HDPE pipe branch connecting to CI pipe

[MonikaSM]

Hello,

I am designing a new 125 OD SDR11 pipe connecting via a tee to an existing 150 dia CI pipe. The test pressure will be 10 bar. The pipe supplies potable water from a borehole site, so temperature variations would be small - say 5 degrees C. I have a problem understanding what will be the combined effect of the thrust at the tee, the Poisson force and any thermal movement. I can't work out what restraint is needed to protect the CI pipe joints and what pressure rating should the joint between the HDPE pipe and the tee be.

Can anyone help by pointing me to a design guidance document or in any other way?

Many thanks in advance.

 

[rconner]

I know there are a few past threads on these forums e.g.

http://eng-tips.com/viewthread.cfm?qid=290415

addressing thrust resisting means and concerns for connections of polyethylene pipe (with its perhaps non-obvious pronounced modulus and thermal nuances etc.) to other types of pipe (you can access these with the advanced "search" feature. Some have links to recommendations of manufacturers etc..

Yours however may have a sort of twist so to speak - while you do not say in which direction you are approaching the tee, I am guessing you are to be approaching at right angle the larger exisiting main (i.e. into the "branch", not the "run").

Is the cast iron tee existing, or are you tapping or cutting in a new one etc.? (If the latter, I guess it would be helpful to know the actual outside diameter of the old cast iron.) If the former, it would be helpful to know the specific type of joints involved with those items. In any case, while I suspect the forces will not be real large with such small pipes, it certainly won't hurt to have a "plan". Like others who have not reponded, will wait on alittl more info.

 

[bimr]

Generally, it is necessary to anchor the ends of a PE pipeline that transitions into an unrestrained gasket jointed pipe system. If the gasket joints are restrained anchoring is unnecessary. See Appendix 3, “Pull-out of Mechanical Joints due to the Poisson Effect” for a complete discussion of the pull-out effect.

https://plasticpipe.org/pdf/chapter07.pdf

 

[MonikaSM]

Hello, thanks for the replies. I saw the thread quoted above, but it doesn't quite give me the answer. The existing pipe is CI, with unrestrained spigot and socket joints, and the new HDPE pipe will come at 90 degrees to connect. A new DI tee will be provided in line on the existing pipe. The joint to the tee could be by a PE stub flange. There will be thrust at the tee resulting in tensile stress in the new pipe but at the same time the new pipe will try to shrink. On top of that, temperature expansion or contraction is possible.
Any expansion or contraction of the PE pipe will make the joints of the existing pipe pull off, so I want to make sure it is restrained from movement. But the question is - what force and in what direction should be restrained? Is it the thrust + thermal expansion force - the poisson force?

Also, what pressure does the connecting flange need to be rated to? I think this needs to be the higher of the thrust or shrinkage force, but there are opinions that it should be the sum of them.

Thanks again

 

[MonikaSM]

I forgot to mention that I have seen the guidance for restraining a PE pipe when connecting in line. But this is a tee connection and therefore there is a thrust force, which doesn't exist when connecting in line.[pre][/pre]

 

[rconner]

The additional information is helpful. There is however actually a thrust force associated with ANY connection with or within hdpe pressure piping, and then of course dead end (PA) thrust force from the 4" direction pushing against the back of the tee in the same or opposite direction, The former are due to a pronounced long-term "Poisson" or Bourdon effects, and that thrust can also be exacerbated by thermal changes (also with very high thermal coefficient) from the time the pipeline is installed to later service. If you are dealing with old gray cast iron piping and you are at least restraining the hdpe connection to the tee(as I suspect), perhaps the risks to the old line or its joints will be the excavation around and/or under/around same to cut in the tee, and then a subsequent primary beam load or joint movements applied when the hdpe-related thrusts (and/o "Bourdon" movements, if the tee in unblocked) kick in.
While 6" gray cast iron piping in good shape may be a more robust system than feared, and the aggregate and perhaps even somewhat rebating thrusts from 4" hdpe might not be large, you are nevertheless prudent to think about same and maybe also one could put a thrust collar and wall of some sort on the new 4" hdpe branch line not far from the tee. If that sort of thing is not available with your hdpe, I guess a section of 4" ductile iron pipe with (I know available) thrust collar could be restrained to the tee branch, and work with/inside a small transverse concrete wall/anchor, thereby at least isolating all hdpe effects. Since you are already cutting into the old 6" line, there are perhaps other options, e.g. removing some of that as well and replacing with new/modern 6" jointing and/or even stronger modern piping for some distance in the run directions etc.

 

[bimr]

A general recommended is that, if the tee does not provide its own restraint, then external restraints should be utilized on each side of the fitting to prevent pullout due to the thermal expansion or the Poisson effect of the pipe. You can use mechanical restraints on the ductile iron pipe.

https://plasticpipe.org/pdf/chapter15.pdf