HDPE to cast iron adaptor

[Torm]

We are planning to install an HDPE fire protection pipeline and there are several tie-ins to existing old cast iron pipe. I've been assured this is cast iron, not ductile iron. The engineer's information indicates the existing pipe conforms to the standard ANSI A21.6, which appears to be an old standard. Pipe size at tie-ins in 10" and 8". There are ductile iron adaptors that use a restraint similar to the 'megalug' style, which have set-bolts that bolt down to the outside wall of the pipe, but I am concerned in using this type of fitting, for fear of snapping the cast iron pipe. Any ideas would be appreciated on a fitting to use for these connections.

 

[MiketheEngineer]

I've seen rubber "collars" used that have circular clamps. Usually used on drain pipes which have little pressure. Not sure they would work in your app.

 

[stanier]

Be very careful with legacy cast iron. Experience has shown that the cast iron can graphatize and be very brittle. The risks for a fire protection system are immense.

In theory the ductile adaptor should work OK if reinforced with thrust block. The downside is the legacy cast iron hasnt read the brochure and will bite you on the backside.

Design very conservatively and then add more. Get the supplier involved with the client and all agree on the approach. Do a risk study and document the agreement. If the fire system fails then the consequences may be horrendous. So in risk terms likelihood of failure moderate to high consequences ? Your guess.

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

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

 

[rconner]

I agree with stanier that it is a good idea to learn as much as possible about the existing system. Due to the reported use of gray cast iron pipe, the existing system is likely 40, or even far more, years old. While this in and of itself does not mean it is in bad condition, I would also include that the type of joints actually employed in the old system (perhaps unseen when the connection point is dug) could nevertheless also now have bearing on effective design of such transitions.
Depending on how hard the polyethylene will tug on the old pipe, as a result of whatever thermal cooling/shrinkage and pressure-related Poisson shrinking and/or recoil etc. actions that will result from the actual installation and service conditions, and particularly if the old system is composed of unrestrained joints, just restraining the transition adaptor joint may not be sufficient to prevent problems caused by the polyethylene in adjacent still buried joints/piping. Therefore, external anchorage, perhaps e.g. by thrust collar/wall and most helpful on the new hdpe side to fairly keep the extra loads off the old iron, is something that might also be included in the design tool box.

 

[77JQX]

Stainer and rconner are right on. The thermal and poisson forces from HDPE can be huge. If this was my job, I wouldn't transfer any forces to the cast iron, and instead anchor the HDPE independently as suggested by rconner. Since the cast iron is existing, you shouldn't need to add any restraint to it, unless you disturb its thrust restraint with the new construction.

 

[racookpe1978]

Failure can come from fire (loss of fire protection water pressure and flow - when needed) and from flooding (excess fire protection pressure and flow when and where NOT needed).

If your fire protection lines are dry (pressurized up upon an alarm, then you reduce the flooding potential from a crack in the old iron, but create the risk of a water hammer/water surge cracking the system when it starts up. If always pressurized, then the flooding risk is always present.

If a fire happens, will the plastic be protected against sagging, melting adequately?

 

[stanier]

Thermo plastic material can be used for for fire sprinker systems in buildings up to 4 storeys in most jurisdictions. The material of choice is PVC-C. The advantage of this material is that the legacy galvanised iorn or steel would rust in a wet system. When it came to be used the rust would clog the sprinklers. PVC-C has to be installed in the ceiling cavity.

http://www.blazemaster.com.au/index_flash.htm

As for buried pipelines PE is commonly used, even in refineries. It has advantages over ductile iron and steel. This is particulalry so if sea water is used for fire fighting. Inr espect of waterhammer the PE presents a lower celerity and hence lower transient pressures. That said, the PE pressure rating is somewaht lower than ductile iron and steel.

Any system can be poorly designed if the engineer does not know all the requirements of codes and design practices. Fire design is something Is teer clear of because the risks far outweigh the rewards.

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

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