HDPE Pipe Faiure at Low Pressure 1

[ram0608]

Dear Sirs,

When we were conducting field pressure testing for 110mm SDR11 HDPE Pipe, it was failed at low pressure i.e 3.0 bar during pressuization. The test pressure was 12.0 bar for 4 hours according to pressure reduction factors with temprature. The temprature was around 50 Deg.C during test. Failed HDPE Pipe was exposed to that temprature and failure mode is in ductile. Please find the attached photo for the same. We would like to know what could be the reason(s) for failure of pipe at low pressure during pressurization only?

Thanks in advance,

RAM

 

[Demon3]

There appear to be scratches on your pipe in the same location and direction as the failure. Any surface scratches will initiate failure at lower pressures.

Is the pipe specified to take that pressure at 50C? As you know, increased temperature will soften and weaken the polymer so it will not take the same pressure at 50C as it would at just 20C.

Polymers are sensitive to chemicals (ESCR) so if the pipe has been e.g. cleaned with surfactant (detergent) or contaminated with oil, that would cause failure under load.

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

Chris's point is very valid.

Also, is the wall section uniform where it failed. Where in the run was pipe tested vs pipe failed. Could conditions have changed to introduce a cold weld from the legs that centralise the torpedo.

Is there any sign of delamination at the failure point. Delamination may be caused by poor purging of maybe PP from the machine.

Regards
Pat
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[ram0608]

Dear Demon,

Thanks for your message. We would like to inform you that there was no scratches on failed pipe surface and it's new pipeline. There was no dimensional defect for the pipe. The design pressure was 16 bar and field testing pressure was 1.25 x DP (i.e 20 bar). Due to high temperature we have reduced test pressure to 12.0 bar as per the pressure reduction factors with temperature speicifed in international standards as well as plastic pipe institute references. so, any furuther reason(s) for failure at low pressure, please let us know?

 

[patprimmer]

Different grades of PE lose properties at different rates when heated.

Even though you claim HDPE, there is a reasonable range of densities in the HDPE range. As a general rule, the higher the density in PE, the higher the degree of crystallinity and therefore the higher the melting point and the better it retains properties up to the melting point.

As well as the molecular structure, the processing conditions have some influence on the crystal structure and therefore density.

The scratches Chris refers to are the striations that always result form the molten plastic passing through the die.

Regards
Pat
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[Demon3]

Hi Pat,

Thanks for explaining about the scratches being striations. Do you think such surface imperfections can act as weak points leading to early failure?

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

Chris

Yes they can and do at times although the magnitude of the loss here is high and unless it is a poor quality die or calibrator, I would not expect that degree of loss of property as in general, striations are not as sharp as scratches as they happen mostly to the melt which self levels to some small extent before cooling

Regards
Pat
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[Demon3]

Thanks Pat. Always a pleasure to learn something from you!

By the level of strength reduction it's probably environmental stress cracking from exposure to some cleaning agent or oil then.

Use of off-spec or recycled PE would account for it too.

Chris DeArmitt PhD FRSC CChem

 

[btrueblood]

Chris,

What cleaning agents cause cracking of HDPE? I know high alkaline solutions (NaOH) can attack it, esp. at higher temperatures, but you mentioned surfactants, which I had not heard of. I am thinking of things like sodium lauryl sulfate (baby shampoo) when I think of surfactants, do these attack PE somehow?

 

[Demon3]

Yes, in the UK anyway washing up bowls are made of PE and fail at the injection point after some exposure to water with detergent (washing up liquid). The surfactant you mentioned is the most common one, found in shower gels, toothpaste and just about all other products. Sodium laureth sulfate is slightly milder to the skin. Baby shampoos use other, even milder surfactants.

Chris DeArmitt PhD FRSC CChem

 

[patprimmer]

I thought washing up detergent was mostly ethylene oxide condensate with fragrances added. Fragrances can certainly cause unexpected results at times. Tepol is an associated trade name from deep in some obscure memory bank.

Anyways, with the lack of detailed data provided, we have no idea what the exposure was other than temperature.

Regards
Pat
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[ram0608]

Dear Pat,
Thanks for information provided and I would like to inform you that the exposure medium was only potable water bought for testing purpose.

RAM

 

[patprimmer]

OK

Tap water pretty well eliminates solvent stress crack. With some polymers, hot water, zinc from galvanised fittings or poor quality brass plus chlorine for health reasons can cause stress cracking, but I never heard of it being a problem with PE.

Regards
Pat
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[Demon3]

Hi Pat,

Yes, washing up liquid does contain chemicals of that nature. Usually they are a mixture of an anionic and a nonionic surfactant plus some salt and fragrance.

Anionic is often sodium lauryl sulfate and the nonionic is a short chain PEO chain connected to an alkyl chain.

Chris DeArmitt PhD FRSC CChem