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Pnuematic Testing of Plastic Pipe

Pnuematic Testing of Plastic Pipe

Pnuematic Testing of Plastic Pipe

(OP)
I'm a construction supervisor being requested by a customer to pressure test a SDR 11 12" HDPE (ASTM 3408) line to 125 PSI.  The Pipe is rated to 100 PSI design pressure.  The system is Natural Gas, so the mechanical code reuie requires that I test it with a 'gas' (not water).  We inthe construction industry consider try to avoid pnuematic tests even when at lower pressures, but 125 PSI concerns me.  The design pressure of the system is only 20 PSI.  Snice this is natural gas, it isn't covered by ASME B31.3, but B31.3 recommends a test pressure of 1.1 x design pressure when pnuematic testing, which would lower this test to a much safer level.  Does anyone have any input that I could use to help 'guide' my customer to a lower pressure, or any thoughts on why this may not be a safety issue?

RE: Pnuematic Testing of Plastic Pipe

A pneumatic test significantly beyond 1.1x MAWP is a bit extreme.  What are they hoping to learn from this test that would not be learned at 110 psi?.  But testing HDPE pneumatically is less of a worry than testing PVC or similar materials which have a tendency toward brittle fracture with the generation of damaging schrapnel.

RE: Pnuematic Testing of Plastic Pipe

I would get ahold of a copy of ASME PCC-2 - 2008 and read part 5. It has by far the best guidence on pnuematic testing I have found to date, as well as very good direction on when pnuematic testing can safley be performed and what is required to perform the test safley. You will need to perform a stored energy calculation to see if the test is feasable.

Additionally you need to be very concerend with brittle fracture of your HDPE material. Be very attentive to the temperature of your plastic pipe whent testing and make sure is at a platic and ductile temperature if you end up doing a pnuematic test.

A question properly stated is a problem half solved.

Always remember, free advice is worth exactly what you pay for it!  

http://www.ap-dynamics.ab.ca/

RE: Pnuematic Testing of Plastic Pipe

LCM1,
I don't really think that you speak for the whole "construction industry".  I consider myself to be in that industry and I've designed and executed hundreds of pneumatic tests without ever hurting anyone or damaging any equipment.  With a proper test design and proper test execution there is no added risk over a hydrostatic test (and you don't have to find a way to dispose of the water).  I've investigated a number of pneumatic test failures, and have found that every single one of them failed because the test was incompetently designed or it was executed by people who didn't follow the procedure.  Every single time.  When a pipe fails an pneumatic test it is exciting and can be dangerous, but the test design needs to take that into account.  Little things like understanding the materials stress response to temperature changes (to avoid the brittle failure that ColonelSanders mentioned) are totally ignored by poor designers, and they often leave the rate of pressurization to the field guys who are being pressured to finish the test and move on.

Is this pipe inside of a plant?  If not then ASME B31.8 is a more appropriate standard than B31.3.  It has lengthy chapters on various kinds of non-metallic piping including HDPE.  Using the equations in B31.8, SDR-11 ASTM 3408 HDPE (in any diameter) has an MAWP in gas of less than 102 psig, so your 100 psig sounds perfect.

From the 125% test factor, I'm going to assume that this is qualified for the Location 1, Division 2 design factor (i.e., less than 10 buildings intended for human occupancy within 500 ft of the pipeline per mile, and you plan to operate it less than 72% of SMYS).  When you look at the accumulated safety factors that go into the 100 psig MAWP, the pipe can actually handle 700-800 psig for weeks--the reason for the low MAWP is the long term response to environmental factors which lowers the strength of the pipe at 20 years to 5-10% of new pipe factors.

Doing an air test on new HDPE to 125 psig is less risky than standing next your garage air compressor.  If a weld-joint is going to fail, it will fail at very low pressure, and since the joints are 4 times stronger than the pipe, if you get past 5 psig you will have a good test.  

As to the "danger" of 125 psig air, I regularly test steel pipe with air, nitrogen, or (sometimes) with natural gas to 150% of the rating of ANSI 300 flanges (test pressure of 900 psig is common).  I've never had a problem yet, although I have occasionally required that a test wait for ambient temperature to be above a specific value which can create havoc with construction schedules (but I would have the same restriction for a hydrostatic test).  

I have staked my P.E. license on this sort of test many times and have never regretted it.  Neither have my clients.

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

RE: Pnuematic Testing of Plastic Pipe

If this is a gas distribution line, it is assumed that ASME B31.8, 842.52 would apply.  

RE: Pnuematic Testing of Plastic Pipe

B31.8 only applies IF it is a pipeline.  See Section 1, SCOPE of application of the code.  It is possible that B31.3 applies within a plant, or compressor station, etc., and that whatever code applies, it would include its testing procedures.

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

RE: Pnuematic Testing of Plastic Pipe

I believe you are being prudent in questioning only high pressure pneumatic testing in this case. I am also curious as to exactly why the line cannot at least first be hydrostatically tested? In the past I had thought some regulators even had become some nervous of gas pressure in plastic pipes beyond 100 psi? In any case, if you absolutely must perform this test as you describe, you might want to obtain the services of the individual who perhaps claims to be confident of the manufacture and suitable factory testing of the pipe involved, that it has not been/is not dammaged or embrittled in any way, that the integrity of the actual joints and restraints etc. you have installed is safe for such testing, and is willing to design and be responsible for such testing (I have seen that some plastic pipe manufacturers appear to heavily caveat pneumatic testing in their literature). Finally, with all due respect I would also be some skeptical and concerned of ramificationa of any (potentially dangerous?) perception that a 12" size plastic pipeline would hold 700-800 psi "for weeks".       

RE: Pnuematic Testing of Plastic Pipe

Be skeptical all you want, but you should really avoid expressing an opinion based solely on your prejudices.  12-inch SDR-11 has a wall thickness of 1.16 inches.  HDPE is amazingly tough stuff.  I would have no hesitation putting money of the outcome of a test of brand new HDPE (ASTM 3408) SDR-11 in a 12-inch to 800 psi for 336 hours (4 weeks).  None.  You set up the test and name the terms and I'm there.  This is of course assuming that the welds hold a 1 hour soak at 15 psig.

David   

RE: Pnuematic Testing of Plastic Pipe

ZDAS04,

Welds in PE are not 4 times stronger than the pipe wall! In fact a well performed butt fusion weld in PE may have 80% of the pipe wall strength.

The rate of application of the strain is criticval. Rapid application results in a strength apparently greater than a slow rate of strain application.

The challenge with pneumatic testing of PE is that the material will creep as the pressure is applied. Thus the volume will increase and the mass of air will need to increase. It is not an ideal strength test for PE. even in a hydrotest environment the volumetric addition of water needs to be accurately calculated to determine the PE stength. (refer AS 2566.2)

If using compressed air direct from a compressor care needs to be exercised as the air temperature could reduce the PE strength to the point where it fails prematurely.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

RE: Pnuematic Testing of Plastic Pipe

The data I have shows that in tension, the pipe will fail significantly before the weld.  I've had failures in the body of HDPE in normal service, but I've never had a weld that tollerated the first pressure cycle fail later.  Several manufacturers refer to the weld being something like 4 times stronger than the pipe (each uses different words).  All I have to go in is fairly liimited personal experience and what I've read.  Maybe it is all wrong.

With any test, the maximumn and minimum temperatures are essential parameters that need to be carefully specified by the design engineer.  That is as true for steel as for plastic.

David

RE: Pnuematic Testing of Plastic Pipe

I have noticed (although I guess it can be argued that it is not the same application, nor I guess necessarily exactly the same pipe) that the Plastic Pipe Institute now actually has an interesting clause 3.03 Testing relating to pneumatic testing of some buried high density polyethylene pipe in their apparently latest blurb I see now posted at http://www.plasticpipe.org/pdf/ms-3_model_spec_pe_pipe_tubing_fittings_water.pdf that reads as follows,

"B. Pneumatic (compressed air) leakage testing of HDPE pressure piping is prohibited for safety reasons."

Where risks of air testing may be otherwise somehow tolerated, there are lists of incredible and arguably quite far-reaching caveats now from a great many manufacturers, distributors, regulators and trade associations etc. in some areas, that perhaps should be read before folks sign off. In this regard I noticed the reference at http://www.arcticinsulation.net/PDF/HDPE_Testing_Procedure.pdf appears also to now try to assign some interesting responsibility in this regard e.g. in clause 3.0 Pneumatic Testing,

"3.1 Guidelines CAUTION: Pneumatic testing should not be used unless the Owner and the responsible Project Engineer specify pneumatic testing or approve its use as an alternative to hydrostatic testing, Piping system pressure testing using pneumatic testing is not recommended."

A few years ago I actually/incidentally watched some fairly high pressure hydrostatic testing of some actual DR11 (and also thicker) hdpe pipe. If I remember correctly the particular 6" and 24" DR11 hdpe pipe I watched tested failed by ballooning/movement and permanent deformation (no more pressure could be achieved in those tests) at 638 psi and 646 psi, respectively. This ballooning occurred in a duration of same half-day testing, not holding for something like 8 weeks! I think I even took a picture of some of this pipe ballooning in this testing.  

I guess about the only other thing I'd say is that any piping could conceivably contain occasional damages and/or defects in manufacture, shipping/handling, or installation etc. (including joining and including plastic pipes as is explained in e.g.  http://www.janalab.com/expertise_testing_hydrostatic.php and many other contemporary sites). If a weld really is "4 times stronger than the pipe", one wonders a little if it has the same elongation or toughness of the more homogeneous poyethylene? In any case, in light of all this I guess it thus may behoove folks to at least be reasonably careful.

RE: Pnuematic Testing of Plastic Pipe

zdsa04,

I have served on standards committees for thermoplastic pipe materials for 20 years or more an have never come across any substantiation of PE welds exceeding the strength of the parent material. The process of a butt fusion weld is basically to cut the polymer chains, re heat them and then bring them together in the molten state, allowing them to cool. the strength of the long chain molecules is lost in the process.

Manufacturers claims are not necessarily backed by science particulalry in the plastics industry. this is an industry where anyone with an extruder believes they can make PE.

I had one particular PE river crossing where the pipe passed the hydrotest but fell apart at the welds some months later. i have experience of butt welded fittings being shipped in containers arriving at their destination in bits!

I would never employ pneumatic testing at such high pressures with PE pipe. If a weld were to fail the pipe could whip aoround and maim anyone nearby.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

RE: Pnuematic Testing of Plastic Pipe

Stainer,
It sounds like your experience trumps the claims of the manufacutrers.  I need to revisit the data I've been looking at.

David

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