Pig trap

[pipe76]

Hello,

I am looking for information about the stresses of the pipe assocaited with a pig trap. I would also like to know how to calculate these stresses and the standards or code in which they are based on.

Thanks

 

[JohnGP]

Stresses associated with a pig trap are calculated from its design pressure, which needs to at least match the design pressure of the attached pipeline and downstream piping. The design code for the trap provides the means of calculating stresses.

Which code? This depends on where the interface is defined between the pipeline to the trap and the downstream piping. If the trap is considered to be part of the pipeline, then it can be designed to ASME B31.4, ASME B31.8, or similar pipeline code. If part of the plant being fed by the pipeline, it can be designed to ASME B31.3 or similar pressure piping code. After all, the trap is basically bits of pipe welded together. You can also treat the trap as a pressure vessel and design to ASME VIII or similar PV code.

 

[pipe76]

Hello,

I am working with the B31.8. Could you tell me where I can find the information.

Thanks.

 

[JohnGP]

Not sure where your problem is so pardon my ramblings, but hoop stress is simply:

Hoop stress=PD/(2t) for each section of the trap - see para 805.233 .

Secondary stress due to beam action and support loads are usually insignificant - just need to check that attached piping is not arranged to exert large reaction forces back into the trap.

Maximum pressure rating can be determined from para 841.11 - stress then will be equal to specified minimum yield strength multiplied by joint, design, and derating factors (but see para 841.111 for limitations). Required wall thickness in each part can also be derived from this formula (and watching para 841.113).

 

[dwedel]

Just courious, why are you using ASME code and not DOT code?

 

[JohnGP]

It's a big world out there. I should have prefaced my response with the fact that my experience with pig traps has been in Australia and SE Asia where (as far as I can recall) traps have been designed to B31.3. ASME VIII has been used for closure design, but I would think trap would only need to be designed to ASME VIII or similar if barrel sections were rolled from plate.

 

[dwedel]

Ah, not US Department of Transportation juristiction. Makes sense. I need to remember that

http://www is

world wide web

 

[1969grad]

Pig traps are considered fabricated piping assemblies and part of the pipeline. Hence the pig trap should be designed to the same code as the pipeline which is usually B31.8 or the equivilent in Australia. Code break to B31.3, if necessary, is made at the side tap valve.

Yes, the closure is usually designed to ASME VIII and the weld end tapered bored to match the pig barrel.

You can design the trap to ASME VIII but you end up with a thicker wall thickness plus other more restrictive issues in Section VIII and hence more expensive.

You can use either code but common practice is to use the pipeline code for design of the launcher/receiver.

 

[JohnGP]

You are right, of course 1969grad. But as I said, my experience has been that B31.3 is also used. It made sense for an offshore platform job I worked on, as the pig receiver was definitely part of the platform piping (designed to B31.3) - the platform and subsea piping were designed and fabricated under different contracts. Thinking about it though, platform piping had to comply with API RP14E (which uses B31.3) and that sets the transition between codes at the first valve onto the platform, which was upstream of the trap.