Page 203 - CW E-Magazine (26-9-2023)

Special Report

Testing Leaks In Pressure Vessels

IntroductIon a pressure vessel is sealed adequately Dr. S.K. ChaKravorty
eakage detectors forms a criti- and ensure the pressure vessel’s fit- Consultant (Plant Engineering)
cal part of a plant safety system. ness for service. Commonly used in Email: chakravorty4410@gmail.com
LWhen there is a risk associated process industries, pressure vessels are
with leakages, a fixed leakage detect- manufactured with the intention that way to measure leak rates, although
ing system must be placed at venerable the pressure inside the container will accuracy can vary depending on the
points. The term “leakage” appears in be different from the pressure outside. size of the part. A larger pressure vessel
the field of engineers working particu- To ensure this pressure differential can can result in less accuracy and a longer
larly with high pressure vessels. A leak occur, a pressure vessel must be com- test time.
explicitly means an unintended hole pletely sealed; if leaks are present then
or crack in an enclosed surface, which the pressure will decay, potentially to Vacuum decay testing
allows the escape of closed medium. the point where it renders the pressure Vacuum decay testing is somewhat
The critical function of a leak detection vessel unusable. Therefore, it is critical the opposite of pressure decay testing.
system is to locate the spot of flaw. This that pressure vessels remain sealed and Rather than being pressurised with a
leak test assures the integrity of a struc- there are no leaks present. fluid, the pressure vessel has all media
ture(1, 2). Pressurised containers need removed from within it until a vacuum
to be properly sealed between external Leak testing is performed to accom- of a specified level is reached. Once the
atmospheric conditions. plish this and ensure the pressure ves- desired degree of vacuum is achieved,
sel’s fitness for service. Leak tests are any increase in pressure is detected.
Most leaks occur in joints where also performed for choosing a fit-for- The flow rate of the leak is then cal-
gaskets are used. Leaks may appear service of lining system for process ves- culated using the amount of pressure
because of various defects within the sels. There are several different types of increase and the time for the increase
material and/or at connecting areas in a tests that can be performed to check if to occur. The vacuum decay test can be
pressure vessel as described below: a pressure vessel is sealed adequately, useful in conjunction with a pressure
Fixed connections by brazing, weld- of which nine: pressure decay testing; decay test just in case a leak opens dur-
ing, or gluing – especially transitions vacuum decay testing; hydrostatic test- ing one test and closes during the other.
between different materials like ing; tracer gas sniff testing; radioiso- Vacuum decay testing is also less sensi-
glass-metal, ceramics-metal, etc.; tope testing; die penetrant testing; ultra- tive to temperature variations than pres-
Pores and hair cracks due to mechani- sonic testing; acoustic emission testing; sure decay testing.
calor thermal stress, which to some and infra-red thermography technique,
extent are always present and there- are described in this article(4, 5, and 6). Hydrostatic testing
fore must be small enough in size and Hydrostatic testing is another com-
number not to cause disturbance; Pressure decay testing mon way to measure a pressure vessel
Flanged connections; A pressure decay test is perhaps leak. Similar to the pressure decay test,
Cold/warm leaks opening up at ex- the most prevalent type of pressure this test also uses a fluid to pressurise
treme temperatures – often being vessel leak testing presently used. For the container in question. However,
reversible; this test, the vessel is pressurised with hydrostatic testing requires an incom-
Virtual leaks, where gas evaporates a fluid (typically air) using a speciali- pressible fluid (e.g., water) to determine
from inner excavations, dead holes, zed test device known as a pressure both if there is a leak and what the leak
etc., leading eventually to long decay tester. Once the target pressure is rate is. Water is often used because it is
pumping times; and reached, a valve is used to shut off the somewhat neutral and rarely affects the
Indirect leaks from supply lines, for fluid supply. The fluid’s pressure is then future service of the vessel. The leak’s
example, for cooling water or gas/ monitored over time. The amount of flow rate is calculated by the amount
liquids (He, N ) of cryogenic sys- pressure loss, or decay, is captured and of liquid that flowed into the pressure
2
tems(3). recorded. From the amount of pressure vessel after the desired pressure was
loss and the length of time for the loss reached during the test.
There are several different types of to occur, the leak rate can be calculated.
tests that can be performed to check if Pressure decay tests are a very accurate Hydrostatic testing is a common

Chemical Weekly September 26, 2023 203

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