pipeline elongation 2

[ercanbaser]

My question is about pipeline elongation & contraction for underground condition.

We are designing a drinking water pipeline. The type of the pipe is HDPE (polythene) and the transport water line length is 2500 m, dia of pipe is 630 mm. Pipe will be buried in nearly 1,5 m. depth. Installation weather temperature is nearly 40C and drinking water temp will be 20C; therefore we are assuming that there will be a 20 C temp. difference.

The line is almost straight and no direction change.

Here is the formula;
?L= k.?T.L where L=length of pipe :2500 m
k=elongation coeff. (0,18 for HDPE)
?T= temperature change (20)

?L=0,18.20.2500= 9000 mm = 9 m.

There shall be 9 m. contraction. The question is what type of precautions must we considered to compensate such a contraction?

I think laying the pipe in a "snake shape" will be enough but I am not sure.

Thanks for the replies...

 

[CPENG78]

Ercanbaser,
Verify the expected elengotion against a couple of HDPE manufaturers and make sure the expected elongation parameteres meet those by the agency that is reviewing your plans.

That is quite a susbtantial elongation, I would suggest looking into using Expansion/Contraction joints at certain intervals. The number of these will depend on their expansion/contraction ability. An example is the EBBA Iron EX-TEND 200. Search fot it at

http://www.ebaa.com.

Verify that it can be used on HDPE pipe. You are using metric units, so it sounds like you may outside the United States (just my gut feeling), so if this is the case look for similar products from pipe manufacturers in your area if the EBBA product is not available. Hope this helps.

 

[ekolog16]

http://www.karasus.com/kompansatorsabitnoktalarvekilavuzlamaeng.html

 

[zdas04]

You calculation works for unconstrained pipe, but not for buried pipe. The ability of buried pipe to expand and contract is very limited. The stress you've calculated will try to shrink the pipe, but the ground is holding it so you create local stresses. Nice thing about HDPE is that it behaves as a plastic under stress (good behaviour for a material made of plastic) so the stress will cause material to flow until the stress is minimized. It will pull in on the bends and where there isn't enough material to move enough the actual pipe wall will flow.

HDPE is buried all the time. Often it will sit in the sun and heat up to over 50C and then be buried in the 15C earth without problems.

Whatever you do, don't install an expansion loop underground. The constraining action of the ground will keep more than a few dozen meters from benefiting from it and the loops often cause low points or high points to trap liquid in a gas line or gas in a liquid line. They do a lot of harm and no good whatsoever.

David

 

[bimr]

I will second zdas04 recommendations. You should review the engineering design links on the website listed below:

http://www.performancepipe.com/bl/performancepipe/en-us/ww/Pages/EngineeringInformation.aspx

 

[ercanbaser]

thanks for the responses.

As a result, I understand that, for a buried PE pipe system, there is no need to take special precautions for elongation of the pipe. Up to certain temperature change, restriction of the soil is enough. But, If temperature change is extreme, low thrust capacity (unrestrained) connections to manholes may require longitudinal force thrust block (in-line anchor) protection (puddle flange application.)

Additionally, connections between other type of pipe (pvc,ductile etc.)might be a problem. When transitioning from PE pipe to bell and spigot style pipes, the combination of thermal change and thrust load from internal pressure may cause sufficient contraction to pull apart the transition joint or other bell and spigot joints in the pipeline.




The most satisfactory explanation that I can found is here;
(thanks for bimr)

http://www.performancepipe.com/bl/performancepipe/en-us/Documents/PP814-TN Thermal Effects.PDF

 

[rconner]

Interesting thread. I am aware of an article talking about some effects on buried plastic pipe entitled "Designing PVC Pipe for Water-Distribution Systems" that was written by Mr. Robert Hucks (who I believe was manager of a Johns-Manville research lab) and was published in the July 1972 “AWWA Journal”. While this article was predominantly concerned with water hammer effects (that I know had caused problems already at the time) Mr. Hucks also reported in same , “Since PVC pipe moves axially as well as circumferentially in response to pressure surges (“Poisson’s ratio effect”), pipe may undergo scratching of the outside when buried and in contact with sharp stones in the backfill. This area requires further study.” It is some interesting that “Poisson’s ratio” effect (on axial length or stresses) was apparently a concern for buried plastic pipes back then, but now potential thermal movements of several meters or whatever is allegedly not a concern for the (I believe even softer) hdpe pipe. [I incidentally agree with zdas04 that your “delta T” for movement and/or stress calculation effects from original installation in hot sun is likely to be far greater than you have thus far assumed].