pipe stress for HDPE pipe - Eng-Tips

Rudiawan

1) Not that I'm aware of, but have you looked in Caesar? The problem with PE is that it is really quite flexible and also creeps under high stress, so there is no repeatability and the pipe simply "relaxes" after a certain time frame. Hence all the programs which rely on non creep materials like steel and probably GRE, don't like that sort of behaviour
2) ASME B31.3 Chapter VII lists some requirements, but defers a lot to the ASTM codes.
3) The key issues of PE above ground is the very large expansion when exposed to sunlight or high ambient air temperatures. This is x10 that of steel. Also your pressure ratings start to reduce after about 20 or 25C down to a factor of 0.4 at 50C. Also you need to use black pipe which has been certified for use and resistant to UV aging and cracking.

I personally wouldn't bother. Just make sure your pipe has somewhere to expand otherwise it will just start to snake around all over the place as its not very strong and won't transmit large axial forces before buckling or snaking.

But yes 500mm or 500m?? Massive difference.

500mm why are you doing this?

500m Bury it. Or use something else. Like lined steel pipe.

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Also: If you get a response it's polite to respond to it. Also be aware that above ground the pipe will need to be fully supported or closely supported as if you lay it on supports with a big gap, after a short period of time it will begin to sag and look terrible... Or you needs supports much closer than steel lines of the same size. Also point loads will create high stress points and start to deform the pipe so you need good supports which spread the support load out.

Have a look at the attached. You can download all the other chapters as well.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.  https://files.engineering.com/getfile.aspx?folder=-07b8-4d23-b06c-46acc9eb7cfb&file=PE_chapter08.pdf
I agree that HDPE piping stress should not ordinarily be necessary. This is because HDPE should be run underground or if above ground should mostly be the above ground part of the underground pipeline that when above ground for any reason is routed mostly straight lengths where epansion joints can be used to take up the thermal expansion without needing a formal stress analysis. When above ground HDPE should not be configured like metallic pipe with headers and branches, etc, but is normally part of a straight underground pipeline that is routed above ground for short distances.

The one Autopipe I did recently was a battery of 4 pumps suction and discharge lines routed to tanks about 50 to 100 feet away over the diked area. This design was already decided upon by the time I was assigned to it and had to live with it, but should have been something other than HDPE since it was used just like regular metallic piping in routing configurations and support. I initially did a manual calculation but later did a formal stress analysis on Autopipe. Good thing as there were a lot of things going on that would have cause big problems due to the flexibility of the pipe and the thermal expansion that I did not catch in the manual calculation.

The other Caesar calculation was a large diameter underground on-site oil pipeline at the Loop terminal in Louisiana. The issue here was the thermal expansion along the length. Modeling as undergroud piping accurately predicted the expansion of underground piping, maximum elastic bend allowed, and need for anchor where pipe came out of the ground to prevent the expansion against the above ground piping. The issue I have with these programs is that they essentially still look at PE pipe like a weaker version of steel.

This is a visco-elastic material which creeps under high load.

So your quote about "elastic bend" is, IMHO, not correct. If you bend PE pipe and fix it in the ground and leave it for long enough ( a few days / weeks) it will creep in place and cease to be "elastic". So dig it up and lift it out and it will not revert to a straight piece of pipe.

Even those programs seem to say you need to put figures in yourself for any acceptable stress other than hoop stress.

The temperature of a black PE pipe in the sun will exceed 50C for empty / static conditions at which point it is at a pressure rating of 40% of its ambient temp rating and has expanded a whole heap and is often not rated beyond 50C.

So a stress program may point out this expansion, especially for long runs on supports with friction involved and more complex geometries, but I would not use it to determine allowable stress in the same way as a steel pipe or even a GRE pipe which behaves in a fundamentally different way.

Anchoring things coming out of the ground normally makes things worse all round. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.