HDPE pipe: permitted depth of scratches or gouges 3

[DavidMack]

Many websites give the rule that HDPE pipe can handle a scratch as deep as 10% of the wall thickness without reducing the pressure rating of the pipe. One source for this is AWWA M-55 ['Manual for the Design and Installation of Polyethylene Pipe' from the American Water Works Association], which apparently says "pipe with gouges or cuts in excess of 10 percent of the product wall should not normally be used."

My question is, does this 10% rule apply also to HDPE pipe for gas transmission, and if so, is there a code or standard to reference for this?

Thank you,
David

 

[rconner]

I don't know what your actual governing code/situation is, but if you have not already seen e.g.

"§ 192.311 Repair of plastic pipe.
Each imperfection or damage that would impair the serviceability of plastic pipe must be repaired or removed.
[Amdt. 192-93, 68 FR 53900, Sept. 15, 2003]"

I suspect this may eventually be a rather critical question, that if taken literally really gets into the heavy-duty realms of determining all manner of loads on and responses of buried pipes, as well as fracture mechanics and specific pipe material science (that along with how this is inspected for or enforced, and maybe particularly in some field circumstances like for pipes installed by trenchless means like relining, HDD or even pipe splitting/bursting etc.!)

That being said, I suspect many utilities probably try to get applicable guidance from the pipe manufacturer(s), regulators and/or just assume the 10% verbiage is appropriate.

 

[BigInch]

I hope you're not using AWWA for gas piping

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[Drazen]

i would say that manual only gave academic guidelines to better illustrate advantages of PE pipes.

while such scratch allowance could be understood for very thick steel pipes exposed to corrosion, over years, i wonder how would you ever be able to measure such scratches over length of PE pipes to be able to give "go-no go" decision?

or even more, what pipe installation planning would you do which would take such situation into account? for gas pipes, you need to protect buried lines with fine gravel, and later countries have different legislation on how to acess usability of old pipelines, all are based on capability to retain pressure. wherever you would see scratches by bare eyes you would be obliged to replace pipes.

so, in general you should forget idea of allowing any scratches on pipelines. if you want to learn about gas installation in depth, you first source should be your local code. you should not try, however, to design gas system yourself before you spend some time with good mentor.

 

[saplanti]

You can find some information in ASME Sect III Code Case N-755 about design, manufacture and unspection.

 

[DavidMack]

Thanks to all the helpful replies. It is indeed a very big undertaking to determine the size of acceptable flaws.

I did find a bit of Canadian code that applies. CSA Z662-11 'Oil and gas pipeline systems' requires the pipe to be "inspected for cuts, scratches, gouges, and other imperfections." Defects shall be cut out. Then it clarifies: "A plastic pipe defect is an imperfection whose depth is equal to or greater than 10% of the pipe wall thickness." (see 12.6.4 and 12.10.9.1)

David

 

[stanier]

Different criteria for internal and external scratches. Internal scratches are far more of a concern.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

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

 

[DavidMack]

Interesting, stanier, thank you. Can you point me towards further information on that?

 

[rconner]

If any have not seen same I noticed there is a article from some folks in the polyethylene pipe industry in the current (September/October 2012)issue and "Pipeline Technology" area of WEF's "World Water". The article promotes yet another new slow crack "growth" test for such pipes, the article says specifically aimed at enabling "...researchers to faster develop grades of PE pipe that can withstand the adverse conditions of trenchless installation". Also with regard to the specific inquiry of this thread I was struck by a statement I read a little later in this brand new article, "With the wrong material, a tiny scratch made on the pipe surface during the laying process can grow over time and cause failure."

While I agree with stanier that interior scratching, or for that matter also interior environmental or oxidative stress cracking of particularly polyolefin pipes including polyethylene that in practical effect perhaps mimics same, is a very serious and developing problem in at least some applications, it sure doesn't sound like in the long run exterior damage is not a significant worry as well!

I tried to explain before is hard to thoroughly answer this question in few words (the code I first referenced in the above surely does this, but it appears basically by trying to put responsibility on someone else). With regard to external damage, I also noticed some time ago there incidentally appears to be a 104 page report of a scratch study (I suspect also done working maybe closely with the European plastic pipe industry) now posted at

http://www.google.com/urlsa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&ved=0CB4QFjAAOAo&url=http://

http://wwwv2.sp.se/publ/ViewDocumen...joD4Dw&usg=AFQjCNEOMw-N9HoyZd3Od-Mg_5s2dIHEog

. This study appears to be looking at least at tests of in actual size fairly slight depth scratches on the outside of very small, and I think including also some especially ballyhooed as most crack resistant "bimodal" pipe material (though because the pipes are small and DR-related wall thickness therefore also rather thin, the scratches could accurately be reported as up to 20% of the wall thickness.) While there are many perfunctory positive comments made in the report, I'm not sure overall one can be greatly reassured in careful reading of the detailed results of this study on page 31, and as follows, that the life of even most modern pipe is not affected by 10% depth scratches:

"It is clear that the effect of scratch damage on a pipe cannot be evaluated only on the basis of the depth and sharpness of the scratch. In order to be able to assess the effect of the scratch on the pipe’s utility, it is necessary also to know what material has been used to produce the pipe and what safety factors have been applied when determining its rating. For pipes manufactured from most modern bimodal PE80 or PE100 materials, scratches up to 10 % depth can be accepted without having to reduce the working pressure. A reduction of the permissible working pressure, or alternatively planing off the scratch, should be considered for pipes made of older materials but having a design safety factor of 1.25. However, for pipes having a design safety factor of 1.6, it should be possible to accept scratches up to 10 % depth without reducing the working pressure.
As mentioned in the introduction, this investigation has been concentrated on understanding the conditions to allow scratched pipes to achieve a service life of 50 years. In addition, the conclusions in this report are based mainly on the results of tests on 32 mm pipes. This means that, when aiming for service lives of 100 years or more,or for larger pipe sizes, it will probably be necessary to apply lower reduction factors in order to ensure the desired lengths of life.
As only a few reference results are available for unnotched 110 mm pipes, the results from the notched 110 mm pipes must be related to those from the 32 mm pipes. All the results show that the performance of 110 mm pipes with 10 % notches is close to, but somewhat below, the corresponding results for 32 mm pipes with the same relative notch
depth, equivalent to a reduction of the reduction factor of 2-5 %. Whether this is a trend that indicates that successively larger pipes require increasing pressure reductions cannot be decided on the basis of this material, but can be ascertained only by tests carried out on larger pipes."

Not sure exactly where that leaves Owners who have trusted the manufacturers earlier and bought at least some of "earlier generation" pipes over the years, with some in critical services. Notice also that the apparent target 50 year longevity for the most modern pipes that are mentioned above is far less than the present life of much at least some utility pipes of legacy-type materials, that whatever flavor of PE pipes have in some cases eventually replaced. It also appears with the passage of just a very few decades, condition assessment is apparently a growing field for plastic pipes, as it has been for other types of pipe materials with far longer service history. While I think some studies have been conducted of existing plastic pipelines, most often by folks with roots in the industry, and at least some have expressed positive sounding results for future life (that justify their livelihood?), I think you will find that some such conclusions are carefully couched with numerous qualifying statements e.g. related to specific type and quality of installed material, as well as joints and gouge or damage depth etc.

Everyone have a good weekend.

 

[bimr]

You need to contact the specific manufacturer of the pipe to answer that question. Nobody will be willing to vouch for someone else's pipe.

 

[DavidMack]

rconner, thanks for the detailed and interesting reply.
I couldn't get the link to work, but the quote was very informative

 

[rconner]

Not exactly sure why link is not coming across -- let's try it again one more time at

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CB4QFjAA&url=http://

http://www-v2.sp.se/publ/ViewDocume...oHoAw&usg=AFQjCNEOMw-N9HoyZd3Od-Mg_5s2dIHEog.

If there is still problem, this study/paper is "Acceptance criteria for scratches and indentations in plastic pipes", by Mssrs.
Gunnar Bergström, Mathias Flansbjer, Linda Karlsson, Sven-Erik Sällberg och Kristian Thörnblom from the SP Technical Research Institute of Sweden (let me know if you have any trouble finding this reference).