type of pressure......difference???? 3

[pipedream]

in a closed piping system with a guage on it......

1. a 200 psi hydrostatic test is applied, the air is vented out and the guage reads 200 psi, what is the pressure inside of the pipe?

2. a 200 psi air test is applied, all liquids are drained out and the guage reads 200 psi, what is the pressure inside of the pipe?

the real question is, is there a difference between a hydrostatic test pressure and an air pressure test?

was told that 5 psi of water is the same as 1 psi of air, i do not believe this is true, to me pressure is pressure.
i do understand the safety differences between the two.

thanks in advance

 

[unclesyd]

As you state pressure is pressure.
One time you will have a difference is when you have a relatively tall pipe or vessel and you will have a head of water which will make the pressure higher at the bottom where as a gas test will show the same pressure.

 

[zdas04]

As unclesyd implied--the only difference is where you are taking your measurement. I've seen several pipes fail in the Rockies during hydrostatic tests because moron engineers put their test gauges at the top of a 1,000 ft hill, test pressure at the top was 440 psi lower than actual pressure at the bottom.

An air test on the same system would have test pressure at the top and test pressure plus a couple of psi on the bottom.

A lot of people in eng-tips.com will tell you that a hydrostatic test is good and a static test with air (penumostatic?) test is bad. ASME disagrees. I've been told that the write-up on my web page under "Samples" presents an unbiased review of the options and issues, but I'm biased about the document. If you want to read more, that might be a place to start.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[25362]

The safety differences are in the energy stored when compressing a liquid (~incompressible) vs a gas (compressible).
Energy of compression = [∫] PdV .

Assuming gases released from an exploding vessel expand
isentropically the work associated with the expansion is:

[∫] PdV = ( P₂V₂ - P₁V₁ )[÷](1-[γ])​

Where [γ] is the heat capacity ratio (Cp/Cv), P is the pressure, V is the volume, and the subscripts 1 and 2 represent the initial and final stages, respectively.

The energy that may be released on pneumatic tests on a sudden bursting of the equipment being tested or failure of the operation of the pressurizing system involves a hazard including, among others, blast effects, shrapnel, fluid jets, etc., with probable personnel (and public) injury, equipment and environmental damage.

That is, in general terms, the reason why hydrostatic pressure tests are preferred to pneumatic.

 

[quark]

When the leak size is very small, pneumatic test can be a better option. Particularly with fermentors (medicinal), I prefer pneumatic test.

 

[StephenA]

An other reason for using water is temperature stability and leak detection, especially in pipelines

A leak in a water test will result in a pressure drop, and the water temperature will stabilise quicker.

A leak in an air system will only show a pressure drop over a much longer period of time. In addition the compression of the air will result in a higher temperature which would have to stabilise.

It is all about "horses for courses"

 

[rconner]

While admittedly sometimes less convenient, “getting the air out” and subsequent hydrostatic testing for many reasons including those expressed above is in general a better and safer acceptance test for installed pipelines than pneumatic (even when the service might be for quite high pressure air or gas, and particularly when other than very low pressures are involved). I believe that is why so many consensus etc. standards recommend and/or require hydrostatic testing.
I guess it might be argued that in a perfect world (and in a stable/at equilibrium system at constant temperature etc.) "pressure is pressure"; however, it is assuredly NOT a perfect world (see e.g.

http://www.workcover.vic.gov.au/dir...sbyUNID/AEAAF1371FD07003CA256FDC0080385F?Open

, the latter part of .ppt presentation including pictures at

http://www.charlottepipe.com/Documents/Plumbing_Safety.PPT

and

http://www.osha.gov/dts/hib/hib_data/hib19880520.html.)

I suspect there are many more examples out there, as it is perhaps unfortunately human nature to sometimes choose what appears to be most convenient, less messy/cleaner, or cheaper over other alternatives.
The initial inquirer did acknowledge “the safety differences between the two”, and in this regard I feel it is also helpful to understand that things do in fact occasionally go wrong for perhaps myriad reasons in the field and to actually see some ultimate consequences when air was the medium.

 

[Ashereng]

Another difference is in how quickly the pressure is relieved.

A hydraulic system will relieve its pressure much faster than a pneumatic one.

There have been other posts that detail the benefits of a hydrostatic test, including effects on the metal itself.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[davefitz]

a) if the gauge reads a 200 psig pressure difference between inside thepipe and ambient, then there is no difference in terms of experieinced pressure , at that elevation, between the air test and hydrotest in terms of pressure experience by that section of pipe. Lower elevation sections would experience a higher pressure for the hydrostatic test, though.

b) the pnuematic test ( air) has a much higher potential for catastrophic failure than the hydrotest. If there is a crack in the vessel, and the test temperature is below the ductile brittle transition temperature of the metal ( or if teh metal has insufficient ductility at that test temperature), then the pnuematic test vessle can explode and cause widespread damage, while the hydrostatic test vessel might only fail locally . There are also many cases of fatalities during hydrotests, when the failed vessel collapsed onto the test workers, so one cannot claim that hydrotest is a fail-safe test. The main advanage of the hydrotest is that it will not explode, just fail locally.Another isseu hith hydrotest is that the water filled piping may be much heavier than original desing and can fail supports in some cases.

The critical issues to avoid such catastrophic failures include (a) all metal must be at a suitably high temp to ensure adequate ductility exists at all sections of tested piping (b) positive control of test pressures and positive avoidance of excessive overpressure (c) code defined quality control at all levels of design fabrication must be ensured (d) a aftey plan for the testing must be developed by a competent and experienced engineer.

 

[Montemayor]

As usual, once one has attracted 25362 into a thread, the result is a succinct, specific, and valued contribution. My thanks and a Star go to his response. This thread may have gotten off course with respect to the original query, but Safety is surely of prime consideration when testing any equipment. I can only offer the following experience when debating between the two type of pressure testing – which has not been mentioned:

It makes a lot of difference what material of construction is being tested. All of us normally seem to assume that the usual material in question is carbon steel – a material that has some elasticity to it before rupturing. However, there are such conventional materials - like Cast Iron - that grant you no forgiveness when they reach their limit. This characteristic is probably why it is the material of choice when manufacturing grenades. Regardless of who states otherwise, I would never think of allowing a cast iron object to be pneumatically tested.

 

[unclesyd]

Montemayor,
CI valves are still leak tested with Air. The pressure is somewhat less than the hydrostatic test pressure.

Anecdotal:
The company that I CO-OP’d with, many years ago, made CI hand grenade shells by the millions during WWII and the Korean Conflict.
The shells were originally 100% air tested, but later they were statistically sampled. When I worked there they still had a number of grenade test stands turned into test stands for AAR fittings. In testing there were no rejects only failures. The shell was installed and air applied, it either held or disintegrated, no leakers. The initial production runs had a extremely low failure rate. Then the army thinned the shell to get more explosive in and the failure rate went up. My boss’s solution to the problem with failing shells was to lower the test pressure, he said they needed the grenades more than they needed the testing. After about 9 months the Army finally got around it authorizing the change. The operators were protected by a 1/2" thick steel plate and sand berms.

 

[pipedream]

thanks for all of the replies

i guess i need to put the question to a different angle.
what i am in question about is a factory fabricted steel tapping saddle made to go onto ductile iron pipe for underground water distribution piping. all of our piping, fittings and valves have to be hydrostatic tested at 200 psi for two hours with no pressure loss. the tapping saddle is for making a wet tap (pressure is not relieved from the main line before tapping). the tapping saddle comes with an outlet to hydo it and the tapping valve after installation and before the tap is made to check for leaks. a 200 psi hydo is acceptable to the manufacturer.
the company that makes the wet taps for us freaked out when they saw what we were doing and said that only a 15 psi air test for five minutes was needed. they said that 1 psi of air is as good as 5 psi of water (although in there theory 15 psi of air would only be 75 psi of water). i then asked why are most valves rated for the same pressure marked WOG (water, oil, gas), and then asked why were the guages for air and water not made different.
there again i understand the safety side of both test but is there a difference in the two test related to pressure? is air pressure five times better than water pressure?

thanks in advance

 

[Compositepro]

The safety issues aside, the physics is: pressure is pressure. I thought that maybe there was some confusion in that one psi is eqivalent to two feet of water column, but that doesn't seem to be the case. One consideration, though is that if you are going to detect a leak by drop in pressure over time one method will be faster. I'm not sure which though.

 

[zdas04]

Pipedream,
I think you are the victum of someone trying to justify a "we've always done it that way" policy with pseudo-science. If the weld passes a test to 200 psig with air or with water the results are the same--the pipe can be rated at the appropriate percentage of 200 psig. The 5:1 ratio that you were quoted is just nonsense.

As I expected, the "NEVER NO AIR TEST, DAMNIT" contingent has made all the same points they always make when this topic comes up. I just want to make the point before someone reading this thread in the future gets the idea that testing a pipe with air is irresponsible that a properly designed pneumatic static test done on pipeworks of suitable material is perfectly safe and is sanctioned by the codes and standards orginazations.

The examples of failed PVC pipeworks above is a great example of why you don't do air tests on many brittle materials (or on carbon steel below a certain ambient temperature), brittle failure is a dangerous thing. A properly designed test (either hydrostatic or pneumatic) would have proscriptions against approaching the brittle-fracture region of materials performance.

In short, doing stupid things will occasionally get people killed. Doing stupid things should be avoided at all costs. A properly designed pneumatic test is far from a stupid thing and can be done safely time after time.

David

 

[Artisi]

"Anecdotal:
The company that I CO-OP’d with, many years ago, made CI hand grenade shells by the millions during WWII and the Korean Conflict.
The shells were originally 100% air tested, but later they were statistically sampled. ......"

Just a thought, why were the casing tested? - aren't they designed to disinergrate anyway. Seems to me the only worthwhile test would be to make sure they failed from the correct amount of internal pressure - but then of course they are no longer of use.

Naresuan University
Phitsanulok
Thailand

 

[25362]

For guidance on the testing procedure visit:

http://www.cheresources.com/airleaktest.pdf

 

[rconner]

pipedream (I like the moniker!),

Thanks for now explaining the actual issue involving some sort of tapping saddle installation. When you mention, “ a 200 psi hyd®o is acceptable to the manufacturer”, I guess you are probably talking about the manufacturer of the “fabricated steel tapping saddle” (please advise if this is not the case). However, it would appear to me there may conceivably also be as many as two other “manufacturers” (that were at one time involved) whose materials also form boundaries of the apparently small test section you are talking about (namely the outside wall of the specific ductile iron pipe you are testing against, and I guess also a blanked end or closed gate of the tapping valve.) You also mention that the parent pipeline will not be shutdown (I assume this means that there will likely be at least some level of working pressure on the inside of the parent pipeline when all this is done.)
I don’t know exactly what under the circumstances you describe may have “freaked out” the “company that makes the wet taps” responding apparently to your high pressure testing spec requirement; however, I would not necessarily discount that it could conceivably be some level of experience or naturally cautious and not necessarily unhealthy awareness/intuition (particularly if these are folks that have been in that business for some time). In this regard, I think it should be kept in mind that if the outer applied pressure against the pipe wall is greater than the inner pressure applied to same during this test (and I wouldn’t be surprised if that is the case with your present specification), the outer pipe wall (specifically within the periphery of the inner/rubber? tapping saddle seal) would in effect attempt to act in this test as described as a sort of convexly curved (beam) blind flange or test blank that may have to support an aggregate effect/unbalanced outer pressure that is arguably trying to collapse (bend) the uncut coupon from the outside in. However, the thickness of this pipe is probably less than that of a commonly supplied ductile iron test flange.
While I suspect there would be little if any concern on the valve side test boundary (at least if this is a 200 or 250 psi rated valve), any limitations on external collapse of say the uncut coupon of say a sizeable branch connection on a very large parent pipeline with limited parent thickness and say a low or no internal pressure might be another matter. I think this also could present a quite complex strength of materials issue to analyze (maybe best with FEA etc.??). Thus, not knowing any more it would appear this could conceivably be a legitimate concern at least if the differential pressure between inside and outside of a large area coupon were extreme etc. (perhaps at some point the differential external pressure would be high enough for the resultant force on the coupon to, of course completely unnaturally of “normal service” condition after the tap is made and all is pressurized?, bend the coupon inward away from the outer tapping saddle seal or actually cause the parent wall to invert or collapse?) If it is on the other hand just a very small tap with a relatively thick/strong pipe wall and low differential pressure etc., this worry of course is likely a non-issue.
While there could certainly be at least a question of very high differential pressure against the outside of a large uncut coupon with limited thickness (as I’ve attempted to explain above), I am not exactly sure from reading your recent more detailed description of exactly how/why air vs water has now entered into the picture (particularly if the manufacturer of the tapping sleeve/seals etc. wants “hydro”??) However, if in the contractors experience he feels e.g. a safe lower pressure water or air test (say with soap over the outside/welds and around edge of the saddle etc.) in this special sort of test will be suitably indicative of performance of the tapping saddle seal for a higher level of hydrostatic service pressure once the coupon is cut, I’m not sure I could prove him wrong as I believe many of these tapping saddle seals are intended to work sort of like lubricated push-on gaskets or o-ring seals that perhaps just get tighter as increasing pressure is applied? Nevertheless, if it doesn’t represent a safety hazard (with pit open etc.) if I were the Inspector I might also want to at least sneak a peek at the sleeve after the hole is cut and at least normal service hydrostatic pressure is put in the new branch piping connections.
As far as any sort of equivalence of one part air to five parts (times that of the air) water for any field pipeline testing purpose, I can’t say as I’ve heard of that specific reference. I am aware however that ISO 2531 and EN 545 standards allow either a one bar (14.5 psi) air test or a 25 bar (362.5 psi) hydrotest for leak-tightness only of ductile iron fittings/specials produced per those standards (the air test can be thought of similar to a worker checking a tire for air bubble leaks after he has repaired or mounted a new one). The conformance of the fittings to material/strength requirements is of course confirmed by original proof-of-design tests of some representative fitting bodies to much higher pressures, physical testing of actual iron from test bars cast when the fittings are cast, and/or other inspections etc. I don’t know the original basis of this ISO air testing verbiage though I believe it goes way back (and this does not surprise me much as I think air molecules are quite small); however, as far as I know it does not have any necessary relevance to acceptance testing of installed pressure pipelines in the field.
I skimmed through the interesting testing spec at the polyethylene pipe site pointed to by 25362. When others read this closely I wonder if the multiple repetitions on this site of the phrase to effect, “WARNING – Death or serious injury and property damage can result from failure at a joint or connection during pressure leak testing... ” etc. might not scare the hell out of many folks!! While this site apparently does mention air testing, it includes other very interesting verbiage e.g., “For safety reasons, pneumatic testing is very strongly discouraged...” and “Do not conduct pneumatic tests unless the Owner and the Owner’s Project Engineer both specify pneumatic testing and approve its use. The test liquid should meet appropriate industry standards for safety and quality so that the environment, system, test equipment and disposal (if necessary) are not adversely affected. The recommended test liquid is water.” And, “Pneumatic testing should not be considered unless one of the following conditions exists:
... When the piping system is so designed that it cannot be filled with a liquid.
... When traces of a liquid will compromise the piping system or its intended use.
... When the system is to be low-pressure air tested using ASTM F 1417 Standard Test Method for Installation Acceptance of Plastic Gravity Sewer Lines Using Low-Pressure Air.”
and, “Before applying pressure, all piping and all components in the test section must be restrained. This means that if piping or parts move or separate during the test, it will not result in damage or injury…”
The latter statement and other copy surrounding it is quite interesting, in that while in their marketing literature I think the polyethylene folks claim their variously heat-fused systems are completely “restrained” or self-restrained, per this document it appears to me they clearly don’t want to be responsible when something/maybe anything flies apart!! While I guess someone could try to use this document to legitimize a choice of pneumatic testing over hydrostatic, I suspect if something very bad happens in a pneumatic test these folks and/or their attorneys would probably at least try to argue this verbiage indicates this testing protocol was clearly not their decision, and “others” should have either designed/specified the system for hydrostatic testing with “water”, or they should have externally hog-tied restrained the piping every which way but loose so it or parts of it could not “move”. In some circumstances in practical application, I think this might be very hard or expensive to accomplish.
While I personally think there are some places for pneumatic if appropriate materials and all safety precautions are employed etc., if you are to read this all literally in the context of most common field pipeline applications, I believe they are extremely limited. Thus, if Mr. zdas04 has to put me in a specific camp I guess he can go ahead and put me in the “do all reasonable in all steps of the process to avoid putting high pneumatic gas pressures on an installed piping system before (air is removed) and as high or higher hydrostatic pressures etc. are applied on pipelines in the field” camp. I don’t care how strong the piping material is, or at least is supposed to be, or was when it was originally manufactured, etc.!! The fact that someone has crossed say an unauthorized walkway/crossing etc. a thousand times successfully in the past perhaps means little, if on the 1001st crossing they get splatted by a piece of equipment or a locomotive.
Sorry for the long post, but there have now been a lot of “issues” touched on in this thread.

 

[rconner]

In looking for another purpose through some other old notes I had, I noticed another case of apparent pneumatic testing gone quite awry recounted by Mr. Joe Facer AFM #237 near the bottom ofthe page at

http://www.eurospares.com/frame8.htm

including the quote, "..An apprentice in my Local was paralysed for life when someone tried to test a plastic piping system with #100 air. The 6" plastic pipe ruptured/exploded and threw him across a room and crushed his spine..."