Is it allowed to have Pig trap with different wall thickness than Pipeline

[Ameen11]

Dear All,

I am in the process of designing a pig trap using ASME B31.8. However, the trap is being designed with C.A. (Corrosion allowance) of 3.2 mm and the pipeline has 0 C.A.
I am not able to find any guideline for designing traps in 31.8 is there an issue of having different ID between minor barrel in main pipeline. will this hinder the pig movement.
thankful for any help in this matter or any reference I should check.

 

[LittleInch]

The ID of trap and pipeline should be the same. If used as a launcher it might not be too bad, but depends on the size of the pipe.

Used as a receiver then this lip could damage pigs, especially expensive intelligent pigs.

You could taper the end or the flange to bridge the gap at 1:4 taper or better.

But why have two different CAs? Makes no sense.

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

 

[1503-44]

The wall thickness of the launcher/trap piping will be thicker than the pipeline because gas pipelines have different DF design factors.

1. It is a fabricated assembly using a minimum design factor of 0.6
2. The design factor could also be 0.5, or 0.4 depending on location class, inside a station, or highly populated area.

I would preferably limit wall thickness differences to the same as those permitted for welding adjacent pipes of different wall thicknesses (see Fig in B 31.8). That usually is only a few millimeters anyway. Use transition pups as required.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Ameen11]

Thanks LitteInch,

I think tapering the end would be the best option.

I will have to check on why it has a different CA.

 

[1503-44]

When tapering the end, make sure that you use the DF of the launcher/trap when you check allowable hoop stress of the taper.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[Ameen11]

Hello 1503-44,

Thanks for your response.

i saw in B31.8 the limitation on end preparation butt welding but i was not sure if this was applicable.


i will make sure check hoop stress of the taper if this the route i go for.

 

[1503-44]

Right, it is obviously not a criteria for pig passage, but it does provide a more restrictive criteria than a pig passage limitation, so it effectively solves your particular problem too.

What is more of a problem for pigs are usually not related to pipe wall differences, probably due to that weld criteria, but more equipment mismatches, or block valves not opening completely, especially on pipelines being modified to permit internal inspection pig passage. Keep an eye out for valves that are not full bore, injection quills, orifice fittings, spectacle blinds, non-full swing check valves, dead end tees at future tie-ins and block valve vents, swing check valves not locked open during a pig run, large diameter outlet openings on tees, especially those without guide bars and of course any short radius ell fittings, esp R < 5D, reducers and those occasional dented pipes.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[r6155]

The diameter of the trap is?

Regards