Cut into an old 15 inch MSCL

[ashtree]

One of our work teams has been tasked with cutting a TEE into a 15 inch MSCL rising main as the first stage in the ultimate replacement of the main. The main was laid in the mid 1950s and is generally in reasonable condition. It is however a critical asset and it has had a few problems in the past so will be progressively taken out of service.

The main is tar wrapped and has a weld along the top of the pipe. The guys who have planned the cut in were going to use a gibault joint and grind the weld back flush with the pipe surface to get a seal with the rubbers. There has been a fair bit of debate about whether the grinding back the weld is the right way to go or not. I am not a pipe expert but as the senior guy in the outfit they have come to me looking for guidance.

At face value grinding the bead off the weld would seem okay, but this may obviously affect the strength of the pipe. I also know that in the past some of the failures have been along the weld, so grinding it back has to increase the risk of failure in the weld at that point.
The pressure probably gets up to about 80psi at times in the area where this cut in will occur.

My preference has been to weld a flange onto the pipe end that is being cut but this will obviously be a specialist job.

Does anybody have experience in this type of work , who may have an opinion.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[bimr]

Grinding the weld flush with the pipe is acceptable and should not affect the pipe. Welding a flange onto the pipe will be more difficult and would affect the pipe more. Note the gibault joint is unrestrained and should be installed in a straight pipe run.

 

[ashtree]

bimr

Thanks for your help. The cut in is in a long length of straight pipe so should not be an issue.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"

 

[rconner]

I believe Gibault joints have been used for decades (and I think most applications most likely successful, when they are used with actual pipe outside diameters/OD's for which they are designed). As others have noted however standard Gibault connections have been unrestrained, meaning you have to be careful if there is any known or unknown thrust focus (valve, reducer, unblocked bend, unblocked dead end etc) near the tie-in, lest without some sort of supplementary anchorage the joints could separate when pressure or valve closure is applied. In any case, good quality and compacted backfill around the sleeve/cut-in is probably adviseable also, as with any even incidental deflection in the Gibault joints they could cause the unrestrained joints to buck sideways and locally leak/separate without same.
As to cutting the old wrapped steel pipe, if this is a welded system the workers need to also be aware that there can recoil or spring-back in random directions axially or laterally due to residual stresses etc. (I think there is a link somewhere in these forums to a rather wild You-Tube video where someone recorded a worker with a cutting torch sort of riding a bucking bronco when the thing cuts loose!) Finally, as to grinding etc. of some old wrapped pipe surfaces smooth it is probably best for the workers to know exactly what they are dealing with as e.g. explained at

https://pgjonline.com/2011/10/25/considering-asbestos-and-old-pipelines/

etc., and follow any applicable safety precautions for dealing with such.

 

[ashtree]

Thanks for your comments.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"