Are there are dangers posed in performing
pressure testing with bottled gas or is another method preferred?
Mr. Art Montemayor addresses this question as follows:
"I do not recommend the use of pneumatic
testing for piping or vessels within the confines of a process plant. With over 40
years of engineering experience - 10 of them in the compressed gas industry - I regard
pneumatic testing as too hazardous and risky. That is my personal stand and it is
based on personal field experience. I insist on the use of Hydrostatic testing of
equipment, rather than pneumatic.
Let me define the two testing methods:
Pneumatic testing is the use of a compressible
gas (usually Nitrogen or air) to fill 100% of the volume to be tested and subsequently
raising the gas pressure to the test level.
Hydrostatic testing is where the internal
volume to be tested is filled 100% with ambient temperature water and subsequently raising
the water pressure to the test level. The means of raising the water pressure is usually
done with a small, positive displacement piston pump but can be also done by imposing a
compressed gas pressure (always through a 2-stage regulator).
I do not recommend usage of a compressed gas
source as the pressure medium for application on a hydro test. Now, allow me to explain my
recommendations. Often times, there could be no other pressure relief on the system
besides the "bottle regulator". The pressure within the gas cylinder could
be many times higher than the test pressure. Otherwise, you couldn't rely on it as a
source of pressue. This is also the source of the hazard! Unless you have
installed a pressure relief valve downstream of the gas cylinder, there is no safety for
the system. All industrial gas cylinders carry a rupture disc within the cylinder
valve that is rated for protecting the cylinder - NOT YOUR PIPE OR DOWNSTREAM EQUIPMENT.
This cylinder safety device is installed for
the case where the cylinder might be exposed to fire or excessive temperatures.
It is relatively risky to test pneumatically
because of the slowness of the system to relieve itself in the event of a failure. A
small crack will only emit the equivalent critical flowrate (reached at sonic -or
"choked"-flow) and this is usually continued to be fed upstream at the pressure
source. This is the critical time when further addition of gas fluid will usually
cause a massive failure of the equipment. Gas regulators are simply that; they
are not pressure controllers nor can they be relied upon to react fast enough to mitigate
a tragic accident. This is particularly dangerous where there are cast iron
component within the tested system.
Testing with water, on the other hand, will be
characteristically much better controlled - especially if the pressure source is an
independent, small and slow rpm piston pump. Additionally, you can set the pressure
relief valve on the pump's discharge slightly above the test pressure for additional
safety. You will discover that a test failure within the system will result in a
spontaneous liquid leak and a similarly rapid decrease in system pressure. As an
added feature, you can easily trend the pressure maintenance within the system to confirm
the success of the pressure tightness achieved in the system. This is considered a
much safer and conservative method by most experienced engineers.
Agreeably, there is a trade-off between both
methods: While being considered safer, the hydrostatic test is slower and requires a
source of clean water and subsequent drainage and dryout. All of these items
represent additional costs over the pneumatic test. Nevertheless, I have always
opted for the hydrostatic test because I sincerely believe there is no substitute for