By James O. Pennock and Dave Murphy
The following statement was made in a communication about welding two fittings directly to each other. This situation also applies to a fitting welded to a weld-neck flange or two weld-neck flanges welded together.
“Fitting to fitting welding is not permissible; a pipe shall be (placed) between fittings.”
This is an example of someone giving wrong information or wrongly interpreted information that has gotten started by some inexperienced teachers/instructors in the past few years.
This specific statement is totally incorrect when applied to 3” (80mm) and larger butt-weld fittings. Any butt-weld fitting or weld-neck flange can be directly welded to any other butt-weld fitting or weld neck flange without the need for any extra piece of pipe (Pup piece) between them. It is just a butt-weld and if I cannot make a butt-weld between these two round, beveled-end, schedule 40 carbon steel (fitting) objects what if I need to join two other round, beveled-end, schedule 40 carbon steel pipes. What do I put between these two pipe objects? Yes! It does sound a bit stupid.
For butt-weld fittings the only exception to this is; when there is an overall dimension requirement that is greater than the combined total dimensions of the two fittings.
The only fittings that cannot be connected directly to each other are screwed or socket-welded elbow and Tee fittings. Screwed and Socket-welded assemblies do require pipe, a pipe nipple or a swage nipple between these fittings.
I suspect that someone, somewhere was listening to instructions about the make-up of piping assemblies for 2” (50mm) and smaller screwed or socket weld piping and then mistakenly applied the instruction to all piping including large-bore butt-weld piping.
This is not the first time I have seen this. Over the years I have gotten many sketches via E-mail asking about some certain aspect of a piping layout. Some of these sketches have shown this same faulty piping error. One such case showed a common control valve station (manifold) which had 11 (eleven) extra and unnecessary “Pup” pieces where butt-weld fittings could have and should have been welded together. It was a waste of money.
When this is done at a point where it is not required, it does nothing to improve the function of the pipe configuration. The only thing it does do is add cost to the job. The added cost comes from both a direct and an indirect factor.
The direct cost includes:
the cost of the pipe
the cost of cutting the pipe and prepping the ends for welding
the cost welding
the cost of extra NDE
the potential cost of a bad weld discovered during hydrotest that needs to be cut-out and re-welded.
the cost of additional insulation and heat losses.
the cost of adding another potential leak point even if the weld passes initial NDE and hydro.
The indirect cost includes:
the cost of the added space required for a complex piping configuration when multiple pieces of “pipe” (Pup pieces) are added between every fitting to fitting make-up.
I strongly urge all who read this,
If you are doing this then STOP! Ask yourself, Why am I doing this?
If you know pipers who are doing this then tell them to STOP! Ask them why are they doing this?
Why should I not do this? Because it is amateurish, wasteful and improper piping.
About the Author
James O. Pennock has more than forty-five years in the process plant design profession. He has been involved in both home office and job site assignments on refinery, chemical, petrochemical, power and other projects. His experience ranges from entry level designer to engineering manager. Much of this was with Fluor. He is also the author of the book "Piping Engineering Leadership for Process Plant Projects." He is now retired, living in Florida, USA and does only occasional consulting work.