Pressure / Strength Testing

That doesn't sound right. The consequences of a vessel failing catastrophically under test with a compressed gas instead of water would be spectacular, if not violent, if not fatal.

So such generalisations should be disregarded. The correct procedure to be adopted for vessel testing should be reviewed and approved by the Engineer/Surveyor for the insurance company responsible for providing burst indemnity insurance before carrying out the test. In every case. So make the call.

Have you observed an air inflated balloon? After a couple of hours or so, you will find that the balloon, goes limp with leakage of air. Now, fill-up a balloon with water & observe after 24 hours. This experiment, I trust, will answer your question.

I hate to disagree but the reason the balloon goes limp is because the pores in the rubber balloon are somewhat permeable to the gas used to inflate the balloon. I would hope that the steel you are using for the pipe or vessel is not even slightly permeable to gas.

The pressure testing of process piping and vessels is essential in the chemical process industries (CPI). In liquefied natural gas (LNG) and other cryogenic facilities, residual water left by hydraulic pressure testing could result in operational problems if not completely removed, and complete removal of residual water can be very difficult and time consuming. Pneumatic pressure testing avoids these problems, and is frequently used for piping and vessels in which moisture is undesirable.

Hydraulic pressure testing with water, however, is much more common than pneumatic pressure testing with a gas because the stored energy of compressed gas can be roughly 200 times the stored hydraulic energy for the test pressures in the range of 100 barg. Therefore, rupture of a piping test system during a pneumatic pressure test can release much more energy. In applications where pressure testing with liquids is undesirable, such as in cryogenic piping systems and vessels, pneumatic pressure testing can only be justified when care in fabrication and in non-destructive examination of vessels and piping reduces the probability of loss of containment to such a small value that risk is acceptable.

We can all google and cut and paste, but it's not big and not clever.

http://www.che.com/extras/pdf/Evaluating-and-Reducing-the-Risks-Of-Pneumatic-Pressure-Testing_6363.html#.UNKHN29vOi0

As PWS said in #1, because the consequence of failure on test is much higher for pneumatic. But 1.1 x for pneumatic test seems low to me, specially if it will be on pneumatic service. I wouldn't be happy near one that had only been tested to 1.1 x. I don't know offhand what the codes say, though I'd think the test pressures would be about the same, but use sensible precautions - do it where damage is minimised if it fails, and don't let anybody near it during test!

Good morning guys, can I just add a little background information which might help someone out there answer the question I posted and stop people from giving me information I am already aware of & didn't ask for, read the question.

I am currently working on the largest LNG project in the world (68 Billion $) and on an average day we carry out 25 pnuematic and 75 hydrostatic large volume tests

PWS: What doesen't sound right? I didn't ask what the consequences of a catastrophic failure would be nor am I looking for burst indemnity insurance. Read the question.

Joshi:I am sorry but your comment has absolutely no relevance to my question either, you appear to be making a reference to the ability of a balloon to successfully contain two different mediums over a specified amount of time. Read the question.

Anon:Everything you have written is perfectly correct but unfortunately like evryone else who has commented on my post you too have have failed to either read or interpret my straight forward question.

Codemaster:When you decided to add your comment to this post you should have either changed your name from Codemaster or not made the statement "I don't know offhand what the codes say" Read the question

Looks like you put yourself in a corner now.

I think you have a valid question there but first of all you and we will have to understand where the numbers come from.

PWSlack was fishing for this but you did bite the balls and not the hook. WRONG!

So who says that this is the accepted procedure? If it is in a Standard, the guys that made it will have a much better answer than this forum here. If it is a internal procedure, the guys who made it (is probably not there anymore) will know.

Anonym gave you a reason that you put to the side but which could be the reason for testing to a lesser value with air.

You do all the pneumatic testing and the hydro test, but why do you use which? Whats the difference in the equipment you test there? Maybe pneumatic test is carried out on equipment where the strength is not essential? We do not know but you could check this. Are you testing pipe and pipe differently? If why? You are a smart guy and can answer a lot of this yourself if you take the time to dig into this. I would be interested to hear your answers.

68 Billion is a lot to blow up. Better make it right!

Maybe I shouldn't have said codes, but there are lots of different codes out there and my "mastery" is elsewhere!

If it's such a big project and you've carried out so many of these, isn't there somebody in your organisation who can give you the answer?

It might be an idea if you read the question, since you've urged others to do that 4 times in 1 post. Your original post refers to pressure vessels and does not mention pipes, but in #9 you say pipes not vessels. Also it refers to strength testing, not leak testing. It's understandable to use lower pneumatic leak test pressure, for the reasons given.

I don't know how HeHound in #7 knew it was pipes being discussed, as I can't see a ref before then. For pipes, it all falls into place.

You didn't say what code was being used (for pipes or vessels). For vessels, ASME 8 obviously differs in some areas, but I have BS5500 (old copy, but I doubt it's changed much) which specifies same pressure for hydraulic and pneumatic tests. It gives a calculation for test pressure, and this can be > 1.5 x design, depending on construction category etc.

At last I have found someone that can read, very pleased to make your acquaintance HeHound. You are the second person on this thread to make a perfectly correct statement but the first to make a correct statement which actually goes a long way to answering my question. Right again HeHound I am on the Gorgon project in Cooec's fabrication yard in Qingdao China and yes our paths have probably crossed before.

Your humble opinion I suspect is not so humble and if not spot on it is very close to the mark, I have been hung up on the term "Strength Test" and could not give a logical explanation for the difference in test pressure for the different mediums. My original field of expertise being N2/He Leak Testing (BJ PPS, NOWSCO in the 90's) I have always considered a Hydro @ 1.5 and a Pneumatic @ 1.1 x design to be a "Strength Test" on process piping.

Don't worry about vessels we have none on this site just 45,000 metres of pipe varying in size from DN50 to DN1800 & up to 78mm wall thickness and plenty of exotics. Well HeHound I thank you for your expert opinion, as for the rest of the guys that thought they had replied to my question, thanks for replying you've been a right laugh.

Kim?

Si Señor

Which version of ASME Section VIII are you using?

Prior to 1999, vessel pressure tests were 1.5 x design pressure. Now they are 1.3 x design pressure. It's never been 1.25 x design pressure. Check UG-99.

Pneumatic tests are 1.1 x design pressure for both pipe and vessels (according to ASME). Check UG-100.

Both of these are minimum design pressures, and both need to be adjusted for different allowable stresses at the design temperature (make sure you check that!). You can go higher, but by going higher, you risk damaging the equipment or creating a safety hazard...it's best to just go up to these pressures.

As mentioned before, the reason for pneumatic tests being set at a lower pressure than hydrostatic is for safety reasons. If the vessel or pipe ruptures during the test, air will release much more energy than water. Basically a pneumatic test creates a potential bomb while hydrostatic creates a potential leak.

If you're using these codes for design, most likely, you'll need an authorized inspector to buy off on the vessels. That person would be a good resource for code interpretations and questions.

I could not agree more. I see hydrostatic tests done on all our boiler vessels from time to time, and they are nothing more than a sophisticated leak test, definitely not a "strength" test.

You haven't provided any test details. However, if both pneumatic and hydrostatic tests are conducted under adiabatic conditions, the pneumatic test will be at a higher temperature because of the heat of compression. This might explain the difference in test pressure.

Hydrostatic testing uses water as the pressurizing media, while for a pneumatic test generally air or nitrogen is used.

You will use the pneumatic tests on lines where the additional weight of the water inside the pipe may cause damage to foundations, pipes, etc.

An example of a pneumatic test are on flare lines which maybe up to 48" lines, to avoid dimensioning the supports or foundations of the pipe rack for the water weight during the hydro you use air or nitrogen which of course allows the pipe to be tested but without the addition of the water weight.