Pressure Design Basis - Corrugated HDPE Pipe 1

[AnotherDirtGuy]

Hi all,

I'm working on a project that's a little out of my wheelhouse--looking for your insight on appropriate method. I am trying to develop a service life (50 year) Pressure Rating / Pressure Design Basis for a large diameter, corrugated, double-wall thermoplastic pipe (HDPE). Due to the large diameter of the pipe in question, it was not feasible to run a full-scale pressure test. So instead, we followed ASTM D2837 and conducted a long-term pressure test on a small diameter, smooth, single-wall pipe made from the same resin. From this analysis, I got a long-term hydrostatic strength (LTHS) for this HPDE.

It's straight forward enough to compute the Pressure Rating for a smooth, SW pipe from this LTHS value. However, can I use this LTHS property to find a Pressure Rating for a pipe with a corrugated double-wall profile? Does anyone have experience with this type of pipe? Is there an established process?

Thanks all

 

[LittleInch]

Is this what you mean by double wall corrugated ?

If so then I've not seen any usage other than atmospheric sewage pipe.
However the ribs of the outer wall will impose a bending and shear load on the inner pipe which is not present in a simple inner pipe model.

Reasonable scale testing ( why not do a series of sizes you can actually test) can give you a means to extrapolate to bigger sizes.

PE has been studied for a long time for long term strength so I'm not sure why you did these tests or indeed exactly what "long term hydrostatic strength" actually means. Any of the design codes such as ISO 4427 give you a means to understand the design parameters much better than I can. D2837 looks similar. But this isn't based on corrugated pipe.



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[AnotherDirtGuy]

Hey LittleInch,

That's exactly the type of pipe I'm talking about (thanks for the picture). We are interested in using it in low-pressure applications (<20 psi), specifically for irrigation conveyance.

D2837 defines "long-term hydrostatic strength" as the tensile hoop stress in a single wall pipe which will induce failure at 100,000 hours. We needed to run the tests because it is material-dependent, so the property will be specific to the HDPE resin mix of the pipe we are buying. Unfortunately, because of cost the type of testing we could contract can test only pipe up to about 4" ID, while the corrugated pipe we are looking at is made in 12" ID and up. Therefore... my current predicament.

Right now, I am evaluating the pipe as single wall per ASTM (which is similar to ISO, as you say) taking the wall thickness as the thinnest portion of the inner pipe profile. That's where the green arrow is pointing on your photo. However, I am getting very low pressure ratings by this method and I am wondering if I am being overly conservative? Do you think the corrugations and thicker valley sections will provide any support against internal pressure or am I right to neglect it?

 

[LittleInch]

To be honest I don't know. The thicker section where the corrugation meets the inner pipe will clearly help the hoop stress of the inner pipe but the bit in between not only has hoop stress but also will have a tendency to bulge into the gap with then a mix of hoop stress, axial and shear stress. You would probably need an FE analysis to work it out.

Or make a section at full size and do a pressure test at 2 x dp for a few days and see what happens.

What size and wall thickness and pressures are we talking about here?

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

 

[cvg]

I'm fairly certain this type of pipe is not rated for any sort of pressure. In fact the joints are generally rated "soil tight" not as "pressure rated, or water tight". The corrugations are intended to provide strength for earth loads causing compressive stresses in the pipe wall, not tensile stress. It is not surprising that your analysis is failing to show adequate tensile strength. PVC or solid wall HDPE (drisco pipe) might be more appropriate for irrigation and you really don't have to reinvent the wheel for those types. It would help if you gave the actual pipe diameter that you are considering.

 

[AnotherDirtGuy]

I appreciate the responses.

cvg - You're right that this type of pipe is generally not pressure rated. However, that doesn't seem to always be the case. Check out the link below for a similar product. We're looking at pipe with ID 24" up to 60", so for relatively low-pressure applications with thousands of feet of pipe there a huge potential cost saving by going with this type of product over a pressure-rated PVC.

The manufacturer is saying the thin section of the wall inside the corrugation is typically 1/8" to 1/4" for the pipe ID's we are looking at.

LI- I agree with you on the combination of stresses that inner liner will see. It's looking like FEA may be the way to go, since there is no standard design approach. At least that will help identify the failure points in the corrugated profile and we can decide if the simplified single-wall analysis is appropriate.