Can Large size but welded valves be welded together without a pup piece in-between them

[checkparvez]

There is a case of two isolation valves in a control valve station and the end connection of valves are butt weld. Is it mandatory to have a pup piece between them or can the valves be welded together direclty. Please share your experience in this case.

 

[DekDee]

checkparvez,
Pipe pup pieces are added for various reasons - the main one is the properties of the valves may be different to the run pipe so a compatible pup piece is added so when the purchaser receives a valve it is easier to be welded into the line with existing WPS's.
The other reason may be clashes between valve handles - if they are too close you may not be able to open/close one of them.
IMHO if they are both of compatible materials and you have a qualified WPS then no problem with welding them to each other,
Cheers,
DD

 

[BigInch]

Not advisable. Too much weld heat may damage a valve and welded together would only complicate repairs. A pup also allows a bad weld to be made, while still being possible to cut out and redo it. A pup would also allow installation of a bleed off between, to vent or drain if desired, should both valves be closed. A pup might also be a good place to locate a support between both valves. Larger valves can get quite heavy.

 

[DekDee]

Big Inch,
If you are worried about heat input what is the difference between welding a pup piece to the valve and welding a valve to another valve ?
Surely the valve will provide a better "heat sink" and absorb heat better than a short pup piece.
Cheers,
DD

 

[pennpiper]

The question:
"Can Large size but welded valves be welded together without a pup piece in-between them?"

The answer:
Yes

Sometimes its possible to do all the right things and still get bad results

 

[BigInch]

DekDee With no pup, you might have two rather large valve wall thicknesses to weld together rather than only the one, probably thinner, pup wall weld to the valve end.

As I seem to say here quite a lot, just because you can do it, doesn't mean it's a good idea.

 

[LittleInch]

Is it mandatory = no. Is it a good idea = no.

As the connection won't be the weld under pressure, you could always weld a WN flange onto both valves.

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

 

[racookpe1978]

Well, yes - Big Inch, both of the two valve bodies will have larger wall thicknesses than the pipe wall, but the two weld "prep areas" on the two valves should neck down to equivalents of their mating pipe wall diameter and thickness.

Thus, since each end of each single valve will be equal in weight and wall thickness of its matching pipe, the two valves should logically be able to be welded together. Weld pre-heat and weld PWHT and metal heatup rates should be calculated by the valve bodies, just like the weld of valve to pipe is calculated.

But, unless space constraints make it absolutely necessary, my instincts and observations say "Don't do it. Unless you have to."

 

[BigInch]

I really don't care if you have a CFD that says it'll work, or if you've been doing it for 100 years with no problems. Look it's fine with me, if you want to weld those two valves together. Just saying (again) that just because you can do it, doesn't mean it's a good idea, so don't do it on my projects. Try prying them apart when the welder messes up.