Butt weld dims and weld gaps

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17 years 2 months ago - 17 years 2 months ago #8591 by ww
Butt weld dims and weld gaps was created by ww
Recently I was reviewing equipment drawings for a project, and made a correction to a piping dimension. The dimension was from the centerline of a 4" tee to the face of a CL 150, raised-face flange (fitting make-up). The dimension was shown as 7 1/4". I corrected it to 7 1/8".

When the drawing came back, the equipment designer had added a note saying the dimension was correct because he had included a 1/8" weld gap. I've been designing piping for over 20 years and have never done that and never seen it done. I've designed many runs of butt-weld fittings that would not have worked if there were a 1/8" gap between welds, and I've field verified that fabricated runs of butt-weld fittings conform to the fitting charts. I've also never had a checker change my dimensions for this reason nor have I ever checked another designer's work that included an addition for weld gaps.

Does someone have an explanation for what happens to the weld gap between butt-welded fittings? One possibility is that the fittings charts add 1/16" of an inch to the actual dimension to account for this gap. Another possibility is that the gap is offset when the piping is prepared for welding. The butt ends have to be ground down to clean metal and this probably equals the weld gap, so the installed fitting nets out to the reference dimension.

I've gone through all my reference materails but haven't been able to find a clear answer. Any help would be appreciated.
Wayne

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17 years 2 months ago - 17 years 2 months ago #3908 by Jop
Replied by Jop on topic ww, You are identified as a
ww,
You are identified as a "junior." I dont know if this is really true or not. Not knowing which is true and your real level of piping training or experiance makes it a little hard for anyone to address this question.

I will answer it this way.

If this flanged connection is a tie-in point for your piping then the location and the specific 7-1/4" dimension might be of interest to you. However, by your statement the difference between their dimension and your dimension is only 1/8 of an inch. You need to understand that the industry tolerance criteria for 12" and smaller piping is plus or minus (+ or -) an 1/8 of an inch (PFI ES-3). You can try to beat the vendor over the head for an eighth of an inch one way on the drawing and then have the fab shop complete the actual work an eighth of an inch the other way.

If this flanged connnection is internal to the vendors piping package and has no bearing on your connection point then you need to be reminded that it is not your responsibility to check "all" the vendors work. Your job is to make sure all the external connections your company/project needs are shown, identified and properly located.

I hope this helps.

Do it once and Do it Right

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17 years 2 months ago - 17 years 2 months ago #3910 by ww
Replied by ww on topic I stated clearly in my
I stated clearly in my post that I have over 20 years of design experience so I'm not sure why you're wondering what to believe. I presume the "junior" under my name is posted by the forum software and refers to my number of posts, not my experience. In any case, my experience has nothing to do with answering the question.

I appreciate your offer of help, but I don't think you understand the intent of my question. I'm not obsessing over 1/8" nor am I in a quandary about what to do about the vendor's drawings. I was just curious if anyone else had any insight on how the weld gap is accounted for in dimensioning butt-welded fittings. I know that there is a gap between the welds and I also know that designers do not include this gap in their dimension calculations.

I got the answer to my question this morning when I spoke to one of the piping foremen at the refinery. Yes, there is a gap between the welds. This gap is accounted for by the pipefitters and adjustments are made accordingly. If they need to eliminate the gap to conform to the overall dimensions, they can do this by grinding down the end of each fitting by 1/16". Some of the gap also goes away when the weld is made (the two fittings are drawn together).

In some cases, they may leave gaps between the fittings, and make up the difference by shortening a pipe spool.

So the bottom line is: 1) yes, there are gaps between the welds 2) butt-weld fitting charts do not account for these gaps 3) these gaps can be ignored by designers because they will be compensated for by the fabricators.

Wayne

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17 years 2 months ago - 17 years 2 months ago #3913 by 11echo
Replied by 11echo on topic ...I have talked to a
...I have talked to a number of welders about this subject, and come up with a LOT of input, but it boils down to this ...YES welders add a root gap to butt welded fittings, but designers should NOT try to incorporate this gap into their dimensions, for a few reasons. First not all welders use the same root gap ...it can vary from 1/16" up. Also (and this is the big one) the welds shrinks! Welding small dia. piping is normally semi fast ...ie more heat into the work piece. When it shrinks (and they all do) this can be to the point where it eliminates ALL of their weld gap! ...or most of it. This shrinkage is varied depending on type of welding procedure, material thickness, and heat put in the work piece. Trying to calculate shrinkage is almost "black magic" ...just depends on too many variables. SO the bottom line to designers is not to worry about it. As was pointed out there is normally a 1/8" tolerance used in piping configurations and this would cover most fitting make-up situations. My $0.02 anyway …good luck!

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17 years 2 months ago - 17 years 2 months ago #3914 by ww
Replied by ww on topic Thanks for your comments 11echo.
Thanks for your comments 11echo. I agree that in most situations the 1/8" would be inconsequential but there are times when it matters. For example, if you have a run of fittings welded together with no pup pieces (as often occurs at control valve stations), and you accumulate 1/8" at each weld, you could end up with a +3/4" or more difference. This can make a real difference when you're trying to fit piping into a tight spot, or even more so, when your new piping has to align with existing piping (eg a tie-in at an existing flange).

Wayne

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17 years 1 month ago - 17 years 1 month ago #3917 by 11echo
Replied by 11echo on topic ww ...I'm not sure you
ww ...I'm not sure you understanding what I'm trying to say here. Normal pipe fitting come out of the box with a nominal shape for welding, but to weld to ASME standards it is a normal to re-dress these ends to do a "proper" weld ...ie reducing the "standard fitting dimension", AND now add the weld shrinkage factor I stated earlier. ...SO say you did weld a string of fittings together, say 7 fittings, with 1/8" root gap added to each weld. This doesn't mean you’re going to get a dimension 3/4" longer then what the normal fitting lengths would total. In fact would probably be lucky to get half of the extra 3/4".
Bottomline, designers do not "guess" at what/how the welders will perform, just total fitting lengths together and document that dimension. IF a dimension is critical, then state the dimension needed on a string of fitting and trust the welder to do his job and do what ever he has to achieve that. Again, my $0.02!

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