Buried weldolet branch in B31.4 pipeline service 2

[processeng2014]

We are considering using 4" WOL fittings to branch off a buried 16" pipeline. The branch will extend 4" piping above ground for future tie-ins from gas/oil fields and will terminate with a blinded valve. The 4" above ground branch will be anchored by concrete.

We have specified all the required parameters for the WOL to be calculated with adequate reinforcement per 31.4

Is is acceptable to use a WOL for a buried branch connection? I have seen saddles required for branches on 31.8 systems for buried pipe, but is this the case for 31.4 too?

Thanks

 

[LittleInch]

From a purely mechanical point of view a weldolet should be sufficient as it incorporates in its design the amount of metal that needs to be replaced. However buried branches like this are highly susceptible to high bending forces at the joint due to movement of the main pipe (thermal and pressure expansion/contraction) with the relatively small branch anchored by the soil or in your case concrete.

Only by examining this branch in a stress analysis program will you find out if you need further strengthening or not. An alternative is to use an outer PE pipe and fill the annulus with gravel or other material which allows the branch pipe to move and greatly reduce the bending forces on the weldolet. 4" from a 16" may be OK, 2" or less often show up as overstressed, but 4" might not. Only be looking at your specific case will you find out.

As gas lines, especially inside or d/s of compressor stations can see temperatures of 50C +, they commonly need to have some extra strength at the joint.

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