NPT backwelding 3

[dcasto]

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?

 

[ash9144]

If you are going to back weld threaded pipe why would you not just socket weld in the first place? I agree that this is not good practice all you are doing is cutting away metal with the threads then socket welding. I've seen threaded pipe back welded but usually just for existing piping until it could be replaced.

 

[zdas04]

I've never dug into the "why's", but three different major Oil & Gas producers I've worked for have explicitly prohibited backwelding threaded fittings. One of them requires an explicit sign-off by the Chief Engineer (in LA) before you can do it, needless to say no one in the Rockies is going to submit that request and try to explain why they want to do it.

David

 

[hacksaw]

backwelding NPT connections (sometimes called called seal welding) is a field expedient, but it does not address safe or leak free piping design.

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.

 

[dcasto]

Thanks for the replies. So geek in safety placed a root cause on a failure as poor engineering design because the 2" threaded connection failed.

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.

 

[zdas04]

Sounds like you need a Mechanical P.E. with a Colorado license to talk to the Safety Guy. I know one.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[Eltron]

You're shameless, David.


Dan

http://www.eltronresearch.com

Dan's Blog

 

[zdas04]

Totally. Things are tough in the Oil Patch and you have to seize every opportunity for self-promotion.

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

 

[SNORGY]

dcasto:

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.