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HDPE-Ductile Iron thrust blocks

HDPE-Ductile Iron thrust blocks

HDPE-Ductile Iron thrust blocks

I just found a similar topic regarding HDPE here:

We are installing 3200 ft of 8-in SDR11 HDPE pipe.  It will connect into a 10-in ductile iron pipe.  Fluid is water (groundwater).  Pipe will be buried minimum of 24-in below surface.

I'm just starting on reading through the links in that post.  But it appears that HDPE does not require thrust blocks. But, I assume a thrust block will be required at the point of connection between HDPE and ductile iron.

I did find "Thrust Restraint Design For Ductile Iron Pipe" by DIPRA (http://www.dipra.org/publications/design.cfm).

Anyone have any additional advice they would like to share in dealing with a HDPE-Ductile Iron transition?

RE: HDPE-Ductile Iron thrust blocks

Correct, the DICL needs thrust block but only if there is a change of direction at a tee of elbow. Flange the PE to the DICL if it is in a straight line and restrained by good compaction or pipe supports.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

RE: HDPE-Ductile Iron thrust blocks

Thanks.  The 8" PE pipe will most likely connect to 10" DI by "T" type connection.  The water flow direction in this section of 10" DI can change, otherwise, I would go with a Y connection.  The DI pipe is part of a loop for collecting water from water wells.

RE: HDPE-Ductile Iron thrust blocks

While welded/fused joint hdpe pipe is in effect self-restrained, as is a ductile iron system with restrained joints, it is quite different from the standpoint of potential movements.  Pulled sections of hdpe pipe sometimes exhibit substantial end movement (in effect a sort of “recoil” or memory behavior) for some time e.g. after some HDD pulling installation.  Additionally, Poissons ratio of plastics are generally higher than metals (resulting in more shortening of the pipe string under pressure?), as is the coefficient of thermal expansion (and perhaps more importantly in some practical applications, contraction?).  As can be seen from ratio(ing) of comparative values shown on page 17-24 at http://www.acipco.com/adip/products/Sect17.pdf, the coefficient of contraction of hdpe piping in response to thermal changes/cycling also can be roughly 15-20 times what it is of steel or ductile iron piping (or for that matter relative to reinforced concrete structures/rock encasement etc.)  All of this makes for significant potential for subsequent end (and/or sideways if not strapped down or backfilled) movement of hdpe, and for that matter also may create issues involving any transverse connections to the piping (such as services etc.) in the area.  I thus believe at least some hdpe manufacturers claim they can provide structures (similar at least in principle to welded-on thrust collars used for steel and ductile iron piping?), that would ostensibly allow for independent anchorage of their piping vs such movements e.g. by the Contractor with transverse concrete thrust walls.  
I would think alternatively asking e.g. ductile iron pipe manufacturers to increase the cost in their piping quotations by providing extra thrust collars or restrained joints purely to control the movements inherent with the specification and adjacent use of their competitor’s (hdpe) piping could conceivably involve questions of engineering ethics or fairness; however, in any case it would at least be responsible/courteous if this is done to advise the ductile iron folks exactly how and with what magnitude of forces etc. the polyethylene piping system will be pulling or pushing on their piping/joints.  There are apparently other perhaps non-obvious issues that could be potentially involved with connecting to, repairing, or cutting into hdpe pipe systems, e.g. as explained by a large coupling manufacturer at http://www.smith-blair.com/PDF/Water%20PDF/high%20density.pdf.  Finally, while I suspect there has probably been some utility to buried flanged connections of hdpe e.g. to metal or ductile iron piping as I believe was one means suggested earlier in this thread, there could conceivably also be some sort of issues with this as well as I heard many years ago third-hand of some problems at such connections involving the hdpe.  I hope this information aids however in better understanding of this issue.    

RE: HDPE-Ductile Iron thrust blocks

On a recent project we joined a new 16"(OD)HDPE watermain to an existing 12" DI. At the connection a thrust collar was welded to the HDPE and a concrete thrust block was poured   around this collar. This was done to ensure there was no axial movement at the HDPE-DI connection. The connection was done using a welded stub-end and DI back-ring 16" to a 16"-12" reducer Flange x Hub and then the existing 12" was tied into that. I would recomend the installation of a thrust block at a connection to DI.

RE: HDPE-Ductile Iron thrust blocks

The HDPE should be connected to the DIP with Harvey Adapter fused to the HDPE the a MJ solid sleve and a megalug on the DIP side. Joint restraints on the DIP should be calculated as though it is a dead end capped piece of DIP.

RE: HDPE-Ductile Iron thrust blocks

It would appear that this last post is alternatively suggesting that perhaps a good many ductile iron pipe joints be restrained, maybe only for the purpose of controlling intrinsic movement or recoil of the hdpe piping end. While I guess it is possible this might work, if let's say larger diameter pressure piping is involved I think this could at least involve some significant expense.  If this is an all new installation, maybe it might make more sense to instead install only ductile iron pipe throughout. If only ductile iron pipe were used, it would appear this ductile joint restraint, "as though it is a dead end capped piece of DIP", expense to the Owner would be unnecessary.      

RE: HDPE-Ductile Iron thrust blocks

Alternatively you could bypass the DICL altogether and take the PE all the way!

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

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