Strength Test After Backfilling 2

[302Hugo]

I am trying to find this answer but I can't. I have been recently asked why we backfill prior to doing the strength/leak tests on a CSA pipeline. I can't recall the definitive answer but all I can remember is that is the way it is. My colleague is saying we would be able to find the leak faster if it wasn't backfilled. I did a quick search on the forums and I saw on another thread that people have said that CSA says you need to backfill first and that it "is clear". I did a quick look and all I see is that clause 8.1.3 says that the test shall be completed after the section is in place and before operation, but doesn't say anything about backfilling.

 

[LittleInch]

I can't quote CSA specifically, but the general principle, IMHO, is that you perform a pipeline test in its "completed" position.

The reason being that there are stresses imposed on the pipe due to soil weight and compaction and also the expansion or contraction of the pipe is different when fully restrained as opposed to being effectively un restrained at the bottom of a trench. Add in the issues about solar effects on the water temperature where a difference of a few degrees makes a big impact on test pressure and the end effect is that you backfill and complete as much of the pipeline as possible then test. Final backfill or topsoil isn't required, but the majority of the backfill is.

Strength tests are there to test the strength of the pipeline and find issues, not to leak test them. Yes if there is a leak it is difficult to locate in a buried line, but this does not create sufficient justification to not backfill as your pipeline shouldn't (and doesn't) normally leak, especially when new.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rconner]

It makes sense to me that a pipeline should generally be tested in the condition it will normally serve the Owner/Public et al, be that exposed or buried. While all other points LI makes are valid as well, if a pipeline meant to be buried were instead tested before backfill, it also would be more subject to really unnecessary deformation. Thereafter also, if a successfully tested pipeline was somehow sabotaged, damaged or weakened in an interim or subsequent backfilling operation, perhaps no one would be well-served if no more testing were required.

 

[bimr]

Before carrying out the hydrostatic test, pipes should be substantially backfilled to ensure the pipes cannot move. Where joints are exposed some movement of the witness mark away from the socket may be apparent due to the “Poisson effect” that is the shortening of the pipes under circumferential working stress.

Placing backfill and compacting will place loadings on the pipe. These loadings may cause leaks as well.

 

[BigInch]

It makes no sense to design the pipeline for restrained stress, then test it under unrestrained conditions. Nor would you want to design for unrestrained pipe, then bury it. Stresses developed in each case are completely different.

Restrained (BURIED) Longitudinal Stress
S_L = S_E + [ν]*P_i*D/2/t + M/Z + F_A/A

Unrestrained Longitudinal Stress
S_L = P_i*D/4/t + i*M/Z + F_A/A

longitudinal stress from pressure in an unrestrained line must include consideration of bending stress or axial stress that may be caused by elongation of the pipe due to internal pressure and result in stress at bends and at connections and produce additional loads on equipment and on supports, things not normally present, or present in much lesser extent, in buried pipe.

 

[302Hugo]

Thanks guys, everything said makes complete scense. I appreciate the help.