What are spacers in casing pipe FOR? 2

[cht13er]

Question 1 of 2 for today...

We are proposing a 250mm (10") dia. PVC carrier pipe inside a larger steel casing. The railroad specifies that the casing should be 100mm (4") larger than the pipe.

Are spacers required? If so, why? All the Googling I can do says that they're for support and to electrically isolate the carrier pipe. In this instance, it's PVC pipe, so that's not an issue - so why not just let the PVC sit on top of the steel casing?

Thanks for your help!
Chris

 

[rconner]

Am curious, what is the service of the pipeline?

 

[cht13er]

My apologies, that's a very important consideration!

The PVC will be carrying gravity sewage.

 

[bimr]

One reason is that the casing can not be installed accurately at the required grade.

Spacers also allow the pipe to be installed easier as they slide in with less resistance. Casing spacers include low coefficient of friction runners on all surfaces making contact with the casing pipe which facilitates easy insertion of the carrier pipe into the casing, an especially important feature when making long pulls/pushes with heavy pipe.

If the pipe is off the bottom of the casing pipe, it is easier to see a leak.

There will also be less corrosion of the casing since the casing pipe will air dry rather than being in contact with trapped water around the piee.

 

[cht13er]

Excellent, as always!! Thanks so much!

 

[rconner]

bimr has given some good comments (and I suspected initially line was probably gravity sewer as you eventually confirmed). There are actually many functions of skids or spacers, including particularly in the case of plastic pipes and joint designs to minimize placement and localized stress damage to such plastic piping.
Just to add a little to what bimr has said, let’s just say you were however to somehow get pipe properly installed initially, pretty much to “line and grade” without a lot of damage, and as you say “sitting on the bottom of the casing” without spacers. If (or when?) water infiltrates whatever end seals the casing might have and gets between the casing and carrier, would it not be true that a lightweight pvc line like this could well at some point "float" (according to Archimedes), and at least the middle part then end up about 4" higher than you think it is or ought to be?? Taking such concerns a wee bit further, and even if somehow such substantial movement were not to cause a structural or joint problem with the gravity piping, Sec. 6.9.1.1 FOG Problem of the ASCE/EPA Sanitary Sewer Overflow Guidance Document posted e.g. at

http://www.pdhengineer.com/courses/en/EN-5001.pdf

and other sites indicates there are further real life issues to some sag and ponding-type appearances, in that fats, oils, and greases (FOG’s) are even reportedly preferentially attracted to some plastic/oleophilic materials e.g. PVC pipe, and can accumulate due to such actions and vertical undulations of such lines.

Finally, though I guess gravity sewer might be argued not within its purview, I thought skids or spacers were at least one time required by AWWA manual M23 for at least "water" pipe?