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Chemical and Process Engineering Resources

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Category: Experimentation and Testing
Question: Are there are dangers posed in performing pressure testing with bottled gas or is another method preferred?
Keywords: v1i1,pneumatic,pressure,testing,hydro,hydrostatic,piping,pipeline
Answer: BACKGROUNDA pipeline is be pressure tested using inert, bottled gas to raise the pressure within the process piping. The piping is not equiped with pressure relieving devices.ANSWERI 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 to make sure we both know what the other is talking about: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) much as you have partially described. 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.1) You state there is no other pressure relief on the system besides the "bottle regulator". I would refer you to the fact that the pressure within the gas cylinder is 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. Unless I'm grossly mistaken or there are other facts I don't know about, you are not testing in a protected mode!2) It is relatively very 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.3) 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 safety.Source: CERP Message Board, Mr. Art Montemayor