High Density Polyethylene pipe transition

[MalcolmA]

I have a question about transitioning from HDPE pipe to steel pipe. I have seen specifications that call for the transition to be above ground and other specifications that call for the transition to be below ground. If any of you have any insights as to why one method would be used over the other, I would appreciate some input.

 

[bvi]

Phillips Driscopipe has or had a good design for this, which included a mechanical interlock for the mechanical joints that prevents joint separation (a collar on the HDPE end is held in place by the gland).

I am not aware of any technical reason for preference for above ground versus below ground, other than the fact that some types of joints (e.g. bell and spigot) may require soil restraint to keep the joint from pulling apart.

 

[CRG]

HDPE will become degraded from sunlight unless it is protected from UV. Typically carbon black is added to give good resistance to sunlight. This amplifies the other problem with HDPE. The temperature of the pipe increases when exposed to sunlight and HDPE pipe starts to loose strength at elevated temperatures. If I recall it is around 70 to 80 degF that HDPE needs to be de-rated due to the decrease in tensile strength. So, the reason for having transitions below grade is to protect the pipe from the sun, both peak temperature and UV.

 

[CRG]

MalcolmA, what is the service type, environment, pressure, and design temperature for your application. Is fire safety a concern? With a little more information, responses may be more relevant.

 

[MalcolmA]

I am preparing specs for underground water lines for the plant where I work. It would be used for firewater, potable water, brine, and wastewater. The maximum pressure would be about 95 psi and temperature would be ambient. Fire safety is definately a concern with the firewater lines.

Thank you!!

 

[BobPE]

Be careful with you temperature assumption MalcomA, it will not be ambient, but rather reflect the temp of the fluid. This may not seem like a big deal, but fluid temps at WTP's can vary significantly and the force from thermal expansion can be a real downer to the unknowing engineer....

When making a material transition, respect this expansion force. Use of a thrust collar may be warranted to keep the pipe where you want it to be.....

let us know how you make out...

BobPE

 

[gmorin]

We generally try to transition above ground. When you're installing HDPE for its corrosion properties, it makes some sense to avoid an underground metal pipe transition. Note that unlike a lot of other HDPE users, we also generally specify flanged transitions instead of mechanical joints. In that case you would want to keep the bolts above ground if possible.

That being said, the temperature sensitivity is something to be concerned about.


There is one other thing you should be aware of when using HDPE in conjunction with carbon steel or other metal pipe. Your transition should probably include a reducer on the metal pipe side. A nominal 8" SDR 9 pipe will have an ID closer to a Std wall 6" pipe than an 8" pipe.

 

[CRG]

For natural gas distribution anodeless pre-bent risers are the standard (transition is below grade). There are many manufactures for these transitions and they been proven to be highly reliable. The risers are available for coastal regions where there is potential for salt spray in the air; however, these risers do not provide protection for the inside of the line (distributed natural gas should be dry). Perhaps someone in the forum might know of a manufacturer of anodeless risers developed for the services you require.

 

[KLee777]

MalcolmA,

We've used Philips' Performance Pipe for our underground potable water/firewater lines, wastewater, and natural gas lines. I assume this is an installation on private property since it's for your plant. Be sure to check NFPA 24 (Standard for the Installation of Private Fire Service Mains and Their Appurtenances) for a wealth of information.

The Plastics Pipe Institute (

http://www.plasticpipe.org)

is also a good source of information for underground installation of HDPE, among other things.

You might want to take a look at Philips' website:

http://www.cpchem.com/enu/performance_pipe.asp

It includes pipe and fitting data sheets, technical bulletins, and (most importantly) their engineering manual, which includes HDPE-to-steel transition information.

Good luck!

Kelly L

 

[stanier]

Why not use a flanged joint? The joint could be wrapped with a barrier tape to avoid corrosion if below ground.