ASNT NDT Handbook Volume 3 Infrared and Thermal Testing

of the center panel of the four units are, because the water is not flowing. The
from the left, 312.5, 313.2, 312.6 and radiation temperature difference ∆T
312.4 K (39.4, 40.1, 39.5 and 39.3 °C; between the shaft and flange is 1 K
102.9, 104.2, 103.1 and 102.7 °F). The (1 °C = 1.8 °F).
panel temperatures of the first, third and
fourth units from left are lower than that Two pumps are always circulating
of the second one, in suspended during reactor operation and the third
operation. The surface temperature auxiliary pump is on standby. When one
difference between the surface of the main pumps stops because of a
temperature of the panel in operation and malfunction, the auxiliary pump
one in suspension becomes 0.7 K (0.7 °C = immediately starts up so that the reactor
1.3 °F), because a panel in operation continues to be operated continuously
releases heat by evaporation. without a decrease in water flow rate.

To summarize, thermographic The temperature distribution of the
temperature measurements are useful for shaft and flange is continuously recorded
qualitative evaluation of the operating by an infrared radiometer. The
condition of the nuclear reactor from the thermographic system is useful for
exterior of the reactor installation. detecting abnormal conditions of pump
Continuous monitoring of the radiation components, like excess heating of the
temperature of the cooling tower unit driving motor, misalignment of the shaft
uses thermography to evaluate the flange, water leakage from the sealing unit
operating condition of the reactor. and so on.
Infrared thermography can also be used FIGURE 5. Thermography of main circulating
remotely to assess the heat removing pump: (a) operating; (b) idle.
capacity of the cooling towers during (a)
operation from a location safe from
radiation.14 (b)

Detection of Overheating
of Main Circulation Pumps

The thermal output of 50 MW is removed
from the reactor core to the secondary
loop through a heat exchanger. Main
circulation pumps with a flow rate of
556 L·s–1 (2000 m3·h–1 = 19.6 ft3·s–1) and a
water pressure head of 60 m (197 ft) are
installed in the secondary circulation loop
of the reactor to circulate the cooling
water. Three pumps are connected to the
main piping system in parallel.

Figure 5 shows the thermography of
the vertical flange of the pump shaft
85 mm (3.4 in.) in diameter, which
connects the motor and pump casings by
a driven shaft. The upward vertical shaft
is connected to the driving motor and
downward shaft of the pump impeller by
a flange. Figure 5a shows the flange and
shaft in operation and Fig. 5b shows them
in suspended operation.

Radiation temperatures of the lower
flange and shaft under the operation are
308.7 K (35.5 °C = 95.9 °F) and 316.7 K
(43.5 °C = 110.3 °F), respectively. The
temperature of the shaft is higher than
that of the flange. The circulating water
absorbs heat by conduction. The
temperature difference between the
temperature of shaft and flange is ±8 K (8
°C = 14.4 °F).

Temperatures of the lower flange and
shaft in suspended operation are 305.3 K
(32.1 °C = 89.8 °F) and 304.3 K (31.1 °C =
88.0 °F), respectively. The temperature of
the shaft is lower than that of the flange

536 Infrared and Thermal Testing

Reactor Canal Wall higher than that of the wall surface in the
Maintenance morning and vise versa in the evening.

The canal pool connects the reactor These streaks on the wall were caused
container to the hot laboratory to transfer by a mortar layer applied during repair
the irradiated fuels and materials through work several years before, because surface
the water canal. The water canal is cracks on the concrete wall had developed
directly connected from the reactor core through environmental deterioration. The
to the hot laboratory. Predictive concrete cracks under the mortar layer are
maintenance work has been carried out to filled with air. Thermal conductivity ka of
inspect for deterioration of the outside air is less than thermal conductivity kc of
concrete wall of the canal. the concrete.

Figure 6 shows the thermography of Thermal image analysis is carried out
the east concrete wall of the canal taken by numerical calculation to solve the
from the outside area of the canal two-dimensional conduction equation.
building in the morning and evening. The The analysis predicts that the temperature
wall is heated by the sun in the morning difference of the streak on the wall is
and cooled as the wall radiates in the caused by the difference of the thermal
evening. The thermal images show several conductivity between the concrete and air
streaks in the radiation temperature on in the crack.
the wall, streaks invisible to the naked
eye. The temperature of the streaks is If the water permeates into the buried
crack through the mortar layer, the
FIGURE 6. Reactor canal wall: (a) at radiation temperature of the streaks is
10:00 a.m.; (b) at 16:40. lower than that of the wall in the
(a) morning and vise versa in the evening,
because the thermal conductivity of the
water is larger than that of the air. The
positive and negative temperature
differences induced on the surface above a
crack in the morning and evening
indicate the location and dimension of
the crack and whether it contains water.

Thermography and the related thermal
image analysis are useful tools of
predictive maintenance for detecting
cracks, determining their dimensions,
determining whether they are filled with
air or water and monitoring crack growth.

(b)

Electric Power Applications of Infrared and Thermal Testing 537

PART 4. Infrared Thermography for Nuclear
Fusion Reactor

Research and development projects have 400 K (127 °C = 260 °F) and at 1200 K
been carried out to develop the (927 °C = 1700 °F), respectively. It is
International Thermonuclear essential to control impurities in the
Experimental Reactor to maintain plasma to maintain the operation
continuity with Tokamak type reactors15,16 temperature. The diverter absorbs gas
and to develop fusion plasma technology. impurities on its surface at high
The armor of the first wall and diverters temperatures.
of the fusion reactor are fabricated from
high temperature materials to be resistant Diverter plates are installed at the
against the plasma of 1 × 108 K (about 1 × bottom of the doughnut shaped vacuum
108 °C = of 1.8 × 108 °F) in a high vessel of the fusion reactor. One design for
vacuum.17 the diverter is for carbon-to-carbon
composite armor tiles brazed to copper
The role of the diverters reduce the heat sink plates. The armor tile is required
amount of plasma flowing directly into to be made of materials with high thermal
the first wall as a result of plasma conductivity and good thermal shock
disruption. Plasma disruption would take resistance. The composite-to-copper joint
place if the plasma in the fusion reaction structure of the diverter is required to
confined by the magnetic field were maintain its structural integrity under
disrupted for any reason. Fusion reaction high thermal loads.
is maintained by controlling the magnetic
field around the diverter in such a way In the event of plasma disruption
that the impurities in the plasma and armor tiles may be exposed to high heat
those generated during the control flux of 100 to 200 GW·m–2 (3.17 × 1010
procedure are removed through the BTUTC·h–1·ft–2) for a very short period,
diverter. suffering some damage. Damaged tiles
must be replaced. Research and
The normal operation period of the development determines the optimum
reactor is 2000 s per pulse. The heat flux design for replacement technology as well
originating from the plasma, which is to as for inspection after the replacement.
be 1 MW·m–2 at its maximum, is injected
into the first wall and the diverter at Infrared thermography can be used for
remote nondestructive testing of the

FIGURE 7. Radiometric test apparatus.

Screen

Heater

Thermocouple

Plate heater Infrared Cathode
radiometer ray tube
~V Black wall
Specimen Central
A Water tank processing
Liquid nitrogen
sensor (mercury unit
cadmium
tellurium)

538 Infrared and Thermal Testing

diverter joint structure.18 Model diverter is, εs + ρs = 1, emissivity is generally
specimens are heated using a halogen obtained from the equation:
lamp, high temperature combustion gas
or hot water. The heating technique using ⎛ Ta ⎞ 4.09
the hot water is best for the ⎝⎜ Ts ⎠⎟
nondestructive test of the diverter joint as −
structure.
εs = ⎛ Ta ⎞ 4.09
Radiation Properties of ⎝⎜ Ts ⎟⎠
Carbon-to-Carbon (2) 1 –
Composites
The relation between the true
A schematic of experimental apparatus for temperature and emissivity is shown in
measuring radiation temperature of Fig. 8 for the carbon-to-carbon specimen
carbon-to-carbon composites is shown in surface. Open symbols represent values
Fig. 7. A plate specimen is placed on the calculated on the basis of the radiosity
plate heater. Temperature of the specimen coefficient for the carbon-to-carbon
Ts is controlled from 303 K (30 °C = 86 °F) composite. The emissivity of the surface
to about 373 K (100 °C = 212 °F) with a decreases with increasing true temperature
gradient of 10 K (10 °C = 18 °F). A water of the specimen. Experimental data
tank with a pyramidal hood is placed in obtained at environmental temperatures
the space between the specimen and an of 298, 303 and 313 K (25, 30 and 40 °C;
infrared radiometer. The interior of the 77, 86 and 104 °F) for the
hood is lined with black velvet to carbon-to-carbon composite are fitted to a
approximate a blackbody surrounding solid line. The solid line in the figure
with a constant temperature. The ambient represents the relationships obtained by a
temperature Ta is controlled by changing least squares technique:
the water temperature in the tank. After
Ts and Ta are kept constant, the radiation ( )(3) εs = 1.001 − Ts 5.052 × 10–4
energy from the specimen surface is
measured to obtain data on its emissivity The radiation temperature differs from
and radiosity coefficient.19 On one side of the true temperature of the surfaces of the
the hood an infrared radiometer whose carbon-to-carbon composite and differs
sensor is cooled with liquid nitrogen also for a surface painted black. Radiation
measures the radiation energy in the temperatures were measured at
wavelength range from 8 to 13 µm to give environmental temperatures of 298, 303
a two-dimensional thermal image on a and 313 K (25, 30 and 40 °C; 77, 86 and
television monitor or cathode ray tube.
The temperature of the specimen surface FIGURE 8. Emissivity of carbon-to-carbon composite.
is determined by using K type
thermocouples embedded to the side of 0.90
the specimen. The energy Js flowed into
the sensor from the specimen surface and 0.88 εs = 1.001 – 5.052 x 10–4 Ts
the surrounding wall is expressed by the 0.86
following equation, taking into account Emissivity, εs
the wavelength dependence of the 0.84
radiometer.
0.82
(1) Js = σ εs Ts4.09 + σ ρs Ta4.09
= σ as Ts4.09 = σ Ts,4.09 0.80
300 310 320 330 340 350 360 370 380
Here, Ts is apparent radiation (27) (37) (47) (57) (67) (77) (87) (97) (107)
temperature, εs is emissivity, ρs is [81] [99] [117] [135] [153] [171] [189] [207] [225]
reflectivity, subscript s indicates the
specimen and σ is the Stefan-Boltzmann Temperature Ts, K (°C) [°F]
constant. Legend

The radiation temperature measured by = ambient temperature Ta of 298 K (25 °C = 77 °F)
the radiometer is determined by the = ambient temperature Ta of 303 K (30 °C = 86 °F)
energy radiated from the specimen surface = ambient temperature Ta of 313 K (40 °C = 104 °F)
which includes both the energy inherent εs = emissivity (ratio)
to the specimen and that incident on the Ts = apparent radiation temperature
specimen from its surroundings. If the
gray color approximation is assumed, that

Electric Power Applications of Infrared and Thermal Testing 539

104 °F) for carbon-to-carbon composite FIGURE 9. Configuration of model divertor specimen: (a) side
and again for black paint. The radiation view of specimen 3; (b) unbrazed ratio of specimen 1;
temperature of the black paint is nearly (c) unbrazed ration of specimen 2; (d) unbrazed ratio of
equal to the true temperature. For the specimen 3.
carbon-to-carbon composite, however, the
difference between radiation temperature (a) Carbon reinforced
and the true temperature increases with carbon composite
increasing true temperature. 25 mm 10 mm
(1.0 in.) Brazed interface (0.4 in.)
Test Apparatus Using Unbrazed
Artificial Heaters 25 mm interface
(1.0 in.)
A configuration of model diverter 35 mm Copper
specimens with a cooling tube is shown in (1.4 in.)
Fig. 9. These specimens consist of
carbon-to-carbon composite tiles brazed 125 mm (5 in.)
to the copper heat sink with different
brazed area ratios. The tile is composed of Cooling
a carbon fiber reinforced composite with tube
high thermal conductivity, as shown in
Table 1. To prepare the joint of (b) 6.35 mm
carbon-to-carbon composite tiles and (0.25 in.)
copper heat sink, the model specimen is
heated in a vacuum furnace at 1120 K (c) 75 percent 0 percent
(847 °C = 1556 °F). The separation area unbrazed unbrazed
ratio is defined by the ratio of the 15 mm
unbrazed area to the total tile area. (0.6 in.) 19.1 mm 6.35 mm
Separation area ratios of the tiles are 100, (0.75 in.) (0.25 in.)
75, 50, 25 and 0 percent. Three types of (d)
model specimens shown in Figs. 9a, 9b 25 percent 75 percent
and 9c are used in this experiment. 18 mm unbrazed unbrazed
(0.7 in.)
The test specimens were heated using a 13 mm 25 mm 13 mm
halogen lamp, high temperature (0.5 in.) (1.0 in.) (0.5 in.)
combustion gas or hot water.20
25 mm
The radiation temperature distribution (1.0 in.)
on the tile surface of the specimen is
measured by an infrared radiometer 50 percent 50 percent
covering a wavelength range from 8 to unbrazed unbrazed
13 µm. The minimum detectable
temperature difference of the radiometer Legend
is ±0.2 K (0.2 °C = 0.36 °F). = brazed part
= unbrazed part
A schematic of the experimental
apparatus for heating with a halogen
lamp is shown in Fig. 10a. The halogen
lamp with a gold plated parabolic reflector
projects thermal radiation on the
specimen. The infrared radiometer
measures the radiation temperature of the
specimen for 1 s with a heating duration
of 8 s after a shutter in front of the
specimen is removed. The lamp provides a
step function heat pulse of about
10 kW·m–2 (3172 BTUTC·h–1·ft–2).

TABLE 1. Thermal characteristics of carbon fiber reinforced composite.

___T_em__pe_r_at_u_re___ Thermal Thermal
_____D_i_ff_us_iv_it_y_____ _______Co_n_d_u_ct_iv_it_y______
Direction K (°C) [°F] 10–4 m2·s–1 (in.2·min–1) W·m–1·K–1 (BTUTC·h–1·ft–1·°F–1)

X-X 293 (20) [68] 3.286 (30.560) 404 (2.336 × 104)
301 (1.740 × 104)
Y-Y 293 (20) [68] 2.550 (23.715) 226 (1.307 × 104)

Z-Z 297 (24) [72] 1.825 (16.973)

540 Infrared and Thermal Testing

A schematic of the experimental the test specimen is about 5 kW·m–2
apparatus for a heating means using high (1600 BTUTC·h–1·ft–2) at maximum.
temperature combustion gas is shown in
Fig. 10b. Combustion gas at 400 to 430 K A schematic of an experimental
(127 to 157 °C; 260 to 315 °F) is directed apparatus for the hot water technique is
onto the specimen by a gas heater after shown in Fig. 10c. The specimen is placed
the shutter in front of the specimen is in contact with the hot and cold water
removed. Heat flux applied to the tile of jackets enclosed by the insulation plate.
The specimen is mounted in front of the
FIGURE 10. Test setups for heating techniques: (a) heating infrared radiometer. The cooling tube is
with halogen lamp; (b) heating with high temperature gas; initially filled with water of 283 K
(c) heating with hot water. (10 °C = 50 °F). The specimen is heated by
flowing warm water of 303 K
(a) Halogen Support (30 °C = 86 °F) in the cooling tube.
lamp Thermal images of the tile surface are
Parabolic Shutter obtained by the infrared radiometer from
reflector Black paint 1 to 8 s after the start of heating.

Cooling fan Model divertor Detection Limit of Brazed
specimen Tile Elements
Cathode
ray tube Halogen Lamp Heating Test

Infrared Thermal images obtained by the heating
radiometer technique using the halogen lamp for the

Personal FIGURE 11. Thermal images of model divertor specimen 8 s
computer after start of heating by halogen lamp: (a) specimen 1;
(b) specimen 2; (c) specimen 3.
(b) High temperature Model (a)
gas injection divertor
Gas heater specimen 0 percent
100 percent

0 percent

Cathode Shutter (b)
ray tube Infrared radiometer
Central 0 percent
processing unit Valve 2
In Out {75 percent
(c) Circulating
pump 0 percent
Model divertor Table Valve 1
specimen Water (c)
tank
Infrared 0 percent
radiometer
{50 percent
Fixture 0 percent

Insulation

Electric Power Applications of Infrared and Thermal Testing 541

tiles with separation area ratios of 100, 75, becomes ±1.4 K (1.4 °C = 2.5 °F) at the
50 and 0 percent at 8 s after the start of maximum. However, ∆T between the
heating are shown in Fig. 11. The temperature of tiles of 75, 50 and
difference ∆T between the radiation 25 percent and that of 0 percent becomes
temperatures becomes 3.8, 1.7 and 0.9 K zero, giving no detectability for these tiles.
(3.8, 1.7 and 0.9 °C; 6.8, 3.0 and 1.6 °F) Moreover, the radiation temperature
depending on the separation area ratios. distribution on the tile surface is not
Difference ∆T becomes larger with uniform because of turbulent jet flow of
increasing separation area ratio because gas. Stable temperature distribution on
the separation layer is filled with air and the thermal image is measured at 60 s
thermal flow through this area decreases after the start of cooling. In this case, the
compared with that through brazed area. temperature of tile with 100 percent
separation area at 60 s after the start of
The radiation temperature distribution
on the tile surface along the cursor line FIGURE 13. Thermal Images of model divertor specimen
has been measured with the heating time heated by hot water: (a) 1 s after start of heating; (b) 8 s
as a parameter. The temperature after start of heating.
distribution of the tile of 100 percent (a)
separation becomes convex at 2 s after the
start of heating and the difference 0 percent
between the maximum and minimum 100 percent
temperatures on the distribution increases
with increasing heating time. 0 percent

Hot Gas Heating Test {75 percent

Thermal images obtained by the heating 0 percent
technique using high temperature
combustion gas for the tiles with (b)
separation area ratios of 100 percent and
0 percent are shown in Fig. 12. The 0 percent
difference ∆T between the temperature of 100 percent
tiles of 100 percent and that of 0 percent
0 percent
FIGURE 12. Thermal images of model divertor specimen
heated by high temperature gas: (a) 60 s after start of {75 percent
heating; (b) 3 s after start of cooling; (c) 60 s after start of
cooling. 0 percent
(a)
FIGURE 14. Radiation temperature distribution of tile surface
0 percent heated by hot water.
100 percent

0 percent

Radiation temperature, K (°C) [°F] 293 (20) [68]

(b) 291 (18) [64]

0 percent 289 (16) [61]
100 percent
287 (14) [57] 0 percent 100 percent 0 percent
0 percent 285 (12) [54] Unbrazed ratio

(c) Specimen edge Distance Specimen center

0 percent Legend
100 percent =0s
=2s
0 percent =4s
=6s
=8s

542 Infrared and Thermal Testing

cooling becomes smaller than that of tile becomes smaller than that of
0 percent separation because of the heat 0 percent and 75 percent separation tiles.
released from the tile to ambient air Therefore, both tiles with separation area
during cooling. ratios of 0 percent and 25 percent are
detectable by thermography. Values of ∆T
Hot Water Heating Test for tiles of 100 percent separation are
3.8 K (3.8 °C = 6.8 °F). Values of ∆T for
Thermal images obtained by the heating tiles of 75 percent separation are 1.0 K
technique with hot water for the tiles at (1.0 °C = 1.8 °F). The tiles with 50 percent
1 and 8 s after the start of heating are and 25 percent separation can be detected
shown in Fig. 13. The temperature from the temperature difference ∆T
difference of the 100 percent separation smaller than that for 100 percent

FIGURE 15. Heat flow and temperature distribution through specimen of material with
conductivity km around internal discontinuity of conductivity kd: (a) heated surface, km > kd;
(b) cooled surface, km > kd; (c) heated surface, km < kd; (d) cooled surface, km < kd.
(a) (c)

Surface temperature Surface temperature
(relative scale) (relative scale)

Point of surface Point of surface
Heated surface
Heat flow lines

Heated surface

Specimen Internal defect Specimen Internal defect
(b)
(d)

Surface temperature Surface temperature
(relative scale) (relative scale)

Point of surface Point of surface
Cooled surface Cooled surface

Specimen Internal defect Specimen Internal defect

Electric Power Applications of Infrared and Thermal Testing 543

TABLE 2. Detector performance for separation area ratio of three active heating techniques, with
temperature difference ∆T.

Unbrazed ______H_a_lo_g_en__La_m_p______ _____W_a_r_m_W_a_t_er_____ ______H_o_t _G_as______
Ratio (percent) Appearance K or °C (°F) Appearance K or °C (°F) Appearance K or °C (°F)

100 visible 3.8 (6.8) visible 3.8 (6.8) visible 2.9 (5.2)
75 visible 1.7 (3.1) visible 1.7 (3.1) invisible ——
50 visible 0.9 (1.6) visible 0.7 (1.3) invisible ——
50 (side) visible, not clearly visible 0.4 (0.7) invisible ——
25 invisible —— visible 0.3 (0.5) invisible ——
——

separation. Values of ∆T are 0.4 and 0.3 K as a parameter. The halogen lamp
(0.4 and 0.3 °C; 0.7 and 0.5 °F) for 50 and technique can detect separation up to
25 percent, respectively. 50 percent. The combustion gas technique
can detect 100 percent separation. The
The radiation temperature distribution hot water technique can detect all the
of the tile surface along the cursor line is separations examined here.
shown in Fig. 14 with heating time as a
parameter. The temperature distribution The heating time is a parameter in
of the tile of 100 percent separation obtaining the absolute value of the
becomes concave at 2 s after the start of difference in the surface temperature
heating and the difference between the between tiles with 100 percent and
maximum and minimum temperatures on 0 percent separations. The absolute value
the distribution increases with increasing has been obtained by numerical
heating time. computation using the difference
technique for the nonsteady heat
Image Analysis of Internal conduction equation. Heating and
Separations in Brazed Tiles boundary conditions of a numerical
computation for a conceptual model is
Image analysis is carried out numerically the same as that of the present
to determine the temperature distribution experiments. Numerical data for heating
around an internal discontinuity in the using the halogen lamp, high temperature
brazed tile element. A nonsteady heat gas and hot water are shown in solid,
conduction equation of a composite broken and dotted lines, respectively. The
model is solved by a differential control temperature difference increases with
volume technique.21 Temperatures of increasing heating time. The temperature
armor tiles with and without the difference obtained by the hot water
separation have been numerically technique is the greatest among the three
calculated and experimentally measured. techniques. The tendency of the absolute
Numerical calculations of temperature difference in the temperature obtained
changes are close to experimental data. from the numerical computation is in
accord with the experimental results of
Figure 15 summarizes the heat flow the detection performance for three
and generated temperature distribution heating means (Table 2).
around the internal separation. The
difference ∆T between the temperature of The hot water technique is the most
the surface above the internal separation appropriate for the nondestructive testing
and that of the surface of the brazed area of the diverter joint structure with cooling
is caused mainly by the difference in pipes, judging from its detectability and
thermal conductivity between the the simplicity of the apparatus for the
composite material km and the separation measurement. This technique is believed
kd. The temperature distribution on the to be applicable to the nondestructive
surface during the heating and cooling testing of the diverter of nuclear fusion
becomes convex and concave, reactors.
respectively, because thermal conductivity
km of the composite is larger than thermal Although only some examples of
conductivity kd of the separation. fusion reactor components have been
described in this paragraph, it is believed
Detection Performance that the present technique can be
with Three Heating Means applicable to any discontinuities in the
weldments and brazed joints in any
Detection performance is summarized in components. In this sense the technique
Table 2 for the three active heating means has a wide variety of possible applications.
where the separation area ratio is chosen

544 Infrared and Thermal Testing

PART 5. Infrared Thermography of Power
Generation Subsystems

Thermography of Boilers22 Insulation

Infrared and thermal testing has the Boiler insulation, called refractory, can fail
advantage that it is noncontacting and in various ways. The refractory can pull
can rapidly inspect large areas of a away from the walls and fall onto the
component. The method is therefore ideal boiler floor. Other times, the brick or batt
for the inspection of boilers and process insulation separates from its wall anchors,
heaters used in steam generation power allowing the hot gases to flow behind the
plants. Here, temperatures are high and insulation.
access is generally limited to one side of
the boiler or boiler tube. When a boiler is inspected with
infrared thermography from the outside,
Infrared thermography has a these failures show up as hot spots,
prominent role in the nondestructive provided there is not an air space between
testing of electrical generating plants. the external cladding and the insulation.
Thermography can detect high resistance Figure 16 shows such a hot spot, which
(faulty) electrical connection problems has been marked with paint on the
and overloads. Thermal techniques are outside of the boiler. This hot spot, will be
used to locate problems in boilers and monitored on a regular basis to track
process heaters. Technological advances in deterioration, which is indicated by an
infrared thermography can save time and increasing skin temperature.
money in the power generation industry.
Engineers set maximum temperature
Infrared cameras are sensitive to limits at which the metal will be
wavelengths of radiation in the 2 to 14 FIGURE 16. Damaged refractory on inside of
µm region of the electromagnetic this boiler has created skin temperature of
spectrum. They convert this heat energy 465 K (192 °C = 377 °F): (a) photograph;
to a visible light display, which a trained (b) thermogram.
operator (thermographer) analyzes and (a)
documents. Qualitative thermography is
used to locate significant heat differences, (b)
whereas quantitative thermography
assigns accurate temperatures to the
problems found. Because many
developing problems in machinery
increase temperature, thermography is an
ideal tool for predicting when a
component is approaching failure.

Boiler Applications

Boilers are vessels used to transfer heat
from the fire to water flowing through the
boiler tubes. As such, their efficient and
safe operation depends on several factors.
First, the boiler must be insulated to
minimize heat loss through the walls and
to protect the tube surfaces on the
combustion side. Second, the vessel
should be as airtight as possible to prevent
uncontrolled air from entering or exiting.
Third, the water in the tubes must be
unrestricted to prevent overheating and to
allow for maximum heat transfer. Infrared
thermography can play a key role in
ensuring the performance of these three
functions of a boiler.

Electric Power Applications of Infrared and Thermal Testing 545

permanently damaged or a burn through ports. Infrared thermography is suited for
is imminent. The goal is to monitor the locating these hot air leaks, too.
problem and keep the boiler safely on line
until the next scheduled shutdown. Boiler Tube Blockages
Before the shutdown, engineers will use
the thermographic data to determine the As mentioned previously, water within
extent of damaged refractory so that the boiler tubes must flow freely to permit
maintenance workers can order repair efficient heat transfer. When foreign
materials and accurately schedule crews to materials inside a tube impede this
minimize the downtime. Savings can be circulation, the tube overheats and, if hot
tremendous and the safety of boiler enough, will burst and force an
operation and attendant workers is unscheduled shutdown. Tube
ensured. temperatures can be monitored when the
boiler is in operation (as discussed later)
Boiler Casing Leaks but often many of the problems occur
when bringing the boiler back on line
Boiler casing leaks can be a serious and after a shutdown.
costly problem for power plants. In
positive pressure systems, ash and coal Failure analysis of many of these
can emerge through tiny pinholes in the failures has yielded some interesting
boiler to erode boiler tubes and reduce findings. Many of the blockages are
efficiency. These leaks are often very caused by improper paper left in the tubes
difficult and time consuming to detect by welders after they have completed
visually. However, these pinholes leak their repairs. When the boiler is brought
very hot combustion gases, which in turn back on line, scale or magnetite is trapped
heat up the external cladding of the by this paper, creating flow restrictions.
boiler. One power company has used
thermography before boilers are brought
Thermographers can detect these hot back on line. Condensate at 366 K
air leaks, which have a different pattern (93 °C = 200 °F) is cycled through the
than conduction problems caused by tubes and viewed with an infrared camera
damaged insulation. The outer covering from inside the boiler. Figure 18 shows a
and insulation are removed and the leak tube that is cooler (darker) than the
is located and repaired, saving significant adjacent tubes, indicating a fluid flow
amounts of troubleshooting time and restriction. After implementing a
increasing boiler efficiency and tube life. thermographic inspection program, the
As a side benefit, before and after company saved millions of dollars because
thermograms can be taken (see Fig. 17) to virtually no premature failures have been
give repair crews proof that the job was caused by ruptured tubes.
(or was not) properly done. Worker
morale and quality of repairs increases Overview of Boiler Applications
when inspection reports are provided with
infrared thermography. Infrared thermography can be used to
increase the efficiency and safety of
Besides casing leaks, air can escape boilers in generating plants. Successful
from the boiler at other locations, such as programs require highly trained infrared
expansion joints, access doors and view thermographers who are knowledgeable
about the construction and operation of
FIGURE 17. Thermograms prove that repairs have corrected
boiler casing leak: (a) before repair; (b) after repair. FIGURE 18. Dark boiler tube in center is
(a) (b) cooler because 366 K (93 °C = 200 °F)
condensate flow is blocked.

546 Infrared and Thermal Testing

these vessels. Some of the applications technology is fast, cost effective,
described are physically demanding and nonintrusive and ideally suited to the
require specialized equipment but the very thermal nature of this investigation.
returns on the investment will pay for the A test was devised to verify the isolation
inspection program many times over. capability of the three-way valve.

Temperature Modulation In the three-way valve, the shell side
Case History of Closed water enters from the left and is
Cooling Heat Exchanger23 modulated straight through (to the right)
into the heat exchanger shell or is
A closed cooling heat exchanger at a diverted to bypass (down). The nature of
generating station did not properly the test was to circulate warm shell side
control shell side water temperature under water with the three-way valve in the full
certain conditions. It was hypothesized bypass position. At the same time cooler
that the source of the problem was a lake water would be circulating through
leaking three-way valve. Infrared the tube side. Infrared imaging of the
thermography was used to confirm this valve body would show the characteristics
hypothesis. The results of the of the resulting temperature differential
investigation proved, first, that the across the valve and therefore the degree
three-way valve was not leaking. Using the of integrity of the valve’s seating surface.
same technology, the investigation went
on to definitively prove that the problem At the time of the test, lake water
was actually the configuration of the temperature was at about 289 K
bypass line from the three-way valve back (16 °C = 61 °F). Before the test, the cold
to the heat exchanger outlet. lake water was allowed to circulate
through the tubes, cooling the stagnant
The purpose of the heat exchanger is to
cool various critical mechanical loads FIGURE 19. Heat exchanger: (a) without shell side bypass;
within the plant, with a minimum (b) with shell side bypass.
required shell side temperature of 286 K (a)
(13 °C = 55 °F). The tube side of the
subject heat exchanger is cooled by Shell in Tube out
circulated water from a nearby lake. This
provides a direct, ultimate heat sink for Closed
the water on the shell side (Fig. 19a). A
recent design change installed a three-way Shell out Throttled
valve for the purpose of modulating shell T ≥ 286 K (13 °C = 15 °F) Tube in
side water flow rate through the heat Tube out
exchanger (the rest bypassing it), thus (b) Shell in
controlling the temperature of the water Shell temperature Tube in
in the shell side. In the winter months Bypass control valve
when the lake temperature is near 273 K
(0 °C = 32 °F) and minimum loads are on Shell out
the heat exchanger, the temperature of T ≥ 286 K (13 °C = 15 °F)
the water in the shell side cannot be
maintained above 286 K (13 °C = 55 °F),
even with the three-way valve in the full
bypass position (Fig. 19b).

Infrared Testing

Initially, the obvious and reasonable
explanation for the anomaly seemed to be
that the bypass valve was not sealing
tightly when fully closed, allowing shell
side water to pass through the shell. This
hypothesis conveniently and easily
explained how the water was being cooled
even when in full bypass. Given the fact
that the bypass valve was new and would
be very costly to replace, the prudent
course of action was to prove this theory
before valve replacement could be
considered.

Thermography was determined to be
the technology that would most likely be
able to validate the theory. The

Electric Power Applications of Infrared and Thermal Testing 547

water in the heat exchanger shell to 289 K pump, starting as a stripe running straight
(16 °C = 61 °F). The shell side water in the up the shell.
rest of the system was also left static and
in thermal equilibrium with the Figure 20a indicates the path that the
environment 294 K (21 °C = 70 °F). The bypass flow had to follow to get back to
thermal imaging system was trained on the shell side return line. The water must
the valve before the test was started, with navigate two quick 90 degree turns to
the analog color output routed to a video make the return. After that first turn, this
recorder. The shell side pump was then high velocity water would more likely
started in full bypass, circulating the take the short straight path straight into
warmer shell side water through the the heat exchanger. Despite the fact that
bypass side of the three-way valve and on this path was dead headed, it is apparent
to the rest of the system. Thermography that this was occurring.
proved that the shell side water was not FIGURE 20. Shell side discharge piping:
flowing into the heat exchanger shell (a) visible light photograph; (b) thermogram
through the three-way valve when in full at –9 min; (c) thermogram at +5 min.
bypass. (a)

This revelation regarding the integrity (b)
of the three-way valve suddenly made the
issue much more complex. What was (c)
causing the cooling of the shell side
water? Fortunately, the answer was not
far away. At the end of the test described
above, a thermal scan was conducted on
the entire heat exchanger and local
piping. An anomaly was discovered at the
heat exchanger outlet that appeared to
hold the answer.

The piping configuration at the heat
exchanger discharge is shown in Fig. 20a.
The shell side discharge is at the right side
of the picture, traversing out and
immediately into a T. The bypass flow
from the three-way valve traverses in from
the lower left of the picture, rising and
turning into the same T. These two flows
join in the T, constituting the shell side
return traversing to the left and then up.

Once again, the 289 K (16 °C = 61 °F)
lake water was allowed to circulate
through the tubes and cool the stagnant
shell side water. And as before, the shell
side water was left at thermal equilibrium
with the environment at 294 K (21 °C =
70 °F). This time, however, the thermal
imaging system was trained at the shell
side discharge. The analog color output
was again directed to video recording. The
shell pump was then started with the
system in full bypass, circulating the
warmer shell water through the bypass
side of the three-way valve and on to the
rest of the system. Figures 20b and 20c
show the thermal image at time
T = –9 min and T = +5 min, respectively.

Upon examination of these thermal
images, there are several observations
worthy of note. The first is the obvious
warming of the shell side return line,
caused by the warmer water traversing the
bypass line. Second, it should be noted
that the overall shell temperature does
not appear to have changed. The third
and most significant observation is the
warm area that appears above and around
the shell side exit pipe. The real time
video tape shows this area forming
seconds after the start of the shell side

548 Infrared and Thermal Testing

Figure 21 represents the phenomenon components and systems that are critical
schematically. It is unknown whether or to a safe plant shutdown.25 Assessing
not mass exchange (water mixing) was integrity of the service water piping
actually occurring because this has been system includes detection and analysis of
argued as a very unusual situation. It is pipe wall thinning. Conventional test
clear, however, that a thermal exchange methods usually entail the time intensive
was occurring between the bypass water process of ultrasonic thickness
and the water in the shell. The magnitude measurements, based on a grid system, of
of this interaction was so significant that the entire pipe length. An alternative to
the shell side return water temperature this process may lie in the use of active
could not be maintained above the infrared thermography techniques for
required minimum in winter. detection of thin wall areas in the pipe.

Case History Summary Infrared thermography has been widely
used by utilities for a variety of predictive
The closed cooling heat exchanger in this maintenance applications including
case history did not maintain shell side evaluation of mechanical, electronic and
water temperature above the required electrical components. These applications
minimum of 286 K (13 °C = 55 °F) when use infrared and thermal testing in a
lake temperature was near freezing. For passive mode to identify anomalies in the
this specific situation, it is definitively characteristic thermal pattern of an
proven that the cause was not a leaking or operating component in order to
malfunctioning three-way valve. It has determine its suitability for continued
also been shown that the cause of the service. Unlike these components, service
problem was the piping configuration at water piping has no faulty electrical
the heat exchanger shell side discharge. connections, misaligned bearings or other
This configuration caused shell side discreet, inherent sources of thermal
bypass water to exchange heat with the energy to be observed. For assessment of
cooler water inside the shell, lowering the service water piping, an active infrared
overall shell side water temperature below technique, thermal injection, can be
the minimum required. used.26,27 This process involves injection
of a controlled amount of thermal energy
Detection of Wall Thinning into the exterior pipe wall. Anomalies in
in Service Water Piping24 the resultant thermal pattern, as detected
by the infrared system, are then evaluated
Service water piping systems at electric to determine their origin. Similar infrared
power plants provide cooling for a variety nondestructive test techniques have been
of safety and nonsafety related successfully used for evaluation of
components and systems. Reliability of composite materials in the aerospace
service water piping systems is a key industry. Application of these infrared
consideration for safe and reliable plant nondestructive testing techniques for
operations. Under accident conditions in material evaluation can provide rapid
a nuclear power plant, the service water screening for identification of thin wall
piping system provides cooling water to areas in service water piping.

FIGURE 21. Thermal mixing in shell of heat exchanger. A preliminary evaluation of this
technology was conducted on mockups at
Shell in Tube out Vermont Yankee Nuclear Power Plant.
Shell temperature Based on the promising results, the
Bypass control valve infrared thermal injection technique was
optimized for service water piping
Shell out applications. The primary goals of this
effort were to determine the practical
Tube in depth resolution capabilities of the
thermal injection technique in carbon
steel and also to minimize the effects of
pipe curvature on detection capabilities.
Both of these efforts were subject to the
constraint that the system be sufficiently
portable for use in an electrical power
plant, where space and access to the pipe
surface is often limited.

A thermographic system was used to
evaluate a 9.525 mm (0.375 in.) thick, flat
carbon steel plate and 0.3 m (12 in.)
diameter, schedule 40 pipe. (Schedule 40
pipe is black, galvanized pipe made for
ordinary uses in steam, water, gas, and air
lines according to ASTM A 53.28) Back
drilled holes in both targets, ranging in

Electric Power Applications of Infrared and Thermal Testing 549

diameter from 1.59 to 6.35 mm (0.0625 to
0.250 in.) and depths representing about
10 to 90 percent through-wall loss, were
used to simulate wall thinning. Evaluation
of the flat plate indicated that targets with
a specified diameter-to-depth ratio can be
reliably detected using the thermographic
system. A similar evaluation of the 0.3 m
(12 in.) diameter pipe was completed to
address curvature effects on detection
capabilities.

Initial investigation of the pipe
mockup indicated a significant drop in
returned thermal energy away from the
longitudinal axis of the pipe and therefore
a significant loss of detection capability
for areas beyond ±30 degrees of top dead
center.

Two modified flash hoods, using
reflective schemes to optimize both
energy input to the off-axis regions of the
pipe and to increase energy input to the
camera from these regions, were
constructed and tested. The larger unit,
using strategically located gold front
surface mirrors for both input and output
coupling, increased the effective detection
angle to ±50 degrees and was able to
remove spatial distortion on the
peripheral regions of the pipe. A smaller
unit that used reflectors for increased
input coupling provided a coverage angle
of only ±40 degrees but without
compensation for curvature effects on the
periphery of the pipe. Although the larger
unit offered better performance with
respect to curvature compensation, the
smaller unit offered better resolution of
deeply buried targets.

As a followup to the laboratory
investigation, this infrared system and
thermal injection technique will be
applied at an operating nuclear power
plant for verification of test techniques
and detection capabilities on service water
piping systems.
550 Infrared and Thermal Testing

PART 6. Infrared Thermography for Distribution
Systems

Applications for In this way it can be determined if a hot
Thermographic Software29 spot adjacent to the transformer is caused
by the transformer or by a poor electrical
Municipal electric power distribution contact inside the elbow. Pattern
systems are often built underground and recognition has been pioneered on the
use different components from those of thermal profile scanned along the center
the bulk power system. Infrared surveys line of an elbow to assess its condition.
for these systems require a portable and
smart infrared instrument. Modeling

Infrared surveys are an increasingly Software has been developed to take
accepted method to provide early thermograms with a computer vision
detection of incipient faults in many system and to analyze them in terms of a
types of industrial plants, including large thermal model. This analysis amounts to
installations in the bulk electrical power a preprocessing stage, which presents
system. In the 1990s, the Canadian condensed results to a decision making
Electrical Association conducted a routine. A decision making routine to
research program to extend the technique classify suspect components into one of
for use in municipal distribution systems, four categories has been tested on actual
which are often built underground and good and anomalous load break elbows.
which use different components from the
bulk power systems. Work has been done on cooling by
wind. A significant effect due to wind
An additional challenge in distribution direction has been noted on insulated
system work is the lack of training of the components in addition to the effect of
linemen in infrared work. To provide wind speed. (For uninsulated, all-metal
assistance to relatively untrained components such as bolted clamps on
operators, the Canadian Electrical overhead power lines, it is expected that
Association sponsored development of a the lee side and the windward side would
computer based smart infrared have the same temperature.) Solar heating
instrument. has also been modeled. Simulations
demonstrate that infrared radiation from
Load Break Elbows the sun is reflected as well as absorbed,
even for the highly absorbing outer
Methodology development focused on material used on load break elbows. These
load break elbows, which are designed to heating and cooling effects will not apply
literally unplug the power to a shopping to components in underground or room
mall or a residential neighborhood. vaults but they do affect temperature
measurements of the insulated
Because the elbows may be unplugged components located under the cover of
with up to 27 kV still applied there is a outdoor pad mount installations.
risk of a safety incident if they are faulty.
Therefore, one objective of the research To obtain a good estimate of
project was to provide advance warning of temperatures before the elbow is exposed
discontinuities so that such electrical fault to sun or wind, the operator may focus
conditions do not occur. the camera on the pad mount cover
before opening it and then capture an
Load break elbows contain metal image as soon as the cover is opened.
conductors inside a thick layer of rubber Heating of a load break elbow by the
or plastic insulation. The insulation is warm transformer on which it is mounted
used for electrical isolation but it also has can also be modeled mathematically. By
thermal insulation properties. Because of this means, for example, it is possible to
the thermal insulation, temperatures distinguish between an elevated
measured on the surface by an infrared temperature in the bushing area because
camera need to be transformed into of heat from the transformer and one due
internal temperatures to assess the interior to high contact resistance in the knurled
condition of the component. joint of the bushing.

Heat conducted into the elbow from A potential confounding influence in
the transformer on which it is mounted is infrared thermography is variations in the
one of several effects that can be taken infrared emissivity of the surface. Such
into account by mathematical modeling. variations throw confusion into

Electric Power Applications of Infrared and Thermal Testing 551

temperature readings, because the 1. Acquire a thermogram (infrared image
brightness of a spot in an infrared picture calibrated in terms of temperature)
of an object at a given temperature is and manually designate two or three
proportional to the emissivity at that key points on it to define a path along
spot. An emissivity study was performed which a thermal profile should be
on 15 load break elbows removed from taken (Fig. 22). This step uses the
service for various reasons. The final human operator to recognize the load
conclusion was that a standard emissivity break elbow. This is a task that a
value of 0.91 could be used for all load human can perform intuitively but
break elbows. The error in surface has been hard to automate.
temperature which results from using this
fixed value of emissivity is expected to be 2. After the path has been designated,
±1 K (±1 °C; ±2 °F) or less. the software gathers a thermal profile
by reading the temperature values
The fact that the apparent temperature along the path. To remove distance
could be ±1 K (±1 °C; ±2 °F) different from effects on the apparent size of the
the true temperature demands a robust object, the position on the object at
analysis algorithm to avoid misleading which each temperature reading is
results. The same statement holds for the taken is scaled as a percentage of the
environmental influences mentioned distance between the key points. The
above that perturb the actual temperature. actual pose or orientation of the object
A further quantity of direct importance is is not critical provided its surface is a
the load current. Load current is difficult dielectric, because then its infrared
to ascertain in some instances. The emissivity is nearly omnidirectional.
approach taken in this project is to blend
two means of analysis: temperature 3. Process the raw data, which consist of
measurements and thermal profile the thermal profile and information
recognition. about the conditions under which the
thermogram was obtained such as
Temperature measurement techniques ambient temperature and wind speed.
in thermographic literature consist of These data are preprocessed into a
absolute temperature assessment and small number of attributes.
relative temperatures, in which two or
more similar components are compared. 4. Classify the repair status of the
These techniques are incorporated in the component given its make and model,
software. The aging properties of the load current and the attributes
materials of construction of load break generated by the preprocessor step.
elbows have been studied to understand
failure mechanisms and support decision FIGURE 22. Load break elbow profile.
making software based on temperature
measurements. 440 (167) [333]
1
Another approach in the electric power
industry uses pattern recognition, where Temperature, K (°C) [°F] 2
the inspector seeks to understand the
shape of the thermal profile scanned 273 (0) [32] 3 Cable
along the center line of a component. Bushing 4
Shape means a scale invariant shape not
influenced by, for example, overall 65
emissivity or load current. The 7
computational feasibility of this
technique relies on separability of the Center line path
problem. It has been shown that the final
mathematical model can accurately be Legend
taken as a linear combination of an 1. Internal temperature predicted with three-dimensional modeling.
independent solution for each source of 2. Temperatures measured along center line path on load break elbow.
heat. With proof that this fundamental 3. Best fit, linear combination of single source contributions from
solution technique is accurate, a relatively transformer, normal load break elbow, bushing resistance and corner
simple linear regression model can be resistance, added to ambient temperature.
used to fit the observed thermal profile 4. Ambient temperature.
and dissect it into the various root causes 5. Bushing — bad contact where probe contacts bushing.
of heating. In this way, anomalous 6. Normal — best fit contribution to skin temperature for single heat
internal electrical connections can be source of normal load break elbow without discontinuity.
detected. 7. Corner — best fit contribution to skin temperature for single heat
source of anomalous electrical connection at corner, where probe
An example of the methodology is connects to copper top connector.
given in Fig. 22, where an anomalous load
break elbow is assessed with a 50 percent
fused analysis based on temperature and
shape of the thermal profile.

The basic steps to perform an
assessment are as follows.

552 Infrared and Thermal Testing

The output statement indicates the 2. An aluminum conductor was
urgency of repair. compressed into an aluminum sleeve.
An expert thermographer knows when On the opposite end of the sleeve was
thermography is being performed outside a 25 mm (1.0 in.) copper adapter. The
of its limits of validity. Software can also copper adapter was then inserted into
provide warning statements. the two-hole compression spade,
bolted to a brass plate and then
Software Summary attached to the rack in a substation.
The termination eventually failed.
Distribution system thermogram analysis
software can incorporate repair urgency 3. A tapoff point joined a 46 kV
guidelines for transmission system aluminum conductor steel reinforced
components where the same components main line and an aluminum
are used in distribution systems. In conductor steel reinforced lateral line.
addition, a technique has been developed The weak link in the installation was
to handle insulated distribution system the 19 mm (0.75 in.) copper adapter
components and has been applied to load that joined the three aluminum
break elbows. Laboratory tests indicate conductors.
that the technique is successful. Dead
break tees, insulated splices and 4. A termination consisted of a 46 kV
essentially all distribution system aluminum conductor steel reinforced
components can receive similar attention. cable inserted into an aluminum
two-hole compression spade. The
Transmission Line spade was then bolted with steel bolts
Failures30 and nuts to a brass plate on the air
break switch. The failure was
In the 1990s an electric utility in attributed to not using a bimetallic
Michigan had a transmission system that plate between two dissimilar metals.
consisted of 6920 km (4300 mi) of 23 and The failed termination was removed
46 kV, 5890 km (3660 mi) of 138 kV lines from an air break switch feeding a
and 2900 km (1800 mi) of 345 kV lines. substation in a highly populated
When a transmission line meets an industrial and commercial area.
obstruction, such as a wooded area or
lake, it is rerouted around the obstruction. 5. Transmission lines constructed several
When the line angle is great enough, the years ago were designed without a
conductors are dead ended and jumpered static wire to protect them from
around. Aluminum strands can be lightning strokes. Later, new lines were
damaged by lightning or vandalism. constructed with a static wire at the
top of the pole. Occasionally for
Before infrared and thermal testing was various reasons, the conductors were
implemented, burnt off jumpers and full dead ended and jumpered around to
tension sleeves were a frequent cause of the other side of the structure. The
transmission line outages. It was decided static wire was not jumpered because
by the transmission department to use an there was no insulation between the
infrared testing company to fly in an conductor and the structure and there
aircraft and test 46 kV lines with a high was continuity through the hardware.
outage history. To no one’s surprise, Return current through the static wire
numerous anomalies were identified and caused failure on the dead end shoe.
later replaced on the 46 kV system only. The small contact where the dead end
Higher voltages were not inspected. shoe touched the clevis pin was relied
However, because of the lack of on for continuity. When there was not
inspections, many outages were related to enough contact the shoe failed.
burnt off jumpers and full tension sleeves. Jumpers were installed to prevent
future failures.
Transmission Material Failures
6. One failure resulted from installing a
The following are cases of failed copper conductor and an aluminum
components detected by infrared conductor and joining the two
thermography. dissimilar metals together in another
copper adapter.
1. One compression sleeve failed as a
result of installing two dissimilar 7. In a corner tower two steel reinforced
materials together in an aluminum aluminum conductors of different
compression sleeve and joining them thicknesses were joined together with
together with a 19 mm (0.75 in.) or a transition sleeve and a copper
25 mm (1.0 in.) copper adapter. The adapter joining the aluminum sleeves.
cable was a 46 kV steel reinforced
aluminum conductor.

Electric Power Applications of Infrared and Thermal Testing 553

8. As a result of one airborne infrared A satellite based global positioning
inspection of a transmission line, the system developed by the United States
following components were removed Department of Defense provides a
from service: a damaged 46 kV steel consistent, accurate technique of
reinforced aluminum conductor navigation. Originally designed for
jumper and a two-hole compression military applications, it also provides
spade 46 kV copper conductor commercial and recreational users with
compressed into an aluminum sleeve. 24 h, worldwide navigation coverage with
The hot spot was attributed to a accuracy to 15 m (49 ft). A thermometer
copper conductor in an aluminum measures outside ambient temperature.
sleeve. The steel core was carrying
most of the current and made the Navigation
jumper very hot.
Each morning, the observer contacts the
Ground versus Helicopter Based transmission maintenance personnel to
Inspection find out if there were any line operations
since close of business of the previous day.
In cities, where helicopters cannot fly at If so, then that line or lines will be
low altitudes, the unit is mounted on a inspected both visually to identify the
four-wheel drive vehicle. The camera is cause for the operation or outage. The line
mounted on an A frame assembly or lines will also be inspected with
attached to a trailer hitch on the rear of infrared thermography. Of course, if the
the vehicle. The vehicle is also equipped line is out of service an infrared test will
with a 12 to 28 V inverter system to not be completed. Following the
power the system’s electronics and inspection of that line, other lines in the
infrared imager. This converter is identical area will be inspected to eliminate
to that used in F-16 jet fighter aircraft. excessive ferry time.
The electronic system is positioned
between the driver and the The average speed during an airborne
thermographer in the front seat. A ground infrared inspection is about 72 to
level infrared testing crew consists of a 80 km·h–1 (45 to 50 mi·h–1) at treetop
driver and thermographer. level on 23 or 46 kV lines and 121 to
129 km·h–1 (75 to 80 mi·h–1) on lines of
Helicopter inspections are more higher voltages lines. The speed increases
economical than ground based on higher voltage lines because these lines
inspections because they can fly close to have fewer corners and have wider and
the line and cover more area in the same clearer rights of way, tapoffs and other
amount of time. In the United States in complications in their directions. Also the
the 1990s, most 23, 46, 138 and 345 kV towers are taller than the trees, so the
power lines were being inspected every inspection can proceed faster. The video
year by helicopter based thermography. recorder records constantly from
substation to substation.
System Electronics
The recorder can be set to record in
The interface distribution processing gray scale or visible light, in either 1× or
module is the central point for transfer of 4× telephoto. It can also record all cockpit
all commands and data. The system communication, vital information such as
control unit features a gray scale (infrared (1) line name, (2) structure number,
white or black hot) switch and focus (3) affected phase conductor, (4) actual
adjustments. The visible spectrum circuit anomalous material and (5) outside
integrates a charge coupled device camera. ambient temperature.

The camera’s precision pointing system All this information is recorded on tape
incorporates a gyro stabilized in both and also documented on the helicopter
azimuth and elevation. The camera has a observer patrol report. The thermographer
germanium lens and is attached to the records the Greenwich mean time on a
belly of the helicopter by way of a notepad. This will assist in the processing
dovetail mount. The imager unit stage of the operation later on.
incorporates a high resolution
monochrome or color charge coupled Total weekly inspection miles may vary
device camera, 4× zoom lens, the infrared depending on ferry time, weather or other
detectors, high speed scan assembly, factors. For example, inspection crews
cooling optics and associated electronics. may fly from 323 km (200 mi) to 805 km
(500 mi) per week. The airplane used is
For video display, two units are in the classified as a light jet. With the infrared
helicopter operation: one on the hand equipment and the thermographer, a full
held system control unit and one on the capacity of jet fuel is impossible. So
console in the front seat for the observer normally, a little more than 2 h is the
and pilot. These units supply the operator norm when flying lines.
with (1) a visible light image or thermal
image and (2) Greenwich mean time. The 46 kV system has been inspected
for four years. The number of anomalies
may vary from as few as zero and as many

554 Infrared and Thermal Testing

as four in one day. The 138 kV system has
not been inspected as often. As many as
12 anomalies have been found in one day.

Most anomalies identified have been
processed on Monday of the following
week. The processing equipment consist
of the following: VHS video
recorder/player, personal computer,
monitor, printer, high resolution 8 mm
video recorder, portable television and
proprietary analysis software,

Infrared tests and replacing or
monitoring the anomalous materials have
effected huge savings in maintenance by
utility companies and service hours for
customers. It does not seem affordable for
a utility not to perform infrared tests as
part of preventive maintenance.

Electric Power Applications of Infrared and Thermal Testing 555

PART 7. Helicopter Based Thermography of
Power Lines

Goals of Testing and 24) and hot tension joints (Figs. 25
and 26) are detectable with thermography.
Because of resistance, electric current Infrared imaging can identify hot joints,
generates heat in conductors, increasing clamps and other fittings and help to
the temperature of the conductor above estimate how critical these hot joints are
the ambient temperature. Joints and for a safe line.
clamps have normally lower resistance
and larger area than the conductor itself The procedures described below are
and are hence colder than the conductor. intended for inspection only of electrical
With the passage of time and for various loaded bare overhead conductors for
reasons the resistance over a joint may distribution lines and transmission lines
increase and cause a temperature higher of 10 kV and above. They are intended to
than the temperature of the conductor. support other data to decide how quickly
Hot joints will deteriorate in time and will a joint should be scheduled for repair.
eventually fail as the mechanical strength Other data such as current load, wind
of the material decreases with increasing speed, geometrical design of the joint and
temperature. Suspension joints (Figs. 23 consequences of phase dropping are
needed to determine whether a joint
FIGURE 23. Hot suspension joint: (a) original needs to be repaired immediately. Do not
thermogram; (b) joint magnified to show use this procedure for inspection of
digitization. Brightest pixel may be used to discontinuities that do not cause
measure temperature accurately. Power line overheating — for example corona or
is easy to see in original image but corrosion insulator discontinuities. Do not
disappears in noise of closeup. attempt this procedure unless at least
(a) three of the conditions listed in Table 3
are fulfilled.

Reasons for Thermographic
Testing

A thermographic test gives information
about the condition of these components
and important information for
refurbishment planning. Three occasions
call for thermographic testing of
transmission lines.

FIGURE 24. Hot joint and warm suspension
clamps close to tower. Induction normally
warms line suspension clamps.

(b)

1
2

Legend
1. Warm suspension clamp.
2. Hot joint.

556 Infrared and Thermal Testing

FIGURE 25. Hot tension joint: (a) one of 1. After a new transmission line has been
several common designs; (b) thermogram; constructed thermal infrared testing
(c) closer view from different angle; can be used as a quality control
(d) closeup. technique to make a delivery test
(a) Aluminum wires in before the line is taken over by the
line owner.
aluminum pipe
2. Aging of joints and fittings is one of
Bolted joint the circumstances that most reduce
between the life of components in a
aluminum transmission line. An inspection using
plates infrared and thermal testing gives
information about the condition of
Aluminum wires in these components and important
aluminum pipe information for refurbishing planning.

(b) FIGURE 26. Tension joints in tower: (a) in
example of symmetry, six equally hot
fittings; (b) one hot tension joint.
(a)

(c) (b)

(d)

TABLE 3. Conditions affecting quality of helicopter based
thermography of power lines.

Condition Poor Good

Wind speed ≥12 m·s–1 calm days
Atmosphere clear and dry clouds
Conductor very reflective dark
Time of day afternoon of sunny day early morning
Current load low inspection at ≥25 percent

Electric Power Applications of Infrared and Thermal Testing 557

3. If the resistance over a joint has steel joint tube and the interfaces between
started to increase it will eventually be the steel joint tube and the steel cores.
so high that the temperature in the
joint will cause a phase drop. The Because of the larger cross section area
consequences are known by all owners of the aluminum path and the much
of transmission lines. The most serious lower resistivity of the aluminum metal
phase drops that must be prevented compared to the steel, the current is
are in urban areas, at street crossings unequally divided between the two paths.
and over distribution lines. The high resistance of the steel path
forces more than 99 percent of the current
The ultimate goal is to inform the line through the aluminum path. An increase
owner where hot joints are located in its in the resistance of the steel path
power lines and if possible estimate the therefore has a negligible effect on the
overheating relative to the conductor. total heat evolved. Despite the low
resistivity of the aluminum metal, the
One very important fact easily large current results in 99.5 percent of the
overlooked is that to determine the heat being evolved in the aluminum
overheating of a joint it first must be strands and the aluminum tube.
observed during flight or during the
examination of the video recording Compared to the conductor, the larger
because it is not practically possible to cross section of the aluminum tube causes
measure the temperature of all individual less heat to be produced per unit length.
joints on the transmission line. The hot The larger diameter and consequently
joint must therefore present a contrast larger cooling surface results in more
with the conductor or the background. If efficient cooling by convection and
the emissivity of the joint is lower than radiation per unit length. Taken together
unity, the cold sky is partially reflected in these two processes result in a joint tube
the joint and thereby sometimes having a surface temperature lower than
effectively masks the anomalous joint. that of the conductor. If the resistance in
a new joint is 20 µΩ the contact resistance
Materials and Failure represents only a smaller part of that,
Mechanisms maybe only 1 to 2 µΩ. The resistance of a
joint is only about 50 percent of the
Power Line Materials resistance of an equal length of the
conductor. When viewed using an
In power lines, there are two kinds of infrared imager, the joints therefore
conductors: appear colder than the conductor.

1 In all aluminum conductors and all Resistance
aluminum alloy conductors, the
aluminum wires carry both the current If the resistance in a new joint is 20 µΩ
as well as the mechanical load. the contact resistance only represents a
smaller part, maybe only 1 to 2 µΩ. The
2. In aluminum coated steel reinforced resistance of a joint is only about
conductors, the steel core carries the 50 percent of the resistance of an equal
mechanical load and the aluminum length of the conductor. That is the
wires conduct the current. reason why the joint is cooler than the
conductor itself.
The joints for these two conductors are
accordingly of different design. The evolved heat is conducted to the
surface so that the outer surface at the
The joints on power transmission lines two ends of the joint becomes warmer
built from aluminum core steel reinforced than the connecting line. Although only a
conductors are more complex than joints fraction of the current is carried by steel,
on all aluminum and all aluminum alloy the hottest part of the joint will be the
transmission lines. The aluminum core steel tube that connects to the two steel
steel reinforced conductor joint consists of cores. Because of the lack of radial heat
at least two concentric tubes. The inner conductivity between the two tubes, heat
steel tube is the joint of the steel core; the from the steel parts of the joint must pass
outer steel tube is the joint on the through the hot contact surface between
aluminum strands. Therefore the joint can the aluminum strands and the aluminum
electrically be simplified to one aluminum tube.
and one steel path for the current acting
in parallel. Some of the heat is conducted away
from the joint along the conductor.
The aluminum path consists of the Therefore, when the conductor nearest to
aluminum strands of the conductors, the the joint is heated up enough to
aluminum tube and the interfaces overcome the shadowing influence of the
between the aluminum strands and the cold sky, bright tails seem to be growing
aluminum tube. The steel path out from the joint. With increasing
consequently consists of steel cores, the resistance the joint becomes warmer and

558 Infrared and Thermal Testing

eventually the whole joint is hotter thanEmissivity ε (ratio)between the two ends of the joint. Tests
the conductor and appears bright. demonstrate that emissivity increases in
corrosive environments (Fig. 27).
Joint Life
The last period in the life of each joint
It is not possible to predict exactly when a is unstable. Periods of rapidly increasing
joint will fracture after overheating is resistance can be followed by periods with
discovered. The strength of a joint no increase or even decreases in
depends on the temperature and decreases resistance. One theory that explains the
with increasing temperature. The fracture phenomenon is that micromelting inside
will occur when the temperature is so the joint in the boundary between the
high that the joint strength has dropped wires and aluminum pipe will build up
to the conductor tension. current bridges over which the current
passes. The resistance will drop. Because
During current cycling tests in the of the relatively high temperature these
laboratory it has been shown that the bridges will oxidize and cut off the
temperature of the joint develops as current bridge. The resistance will increase
follows. (In the laboratory, one cycle is until a new current bridge has developed.
full current load during 1 h followed by
1 h cooling period.) During the first long Procedure
period of the joint’s life the temperature is
very stable. As the joints are aging the Safety
resistance and hence the temperature will
increase slowly in some of them. This Special rules for airborne surveys of
process is not stable. Melting at a transmission lines differ from country to
microscopic scale inside the joint in the country and must be followed.
boundary between the wires and
aluminum pipe will from time to time To avoid collision all crossing power
build up current bridges where the current lines and towers higher than the line to
passes and the resistance will drop until be inspected must be identified before
that bridge has oxidized and the current inspection. Crossing lines must be
will find other paths. Later in the joint’s identified during the briefing with the
life these changes seem more dramatic. pilot.
The inspector does not know the entire
temperature history and so cannot judge Keep contact with a power company
the remaining lifetime of the joint. representative during the inspection. This
person must be contacted immediately
The aluminum-to-aluminum interfaces before takeoff and after landing. This
oxidize or corrode as a line ages. The person must also know which part of the
result is an increase in the resistance of line will be inspected and also the time
the aluminum path and heat being expected for landing. If there is no
generated at the surface between the contact within 30 min after expected
aluminum strands and the aluminum landing time the helicopter is supposed to
tube. Usually the contact resistances differ be in emergency.

FIGURE 27. Emissivity increases for joints exposed to corrosive Airborne thermographic surveys are
environment. performed only in daylight to reduce the
risk of collision with crossing lines, with
1.0 cables that anchor towers and with other
elevated structures.
0.8
Training and Equipment
0.6
Details of the infrared imaging system are
0.4 given in Table 4. The camera is mounted
on a platform under the fuselage of the
0.2 helicopter (Fig. 28). It is recommended to
use long wave equipment, 8 to 12 µm.
0.0 During high humidity conditions, rain,
0 10 20 30 40 50 snow and fog the absorption of the
Age (yr) infrared radiation in the atmosphere is
not as high for long wave detectors as for
short wave detectors.

Spatial resolution is the most
important criterion for equipment
settings. A conductor is a very lean
component, with diameter from 10 to
40 mm (0.4 to 1.4 in.).

Electric Power Applications of Infrared and Thermal Testing 559

Preplanning 10. Read in time and date; inspector’s,
navigator’s, pilot’s and customer’s
The following steps are among those names; code for the lines to be
needed for someone planning inspected; and current.
thermographic tests of power lines from a
helicopter. 11. Press stop, rewind and play the
recording. Check that both voice and
1. Persons should not attempt to perform image have been recorded. Press stop
this inspection unless they are after passing the end of the recording.
properly trained and certified. Some systems use flash memory
instead of requiring this step.
2. Get a list of all transmission lines to
be inspected and line maps covering 12. The system may now be switched off.
these areas. Having performed these steps, the
inspector can be confident that the
3. Give the customer the criteria for a inspection will proceed as desired after
successful inspection. takeoff.

4. Confirm with the customer the day or FIGURE 28. Helicopter installation of thermographic system:
week for the inspection. (a) diagram of components; (b) camera mounted on
platform under fuselage of helicopter.
5. Ask for a contact person and phone (a)
number.
System control panel —
6. Write down voltage; conductor area joystick pan/tilt, zoom,
and type (aluminum coated steel focus, offset, gain
reinforced or aluminum alloy); line
configuration; installation year; and Video cassette Compressed air
expected current load during recorder
inspection.
Power supply from Electronic
7. Check that the system has sufficient helicopter 24 V processing unit
cooling capacity for the inspection. direct current

8. Use the thermographic system’s
control panel to check that the
detector is sufficiently cooled. When
the system test is activated a gray scale
is visible on the monitor. The gray
scale on some systems is used to adjust
the brightness and contrast on the
monitor. The controls must never be
adjusted on the monitor to improve
the picture during testing without
using the system test. This step applies
only to certain system designs.

9. For some systems, turn offset and gain
on the system control panel until
optimum brightness and contrast are
reached.

TABLE 4. Details of infrared thermal imager. (b)

Parameter Specifications

Waveband long wave, 8 to 12 µm

Filter none needed

Temperature measurement recommended for more reliable
evaluation

Thermal resolution minimum resolvable temperature
difference = about 0.2 K at 293 K
(0.2 °C at 20 °C; 0.36 °F at 68 °F)

Spatial resolution instantaneous field of view =
2.727 mrad without front end
optics (field of view of 60 degrees
horizontal)a

Power requirement 24 V direct current

Portability helicopter mounted

Color monitor required for image processing

Computer processing offline, letting user make quantitative
assessments

a. Because of resolution, it is important to resolve detail of conductor from
distance of 50 to 100 m (165 to 330 ft) or closer.

560 Infrared and Thermal Testing

Steps before Takeoff 7. While getting a closer view, the
inspector should check that the video
1. Check installation and wire is running, record the tower number
connections. Check that the camera is and section and record oral
properly mounted to the fuselage and observations. For later convenience it
that cooling capacity is sufficient for is recommended that the inspector
the testing. Remember to remove the make a written note at what band
lens cap. time each such observation is made.

2. Confirm with the contact that the 8. If the system includes measurement
helicopter is ready for takeoff and ask instruments it would be good to get
for air current at that moment. some pictures of the sky. This will help
to get correct values for the
3. As soon as the helicopter engine is background temperature. When the
started turn on the camera system and helicopter is turning around point the
video recording system. camera to the side that faces the sky.

4. Insert the video tape in the recorder, 9. When the last tower of the line is
press counter reset to zero and start passed rewind the video tape and take
the recording. Make sure the it out, turn of the systems and return
remaining tape length is adequate. A to base.
total of 60 min will, as an average, last
for a 50 km (31 mi) inspection. 10. Mark the video tape with the proper
identification.
5. If the camera is remotely controlled
see that it follows the inspector’s 11. Immediately after landing call the
signals. ground contact and confirm the safe
landing.
6. Inform the pilot that the inspector is
ready for takeoff.

Steps in Air FIGURE 29. Power lines visible from moving
helicopter: (a) each part of conductor in
1. As the helicopter approaches the monitor for 1.6 to 1.8 s; (b) example of
power line to be inspected, start the indication.
system once again. Target distance (a)
could be as far as 200 m (650 ft).
1.6 to 1.8 s
2. Turn offset and gain until optimum
brightness and contrast is reached. (b)

3. Record weather conditions,
temperature wind speed and direction,
atmospheric conditions; type and
amount of clouds, lines to be
inspected and the first tower number.
This must be the tower being
simultaneously watched in the
monitor.

4. The inspector and pilot must now
choose the optimum camera angle and
speed over ground for an inspection
that is both safe and reliable. As a
guideline every part of the conductor
should be at least 1.6 to 1.8 s in the
monitor to not fatigue the inspector’s
eyes too much (Fig. 29). Usually all
phases can be inspected
simultaneously. The maximum speed
of the helicopter should not exceed
56 km·h–1 (35 mi·h–1). A conductor is a
narrow component, with a diameter
from 10 to 40 mm (0.4 to 1.6 in.) and
should be inspected from a distance
not closer than 40 m (132 ft),
generally from a helicopter traveling at
48 to 64 km·h–1 (30 to 40 mi·h–1).

5. Record at least every fifth tower
number and identify it by number on
the video recorder’s audio channel.

6. As soon as a hot joint is observed turn
around the helicopter to get a view
from different directions. Try to hover
to get as close a look as possible of the
joint for later analysis.

Electric Power Applications of Infrared and Thermal Testing 561

Documentation of Results 5. On the X axis mark the actual current
load and on the Y axis the length of
Make copies of the data sheet, one copy the heated zone in joint length units.
for each indication or location. Every data
sheet shall contain following information: 6. If the wind is stronger than 8 m·s–1
name or symbol of the inspection (18 mi·h–1) add one unit in the
company, customer’s name, inspector’s temperature grade (alt class). Write the
name, date of test, line identification, temperature class in the data sheet.
type or area of the conductor, conductor
configuration (simple, duplex, triplex or Quantitative Evaluation Steps
quadruple), year of construction, voltage,
current load, maximum load for the Although qualitative evaluation often
conductor, wind speed, ambient gives an idea about the condition in the
temperature, atmospheric conditions joint it is not possible to judge how many
(percentage of sky covered by clouds or if degrees overheating it represents. This
the atmosphere is diffuse what percentage quantitative figure gives a stronger
of blue sky color is absorbed). indication of the urgency of repair and is
the most important criterion for the
If a video print will simplify location of equipment to use.
hot joint, use it. Documentation with
video printout is recommended if there 1. Select the most likely emissivity factor
are suspension towers with more than one and enter the value in the computer.
line (Figs. 24 and 26).
2. Fast forward the video tape to an
Infrared and thermal testing is useful image of the sky.
in detecting joints and fittings with
increased temperature. The summary 3. Make a measurement of the average
should include the number of hot joints temperature over a representative
discovered and an explanation if no frame.
indication was found.
4. Select the measured temperature and
Qualitative Evaluation Steps enter the value in the computer.

To classify a hot joint according to 5. Choose the frame where the hot
different degrees of overheating requires observation is.
long experience and much feedback from
resistivity measurements from detected 6. Measure the temperature at the hottest
joints. point and note the temperature.

1. Insert the video tape in the recorder, 7. Measure the temperature of the
reset the counter to zero and press conductor at an unaffected point.
play. Complete the data sheet with
information recorded on the tape. 8. Fill in the data sheet for the
Check the line map to check the overheating ∆T measured (kelvin).
correct tower number.
9. Solve the ∆T at maximum load (Eq. 4).
2. Identify the hot joint or fitting. 10. Fill in the data sheet for overheating at
3. Now analyze the thermogram to
maximum load.
decide if this observation is a hot joint
or a joint that looks hot because of FIGURE 30. Warm dead weights glow
other effects — for example, dead because of induced current.
weights warmed by induction
(Fig. 30). Examine all fittings in a
tower or several joints if one line span
looks hot. If the temperature over
joints and fittings changes stepwise
the apparent overheating is probably
due to differences in emission factor
between conductor and joint. Make no
registration on these joints.
4. If the differences in temperature can
be attributed to differences in
emission factor, it is not a hot
observation. If it is not possible to
measure the hot spots, then a
qualitative evaluation may be
performed. Observe that this
estimation only gives a very rough
idea of how hot the joints are. The
two parameters are the current load
and the length of the warm zone in
the conductor.

562 Infrared and Thermal Testing

11. Mark on the data sheet at what tower appears hot because of other effects. On
or between which towers each hot very reflective conductors joints may look
fitting or joint is located. Indicate the warm because of effects other than
phase and line (if more than one in resistive heating. In one case of a duplex
each section) in the data sheet and line, two joints appeared to be hot joints.
mark with an X the relative position The conductors and the joints had a
of the indication (Fig. 31). The surface as delivered, very reflective. The
inspector may add other figures for test was made in the afternoon a sunny
the tower in the inspected line. and clear day. The sky temperature was
between 233 K (–40 °C = –40 °F) and
Verification 223 K (–50 °C = –58 °F). The joints were
reported as hot but the resistance
Infrared and thermal testing has proven measured one week later showed normal
very reliable but precise verification of a values. The conditions for a reliable
thermal test is impossible because the inspection were in this test not fulfilled.
many variables cannot be controlled. It is There are however some circumstances in
recommended however that the line the thermogram that indicate that these
owner should measure the joint resistance are not hot joints.
over both halves of the joint and compare
it to the total joint resistance when the In the case of a rapid temperature drop,
replacement is made. there is no temperature rise in the
conductor outside the joint. The
To test one certain joint where the line temperature in a warm joint should
crosses a highway or distribution line, a change gradually from the hottest area on
place where people stay or for other the joint to the conductor’s normal
reason an alternative means is to use a working temperature. A steep change in
hand held camera and inspect from the radiosity is normally caused by
ground. That test procedure is not contrasting emissivity. If the warm zone
described in this procedure. looks like a rocket fume it is likely a hot
joint.
It is also possible to measure the actual
resistance using a microohm meter. This Although the experienced inspector
technique is recommended for correction will discover all hot spots in an
with replacement of hot joints. It can also transmission line it is difficult for the line
be used as a first check if no other owner to decide what joints to repair first
techniques are available. The ultimate and how critical they are.
goal is to help the line owner decide
whether the joint needs maintenance The decision to test or not depends on
work and give an indication about how external conditions that may be
critical the joint is. quantified. A simple mathematical
formula incorporating some yes or no
Results of thermal testing on a joint conditions would be of great help.
must be compared carefully with results of
a resistance measurement perhaps one Indications That Cause
year later. The temperature measured Temperature Changes
depends on many parameters, few of
which the inspector can control. Several discontinuities in power lines
cause temperature changes in
Indications components. Broken sheds cause a
temperature rise in line post insulators.
Evaluation of Indications Cracks in pin post insulators cause a
temperature drop on insulator surfaces.
Thermograms must be analyzed to decide Other discontinuities such as strong
if each indication is a hot joint or merely corrosion attack in aluminum core steel
reinforced conductors may also cause
FIGURE 31. Example of chart that inspector temperature rise.
may use to mark approximate location of
indication relative to towers. The tension joint, used in tension
towers, can have a large variety of designs
Direction of flight (Figs. 25 and 26). In some lines, tension
joints are more likely to be hot than
Tower number Tower number suspension joints. There are three areas
where the resistance may increase in a
tension joint: in the span conductor side,
in the bolted connection and in the
jumper conductor. It is often possible to
determine in which part the heat is
produced by seeing how the heat spreads
from the hot part.

Figure 24 shows a hot joint close to a
tower with warm suspension clamps.

Electric Power Applications of Infrared and Thermal Testing 563

Nonindications That Cause 3. For afternoon on a sunny day, the
Temperature Changes sensitivity (gain) during the test
depends on the temperature range in
The anomalies above occur only in joints, the surroundings. The gain setting
clamps and fittings where a temperature must cover from the hottest to the
rise is caused by an increase of resistance, coldest area the camera is facing. In
detectable from a distance up to 200 m the afternoon rocks and stones on the
(656 ft). There are also natural ground may be warmer than the
temperature rises in components such as conductor and will therefore appear
the temperature rise in dead weights on brighter on the monitor. The chances
jumpers and heat produced by wind of discovering hot joints has now
induced vibration. For example, when dropped dramatically. The inspector
wind causes conductor vibrations where should consider stopping the test
the conductor enters the suspension or because it is possible that joints must
tension clamp sleeve, then friction makes be extremely hot to be discovered.
wire temperature rise.
Great Temperature Differences in
Neighboring joints are called Ground
symmetrical if they appear to have exactly
the same temperature. They also look the Reflective conductors require optimal
same at both ends and seem to have same external circumstances. Atmospheric
temperature along the whole length of conditions have an especially strong
the joint. Figure 26a shows a case of influence on test quality as the sky reflects
symmetry for all six fittings in a tower. in the conductor.

Dead weights on jumpers may become Clouds are major sources of infrared
warm if they are made of a magnetic radiation in the sky. The clouds adopt the
material. Figure 30 shows dead weights same temperature as the temperature of
warmed by induced current. Bright the surrounding air. Therefore depending
jumpers can be used to confirm that the on the height of clouds the radiation from
line is electrically loaded. Loading can be the clouds have different temperatures.
checked this way during testing of two On a cloudy day with low ceiling the
parallel lines when only one is loaded. radiation from the sky is almost the same
as from the ground but on clear winter
Weather days it may be 223 K (–50 °C = –58 °F). If
there is an overcast sky the temperature
The inspector needs to describe the could be only a few degrees below the
atmospheric conditions. Cloudiness is ambient temperature and this is much
quantified from 0 to 100 — from 0 for more favorable for the test.
completely clear blue sky to 100 percent
covered by clouds. If there are cloud of The temperature difference needed
cumulus type, the inspector looks at the varies with emissivity of the joint and
sky and estimates how much of the entire blackbody temperature of the radiation
sky is covered by clouds. If there are cirrus from the sky. Because the joint emissivity
clouds, which are more transparent than is lower than unity, the cold sky is
cumulus clouds, it can be estimated how partially reflected in the joint and so can
much of the blue color is absorbed by the sometimes effectively mask the
clouds. anomalous joint.

The time of day affects the amount of To show this effect, critical overheating
thermal clutter in the image, from the can be plotted against the
ground. undertemperature of the sky. The
undertemperature of the radiating sky is
1. On a typical morning, all details in the blackbody temperature difference
nature have about the same between the radiation from the ground
temperature, so it is possible to use and the sky. It is possible to plot diagrams
high sensitivity on the camera; the showing the relationship between the
gain covers from 280 to 286 K (7 to undertemperature of the sky and the
13 °C; 45 to 55 °F). overheating at which the anomalous joint
can be observed. By plotting these
2. By midday, when the sun has warmed parameters another benefit is achieved;
up the environment, the temperature the diagrams are almost identically
range is much larger. For example independent of ground temperature.
from 291 to 301 K (18 to 28 °C; 64 to
82 °F). At this time the inspector must Very dry and clear days may have sky
use a lower sensitivity, covering from undertemperatures as low as 233 K
291 to 301 K (18 to 28 °C; 64 to (–40 °C = –40 °F). On such days, critical
82 °F). overheating of the joint can be as great as
40 K (40 °C = 76 °F) before it can be easily
detected.

564 Infrared and Thermal Testing

Clear Days Intensity (relative scale)causes of degradation of image quality
and measurement accuracy.
Clear, cloud free days are excellent for
helicopter flying but in general are not 1. Resolution may be limited by optical
favorable for power line surveys. This effects (point spreading) and
atmospheric condition is the factor that limitations in the detecting system.
most limits test times. Clear days are also
deceptive because the improvement in 2. Image quality is degraded when the
picture quality in the monitor does not image is digitized for analysis,
also give unambiguous indications. transmission or storage.

If a clear day is expected the test The magnitude of the digitization error
should start immediately at sunrise when can only be determined by measuring the
the surroundings are in thermal balance, temperature at neighboring picture
that is, when all details have same elements.
temperature after the night. During these
early hours it is possible to use a high It must be understood that there is a
gain, which means that the temperature fundamental difference between what can
window is very narrow, from 2 to about be detected on the image and what can be
8 K (2 to about 8 °C; 3.6 to about 14.4 °F). measured accurately. To be able to see an
During these unfavorable conditions the object reasonably clearly 10 percent of the
inspector can only be sure to find contrast may be enough but to measure
anomalous joints with an overheating the temperature accurately more than
greater than 12 K (12 °C = 22 °F). All 90 percent of the contrast must be
details on the ground appear with low transferred in the image.
contrast as they have almost the same
temperature. Because of sky The limitations in resolution cause a
undertemperature, a joint with only a few small sharp dot to be depicted as a diffuse
degrees overheating will shine very blob (Fig. 32a) and a sharp edge as a
brightly in the monitor and hence be easy gradual increase in intensity.
to detect.
The digitization is ideally a process that
Later in the day when the sun has been forms a mean value of intensity over each
up for some hours the inspector has to picture element, or pixel. Depending on
reduce the gain to get an acceptable the system such a digitized picture usually
picture quality. The window must increase consists of 100 × 100 pixels to 500 × 500
from 283 to 313 K (10 to maybe 40 °C; 50
to 104 °F) to give a picture acceptable to FIGURE 32. Effects of limitations in resolution:
examine. Vegetation, stones and other (a) small object appearing as diffuse blob;
details in the background will now appear (b) digitization resulting in image consisting
brighter and sharper in the monitor. of squares each with uniform intensity.
Stones will shine because the sun has (a) Small object
warmed them up to higher temperature
than the conductor. At this time hot Projection
joints are less likely to be detected, so the
inspector has a strong reason to stop
testing.

Errors Position (relative scale)

There are two groups of errors connected (b)
to the measurement of a joint’s
overheating. The first is the measurement
of the radiation temperature and the
degradation of image quality because of
limitations in the system. The second is
errors in the estimation of emissivity and
sky temperature when the observed
radiosity is translated to temperature.

Resolution Related Errors

Errors in the measurement of the
radiation temperature of the joint and the
conductor are due mainly to limited
resolution and digitization effects that
tend to shift the measured temperatures
toward the temperature of the
background. There are basically two

Electric Power Applications of Infrared and Thermal Testing 565

pixels. The more pixels, the smaller the the image in the same way as defocusing
area averaged to form the pixel. It can a camera lens and thereby decreases the
also be a sampling of the intensity at measured temperature difference. It is
different positions on the image. The final furthermore difficult to focus a high
result is the same; a picture built up from resolution scanner to its ultimate
small squares with uniform intensity over resolution because of the limited
the area of each square. In Fig. 32b, the resolution of the monitor. A phenomenon
original picture is a diagonal line from top related to incorrect focusing is that, when
left to bottom right. the lens is focused on the joint, the
background is not in focus and its
The resolution of the imaging system intensity has a tendency to bleed slightly
and the resolution of the digitization are into the sharp image and thereby reduce
two different entities, with the the contrast (Fig. 34).
digitization convoluting the original
image. Therefore the only way the Because of spreading, there is no way
digitization can contribute to the image to tell the exact temperature of a very
detail is to the worse. If the resolution of small object because the measured
the imaging system is higher than the temperature difference for those objects
resolution of the digitization, the will be the product of area and
intensity of the object is smeared out over temperature. At a great distance, the
the pixel in proportion to the amount measured temperature will be lower than
covered by the pixel. If the resolution of the true temperature. The relative
the digitization is higher than the seriousness of the problem joints will not
resolution of the imaging system, the be changed and, because of relatively
smooth intensity profile acquires a step constant viewing parameters, the
shape that can give a false impression of measured temperature will be directly
high resolution (Fig. 33). proportional to the true temperature (see
Fig. 23b).
For these reasons when images are used
for measurements high resolution is For example, the digitized image of a
needed for both the imaging system and thermal imaging system consists of
the digitization. 520 × 520 pixels. Using the longest focal
length at 20 m (66 ft) distance each pixel
Other factors that depend more on the corresponds to a 4 × 4 mm
viewing situation than on the thermal (0.16 × 0.16 in.) square. Objects this small
imaging system and influence the cannot be measured. However
resolution are characteristics of the zoom experiments with slits show that at 20 m
lens, the focusing and to a slight extent (66 ft) an object with 27 mm (1.1 in.)
also of the differences in distance between cross section can be measured with good
the imager and the object and between accuracy (90 percent of the true
the imager and the background. The temperature difference between object
ability of the zoom lens to transfer the and background). This cross section is
contrast varies somewhat with the focal about as large as a cross section of the
length setting. This means that a 4× zoom conductor and half the size of the joint.
lens does not make it possible accurately
to measure objects 4× smaller with the Figure 23b shows the joint in Fig. 23a
telephoto setting than with the wide magnified to show the digitization. The
angle setting. Incorrect focusing spreads brightest pixel will give a sufficiently
accurate temperature. Even though the
FIGURE 33. Digitization can create false appearance: FIGURE 34. In example of overbleeding,
(a) smooth intensity profile; (b) digitized illusion of high warm roof increases apparent temperature
resolution. of conductor.
(a)

(b)

566 Infrared and Thermal Testing

conductor is easy to see on the original low rather than too high. The error is
image, it disappears in the noise. influenced more by sky temperature if the
estimated emissivity is lower than true
The radiation from a warm structure on emissivity.
the ground can overbleed into the image of
the target. Figure 34 shows an example of The diagram in Fig. 36 shows the error
overbleeding, in which a warm roof in the measurement as a result of incorrect
increases the apparent temperature of the estimation of the blackbody temperature
conductor. of the sky for different emissivities of the
joint. The true blackbody temperature of
Errors in Emissivity and Estimated the sky is 268 K (–5 °C = 23 °F). Negative
Sky Temperature values mean that the measured
temperature is lower than the true
When analyzing the temperature image, temperature. Overlaid in the diagram is an
errors in the emissivity and error in the estimation of sky temperature to be 288 K
estimated sky temperature causes error in (15 °C = 59 °F) when the true sky
the determined overheating of the joint. temperature was 268 K (–5 °C = 23 °F),
Usually the measurement of the joint which results in a measured temperature
overheating is performed using the 0.83 K (0.83 °C = 1.5 °F) lower than the
conductor as a reference. true joint temperature. Note the relatively
small effect on the measurement from the
The most common source of error is the error of estimating a too low blackbody
estimation of material emissivity. The temperature of the sky.
diagram in Fig. 35 shows the measurement
error due to incorrect guesses for different Figures 36 and 37 show some important
emissivities. Diagrams such as the one in properties of the error introduced when
Fig. 35, plotted for different temperatures, estimating the sky temperature. The slope
show some important properties of this of the curves are steeper on the high
error. A lower background temperature temperature side which means that it is
results in a marginally smaller error due to better to use a sky temperature too low
incorrect guess of emissivity. A larger error than too high. Below the correct sky
is introduced by guessing an emissivity too temperature, to the left in the diagrams,

FIGURE 35. Error in measurement because of incorrect guess of emissivity for different true
emissivities. Difference is temperature obtained with guessed values minus temperature
obtained with true values. In this case, joint temperature is 283 K (10 °C = 50 °F), conductor
temperature is 273 K (0 °C = 32 °F) and temperature of sky is 268 K (–5 °C = 23 °F).

Temperature difference, K (°C) [°F] 20 (20) [36.0] Note Note
18 (18) [32.4] 0.4 0.6 0.8 1.0
16 (16) [28.8] 0.2
14 (14) [25.2]
12 (12) [21.6]
10 (10) [18.0]

8 (8) [14.4]
6 (6) [10.8]
4 (4) [7.2]
2 (2) [3.6]
0 (0) [0]
–2 (–2) [–3.6]
–4 (–4) [–7.2]
–6 (–6) [–10.8]
–8 (–8) [–14.4]
–10 (–10) [–18.0]

0.0

Estimated emission factor (ratio of one)

Legend
= 0.8 true joint emissivity
= 0.6 true joint emissivity
= 0.5 true joint emissivity
= 0.4 true joint emissivity
= 0.3 true joint emissivity
= 0.2 true joint emissivity

Note = Emissivity has been estimated to be 0.45 but may have been as low as 0.3. If true emissivity is 0.3, then
true overheating of joint is 3 K (3 °C = 7.2 °F) higher than measured.

Electric Power Applications of Infrared and Thermal Testing 567

the shape of the curves are almost normally occurring values the spread in
identical, independent of the sky the difference is less than 0.2 K
temperature and the temperature of the (0.2 °C = 0.36 °F) because of this very
joint, if the temperature difference constant shape, which means that ideally
between the line and the joint is constant. one diagram covering all usual situations
The error is furthermore almost linearly could be used.
proportional to the temperature difference
between the joint and the conductor. For

FIGURE 36. Diagram showing error in measurement as result of incorrect estimation of
blackbody temperature of sky for different emissivities of joint. True blackbody temperature of
sky is 248 K (–5 °C = –13 °F).

10 (10) [18.0] Note
8 (8) [14.4]
Temperature difference, K (°C) [°F] 6 (6) [10.8] Note
4 (4) [7.2] 283
2 (2) [3.6] 233 243 253 263 273 (10)
0 (0) [0] (–40) (–30) (–20) (–10) (0) [50]
[–40] [–22] [–4] [14] [32]
–2 (–2) [–3.6]
–4 (–4) [–7.2]
–6 (–6) [–10.8]
–8 (–8) [–14.4]
–10 (10) [–18.0]

223
(–50)
[–58]

Estimated sky temperature, K (°C) [°F]

Legend
= 0.8 true joint emissivity
= 0.6 true joint emissivity
= 0.5 true joint emissivity
= 0.4 true joint emissivity
= 0.3 true joint emissivity
= 0.2 true joint emissivity

Note = Estimation of sky temperature at 258 K (–15 °C = 5 °F) when true sky temperature was 268 K (–5 °C = 23 °F),
resulting in measured temperature 0.83 K (0.83 °C = 1.49 °F) lower than true joint temperature.

568 Infrared and Thermal Testing

References

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Infrared Thermography Applied to p 36-38.
Power Generation Facilities — A Case
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Quality Testing Show [Dallas, TX]. Electrical Equipment Maintenance.
Columbus, OH: American Society for Quincy, MA: National Fire Prevention
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11. Naka, M., K. Kanaya, S. Hirohara,
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12. Department of JMTR Project. JMTR
6. Finneson, S. “Correlation of Data: Handbook. Tokyo, Japan: Japan Atomic
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p 146-173.

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and Y. Okamoto. “Study on the Visual Thermography for Distribution
Detection of Defects in Divertor Systems.” ASNT Fall Conference and
Structures for the Fusion Reactor by Quality Testing Show [Dallas, TX].
Means of Infrared Radiometer Columbus, OH: American Society for
(Influence of Heating Methods on Nondestructive Testing (October
Visual Detection).” Journal of the 1995): p 144-146.
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No. 67. Tokyo, Japan: Visualization 30. Rayl, R.R. “Transmission Line Infrared
Society of Japan (1997): p 36-42. Procedures.” ASNTs Infrared
Thermography Topical Conference
21. Ishii, T., M. Eto, Y. Okamoto, [Cleveland, OH]. Columbus, OH:
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Y. Okamoto. “Detection of the Testing (June 1997): p 117-127.
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Model Analysis.” Heat Transfer —
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York, NY: Wiley Interscience, John
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22. Grover, P.E. and R.J. Seffrin. “Infrared
Inspection of Boilers and Process
Heaters.” Materials Evaluation. Vol. 49,
No. 10. Columbus, OH: American
Society for Nondestructive Testing
(October 1991): p 1272-1274.

23. Bosworth, B.R. “Closed Cooling Heat
Exchanger Temperature Modulation
Investigation.” ASNT’s Infrared
Thermography Topical Conference
[Cleveland, OH, June 1997].
Columbus, OH: American Society for
Nondestructive Testing (1997):
p 129-132.

570 Infrared and Thermal Testing

CHAPTER

Chemical and Petroleum
Applications of Infrared

and Thermal Testing

Maurice J. Bales, Bales Scientific Incorporated, Walnut
Creek, California (Part 3)
Pier G. Berardi, University of Salerno, Fisciano, Italy
(Part 4)
Clifford C. Bishop, Concord, California (Part 3)
Gennaro Cuccurullo, University of Salerno, Salerno,
Italy (Part 4)
Paul E. Grover, Shelburne, Vermont (Part 1)
Russel T. Mack, Mack Inspection and Thermal
Technologies, Fresno, Texas (Part 1)
Thomas G. McRae, Laser Imaging Systems,
Incorporated, Punta Gorda, Florida (Part 2)
Michael W. Pelton, Dow Chemical USA, Freeport,
Texas (Part 1)
R. James Seffrin, lnfraspection Institute, Burlington,
New Jersey (Part 1)
Gary J. Weil, EnTech Engineering, Incorporated,
Saint Louis, Missouri (Part 2)

PART 1. Thermographic: Inspection of Process
Furnaces1

Infrared and thermal testing has a In nondestructive testing, infrared
prominent role in the nondestructive cameras are sensitive to wavelengths of
inspection of petrochemical plants. radiation in the 2 to 14 pm band of the
Thermography can detect high resistance electromagnetic spectrum. They convert
electrical connection problems and this heat encrm' to a visible light display,
overloads and can locate problems in which a trained thermographer analyzes
petrochemical process furnaces. and documents. Qualitative
thermography is used to locate significant
Thermography has the adyantage that heat differences, whereas quantitative
it is noncontacting and can rapidly thermography assigns temperatures to the
inspect large areas of a component. 2 It is problems found. Because many
hence ideal for the inspection of boilers developing problems in machinery
and process furnaces in petrochemical increase temperature, thermography is an
processing plants. Here, temperatures are ideal tool for predicting when a
high and access is generally limited to one component is approaching failure.
side of the vessel. A maintenance program
of thermographic inspection can save
time and money in plant operation.

FIGURE 1. Thermographer looks through flames and detects tube areas overheating by internal
coking: (a) visible light photograph of flames; (b) in thermogram, white areas on dark tubes
reveal hot spots; (c) coking problems in process furnace tubing.

(a) (b)

(c)

1073 (800) (1472]

1023 (750) [1382)

[L

/'_. 973 (700) [1292]

G 923 (650) (1202]
'-'
'"'~- 873 (600) [1112]
~; '

e3 823 (550) [1022]
a~.
E 773 {500) 19321

~ 723 (450) (842]

673 (400) (752]

572 Infrared and Thermal Testing

Process Furnace the decoking procedure to continuously
Components monitor the tube temperatures.
Temperatures are relayed back to the
Tube Fluid Flow operators, \Vho then adjust the mixture of
steam and air to create a controlled burn.
Process furnaces used in the Although it is a demanding application,
petrochemical industry are similar in infrared thermography can be of
function to boilers but with some tremendous value in ensuring that the
important exceptions. First, the heat process furnace tubes are not overstressed
exchanging tubes within the process and damaged.
furnace usually contain a petroleum
product derivative or a similar flammable Insulation
fluid. If a tube overheats and bursts, the
result can be catastrophic to workers and In many ways, proper insulation is. more
equipment. There is a need to monitor critical in process furnaces than it is in
the heating of the tubes on line, boilers. Many processes are temperature
preferably under full load. This is a more critical. In some cases, a variation of only
challenging application than viewing a few degrees will compromise the quality
boiler tubes from the inside of the boiler of the product and render it useless or
during shutdown. But the payoffs more reduce its value. Infrared thermography is
than compensate for the effort. the ideal technology for assessing the
quality and quantity of thermal resistance
To see the tubes, thermographers need provided by the insulation, whether in
to view through the fire. Jt is necessary to process furnaces or around pipes. The
use a special fiJter in the 3.8 to 3.9 pm process furnace shown in Fig. 2 is a good
waveband to minimize the effects of the
hot gases and combustion byproducts. FIGURE 2. Coking unit: (a) visible light
Neutral density filters may also be needed photograph; (b) white in thermogram
to attenuate the signal, putting it within shows excessive heat loss caused by
sensing range of the detector. \•Vorkers damaged insulation.
also need to use special heat shields to
protect their cameras from the intense (a)
radiated heat and the gases coming from
open viC'\V ports. Depending on the (b)
thermographe(s distance from the tubes,
telephoto lenses are often required to
accurately measure the tube surfaces.
lvfeasurements are complicated because of
atmospheric attenuation, unknown tube
emissivities and hot refractory wall
reflections.

Flow restrictions and blockages in
process furnace tubes are most often
caused by internal deposits of coke; the
process accelerates when a tube is
overheated. Overheating can be caused by
flame impingement, improper firing of
burners and a variety of other causes.
Figure 1 shows tube overheating because
of internal coking. \.Yorking closely with
engineers and temperature design ratings
for this metal tube, the thennographer
will periodically monitor the problem.
The goal is to extend the process nm
without jeopardizing the safety of the
workers and plant.

Process Furnace Decoking

Once a fired process furnace is taken out
of service because of coked tubes,
operators must burn out the coke by
using a carefully monitored mixture of
steam and air. Again, there is a danger of
overheating the tubes, thereby
compromising the structural integrity of
the metal. Thermography is used during

Chemical and Petroleum Applications of Infrared and Thermal Testing 573

example. On the basis of the findings of reduced tube life by about 40 percent for
the thermogram in Fig. 1b, engineers were modified austenitic nickel chromium high
able to replace only the damaged portions temperature alloys and about 55 percent
of insulation wther than reinsulate the
entire vessel. for Unified Numbering System J 94204

Temperature Monitoring alloys.H Tube life is reduced further at
of Process Furnace Tubes3 even higher temperature increases. This is
of great monetary concern because excess
In high temperature petrochemical tube temperatures can cost a company
process furnaces, the importance of the anywhere from $200 000 maintenance per
tube surface temperatures must not be process furnace because of coke buildup
underestimated. Operation of furnace to several million dollars as a result of
tubes slightly above or below design tube rupture. Furthermore, lower than
temperatures can he costly. Thus, the optimum temperatures will result in
accurate measurement and monitoring of incomplete product conversion, thus
process furnace tube temperatures is reducing yield. This reduction occurs
critical for optimum cost efficiency, as whether optimum is judged to be near the
well as safety of plant personnel. Accurate creep rupture design temperature or at a
measurement of temperatures is difficult. reduced temperature (sometimes chosen
The problems associated with such to increase yield of a useful byproduct).
measurements and the advantages and
disadvantages of available techniques Techniques for
deserve special consideration by Temperature Measurement
inspection personnel.
Industry uses thermocouples, spot
A direct fired process furnace is pyrometry and thermal imaging to
composed of a bank or banks of process n_1easure tube temperature.
furnace tubes typically ranging in size
from 31 to 200 mm (1.25 to 8 in.) Thermocouples
diameter, made typically from a high
temperature alloy (such as Unified Thermocouples have been widely used to
monitor and measure tube temperatures,
Numbering System J 94204 or austenitic as well as temperatures of products
flo·wing through the tube. Properly
nickel chromium high temperature steel) calibrated thermocouples will provide
in a light hydrocarbon furnace; or various accuracies to within a few kelvin, so if
forms of carbon steel and stainless steel - special steps are taken, thermocouples can
along with chrome half molybdenum, provide accurate spot temperatures for
nickel chrome or niobium alloys - in various applications. However, because
power generation, styrene, ethyl benzene often several hundred of these
and vinyl chloride process furnaces. instruments are placed in a single furnace
and are usually some of the last
These tuhes have external surface components installed during
temperatures from 420 K (ISO oc ~ 300 oF) construction, accuracy is often
to 1350 K (1100 oc ~ 2000 oF) in a light compromised because of poor installation.
hydrocarbon furnace. The flames that
burn in these process furnaces can achieve Also, after extended use in a process
temperatures as high as 2330 K furnace the accuracy tends to irreversibly
(2060 oc ~ 3740 oF), where hydrogen is deteriorate partly because of oxidation
burned. Surrounding these tubes is a formation or coke buildup from the
refractory ·waH (brick lining) made of combustion gases in the furnace. In
materials such as alumina, silica and addition, a thermocouple can measure
magnetite, which do not deform and are only a single point and even ·when
not chemically altered at high multiple thermocouples are installed, tube
temperatures. temperatures can differ greatly among the
monitored points. Furthermore, because
It is very important that these tubes these <Ire contact instruments a change in
operate at their designed optimum heat flux often occurs at the point of
operating temperatures. If a tube is contact (especially for high temperature
operated above its creep rupture design applications) causing bias error. Lastly, ga~
temperature for any sustained period of flow at and near the point of contact is
time, then the life span of the tube will be not typical of that along the tube, also
greatly reduced. The significance of this causing bias error. Therefore,
can be seen in the example, reported by thermocouple measurements may not
C. E. Jaske and F.A Simonen, of steam represent tube temperatures with the
reformer tubes operating at about 3.5 1\1Pa accuracy needed.
(500 lb1-in.-2) and with tube wall
temperature of 1140 K (870 oc ~
1600 °F).4-7 Furthermore, a 28 K

(28 oc =SO °F) rise in tube temperature

574 Infrared and Thermal Testing

Optical Pyrometry and other process furnace tubes acting on

?Plical pyrometry, or hut wire pyrometry, the target tube and has the same inability
1.1. another means by which tube
temperatures can be determined. These to filter out the effects of process furnace
ins~nnnent~ are si~nple to use and require
no mstallatJon - mstallation would gases between the pyrometer and the
require modifications to the furnace.
Because this is a noncontact technique of target. Furthermore, standard infrared
temperature measurement, no
pyrometers cannot distinguish variable
interferences exist as with thermocouples.
Other advantages are that an optical emissivity from temperature differences
pyrometer can be used to survey
temperatures at all visible sites within and cannot determine unknown
proce.ss furnaces, ~t is not limited to only
one flxed spot as JS a thermocouple. In emissivity.
addition, the calibration can be checked
periodically at the discretion of the A special infrared pyrometer known as
operator.
a gold cup pyrometer is sometimes used to
The optical pyrometer"is not without
its problems. The accuracies of the try to reduce the sources of error in
readings depend on the operator's ability
to discriminate different shades of red process furnace tube temperature
orange and yellow. This operator prec{sion
error band is inherent in the system measurement. A gold cup pyrometer is a

design and is further magnified because of close focus pyrometer reading through a
inconsistencies in discrimination bet\\'een
several operators. Also, at process furnace small hole in a hemisphere. For furnace
temperatures the energy peak is \\'ithin
the infrared spectrum. Because this use, this assembly is often affixed to the
instrument operates in the visible
spectrum it is not as sensitive to end of a long metal rod, for insertion

temperature differences as it would be if it through process furnace fire doors. In
operated in the infrared spectrum. Thus
there is an inherent instrument precisio;1 operation, the reflective hemisphere ('\old
error under these conditions.
wjJ) is placed over the target area of a
Furthermore, the instrument is subject to
a theoretical bias error. This bias error is furnace tube, creating in effect a blackbody
due to reflection from flames, the
refractory wall (wall slliue) and the other (effective emissivity about 1.00) so that
tubes acting on the target tube. Also, the
process furnace gases in the path of the pyrometer reads true surface

measurement (distance between the lens temperatures. The gold cup has many of
and the target) affect the measurement.
Finally, an optical pyrometer does not the advantages of standard infrared
provide feedback for process furnace
control. pyrometcrs. Also, in some cases the gold

Infrared Pyrometry cup has been used to eliminate or

Another technique for measuring the minimize the error effects of emissivity,
temperatures of process furnace tubes is
through the implementation of a standard furnace gases and surface reflections. The
infrared pyrometer. This instrument
technique must be used carefully,
possesses the same advantageous features
as the optical pyrometer and a few others however, or the gold cup itself will induce
besides. There is a digital readout that
redu_c~s the magnitude of built-in operator interference errors like those caused by
precrswn error. Also, an infrared
pyrorneter is very sensitive to slight thermocouples. It is also quite expensive
temperature differences in process
furnaces. This instrument can also be and cumbersome. Because the gold cup
permanently mounted to provide
feedback for process furnace regulation. must contact the target surface the
However, the infrared pyrometer is subject
to nu~ch of the same theoretical precision number of candidate targets is usually
and b1as error as the optical pyrometer. It
has the same problem of detecting limited. .
reflection from the flame, refractory wall
There arc special process furnace

infrared pyrometers on the market at th1s

time designed for the special needs

associated with process furnace tube

temperature measurements. These

pyrometers include special features that

help to overcome the effects of reflection

and the attenuation of the signal between

the target and the instrument. Because

process furnace pyrometers are relatively

difficult to operate correctly, the readings

are more subject to variations among

operators' sampling practices. Thus,

repeatability is somewhat less for this

instrument than for a standard infrared

pyrometer. Furthermore, this instrument

still contains the same error of variable or

unknown emissivity as that demonstrated

by a standard infrared pyrometer.

A ratio, or two-color, infrared

pyrometer possesses a1! the conveniences

of a standard infrared pyrometer with a

few added features for special temperature

measurement needs. This infrared

pyrometer uses the ratio of two different

narrow spectral bands to accurately
deten!1ine ~arget temperatures. lly this

techmque, 1f the response signals are

weakened - as sometimes happens

because of smoke or furnace gases - both

Chemical and Petroleum Applications of Infrared and Thermal Testing 575

bands are equally reduced and the ratio of accurate and reliable tube temperature
the two signals is still the same. Thus, the measurement technologies. One such
temperature readings arc not normally study was by the Van Swinden Laboratory
affected by typical impurities in the path (Dutch :t'v1etrology Service) working with
of the target and the instrument. The process furnace operators in Terneuzen,
ratio principle also applies to \'ariable or Netherlands? The report suggests that
unknown emissivity. However, a model quantum logic laser pyrometry has
with built-in features to eliminate the considerable potential for accurate tube
error due to reflection from the other temperature measurements despite
components or flames has not been potential error sources. The report also
available. Also, evidence suggests that provides information ahout tube
process furnace tubes and their surface emissivity changes due to aging in harsh
deposits sometimes exhibit emissivity environments.
characteristiCs that cause ratio pyrometers
to produce even larger errors than Summary
standard infrared pyrometers.
Infrared thermography can be used to
laser Pyrometry increase the efficiency and safety of
process furnaces in petrochemical plants.
The laser pyrometer has features of a Successful programs require highly trained
standard infrared pyrometer plus a few infrared thermographers knowledgeable
additional features. The laser pyrometer about the construction and operation of
has a special built-in feature that allows it these vessels. Some applications are
to eliminate much -of the effects due to physically demanding and require
reflection and furnace gas attenuation as specialized equipment but returns on the
discussed previously. There is also a investment will pay for the program
built-in laser reflectometer that when many times over.
correctly used can help to correct for
variable or unknown emissivity. Because
the laser pyrometer is difficult to operate
correctly, it is more susceptible to operator
error and inconsistencies. It is also
comparatively expensive.

Thermal Imaging

The infrared thermal imager provides the
owners with a two-dimensional picture of
process furnace teml1erature profiles for
maximum understanding of relative
internal temperatures. The temperature
profiles can be observed in the field and
changes can be noted instantaneously.
This system contains all the helpful
features of a standard infrared pyrometer
along with special filters that help to
eliminate the signal attenuation due to
furnace gases. However, the imaging
system has the same problems with
reflection and variable or unknown
emissivities as a standard infrared
pyrometer. Also, infrared thermal imaging
systems arc relatively expensive.

Retter understanding of the effects that
occur within direct fired process furnaces
is helping to establish reliable techniques
for producing accurate, repeatable tube
temperature measurements. Signal
absorption by smoke and process furnace
gases is corrected with special, narrow
band pass filters. The effects of reflected
radiation are cancelled through energy
measurements of the radiant surroundings
of the tubes. Varying emissivity remains a
problem and only time and more research
might help to solve.

Studies have been conducted in various
laboratories to determine the most

576 Infrared and Thermal Testing

PART 2. Passive Thermographic Detection of

Chemical leakage from Pipelines and Storage
Vessels10

Thermographic Leak Their differences come into play in the
Testing types of leaks they are used for and the
auxiliary equipment used with the basic
Infrared thermographic leak testing infrared thermographic imagers.
techniques are accurate and cost effective
processes for water, sewer, steam, The first category, based on infrared
petroleum, chemical and gas pipeline emission pattern techniques, uses an
rehabilitation programs and for locating infrared imager to view large ground
leak discontinuities in storage facilities surface areas and lets the operator look for
and manufacturing programs.JO-J<J These general thermal anomalies, either hotter
techniques have been used to test or colder than the surrounding
petroleum transmission pipelines, background surfaces, that could indicate
chemical plants, water supply systems, subsurface pipeline leaks. This technique
steam power plants, natural gas pipelines can be used with portable imagers, truck
and sewer systems. mounted imagers or helicopter and fixed
wing mounted infrared imagers. The
Thermographic technology makes it decision whether to look for anomalies
possible to inspect large areas from remote hotter or colder than the background is
distances with 100 percent coverage. In determined ·with knowledge of the type of
addition, certain infrared thermographic leakage sought, the ambient conditions
techniques can locate voids and erosion and the time of day. This technique has
areas surrounding buried pipelines, been used to investigate up to 800 km
making their testing capabilities unique (500 mi) of pipeline daily for leaks.
and highly desirable.
The second category, based on the
Three approaches of infrared absorption of specific infrared frequencies
thermographic leak testing are the in the thermal spectral bands (emitted
following: (1) infrared emission pattern from a combination infrared emitter and
techniques,10- 15 (2) infrared absorption infrared imager) uses the infrared imager
techniques 10,H-IH and (3) infrared to view small and medium size. areas and
photoacoustic techniques_IO,I9 To observe lets the operator look for areas where the
image is black or missing, because of the
the characteristic absorbance of the absorption of the visualizing energy.
radiation, the first two techniques rely on Imagers can be hand carried or can be
an infrared thermographic imager either mounted on inspectors or trucks. This
to image the infrared energy emitted by technique is specifie<llly designed to locatt'
leaking fluid and its effect on its leaks in a variety of situations, such as
surroundings or to image the leaking fluid locating fugitive emission leaks in
as it is irradiated with a specific frequency chemical plants or small gas le<~ks in
of infrared energy. Both techniques have manufacturing and assembly operations.
the foJJowing aspects in common.
The third category is based on using a
1. They are accurate. tuned laser to excite a specific leak testing
2. They are noncontacting and gas in a repetitive manufacturing process,
such as air conditioning heat exchanger
nondestructive. testing. The excitation of the gas by the
3. They are used to inspect large areas as tuned laser causes the tracer gas to emit a
specific acoustic signature that can be
well as localized areas. picked up by nearby microphones. From
4. They are efficient in terms of both the information gathered, the exact
location of the leakage can be accurately
labor and equipment. determined. 19
5. They are economical.
6. They are not obtrusive to the

surrounding environment.
7. They dO not inconvenience the

pipeline's users or the production
process.

The third technique is based on using a
laser with a specific frequency in the
infrared spectrum to cause leaking gas to
emit an acoustic signal.

Chemical and Petroleum Applications of Infrared and Thermal Testing 577

Passive Thermographic infrared thermographic equipment was set
Case Histories up to locate areas cooler than normal,
under the hypothesis that pinhole leaks
Buried Natural Gas Pipeline in a pressurized natural gas pipeline
would cool the surrounding soil because
Jn- 1985, an investigation of 3.2 km of the venturi cooling as the gas escaped
(2.0 mi) of six·lane concrete pavement from the pipeline and expanded.
was conducted through the main
downtown area of Belleville, Illinois. The The entire field portion of the project
main purpose of this inspection was to took only one night and located, along
locate any anomalies (for example, utility with other anomalies, two natural gas
pipeline leaks or voids beneath the city pipeline leaks, one of which is shown
street) that could collapse or cause the in Fig. 3.
need for repairs after a proposed street
resurfacing project took place. Several Buried Petroleum Pipeline
utilities were located beneath the city
streets including sewage, water and In November 1990, infrared
natural gas. thermography was used to inspect a
7.3 km (4.5 mi) section of subsurface oil
\,Vhile the areas containing buried supply pipeline for a large Illinois refinery.
natural gas pipelines were inspected, the The purpose of the investigation was to
locate the cause of a drop in line pressure.
FIGURE 3. Buried natural gas pipeline and The sudden drop in line pressure was
pipeline leakage in downtown Belleville, believed to be caused by a leak in the
Illinois: (a) visible light photograph; subsurface oil transmission pipeline
(b) thermogram. system.

(a) Because of the rough terrain, the
investigation was performed from a
helicopter at an altitude of 300m
(1000 ft). With the aid of telephoto and
wide angle optics, the 7.25 km (4.5 mi)
section of pipeline was field inspected in

fiGURE 4. Buried oil pipeline, pipeline
leakage and leakage plume: (a) visible light
photograph; (b) thermogram.

(a)

(b)
(b)

578 Infrared and Thermal Testing

Jess than 30 min. The results of the above.ground pipeline used to transport
inspection included several small oil line liquid sulfur from a petrochemical
leaks and one substantial pipeline leak refinery in Carter Creek, \Vyoming. The
estimated at 4.1 L·s-1 (65 gal·min-1) 34 km (21 mi) pipeline across the
(Fig. 4). In addition to locating the leak badlands of \•Vyoming was critical to the
precisely, the infrared thermographic uninterrupted output of the refinery.
techniques helped determine how much
soil had been contaminated and what the Two four-wheel vehicles were used to
rate of contamination spread was over carry engineers and equipment. One was
time. used as a main vehicle and the other as a
safety vehicle to help get the team over
Underground Storage Tank rough areas and out of waist deep mud
holes caused by intermittent rains. The
In 1986, infrared thermographic four-whee] drive vehicles were used
techniques were developed to investigate because weather conditions did not
3.2 km (2 mi) of a six·lane concrete permit a helicopter.
pavement through the main downtown
area of Belleville, Illinois. The purpose of During the investigation, which took
this inspection was to locate any about four days because of rain and rough
anomalies underneath the street that terrain, several small leaks and insulation
might cause future problems after the problems were located by their elevated
street was resurfaced. temperature profiles (Fig. 6). Problems
with the heat tracing equipment were
During the investigation several
anomalies ·were located including an FIGURE 6. Thermogram of leakage in sulfur
abandoned and leaking gasoline tank pipeline, Carter Creek, Wyoming.
about 3 m (10ft) below the surface. The
thermogram illustrating the tank and leak
plume shmved cooler areas where the
chemical plume and tank were located
(Fig. 5). When the tank was dug up and
removed, it showed large areas of rust, a
hole in one side about 350 mm (14 in.)
from the bottom. It still contained about
750 L (200 gal) of petroleum materials.

Aboveground Chemical Pipeline

In 1985, infrared thermography was used
to locate small pipeline leaks and
insulation problems in the world's longest

FIGURE 5. Thermogram of abandoned, buried gasoline tank, Belleville, Illinois.

Person standing on Infrared image of
street surface buried lank

Cooler areas indicate
subsurface leakage
plume

Chemical and Petroleum Applications of Infrared and Thermal Testing 579

located by its lack of heal in certain Figure 7a shows the passive method of
cables. Electrical panels supplying power gas absorption. The background plate is
to the outside heat tracing equipment 100 x 100 mm (4.0 x 4.0 in.) and coated
were also inspected for loose connections by a near black matte paint. 'fhe
and anomalous or fatigued components as temperature of the background surface
evidenced by their elevated temperatures. heated by the electric heater is kept over

Infrared Absorption of 323 K (50 •c = 122 •F). The infrared
Leaking Gas
energy radiated from the heated surface of
The primary applications of the infrared the background surroundings is absorbed
camera have been found in fields such as in the gas. The infrared intensity
night security surveillance, nondestructive difference between the intensity I
testing and process control in industrial transmitted by the absorbed gas and the
plants. In such applications, infrared intensity /0 that does not transmit the gas
video cameras have been designed to is measured by the infrared camera and is
maximize the amount of backscattered displayed on a real time image monitor to
infrared and laser radiation collected from reveal the leaked gas.
the observed object.
Figure 7b shuws the active method of
The infrared camera can be used to gas absorption. Ten percent of the infrared
observe the absorption of infrared energy energy radiated from the infrared halogen
by a gas over a spectral hand. The infrared lamp is reflected from the pure black
camera is introduced to detect the surface of the background and is absorbed
invisible gas leaked from a vessel or by the gas. The input capacity of the
pipeline. The objectives of the invisible infrared halogen lamp is 270 \~'.The
gas detection system are (1) to measure temperature of the lamp is 2400 K (2127
the intensity difference between "C = 3861 "F) and r<1diates the infmred
background radiation that passes through energy at 2.5 to 3.5 pm. The ernissive
the gas from radiation background that pattern of the lamp spans 41.1 degrees.
does not and (2) quantitatively to
determine the absorbed gas distribution of FIGURE 7. Detection of leaked gas: (a) passive technique;
the leaked gas density. The laser excitation (b) active technique.
of the absorbed gas is extensively applied
to detect the location of the leaking gas at (a)
distances over 100m (over 300ft).
DQ
The technology described is applied to
detect and locate methane gas leakage Background surroundings
into the observed environment by passive
and active thermal techniques_14,15 (b)

Imaging of Leaking Gas

Diatomic gas has a spectral signature due
to the vibration of molecular compounds.
Compounds of carbon and hydrogen such
as methane have specific absorption
bands in the infrared wavelength. Table 1
shows the peak absorption bands of the
diatomic gases. The peak absorption
wavelength is different for various types
of gases.

There are two infrared techniques for
visualizing a gas of interest by its
absorption characteristics.

TABLE 1. Peak absorption of gases. Infrared camera

Peak Absorption Background surroundings
Gas Ballds (l-Im)
legend
C02 (carbon dioxide) 2.0, 2.7, 4.3, 15 I oo- radiation intensity
CH4 (methane) 1.7, 3.4, 7.0 /0 "' intensity of radiation transmitted and partially absorbed by gas
NO (nitrous oxide) 2.7, 5.3
1.4, 1.9, 2.7, 6.3, 20
H20 (water)

580 Infrared and Thermal Testing

Figure 8 shows the schematic ilpparatus infrared intensity irradiated from the
of the gas injector. The metll<me gas is background surroundings and reflected
stray disturbance other than the infrared
injected from a gas pipe nozzle measuring halogen intensity. The infrared image B of
10 mm (0.4 in.) in diameter. The velocity 3.6 to 3.8 ~1m comprises the nonuniform
of the gas is 50 mm·s-1 (3 m·min-1) with irradiation intensity reflected from tile
the passive method and 17 mm·s-1 halogen lamp. The infrared image C of
(1.0 m·min-1) with the active one. The 3.2 to 3.4 pm comprises the reflected
flow rate is measured by a mass flow infrared intensity affected by the gas
meter regulated by the regulator valve at absorption and the irradiated intensity of
the outlet of the pressurized bomb. the background object.

Figure 9 shows the image processing The intensity distribution of each of
system in the active technique to correct these measured images is stored in the
computer. The subtraction of the intensity
the nonuniform reflected radiation of the image A and that of B shows the
energy. The sensor of the infrared camera reflected intensity of the irradiated
infrared energy. Signal difference and
is sensitive to ·wavelengths from 3 to 5 reflected nonuniform intensity is
pm. Two bandpass rotating filters are corrected using the matched image C as
shmvn in Fig. 9.
installed in the infrared camera at wave
bands of 3.6 to 3.8 pm and 3.2 to 3.4 pm, Jf the lnfrared energy is transmitted
through the absorbed gas, the injected
respectively. Three different bandpass intensity versus transmitted intensity l·lo1
is related to the thickness L, spectral
images are measured to extract the absorption value a. and density C of the
specific absorption image. gas media, according to Lambert-lleer's
law:
The infrared image A of 3 to 5 pm
comprises the normal image of the (1) I ~ /0 exp(-aCL)

FIGURE 8. Schematic of apparatus for gas injection. The spectral absorption value a. is
calculated from the above equation. It
Y axis Regulator should he noted that the objective of this
Measurement study is to measure these gases in air
composed of about 80 percent nitrogen
point and about 20 percent oxygen. lloth
nitrogen and oxygen gases have no
Surface area absorption hand in the same spectral
range. For methane gas, the absorption
Methane value is a= 9.886 x 10~4 at 3.368 J-1111.
9"
To simplify the GtlcuJation of the gas
Gas tube ~b.---j Masi flow density, several assumptions are made.

meter

FIGURE 9. Image processing system.

""

)*l ~ I Spatial reflected
light intensity
I Image B ~8+Corrected
Gofofs;net ,_
image B
?

Image C .correctio~ Corrected - _to,. Extracted ,
image C differential
...-
y image
y 'I
y

Gas drn~ity

Chemical and Petroleum Applications of Infrared and Thermal Testing 581

The friction force F of the gas is assumed FIGURE. 10. Passive technique for three-dimensional profile of
to be proportional to the gas velocity gas deno;ity: (a) theoretical; (b) experimental.
F cubed:
(a)
(2) F = 1'3
t;n 1500
It is assumed that there is no air current
and that gas density D(r) is inversely .3 1200
proportional to the surface area S(r). The
surface area S(r) is expressed as follows: 900

(3) s(r) 2n(l- cos8)r2 600
2n(l- cose)(x2 + Y 2) 300

0 60 (2.36)
-16.0 {-0.63)
And the gas volume density D(r) (percent)
as a function of r can be calculated: -9.6 (-0.38)
-3.2 (-0.13)
(4) D(r) F0S0 D0 +3.2(+0.13)
+9.6 (+0.38)
V(t)S(r) +16.0(+0.63)

Passive Technique Experiment X dimension, mm (in.)

In the passive technique experiment, a gas (b)
vapor cloud is exhausted from the nozzle
of the pipeline and diffuses upward and 1500
radially. The gas density is calculated 1200
using the signal-to-noise ratio of the
infrared camera; the two-dimensional 900
distribution of the gas intensity is 600
obtained from this image.
-16.0 (-0.63) 15 (0.59) E
Figure 1Oa shows the theoretical -9.6 (-0.38)
three-dimensional profile of the gas -3.2 (-0.13) E
volume density. However, the evaluated +3.2 (+0.13) c·
gas density data region is 60 mm x +9.6 (+0.38) ·;0;;
±16 mm (2.4 x ±0.6 in.) clue to the size cw
limitation of the blackbody source. +16.0 (+0.63) E

Figure lOb shows the experimental X dimension, mm (in.) ',.6_
three-dimensional profile of the gas
volume density. Table 2 shows the
experiinental and theoretical data of the
gas volume density. Experimental
correlated and theoretical simulated data
are generally quite consistenti the area at
the bottom of the cloud is an explainable
exception and does not constitute a

TABLE. 2. Experimental data of gas volume density (percent) measured with passive technique, versus measurements
with conventional gas detector. See Fig. 10.

X Dimension, mm (in.)

Y Dimension -SO (-2.0) -20 (-0.8) 0
(mm) (in.) Experimental Conventional Experimental Conventional Experimental Conventional

0 (0) 0.0 0.0 0.0 0.0 100.0 100.0
5 (0.2) 0.0 0.0 0.0 0.0 42.68 40.0
10 (0.4) 0.0 0.0 18.46 7.0 18.84 20.0
20 (0.8) 0.0 0.0 7.47 4.0 10.0
30 (1.2) 4.19 1.5 4.26 3.0 7.52
40 (1.6} 2.80 1.5 2.83 3.0 4.27 4.0
50 (2.0) 2.04 1.5 205.0 1.5 2.85 3.0
206.0 1.5

582 Infrared and Thermal Testing

significant error in the concept. The FIGURE 11. Active technique for detection of leaking gas:
results are comparable to gas volume data (a) infrared image of gas intensity; (b) experimental
measured by conventional gas detectors. three-dimensional profile of gas density.

The intensity of input infrared (a)
radiation with the passive technique is
too low in normal conditions, where the (b)
temperature of the background is nearly
equal to that of the gas cloud. If tile 1500 60 (2.36)
background surrounding reaches 1200
temperatures over 323 K (SO "C ::: 122 oF) (1.77) c
it is possible to detect the presence of gas 900
leakage. With the passive technique the 600 c
detection sensitivity increases with
increases in uniform radiant heat flux -16.0 (-0.63) E
from the background wall. -9.6 (-0.38) E
-3.2 (-0.13)
Active Technique Experiment +3.2 (+0.13) c'
+9.6 (+0.38) 0
To detect and locate a gas, it is helpful to +16.0(+0.63)
increase the intensity of incident -~
radiation by using an iHumination source. X dimension, mm (in.)
The active method makes it possible to cw
detect methane concentration at norm<1I E
environmental conditions if the infrared '5
emitted ener&'Y using the halogen lamp is >-
reflected by the background.

Figure 11 a is an infrared image of gas
density by the active technique. The gas
vapor cloud is exhausted from the nozzle.
Figure 11 b shows the active, experimental
three-dimensional profile of the gas
volume density. To determine the
accuracy of these techniques for gas
detection theory, the gas volume densities
are calculated from experimental density
data. The corrected data are more accurate
than uncorrected data.

Table .1 shows the experimental and
theoretical data of the gas volume density.
Theoretical and measurenlent data are
quite consistent (except for the bottom
area) and indicate no major errors in the
core area of the injected flow. The result
shows that it is possible to compare them
to gas volume data measured by
conventional gas detectors.

The active system makes it possible to
detect methane concentration at normal
environmental conditions if the infrared

TABLE 3. Experimental data of gas volume density (percent) measured with active technique, versus measurements with
conventional gas detector. See Fig. 11.

X Dimension, mm (in.)

YDimension -10 (-0.4) -5 (-0.2) 0
(mm) (ln.) Experimental Conventional Experimental Conventional Experimental Conventional

0 (0) 0.0 0.0 0.0 0.0 100.0 100.0
10 (0.4) 0.0 0.0 0.0 0.0 53.9 55.0
20 (0.8) 0.0 0.0 30.6 37.0 39.4 38.0
30 (1.2) 0.0 0.0 22.3 28.35 29.0 29.0
40 (1.6) 19.8 23.0 18.5 23.0 23.9 23.0
50 (2.0) 8.4 19.5 14.6 19.5 20.2 19.5
60 (2.0) 7.9 16.5 14.2 17.0 16.3 17.0

Chemical and Petroleum Applications of Infrared and Thermal Testing 583

TABLE 4. Infrared radiation absorption of detectable gases. Safety laser Detector
Threshofda Wavelengthb
Gas Chemical Formula Sensitivity
(~<L·L- 1 ) (~m)
--------

(~ll·l-l·m)' ( k g - y r 1) '

Acetaldehyde C2H40 25 9.21009 436 297
Acetonitrile CH3CN 40 9.293 79 1000 636
Acrolein CH2 :CHCHO 10.28880 128
Acrylonitrile CH 2CHCN 0.1 10.303 47 148
Allyl alcohol C3H60 2 9.694 83 86 71
Ammonia NH3 2' 10.333 70 69 62
Amyl acetate 25 9.45805 13
Arsine C7H1402 100 10.51312 46 4
Benzene AsH 3 0.05 9.63917 79 93
Butadeine C6H6 10 11.005031 208 95
Butane 10 10.34928 45 251
T-butanol C4H6 800 10.74112 772 94
Carbonyl difluoride C4H10 100 I 0.23317 108 694
Chlorobenzene (CH 3),COH 2 9.200 73 76 124
Chloroprene COF2 10 10.260 39 82 78
Cyclohexane C6H5CI 10' 9.621 22 46 142
Cycfopentane C4H5CI 300 10.74112 1000 63
0-dichlorobenzene 600 9.26053 4380 1302
Trans 1,2-dichloroethylene C6H12 25 10.76406 79 4752
Dimethylamine 200 9.75326 160 179
P-dioxane CsHw 5 9.21009 485 238
Ethyl acetate 25' 9.45805 190 338
Ethyl acrylate C6H4CI2 400 9.31725 34 259
Ethyl alcohol C2H2CI2 5 9.503 94 57 46
Ethylene (CH 3) , N H 1000 10.53209 61 91
Ethylene chlorohydrin C4H80z 5500 9.24995 15 43
Ethylene dichloride CH 3C O O C 2 H5 ]' 10.494 49 45
Ethylene oxide CH5 :CHCOOCH 2 CH 3 10 10.859 78 1895 6
Ethyl ether C2HsOH 1 9.21009 651 56
Ethyl mercaptan 400 10.19458 119 1850
Formic acid C2 H2 0.5 9.21969 730 445
Furan 5 10.18231 24 107
Germane C2H5CIO 10.69639 100 702
N-hexane CzH 4CI 50 9.341 76 219 16
Hydrazine (CH 2) , 0 10.44059 2205 105
Hydrogen selenide C2H60 o.o1e 9.15745 55 254
Isopropanol C2H5 SH 10.49449 758 2939
Methacrylonitrile HCOOH 0.05 10.78516 I 10 27
Methanol 9.675 97 31 905
Methyl acetate C4H40 200e 9.51981 19 102
Methyl bromide GeH 4 200 10.69639 51 32
Methyl chloride C6H14 9.60357 402
Methyl chloroform 5 9.200 73 1020 9
Methylethylketone N2H4 50 10.591 04 26 58
Methyl methacrylate H2 Se 350 10.611 39 343 586
Monochloroethane (CH 3),CHOH 200 10.274 45 62 791
Monomethylamine CH2 :C(CH 3)CN 100 9.21969 126 53
Monomethylhydrazine CH 30H 10.333 70 174 383
Orthodkhlorobenzene 0.1 9.621 22 120 85
Ozone C3H60z 600 9.503 95 54 125
Pentane CH 3Br 9.675 97 33 84
Perch!oroethylene CH3CI 25 10.74112 4240 84
Phosgene 0.1 10.23317 85 122
Phosphine CH 3 CCI3 0.3 9.694 83 318 25
CH 3 COC2H5 104 4732
CH2C(CH3)COOH 3 217
C2H5CI 509
CH3NH2 55

CH 3 NNH 2
C6H4CI2

o,

C5H12
C2CI4
COCI 2
PH 3

584 Infrared and Thermal Testing

TABLE 4, CONTINUED. Infrared radiation absorption of detectable gases. laser Detector
Wavelengthb Sensitivity
Safety
Thresholda (pm) --~~-"

Ga• Chemical Formula (~tl"L~I) (Jll·L- 1·mY (kg·yr 1)'

Propane C3Hs 20 10.81111 2900 2000
Propylene 1000 10.67459 174 113
Propylene oxide C3H6 1000 10,51320 332 175
Refrigerant-11 C3H60 1000 12 25
Refrigerant-12 CCI 3F 1000 9.22953 9 17
Refrigerant-13 CF2CI 2 1000 10.76406 336 542
Refrigerant-22 CCIF 3 1000 11.085 631 564 752
Refrigerant-1381 CHCIF 2 1000 10.832931 3 7
Refrigerant-11 3 CBrF 3 21 61
Refrigerant-114 C2CI 3F3 50 9.21969 15 40
Styrene (CC~f 2), 2 9.60357 152 245
Sulfur dioxide C 6 H5CHCH 2 9.50394 3759
Sulfur hexafluoride 1000 10.85811 3790
Sulfuryl fluoride so, 5 9.21969 0.4 3543
Toluene 50' 10.55140 887
1,1,2 trichloroethane Sf6 10' 9,24995 2241 67
Trichloroethylene 50 9.62122 622 66
Trimethylamine F20 2S 5 9.23961 34 92
Unsymmetrical dimethylhydrazine 10.59104 33 99
Vinyl acetate C6H5CH 3 o.ole 9.58623 101 75
Vinyl bromide CH 2CICHCI2 10.835 24 106 168
Vinyl chloride C 2 HCI3 10 9.71400 44 46
Vinylidene chloride (CH,) 3N 5 10.611 39 102 46
Xylene (CH3) 2NNH2 5 10.611 39 48 787
C H 3C 0 2CH:CH 2 5 9,21009 31
9.53597 479
C2H3Br 100

C2 H3CI
CH2:CCI2
C6 H4(CH3h

a. Threshold limit value (TLV) expre~sed as a time weighted average {TWA), according to American Council of Governmental Industrial HygienistsY· 18
b. 12C01602 laser unless otherwise noted.
c. Average concentration for a 1 m (40 in.) thick cloud.
d. Minimum observable leak rate for gas at standard temperature and pressure; airspeed= 50 mm·s-1 (10 ft-min- 1), range= 5 m (f6.4 ft), right angle viewing

and uniform background.

e. Threshold limit value for skin.
f. 1l(16Q2 laser.

emitted intensity can be reflected hy the Infrared Absorption for
background. Gaseous Leak Location 1o,16

By using a radiation source to increase Principle of Operation
the incident infrared intensity, the active
technique makes it possible to detect the The concept of using
lower density of the methane gas than backscatter/absorption gas imaging (BAG I)
that by the passive technique. It is was developed by the United States
possible to detect methane concentration Department of Energy and transferred to
as low as 1 pg·g-1 by using the the private sector for commercialization
illumination lamp source to increase the in the late 1980s. This technique is
intensity of incident radiation. However, designed to locate leaks by making the
the lamp source of the active system normally invisible gas leakage visible on a
needs to have 'il uniform heat flux. standard video display of the region of
Software is required to correct the interest. This image of the escaping gas
uniformity of the lamp irradiation. lets the operator quickly identify the
location of the leak. The system is not
designed to determine the gas
concentration values of the leakage.

The principle of operation of the
technique is the production of a video
image by backscattered laser radiation

Chemical and Petroleum Applications of Infrared and Thermal Testing 585