Butt welded piping fitting to fitting make-up weld gap 2

[Kevin111]

Folks,

In my neck of the woods this has been an age old question that rears its ugly head on occasion. When you have two fittings being welded together, (for this discussion a 6" LR 90 Ell and a 6" 300# RFWN) should the dimension include the required weld gap? Example: 6" LR 90 = 9" + weld gap of 1/8" + 6" 300# RFWN thru hub of 3 7/8" = 13" dimension. The argument is made that the gap "disappears" during welding but some measurable gap remains. As an engineering contractor we have clients that prefer it one way or the other. Just wanted to see what other thoughts were on the matter. I'm doubting this is the first thread of this nature so the simple solution may be to direct me to a previous thread. Thanks in advance for your comments!

 

[nickelkid]

Fitting make-up does not include the weld gap. Weld gap is per the weld procedure. In general, specifying a weld gap will cause nothing but trouble on a piping drawing.

 

[Kevin111]

Robin, We have been doing this for a number of years for numerous clients, in fact some clients have it written into their design specifications. My experience has been that the clients that do not include it in the dimension have had more trouble. One example that comes to mind was a 12" header with several flanged valves and tees. The drawing dimensions omitted the gap (per client) and the final vert. tee was severely misaligned. In other words the fabricated spools, once bolted together, were longer than the stated dimension. Were these off spec fitting? Possible but doubtful. I always argue both sides and try to meet in the middle. That being said, if the gap is in the weld procedure, it is there in real life also. That cannot be ignored. The question becomes, "How much of the gap shrinks away during welding and how much is left?"

 

[pennpiper]

Kevin111
My experience includes many years in the design room and a few years as the Engineering Manager in a Pipe Fabrication Shop. That said I stand with Robin Handy. Do not show the gap and do not add the gap dimension in the overall dimension.

The accumulative error you referred is a different problem and must be taken care of in a different way.


prognosis: Lead or Lag

 

[berkshire]

Give the welder/Pipe fabricator the finished dimensions you want, and let them figure out how to do it.
Weld gap is an item that varies according to the personal habits of the welder, and can vary even by the temperature of the day. Sometimes the gap will close while welding ,othertimes not.
It is far more important to have the welder properly certified for the type of weld, than to tell him/her how to do his/her job.

Robin Handy has it right.

B.E.

"A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government."
-George Washington, President of the United States----

 

[racookpe1978]

However, on the assembly drawing, DO include the 1/8 (typical) gap for the gaskets, but be explicit about whether you are including that gap on which side of the two spool drawings.

 

[Krausen]

I'm in complete agreement with RobinHandy & berkshire. As for gasket thicknesses, I would strongly recommend that the same concept also apply. Talk to the veterans in the industry about this topic, and about every one of them will tell you the same thing - provide the overall dimension they need to achieve, and let them figure out the best way to do it. At my office, we provide the overall dimension, along with other critical intermediate dimensions, and finally a stategically-placed "F.T.D." dimension. This "F.T.D." designation stands for "Field to Determine" (i.e. It doesn't matter, so long as the critical dimensions are met).

 

[Kiwi2671]

Kevin,
Having spent many years as a pipewelder and/or pipefitter before I moved into quality control I agree with all the comments listed above.
Gasket measurements should be included as they are a constant measurement whereas weld gap can change markedly based on welding process, shrinkage rate,shrinkage rate based on material (c/s versus s/s)etc etc.
Hypothetically we have an isometric showing a pipe spool that has two x 300 nb 90 degree elbows, two x 300 nb RFWN flanges, two x 300 nb pipe pieces and 1 x 300 nb Ball valve.
The isometric should show the measurement from the centre of one elbow to the flange face, then the valve length should be shown which includes 6 mm (2 x gaskets)and then the measurement from the flange face to the centre of the remaining elbow.
It should also show the critical measurement of centre to centre of the two elbows (which should be the same as the sub totals combined).
Now the onus is on the contractor to calculate the pipe lengths required based on welding process, initial root gap, calculated weld shrinkage etc and these lengths are then given to the cutting team and hopefully when everything is fabricated it is within allowable tolerances for length,
Regards,
Kiwi

 

[Kevin111]

OK...just for the sake of argument.....and by the way I do appreciate the comments, are we saying that in my example that the fabricator would dress the fittings in the shop to achieve the 12 7/8" dimension of just the fittings? In other words the shop would knock off around a 1/16" from each fitting prior to beginning the weld out. On the gasket comments, that is a huge pet peeve of mine. DO NOT include the gasket thickness in the individual spool length. Why make the shop do additional math, its just another chance for an error to be made. Many thanks for the comments.