel Gas Systems for valve, except where cylinders are
Welding, Cutting, and Allied Processes.13 attached to a manifold, in which case
the regulator should be attached to
Storage rooms for cylinders containing the manifold header.
flammable gases should be well ventilated 6. Before making connection to a
to prevent the accumulation of explosive cylinder valve outlet, except that of a
concentrations of gas. No source of hydrogen cylinder, crack the valve for
ignition will be permitted; smoking must an instant to clear the opening of
be prohibited. Wiring should be in particles of dust and dirt. Always point
conduit. Electric lights should be in fixed the valve and opening away from the
positions and enclosed in glass or other body and not toward anyone else.
transparent material and equipped with Operators should wear safety glasses.
guards to prevent breakage. (Note that 7. Use regulators and pressure gages only
glass enclosures, electrical conduit and with gases for which they are designed
conventional switch and receptacle boxes and intended. Do not attempt to
used in electrical wiring systems do not repair or alter cylinders, valves,
prevent entry of gases into their regulators or attachments. This work
enclosures.) Therefore, electrical switches, should be done only by the
which are subject to sparking or arcing manufacturer.
during operation, should be located 8. Unless the cylinder valve has first been
outside the room in which flammable closed tightly, do not attempt to stop
gases are stored. a leak between the cylinder and the
regulator by tightening the union nut.
Precautions in Outdoor 9. Combustible gas cylinders in which
Storage of Gas Cylinders leaks occur should be taken out of use
immediately and handled as follows:
One common type of storage house (a) Close the valve and take the
consists of a shed roof with side walls cylinder outdoors well away from any
extending about halfway down from the source of ignition. Properly tag the
roof and a dividing wall between cylinder and notify the supplier. A
cylinders of one kind of gas and those for regulator attached to the valve may be
another gas. To prevent rusting, cylinders used temporarily to stop a leak
stored in the open should be protected through the valve seat. (b) If the leak
from contact with the ground and against occurs at a fuse plug or other safety
extremes of weather, accumulations of ice device, take the cylinder outdoors well
and snow in winter and continuous direct away from any source of ignition,
rays of the sun in summer. open the cylinder valve slightly and
permit the gas to escape slowly. Tag
Safe Procedures for Using the cylinder plainly. Post warnings
Cylinders of Compressed against approaching with lighted
Gases cigarettes or other sources of ignition,
promptly notify the supplier and
Safe procedures for compressed gas follow its instructions for returning
cylinders include the following. the cylinder.
10. Do not permit heavy objects, sparks,
1. Use cylinders in the upright position molten metal, electric currents,
and secure them to prevent them from excessive heat or flames to come in
being accidentally knocked over. contact with cylinders or attachments.
11. Never use oil or grease as a lubricant
2. Unless the cylinder valve is protected for valves or attachments of oxygen
by a recess in the head, keep the metal cylinders. Keep oxygen cylinders and
cap in place to protect the valve when fittings away from oil and grease and
the cylinder is not connected for use. do not handle such cylinders or
A blow on an unprotected valve might
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apparatus with oily hands, gloves or different colors. Cylinder valve outlet
clothing. Signs should be posted threads have been standardized for most
where oxygen is stored, prohibiting industrial and medical gases by the
oil, grease or other lubricants on American National Standards Institute,
oxygen equipment. recommending different combinations of
12. Never use oxygen as a substitute for right hand and left hand threads, internal
compressed air in pneumatic tools or and external threads and different
to start internal combustion engines diameters to guard against wrong
or for pressurizing a system for testing connections. Standards are being rapidly
or for dust removal. Use it only for the adopted whenever gas manufacturers and
purpose for which it is intended. industrial users reach agreement to
13. Never bring gas cylinders into vessels change both valve outlets and regulator
or unventilated rooms. connections. Adaptors are used in the
14. Do not fill cylinders except with the interim until the changes are completed.
consent of the owner and then only in
accordance with regulations. Do not The regulator is a delicate apparatus
attempt to mix gases in a compressed and should always be handled carefully. It
gas cylinder or to use it for purposes should not be forced, dropped or
other than those intended by the pounded. Regulators should be sent to the
supplier. manufacturer for repairs and testing by
15. Secure all gages and hoses with proper skilled personnel.
size wrenches, not slip jaw pliers.
16. Do not overtighten or strip threads on Safety Procedures for
cylinder attachments. Leaky or Anomalous
Regulators
Safety Precautions with
Valves or Regulators on Leaky or creeping regulators are a source
Gas Cylinders of danger and should be withdrawn from
service at once for repairs. If a regulator
Regulators or reducing valves must be shows a continuous creep, indicated on
used on gas cylinders to maintain a the low pressure (delivery) gage by a
uniform gas supply. Technicians should steady buildup of pressure when the
stand to one side and away from regulator outlet valves are closed, the cylinder valve
gage faces when opening cylinder valves. should be closed and the regulator
Always wear safety glasses to protect eyes removed for repairs. If the regulator
from ejected particles. pressure gages have been strained so that
the pointers do not register properly, the
Only regulators listed or approved by regulator must be repaired at once. When
agencies such as Underwriters’ regulators are connected but are not in
Laboratories, Incorporated, should be used use, the pressure adjusting device should
on cylinders of compressed gas. Each be released. Cylinder valves should never
regulator should be equipped with both a be opened until the regulator is drained of
high pressure (contents) gage and a low gas and the pressure adjusting device on
pressure (working) gage. the regulator is fully released.
Safety Precautions with
Oxygen Pressure
Regulators
High pressure oxygen gages should have
safety vent covers to protect the operator
from broken glass in case of an internal
explosion. Each oxygen gage should be
marked OXYGEN—USE NO OIL.
Serious, even fatal accidents, have
resulted when oxygen regulators have
been attached to cylinders containing
combustible gas or vice versa. To guard
against this hazard, it has been customary
to make connections for oxygen
regulators with right hand threads and
those for combustible gases such as
acetylene with left hand threads, to mark
the gas service on the regulator case, and
to paint the two types of regulators
132 Leak Testing
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PART 7. Safety Precautions in Pressure and
Vacuum Leak Testing
Safety Considerations in Pressure Vessel Code
Leak Testing Requirements for Safety
Procedures
When a pressure or a vacuum vessel is
fabricated, some means of testing must be The degree of safety precautions necessary
used to predict safe performance of the during leak testing varies greatly with the
vessel. It is sometimes necessary to exceed type of system being tested. In the case of
the designed operating conditions during hydrostatic and pneumatic tests of
initial pressure testing. This requires many pressure vessels, the ASME Boiler and
safety considerations to ensure proper Pressure Vessel Code outlines the minimum
protection of personnel. (Hazards related safety procedures to be followed during
to toxic or flammable solvent vapors and pressure testing. The ASME Boiler and
tracer gases in leak testing should also be Pressure Vessel Code and other applicable
given careful consideration.) specifications should be followed with
care to ensure safety in all operations to
Explosion and Implosion which they apply. However, often it is the
Hazards in Pressure and rather subtle hazard that may be
Vacuum Leak Testing disastrous. Potential hazards should be
taken into account both when preparing
Pressurized vessels can fail by explosion for or performing leak testing. These
because of the energy stored in air or include tracer gas safety aspects such as
nonflammable gases used to pressurize flammability, asphyxiation or specific
systems during leak testing. In systems physiological effects as well as the
that are evacuated during leak testing, possibility of pressure vessel explosions.
implosion (violent collapse) failures can
result from external (atmospheric) Protecting Test Personnel
pressures applied to structures not during Pressure Testing
designed for such loading. Where
flammable tracer gases are used in leak Greater respect for high pressure testing
testing in the presence of air or oxygen, has led to increased emphasis on safety,
violent combustion or explosive chemical with the result that overall safety
reactions can occur. These hazards must experience has been very good. This
be foreseen and carefully controlled to respect is well justified when one realizes
ensure safety during leak testing. that a valve stem operating at 200 MPa
(3 × 104 lbf·in.–2) that fails and is blown
Precautions in Selecting out is propelled under conditions similar
Sites for Leak Testing to those of a bullet fired from a high
powered rifle. The energy released from a
Major factors determining the size, shape completely liquid system should not be
and type of buildings and structures to be underestimated either. Compressed liquid,
used for leak testing of components need although smaller volumetrically than
to be investigated. Catastrophes resulting compressed gas, is very much to be
in large loss of life and heavy property reckoned with in considering potential
damage often are due to inadequate forces to be handled when pressure is
planning stage considerations. High released. For example, a gasket 0.4 mm
hazard leak testing operations should be (0.016 in.) thick, blown between split
located in small isolated buildings of flanges under a pressure of more than
limited occupancy. Buildings can be 10 MPa (more than 2 × 103 lbf·in.–2), will
designed so that internal explosions will release a thin sheet of water like a knife
produce minimum damage and minimum edge that could cause injury, eye damage
broken glass. Lower hazard operations can and loss of sight.
justify large units.
Successful personnel protection during
pressure testing involves not only
mechanical devices to guard against
Safety Aspects of Leak Testing 133
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injury should failure occur, but thorough solids at various temperatures and at
training of people, establishment and various pressures, ranging from absolute
enforcement of rigid safety rules and pressures of nanopascals or lower
necessary disciplinary action when pressures to tens of megapascals (10–9 to
justified. Without the proper attitude and 107 Pa).
respect for what is being handled, trouble
is sure to occur. Some common causes of failure in
pressure vessels are the following:
Safety with Scaffolds (1) errors in design, construction and
nondestructive testing; (2) improper
A scaffold is an elevated working education of testing personnel;
platform, usually temporary, for (3) mechanical breakdown such as failure,
supporting both men and materials. For blocking or lack of safety devices; (4) poor
safety’s safe, scaffolds should be designed visual inspection before pressurization;
to support at least four times the (5) improper test procedure; (6) improper
anticipated weight of men and materials application of test equipment; (7) blocked
to be placed on them and all elevated or dysfunctional gages; (8) test flanges or
working platform areas should be guarded valves of wrong material; (9) improperly
(as by railings) on all exposed sides. designed test flanges; and (10) test
Working scaffolds should not be used as a pressure too high. These causes of
platform for jacking or leverage purposes potential failures should be anticipated
without proper allowance for the added and avoided insofar as possible. Before a
loads and stresses. pressure vessel is tested, three questions
should be answered about its design.
Barricades, Protective
Walls and Distance for 1. Can the filled vessel carry the weight
Safety during Leak Testing of its contents in addition to the
internal pressure without undue
Based on safety experience accumulated strain?
during laboratory operations and on
sound design principles, a custom vessel 2. Can the support structure and
can be built with reasonable assurance building floor carry the weight of the
that it may be leak tested or pressure filled vessel?
tested safely. While complete isolation
usually is not required, certain pieces of 3. Can the vessel withstand any vacuum
equipment may need barricade and not collapse under external
protection. Access to the test area during atmospheric pressure that may be
testing should be restricted to minimize created either accidentally or
exposure of personnel to hazards. intentionally?
Remote control and observation may It is imperative that any safety enclosures
be used where possible during leak be designed to withstand the worst
testing. Periscope techniques, shatterproof possible conditions of failure; otherwise,
glass windows and industrial television protective walls may break and lethal
offer opportunities to check on operating fragments of metal or concrete can be
equipment without exposure. Instrument blown outward. Vented roofs or pressure
data can be transmitted electrically or by testing below ground level should be
low pressure pneumatic systems to a considered when pressure testing with
separate control room. Valves that are not compressed air or gases.
controlled automatically can be operated
by rods or shafts extended through a Precautions for Protection
barricade gage board combination with against Equipment Failure
proper seals. from Overpressure
Pressure Vessel Design and Safety precautions to protect personnel
Causes of Failures and equipment from failures during
pressure testing include the following.
Fired and unfired pressure vessels of many
types are in common use in industrial, 1. Ensure that the test equipment and
commercial and public buildings for space vessel under test are properly designed
and process heating and heat exchange; and constructed in the first place.
for processing food, chemicals, petroleum
and other industrial products; and for 2. Before pressure testing, ensure that
processes involving nuclear energy. These equipment is properly assembled to
vessels hold gases, vapors, liquids and avoid overstressing. This includes
proper bracing and shoring under
pressure vessels to support critical
points. Otherwise it is possible that
failure may actually be started before
or while equipment is being set up for
the test.
3. Be sure that careful visual and other
inspections are done during
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construction and testing to guarantee placed in circular grooves and compressed
compliance with design, proper to form tight seals between mating parts
manufacturing procedures, material of pressure or vacuum systems. They can
choices and workmanship standards. have any cross sectional area required for
4. Watch out for areas where stress protection and can be designed for any
concentrations or nonuniform loading relieving pressure. The O-ring seals should
in enclosures, pump and compressor be made of material that will not fail or
cylinders, valves etc. may cause deteriorate from the test medium used.
sudden or gradual failures.
5. Install preliminary warning devices Pressure Gage Calibration
that alert the leak testing technician and Safety Applications
when test pressures are increasing too
rapidly or when pressurization is One of the best means of protection from
approaching an excessive level. These overpressure is to use an accurate gage. To
devices can call attention to an ensure accuracy of a pressure gage, it must
abnormal situation before a pressure be periodically checked against some
relieving device is set off. Prompt known standard pressure. Dead weight
correction of trends toward excessive testers are used for calibration and
pressure can often forestall the checking of the elastic gages for pressures
actuation of emergency pressure relief exceeding approximately 100 kPa
valves. This is most valuable in (15 lbf·in.–2) and extending to 70 MPa
extreme pressure work. (105 lbf·in.–2) or even higher. Dead weight
6. Check temperature of test water or gages are used for the precise
other test medium for compliance to determination of essentially constant
test procedure. pressures maintained in a vessel by some
7. Assure the availability on test site of pressure generating mechanism. The dead
approved written test and safety weight tester and the pressure gages
procedures for all test personnel. should both be calibrated over their full
scale. Pressure gages should be calibrated
Pressure Relieving Devices in Pressure both before and after testing on critical
high pressure tests. Gage calibration
Leak Testing should follow approved written
procedures.
Spring loaded relief valves are used up to
100 MPa (1.5 × 104 lbf·in.–2) as pressure Care, Handling and
relieving devices. They are quite reliable Storage of Pressure Gages
for nonpulsating operations at 15 to
20 percent above working pressure, but Handling and storage should be done
cannot be completely relied on to reseat with the knowledge that a gage suitable
without leakage. Shear rupture disks, for accurate pressure measurement is as
made of bronze, stainless steel or other delicate as a watch. Its removal and
metals, depending on service conditions, replacement for calibration purposes (and,
are suitable for nonpulsating operations at of course, its installation and use) should
test pressures up to 20 to 30 percent be entrusted only to persons who can be
above working pressure. Formed heads depended on to avoid dropping or jarring
failing in tension have been applied to the gage or subjecting it to rough
appreciable pressures but do not possess treatment. The gage should always be
the accuracy required at higher pressure attached by using a wrench on the flats
up to 70 MPa (1 × 104 lbf·in.–2). provided on the connection. A gage must
never be screwed or unscrewed by using
Sometimes relief valves and rupture the gage casing. If the gage has the proper
disks are used in parallel. In this case, the tolerances and is handled correctly, the
relief valve is set to open at a lower gage corrections determined before and
pressure. This warns test technicians that after the pressure test for each test for
prompt corrections may be necessary to which the gage was used should agree
avoid rupture disk failure, with resulting within specified calibration accuracies. If
lost time. Rupture disks and relief valves the gage is not handled properly, there is
are also used in series, with one or the a chance that the calibration and
other in the upstream position. In this corrections determined before and after
series case, unless a small vent hole is the test will differ appreciably. In such
used between the two to prevent seepage, events there is no sure way to know
the back pressure caused by seepage can which correction to use and the result of
force the failure pressure on the upstream the test will be in doubt.
unit to rise to a dangerous value equal to
the relieving pressure. Any accident to a gage requires that
the gage be given a complete calibration
Hydraulically loaded plugs using O-ring
seals are dependable and will relieve at
test pressures closer to the working
pressure than other devices. O-ring seals
are typically flexible ring shaped inserts
Safety Aspects of Leak Testing 135
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and correction test before further use. This would be necessary during pressurizing.
would apply also if a gage shows obvious However, the damage from rupture of a
evidence of prior damage. In the event gas filled volume results from the total
that a gage is mishandled by dropping it, amount of gas it contains. Therefore,
exceeding its pressure range or exposing it either a small system under high pressure
to vacuum (unless it is an absolute can be as dangerous a large system under
pressure gage), the gage must be repaired lower pressure. The energy stored in a
and recalibrated by its manufacturer, a pressurized gas volume is equal to the
qualified laboratory or equipment product of its pressure and its volume.
manufacturer with proper calibration The pressure in pascal or newton per
facilities. square meter multiplied by the volume in
cubic meter results in energy in joule,
Hazards of Pressurized (N·m–2) × m3 = Nm = J. By comparison,
Test Systems 1 kg of gasoline contains about 44 MJ,
enough to blow up a tank.
The necessary safety precautions vary
greatly with the type of system being leak When pressurizing a system, a pressure
tested. Some general types are listed below regulator fitted with a safety overpressure
in ascending order of the potential danger release device should be installed so that a
involved. pressure in excess of the design pressure
can never be applied to a vessel or system
1. With small hydraulic systems of under test.
moderate pressure, the major hazard is
from a jet of the liquid either from a Rupture Hazards in
leak or failure. Occasionally, the Pressure Testing
necessity to include a brittle material
such as a sight glass or glass flow Although the prevention of clogged leaks
meter in the system adds the hazard of dictates that leak testing with gaseous
flying particles. tracers should be done before contact of
the system with liquid, the need for safety
2. Low pressure systems involving might overrule this procedure.
nonreactive gases or liquids above Pressurizing a system with a liquid does
their boiling point involve little not create the explosion hazard involved
hazard if correctly handled. However, with gases under high pressure. Therefore,
it is important to have the proper safety requirements may dictate pressure
relief valves, rupture disks and testing of a system with a liquid before
pressure regulators to maintain safety gases are introduced for leak testing. An
in low pressure systems. The hazard of alternate preliminary leak test might be a
low pressure systems can be higher if low pressure, high sensitivity mass
large volumes of gases are involved. spectrometer leak test using helium as the
tracer gas.
3. Systems involving flammable gases or
liquids (such as kerosene) as the The amount of energy stored in a tank
pressure testing fluid involve major pressurized with gas is a function of the
hazards, including those of fires or quantity and type of gas contained in the
explosions resulting from leakage or vessel. Because of this, a high volume, low
failure of some component. pressure vessel can contain the same
stored energy as a low volume, high
4. The hazards of high pressure hydraulic pressure vessel. Therefore, each presents a
and inert gas systems increase with the hazard of similar magnitude.
increase in pressure, the
compressibility of the testing media The exact rupture hazard involved in
and the volume of the system. There is pressure testing is difficult to define,
an increasing probability that although the structural burst limit is a
equipment in the higher pressure reasonably predictable design factor. Any
ranges will not permanently resist the damage incurred during fabrication,
effect of pressure. erection, testing or service by a vessel
under pressure, such as weld undercut or
Explosion of Systems or a deep nick or gouge, may cause explosive
Vessels Pressurized for failure if the damage is severe. Small flaws
Leak Testing can be progressive depending on metal
strain and the type of load. Surface stress
If a system to be leak tested is pressurized concentrations caused by vessel damage
with tracer gas or gas mixtures, rupture of may not result in immediate failure, but
its containment walls or pressure may progress and cause failure later.
boundaries could produce considerable When a skin puncture takes place, it
damage. If the system being pressurized is results in a tearing action that tends to
small, it might seem as if few precautions enlarge the hole. An inspection of a failed
vessel will show tears extending across the
entire face of the skin.
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Effects of Leak Size and Shape of pressure P2 equals 100 kPa (one
the Opening on Failure atmosphere), the energy released can be
Mechanisms computed as follows. Because PVk is a
A smaller leak will dissipate the same constant,
energy as a larger leak, but over a long
period without the explosive effect. The 1
critical leak size is related to the tensile
strength of the enclosing skin. The critical (5) V2 = V1 P1 k
point of explosive pressure release is P2
reached when the force of the gas
escaping from the hole exceeds the force
that can be withstood by the skin. 1
Another factor influencing failures is = 15 000 1.4
the shape of the leak opening. An 0.04
irregularly shaped opening, with many 100
microscopic irregularities, each providing
a stress point, offer an ideal starting point = 1.43 m3
for a tearing and shredding action. As the
pressure in a tank increases, a critical Substituting in the work equation:
pressure is reached where the stress exerted
by the confined gas exceeds the strength of 1W2 = ( )( ) ( )( )100 1.43 − 15 000 0.04
the metal surrounding the failure. This
causes an explosive disintegration. 1 − 1.4
Variation of the critical point of explosive
pressure release can occur with conditions = 1.142 MJ
of service, vessel shape, size and wall
thickness and material, fabrication This energy (somewhat more than
methods and type of failure. 1 MJ) could be evaluated in comparison
with the 44 MJ of energy available by
Energy Contained in Pressurized combustion of 1 kg of gasoline, of 38 MJ
Vessels from 1 m3 of natural gas or of 32 MJ from
1 kg of coal.
The work done to compress gas in a vessel
is stored in that gas. It is normally Evaluating Hazards of
returned by propelling the gas to places Explosive Pressure Release
where it is needed. However, a rupture in
the tank may suddenly release all the The critical point of explosive pressure
energy at once as an explosion. An release is a very important factor in
explosion is so fearsome because of the determining the hazard magnitude of
short duration of the energy release that high pressure leak tests. However,
can be calculated by means of equations calculation of available stored gas energy
for isentropic processes: is necessary for a thorough analysis of the
potential hazard. This calculation includes
(3) PV k = constant two important considerations: (1) the
amount of energy stored in the
and for work 1W2: compressed gas and (2) the rapidity with
which this energy is released.
The amount of energy stored in a
noncombustible compressed gas can be
approximated by Eq. 6:
(4) 1W2 = P2V2 − P1V1 K −1
1− k
(6) E= P1V1 P2 K
1− K P1 − 1
Equations 3 and 4 take into account
any sudden temperature change during
the explosion. In this equation, k = 1.4, a
constant. The work done (energy released) where K is the ratio of specific heat Cp at a
is that resulting from a change from constant pressure to that of a constant
conditions identified by subscript 1 to
those identified by subscript 2. As an volume Cv, where P1 is initial absolute
example, compute the energy released pressure, V1 is initial volume and P2 is
when a small pressurized tank is ruptured final pressure (100 kPa or 1 atm). This
and compressed gas escapes to
atmospheric pressure and temperature. If equation is based on the ideal gas law and
the internal pressure within the tank is
P1 = 15 MPa (150 atm), the volume of the isentropic expansion. At high pressure
tank is V1 equals 0.04 m3, and the
compressed gas escaping to atmospheric (e.g., above 20 MPa) where the deviation
from an ideal gas may be appreciable, the
equation is still valid provided one divides
the right hand side by 2, the so-called
compressibility factor found in gas
handbooks.
Safety Aspects of Leak Testing 137
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