Casing Spacers

[JCB1]

We have an 8 inch PVC water line in a 12-inch ductile iron pipe. The contractor installed the casing and the water pipe without using spacers so the carrier pipe is laying on the bottom of the 12-inch pipe. The owner wants spacers in the carrier pipe. The pipe spans about 14 feet PVC DR-18. Why use spacers ? The contractor does not think they are needed. The pipe is installed already- is there any purpose/ reason to try and put spacers in now ?

 

[rconner]

AWWA manual M23 PVC Pipe - Design and Installation says, "PVC pipe in casings should not rest on bells."

 

[auzie5]

What about pipe with no bells/spigots? We've seen lots of spoolable non-metallic composite pipe pulled through older steel pipelines with no casing spacers. As long as there are no connection fittings on the drag section, the carrier pipe is usually resting on the bottom of the old steel pipeline (i.e. no casing spacers). If it's a longer pull that requires connection fittings, perhaps casing spacers are warranted.

Typically run an aluminum gauge plate first and inspect at least the leading 10m of carrier pipe to confirm no damage during pull.

I can see how casing spacers with runners that extend to fix the carrier pipe in place inside the conduit may be required when there is risk of the carrier pipe "walking around" in the conduit due to operating conditions (e.g. liquid lines with aggressive slug flow?).

 

[cvg]

you can glue pvc. but if you are going to pull it out and reinstall, just use spacers.

 

[bimr]

8-Inch PVD pipe has a bell OD of 11.75-Inch. Bell and spigot pipe may not have been used.

Spacers provide an easy insertion of the carrier into the casing pipe preventing damage of carrier pipe coating. Spacers are designed and tested to maintain continuous and long term support for the carrier pipe and its contents. Casing spacers are used on steel carriers when installed within steel casings to prevent electrical contact and shorting out the cathodic protection.

With the installation already completed, there is not much point to installing spacers. However, if the Owner is a railroad, don't believe the railroad will be persuaded.

 

[JCB1]

It's a straight length pipe -no bells- The pipe is about 14 feet long inside the casing. Joints are MJ outside the casing pipe. PVC is DR-18.

 

[rconner]

This is probably a better situation than if the pipe was riding with basically a discrete point load on the the outside of a Reiber bell. However, while AWWA M23 is silent on such application it does say "skids may be required to prevent damage to the pipe and to provide proper long-term support", and also to "properly position the pvc pipe in the casing". You don't mention what you are crossing (i.e. reason for the casing), but something else not mentioned in standards is often, e.g. particularly in jack and bored road crossings etc. there are likely some variable, and perhaps even highly variable load/support conditions due to a different soil envelope around the carrier pipeline where it is directly in the ditch soil away from the roadway, as opposed to the cased line through a very highly compacted (and with select material) roadbed. While you perhaps don't have the following situation as this is a very short casing, this can be aggravated when there is over excavation below the invert where the carrier enters and exits the casing e.g. for deeper jacking or exit "pits". This can be argued at least similar to the situation where a pipeline enters and exoits any other structure. In an arguably more critical service this reality, born of unfortunate experience, is not lost however by gas pipeline regulators in the Code of Federal Regulation where Sec. 193.321 now requires of plastic pipe:
"(b) Plastic pipe that is installed in a vault or any other below grade enclosure must be completely encased in gas-tight metal pipe and fittings that are adequately protected from corrosion.
(c) Plastic pipe must be installed so as to minimize shear or tensile stresses." and
"(f) Plastic pipe that is being encased must be inserted into the casing pipe in a manner that will protect the plastic. The leading end of the plastic must be closed before insertion." [Translation, if a significant enough shear load is applied it is not going to be the very strong steel or ductile iron pipe that shears, and the specific means and methods on how to accomplish the minimization of stresses to the weaker plastic are not supplied.]
Finally, I'm going to hazard a guess you now have (assuming an empty pipe is not now floating in a casing full of groundwater) more than 3" of space between the OD (9.05"?) of the 8" pvc barrel and the (normally larger than nominal) ID of the 12" ductile iron, and this should be understood.