Rocker pieces 2

[EngAzat]

In case of HDPE pipe line to be connected to a valve chamber (DI connection in and out of it),is it advisable to use rocker pieces upstream & downstream the valve chamber.
Is there a standard supporting this issue?

 

[rconner]

I believe you ask at least a very good question, and there have reportedly been some quite serious problems apparently where very strong pipes are joined to or in contact with much weaker pipes like polyethylene (and there is maybe eventual relative movement e.g. see

http://www.pstrust.org/library/docs/P98_1_5.pdf,

and I believe subsequent advisories that were spawned by same)

 

[EngAzat]

Thank you.

 

[stanier]

Rconner,

The paper you refer to investigated the failure of a material of some decades ago. It is misleading in terms of this question.

PE 100 has been on the market for some time and has improved ductile to brittle characteristics. It is a medium density polyethylene rather than a HDPE.

That said differential settlement between structure and pipe can cause failure due to shear. The settlement of structures can be provided by the civil/geotech engineer for your project. From this data you can determine the need for rocker joints.

Generally for buried PE100 pipelines rocker jonts are not used as there is adequate capability to deflect without shear. For materials with low deflection limits such as concrete, vitrified clay, DICL and GRP then rocker joints are the norm. But you have to do the number syou cannot guess.

"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.

http://waterhammer.hopout.com.au/

 

[rconner]

I believe the first unfortunate incident in the article I referred the OP to occurred reportedly with a type of “medium density polyethylene pipe” just 17 years ago, on October 17, 1994 (and incidentally very close to where I played ball as a kid and just a few miles from where I grew up). While I guess it can certainly be argued there has been some improvement in resins and pipe since, this is still reportedly the worst pipeline accident in recent memory in the state of Iowa. Other incidents referred to have occurred since. In any case, some technical lessons are arguably timeless (lest history is to repeat itself?)

“PE100” is a polyethylene pipe that it appears as per the reference at

http://www.conind.com/cont/pdf/LITE...EC SHEETS/BimodalPolyethyleneforPE100Pipe.pdf

may now be produced in a gas process that involves “chrome calalysts”. Indeed, it is purported to have some improved and higher strength characteristics, and in the last few years and is being promoted by some in the plastics pipe industry and per this reference quite clearly to allow thinner walls (i.e. less manufactured material) for the same low pressure applications, and/or expand the applications into higher pressure areas than the traditional very low pressure distribution market. Nevertheless, PE100 is not nearly as stiff nor strong as steel or ductile iron, and I believe e.g. if enough shear load is applied across a connection between the two I have a sneaking suspicion what material would fail first. Additionally, up until at least very recently, I believe there has been very little use of this at least some higher stressed pipe in the USA. See the caveats explained in the aforementioned linked document including,

“Keep in mind, for now, a U.S. Department of Transportation (DOT) waiver is required to use CONTINUUM DGDA-2490 and DGDA-2492 Bimodal Polyethylene Resins for natural gas pipe, in order to obtain the 45% pressure rating advantage PE100 resins offer over PE3408 resins.”

And in more fine print at the very end,

“NOTICE: If products are described as “experimental” or “developmental”: (1) product specifications may not be fully determined; (2) analysis of hazards and caution in handling and use are required; and (3) there is greater potential for Dow to change specifications and/or discontinue production.”

While “rocker pipes” are not required per e.g. AWWA standards for steel or ductile iron pipes, it is true some designers take advantage of the availability of modern flexible joints and variable fabricated lengths at least for such piping in this form, at least where there might be significant relative settlement or seismic etc. movements between the structure and the exterior soil/embedded pipes. Such structures in effect act as structural hinges to absolutely minimize extra beam loads and strains on pipe barrels and joints.

It is my opinion that in such cases some effective means of minimizing extra shear and bending duress on the pipe barrel or shear penetration/joints (otherwise not often specifically three-dimensionally “designed” for, nor covered in e.g. any AWWA standards?) could in general be argued advantageous, regardless of the pipe material employed.

Everyone have a good weekend.