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pennpiper (Mechanical) (OP)
1 Mar 09 13:20
The following was recently posted on one of the piping forum web sites.
"Fitting to fitting welding is not permissible; a pipe shall be (placed) between fittings."
(Posters name withheld)

This is an example of 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 swedge 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 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,
o    if you are doing this then STOP!  Ask yourself, Why am I doing this?
o    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 is that it is amateurish, wasteful and improper piping.
 
jte (Mechanical)
1 Mar 09 23:11
pennpiper-

Nothing like folks who believe everything they've learned - even if their teacher didn't mean to teach them what they learned.

I'd add a few additional costs:
* 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.

A well written post... I suggest you gussy it up a bit and consider submitting it as an FAQ on this site.

jt
Helpful Member!  pennpiper (Mechanical) (OP)
2 Mar 09 12:02
jte,
What do you mean by this?
"I suggest you gussy it up a bit "
jte (Mechanical)
4 Mar 09 10:36
pennpiper-

Simply that you clean it up a bit to read more like an article, a bit less like a rant (which, in the original context of the post is fine). Remove the reference to another site, etc.

Consider adding a discussion on the minimum seam to seam distance if a pup piece is added - this is certainly related and a relatively common thread on Eng-Tips. Do I sound like an editor yet?

jt
racookpe1978 (Nuclear)
5 Mar 09 15:05
On the other hand, I have had times when I've needed as little as 5/8" gap between the ends of two 8" diameter Sch 40 elbows that had to be "closed up" - in that case, I required a "pipe" be inserted, not just a "looong" butt weld between the two 90 elbows.
IFRs (Petroleum)
5 Mar 09 17:21
I such a situation I use a "long tangent elbow" and trim the tangent to the length needed.
racookpe1978 (Nuclear)
6 Mar 09 11:24
Good idea - I'll keep that in mind.  

And make sure I get enough time in pre-planning before the outage to order the special elbow.  8<)
sjrfc2 (Mechanical)
7 Mar 09 18:30
Just a note regarding welding socket fittings together: Recently I ran across ASTM F722 that shows an arrangement that includes two socket fittings welded together. It uses a short piece of pipe as a backing ring and fills the groove between the socket fittings with weld. I had never seen it before so it took me by surprise. Its Figure 15 in that spec.

Fig 17 also took me by surprise a little as it puts a fillet weld in the curvature of a pipe bend.

Regarding the long tangent elbow I know that I've seen elbows routinely spec'd to B16.9 with a 1/2" length beyond tangent to account for a backing ring joint.

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