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Hello there, I would appreciate your thoughts on this one. There is a rule of thumb that says that you should not make a straight pipe run smaller than 1 diameter, in fact that is one of the first things you learn, no one tells you that you do that to avoid HAZ and SCC. Now most people knows that with bigger diameters you don’t have to follow that, some says that you can use half the diameter from 14" and above. In my experience that is something that everybody knows but cant support with codes, when I was working in Fluor some guy said to me that there was a British code that states that the smallest possible pipe run between two welds is 5 times the wall thickness, I personally think that is too little.
What I thought was to check in ASME B31.3 for miter bends, I did, please check figure 304.2.3 in page 21, there you get the formula to obtain “m” dimension for the small radius, therefore you get a dimension backed up by B31.3. Now I’m not sure if that applies for sour service (guess I’ll have to check NACE) someone told me that it doesn’t matter because you can solve such problems with PWHT, I don’t think so, specially on site.
Why all the fuss? There are some old guys in Pemex (my current client) that object to some design I made, they say that you should follow that rule of thumb of one diameter “no matter what”. The thing I like most about this job is that the phrase: “no matter what” haves little value” (as long as you can back up what you do with codes).
Any thoughts please?
TIA
Cheers!
Eliut
Well if I understand your question you're asking what is the min. distance between welds!?? ...I worked at "mother Fluor" and they told us it was 3", no matter the size of pipe ...and was pointing at mitered elbows at the time. I later worked with a welding engineer and he said 3" was a good "rule of thumb" ...BUT if you were fighting 1/16" of and inch you could squeeze that down to 2" ...without post heat treating the weld ...then it didn't matter.
However I'm sure there are probably company standards you may have to deal with, along with some "old hands" that have other ideas. All apart of a piping designers job! *L* GOOD LUCK! ...Mark
EliutBB, This subject has been covered twice before.
See;
"PUP Piece-How long?"
and
"Pup"
Both of these postings have some good answers for you.
My answer for you at this time is that the requirement of "one pipe diameter" is only valid when the third "Rule-of-Thumb" is applied.
[color=darkred]PIPING RULE-OF-THUMB
#1 There is a rule for everything.
#2 There will be an exception to every Rule
#3 The Boss is still the Boss
#4 Remember Rule #1[/color]