NPT backwelding
NPT backwelding
(OP)
I have always been taught that backwelding threaded pipe actually does more harm than good. If you cut and make the threads per ASME B1.20.1, then back welding should not be required. The theory I was told was that the welding just creates a heat affected zone and you end up with lower strenght of the pipe and joint.
Is it good engineering practice to not backweld or good engineering practice to backweld 2" pipe in natural gas service?
Is it good engineering practice to not backweld or good engineering practice to backweld 2" pipe in natural gas service?





RE: NPT backwelding
RE: NPT backwelding
David
RE: NPT backwelding
It is not recommended as an engineered design. Where it has been used the welds don't last with process leaks following anyway given time and temperature cycling.
RE: NPT backwelding
Inspection of photos indicate there were less tha 5 rounds made up and the system had the connection in a high bending moment, Because of this recommendation, he wants all threaded connections 1 1/2" and larger back welded.
I'll buy doing a inspection of connections in a similar high bending moments and use a flange connection, beter yet get rid of some of the moments.
RE: NPT backwelding
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: NPT backwelding
Dan
www.eltronresearch.com
Dan's Blog
RE: NPT backwelding
DCASTO lives about 40 miles from me and we've been going back and forth on eng-tips.com for years, we may meet someday, but probably not for work I don't think he needs a lot of what I can offer.
David
RE: NPT backwelding
Back-welding screwed connections has the same effect as not gapping a socket weld properly at fit-up. You end up pre-stressing the weld and leaving it with residual stress. Not desirable in sour service. For screwed valves in sour service, you are better off machining the FNPT threads out and going to a straight socket weld onto a PE nipple.
Regards,
SNORGY.