Steel casing inside a steel casing - good idea or bad?

[cht13er]

Question 2/2 for today!

Here's the situation: we've got a CSP carrying sewage inside a steel casing. The CSP is old and CCTV shows that the ceiling of the pipe is completely corroded out - but that the steel casing may or may not be OK to stay.

In order to be safe (e.g. sections of the casing pipe that we can't see that are corroded or weak), we'd like to propose a new steel casing inside the existing casing, and inside that new casing a PVC carrier pipe. Due to the mannings coefficient difference between CSP and PVC, we can drop down a pipe size with the new PVC with slightly better capacity than the existing CSP. As a result, there should be enough room for a new casing and a new pipe to fit inside the existing casing!

Have you ever heard of this being proposed/implemented? What issues do you anticipate, and what negative effects should we consider?

Thank you very much for your help!
Chris

 

[rconner]

I have noticed noone has replied to your inquiry after many days. It might therefore help to provide a little more information about your application (e.g. what was purpose of original casing, what is above same, etc.) Not knowing the condition of your casing, you nevertheless may want to consider that there could conceivably now be voids around the casing, that could arguably cause future problems, of course depending on their extent and the specific crossing details involved etc. Finally, you may want to make sure that you are assuming a flow coefficient of the reduced internal diameter that is proven realistic for long-term service in the sewage application.