Page 205 - CW E-Magazine (26-9-2023)

Special Report

the proper radioisotope, a radiation the pressure vessel (or inside, depend- Figure-3 shows the detection of high
detection device is used to determine ing on where the inspection is occur- pressure tube leakage being conducted
if any of the radioisotopes escaped the ring). A dye is applied on the opposite in a tube bundle.
pressure vessel via a leak. While this wall. The developer is usually a white,
method can detect very small leaks, it chalky substance that creates a good
is relatively expensive compared with contrast, while the dye is typically
other methods. bright or fluorescent so that it can be
easily detected. If there is a leak, the
Radiotracers are the most sensitive dye will penetrate through the vessel’s
and competitive tools largely used for wall and the developer, and will appear
on-line leak detection in critical high quite obviously on the other side on the
pressure heat exchangers. The success developer. This method is useful for
of radiotracer application for leak determining whether or not leaks exist,
detection rests upon their high detection but it is unable to measure the flow rate
sensitivity for extremely small concen- of the leak.
trations: for instance, some radiotracers
may be detected in quantities as small ultrasonic testing Fig. 3: ultrasonic Leak detector for checking
as 10 gms (the amounts of radiotracer Oftentimes, if the media within a high pressure tube leakage
-17
used are virtually insignificant). That is pressure vessel is able to escape via If the leak rate is less than the
why, when injected they don’t disturb a leak, then that leak can also have threshold level, ‘Liquid Leak Ampli-
at all the fluid dynamics inside the heat an effect on sound waves transmitted fier’, may be used. This incorporates
exchanger/pressure vessel under inves- through the vessel. For the ultrasonic the use of a surfactant with low surface
tigation, and don’t spoil the product test, equipment is used to send ultra- tension. The pressure vessel or heat
quality. It enables the measurement of sound waves through the vessel where exchanger is pressurised and the liquid
extremely small leak flows up to 10 -10 they can potentially be disrupted by a is applied to sections of the welded
mbar.L/s. leak. If a disruption occurs, then the joints or tube sheet. This is similar
ultrasound equipment can detect it and in nature to the typical “bubble test”
Gamma radiation emitted by the display it on a screen for an inspector with one exception. The fluid used has
radiotracers penetrate the walls of the to interpret. The inspector can then low surface tension so that a low-flow
heat exchangers and provide informa- pinpoint the location of the leak and leak will form a bubble that will burst
tion about very small leaks even when estimate its size. Ultrasonic testing, almost immediately. This produces
direct access to the heat exchanger is while somewhat accurate, does not a detectable ultrasound, which can
not possible due to envelops or other give information about the leak’s flow be detected by the ultrasonic leakage
barriers. The emission of radiation is rate. detector easily.
a specific property of the radioisotope,
not affected by the interference from Typically, ultrasound leak detec- Acoustic Emission (AE) testing
other materials in the system (radio- tion is used to locate leaks where Acoustic emission is a phenomenon
tracers have strong resistance against the pressure differential is enough to of sound and ultrasound wave radia-
severe process conditions of heat ex- produce a turbulent flow as the gas tion in materials undergoing deforma-
changers). As the characteristics of moves from the high-pressure to the tion and fracture processes as shown in
radiations differ from one radioisotope low-pressure side of a leak. Most often Figure-4 and 5. In 1920, Abram Joffe
to another, multiple radiotracers may any leak with a rate below 1x10 std. (Russia) observed the noise generated
-3
also be employed and measured simul- cc/sec will not generate a detectable, by deformation process of salt and zinc
taneously if needed to locate the leaks. turbulent flow. For this reason, the crystals. In 1948, Warren P.Mason,
majority of leak applications for ultra- Herbert J. McSkimin and William
dye penetrant testing sound are limited to leaks above this Shockley (US) suggested measuring AE
Dye penetrant testing is another threshold. One of the advantages of to observe the moving dislocations by
method that can be used to determine ultrasound is that leak detection is not means of the stress waves they genera-
whether a leak exists in a pressure ves- limited to a specific fluid. The techno- ted. In 1963, Dunegan suggested the use
sel. To perform a dye penetrant test, a logy is open to identifying leaks in all of AE for examination of high-pressure
developer is applied to the outside of types of gas and even fluid systems. vessels and in 1969, Dunegan founded

Chemical Weekly September 26, 2023 205

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