Socket welding underground threaded pipes

[engr2GW]

Hello all,

would there be any problems to watch out for is socket welding threaded pipes to be buried?

 

[StoneCold]

If you have not made the pipe already, you should use socket welded fittings and not weld threaded fittings. Then you should probably have a coating on the pipe and fittings to minimize corrosion. Threading the pipe makes it extremely thin, espectially on sch 40.

My two cents

Regards
StoneCold

 

[metengr]

It is poor practice to socket weld any threaded connection. Why? Because you have an inherent stress riser along the edge of the weld - the thread root.

 

[engr2GW]

Thanks everyone,
So it is generally better to socket weld it (without threads) right?
ALSO
do I have to worry about corrosion coating if this is for dry gas service or oil/condensate. What about produced water.
FINALLY
B31.3 made some confusing comments, can you please help me clarify
in the latest version of B31.3
Para 311.2.5: it says use it but avoid corrosion and sizes > 2"
Para K306.1.2: it says don't use it for fitings, bends, and branch connections
Para K308.2.2: it says don't use it for flanges, blanks, facings, and gaskets
Para K311.2.3: it says don't use it for weld joints

It almost sounded like use it but don't use it...can anyone help clarify the "not permitted" parts please

Thanks a lot.

 

[Duwe6]

First, ignore all the K3xx citations. The "K" section is for extremely high pressure -- above Class 2500# flange allowable pressures.

Unless Crevice Corrosion is a problem in your service [rare] there is no preference between socket-weld and butt-weld fittings, in smaller sizes.

3"NPS and larger socket-weld fittings are available, but unless it is impossible to hire a decent pipe welder, buttweld those larger sizes. There are arguments for both socketwelding and buttwelding 2"NPS. I currently prefer buttwelding.

Threaded pipe can be readily "backwelded", and turned into a socketweld connection. But it is expensive and time consuming. If this is an existing threaded connection, it will have to be disassembled and both the male and female threads cleaned of all the pipe dope [thread sealant]. Then all malleable iron fittings must be discarded and replaced with forged steel. Then the weld is made, and ALL the exposed threads must be 'buried' with weldmetal so bring the pipe back up to full wall thickness.

Just buy Class 3000 socketweld fittings made to SA-105. Socketwelding is actually faster than threading, if you have a good pipefitter and a good welder.

 

[MintJulep]

The coating is on the outside to attempt to reduce corrosion from water in the soil.

Depending on a whole bunch of things that I don't fully undestand you may need cathodic protection too.

 

[engr2GW]

Thanks a lot for the useful information.

 

[engr2GW]

@ Metengr:

could you please explain how the back welding would cause "an inherent stress riser along the edge of the weld - the thread root" please.

Thank you.

 

[Duwe6]

Assuming that all the exposed threads are buried in weld, there is no stress riser.

Any exposed threads have a sharp, 60° included-angle stress riser at the thread root. Bad thing to have.