B31.3 doesnt prohibit such practice. Because it’s not a cook book type of code. It’s not standard industry practice, perhaps, but that doesn’t mean that should hold you from doing it. We do it sometimes on our piping, and on NPS2” and smaller, there’s no real drawback to it, honestly.

Just weigh the pros and cons for your application.

Agree with XL83NL - no impact on the code

The same issue is often posed as a "how small can the pipe pup be", for which there is no answer, only a significant number of opinions - just search on this here to see.

In essence you have the minimum length pup piece physically possible, but how have you got into such as situation? Can't you move one or other of the flanged items closer together or further apart?

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

Nothing wrong with butt-welding any two fittings- including weld-neck flanges- to one another.

Definitely a problem if you have to TRIM either piece to make the length of the assembly shorter, though.

In the end you likely need to achieve a joint that you can inspect by radiography, and this may limit the dimensions you can achieve.

Be aware that you are making an extremely stiff piping piece.

This will not allow any field adjustment by pipe fitters to align for welding or bolt-up....

MJCronin
Sr. Process Engineer

Everyone,

Thank you for responses. They all have been informitive. Over twenty years doing piping I have never done this if we did this we would have a 3" pup piece between to get a good weld. I feel welding back to back we are only really getting a root pass weld. With piping raging from 350 to 400 F this looks like a weak area on the pump flange.

Any who have some thought it would be informitive to tell the client this not good piping practice and go with my orginal design is a stubin then the weldneck.

With this unusual configuration you may find problems with clearances for the studs and bolts ...

Have you checked how this component will be assembled into the finished piping system ???

Please respect us and finish this thread .....

MJCronin
Sr. Process Engineer

I've got 40+ yr.s of piping design behind me and doing back to back weld neck flanges is usually indicates a poor design. I had to do this only once when we were hooking up a 600# system to a 900# and space was limited, and that was it.

What's the piping design application that leads to this setup? Perhaps we can assist with proposing alternate designs that still meet the original intent.

As others have mentioned, there's nothing in code that prohibits such an assembly, but NDE and welding may become more challenging, depending of course on flange size and rating.

The only time I think I've come across this at my shop is for retrofits where existing piping can lead to unusual piping design choices.

Are the two flanges identical, or different in some way? If you're just needing to make up a certain dimension that prevents you from using a pup in between, we've sometimes used very thick bleed rings (with or without taps), and assembled a wafer style joint. This may or may not make sense depending on your application, but at the minimum, it eliminates one more weld that needs to be tracked and inspected.

Agree with XL83NL and LI,
Your previous comments lead me to believe you want a "pup" piece and the client wants two flanges welded together.
Give the client what they want - get it in writing so if there are issues down the track you are covered.
Depending on size of your pipework RT may be more difficult than if you had a pup piece,
Regards,
Shane

I've done this to transition between bolts straddle centerline to bolts on centerline for a new piece of equipment that could not be turned being attached to existing piping. Also to introduce line breaks for future LOTO where the owner wants an air gap to demonstrate isolation.

Have done this for some experimental small vessels for an experimental chemical process. Client wanted vessels U-Stamped. We did.

Thank you every one, this was a losing battle. They don't understand good piping practices and proper welding procedures. "We did this other plants". They don't want to do it properly then I am not going to fight the battle. It will show in Caesar that we have an issue.


Thanks again for all your comments they all were very helpful.