Necessity of "pup" pieces for a fitting to fitting weld. 5

[gbratis]

Hi everybody,
Is it permissible to weld an elbow to an elbow without the use of a pup (spool) piece during an ASME B 31.4 pipeline design.? A client commented " Elbow 90 degree cut through 45 should have min. 0,5 m pup piece prewelded on to bend at workshop/ factory." Is the clause valid.? A general rule of thumb indicates a 1/2xD pup piece. Could someone assist me on the source-code from which the necessity of such a pup pieses is clearly stated.? Does it have to do with the welding heat effecting zone of the circumferential weld joints

 

[BigInch]

Its not a code issue. Its a line-up and welding issue. My rule of thumb is 1/2 pipe diameter, minimum 6", straight length between fittings.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[moltenmetal]

I am not familiar at all with B31.4.

Perhaps I don't understand your client's English, but what I hear them saying is that you can't cut part of a 90 degree butt-welding elbow, grind a new bevel on that part, and weld it directly to another fitting without an intermediate piece of pipe. I can see that due to potential ovality between the finished ends of the elbow. The same issue might be encountered if you attempt to weld too close to a bend.

If we weren't permitted to weld butt-weld fittings to other butt-weld fittings directly without welding pup pieces between them, we'd be in serious trouble! It seems we never have enough room the way it is- and the welding and NDE is expensive enough without essentially doubling a significant portion of it!

 

[BigInch]

MM, true, it can be done, but if there's any way possible to avoid it, most clients will at least make a negative comment.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[pennpiper]

This is simply a case of bogus logic that seems to come up from time to time, put out mostly by "inexperienced experts" from some other discipline.
After more than 45 years in the piping game and some of those years as the Engineering Manager in a pipe fabrication shop I say:
--You can/may weld an elbow to another elbow WITHOUT a pup piece.
--You can/may "trim" an elbow (to a different angle) and weld it to another elbow WITHOUT a pup piece.
--There is nothing in the B31 codes that says otherwise.

 

[BigInch]

That is absolutely correct. It is permitted by all codes I know of.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[pipingdesigner]

It is permissable to weld elbows on a fitting to fitting basis, the problem of minimum length only arises when you actually NEED a pup or spool to suit the design.

The bare minimum you can make a pup can be determined through a simple calculation.

L = 4xt

where....

L = the length of pipe between the edge of welds
t = the pipe wall thickness

Note - as this formula gives you the distance from the edge of the welds, you'll have to do a little trigonometry to determine the edge of weld to center of weld dimension.

Of course, there are many other rules of thumb:
- 1.5dia or min 150mm (6")

If you are using a pup and are woried about heat transfer along a short pup during welding, the trick used was to make the first weld to a longer piece of pipe, then cut the pipe to length, then weld the second elbow on. That way you get a "normal" heat pattern in both welds as there's more material to carry the heat away.




pipingdesigner

http://www.pipingdesigners.com

 

[Zapster]

Welding fitting to fitting has some implications in the SIF. I read somewhere that there was a change in the last version of B31.3 regarding fitting to fitting SIF; however, the original post deals with B31.4. I agree with pennpiper about the ability to weld fitting to fitting.

 

[Hench]

gbratis,

Are you talking about purchased LR 90deg elbows or 90deg bends?

Hench

 

[Cockroach]

PipingDesigner is absolutely correct, there is nothing in the code. The issue of pups arises by end user conditions and possible flow implications.

For example, gate/ball valves use pups on pipelines in an effort to keep circular welds a certain distance from the heat affected zone. Otherwise the risk of burning out the soft seat elements during installation would render the valve assembly dysfunctional.

It may also arise from basic design principles, a pipline installation gives a prescribed gap for future placement of a valve assembly. This is common in block houses for example, I may wish to place an orifice fitting inline at a pre-specified point. The pup is a way of making up length, not too dissimilar from a spool in this case.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[nycpipewelder]

As a pipe welder myself in plumbers local one nyc I have never been called on welding a fitting to fitting. My experenice is on new construction high rise's and municipal projects

 

[Zapster]

The following references were from Ken A. Nisly-Nagele on another site:

B31.3-2004, Table D300, Note 13: SIFs for branch connections are based on tests with at least two diameters of straight run pipe on each side of the branch centerline. More closely loaded branches may require special consideration.

B31.1-2001, Table D-1: No note similar to above. (I couldn't access 2004 edition, perhaps it has been added.)

B&PV Code-2004, Fig. ND-3673.2(b)-1, Note 10.(c): Similar note to B31.3 but in terms of arc distances in longitudinal and circumferential direction.




Has there been a change in the notes for SIF in the B31.4 2002 issue? If not, there may be some coming in the next issue regarding this.

 

[zuccus]

If you couldn't weld fitting to fitting, most of the offshore skids I have worked on would double in size. In matter of fact depending on what industry your in and what material your working with it is prefered to save a weld.

On the same note though we try to avoid it unless neccessary. For some reason lately we have a couple of cases where elbows have come in a little out of perfectly round but I believe still within tolerance. Then we avoid welding these elbows together. This is in low pressure water service.

We even had a couple come in quite a bit out of wack. I have never seen this before. Those we have sent back.

Zuccus

 

[legs]

I think I see two or maybe three different conditions being addressed here. Zapster is talking about a 'branch connection' which does require certain distance between welds. The original question was regarding welding fitting-to-fitting. This is not the same thing. There is nothing wrong with fitting-to fitting welding. It actually can be a smart choice if you have limited space or you want to reduce the number of welds. Why would you add a pup (and its' welds) if you do not need it? As for the question concerning the heat deformaties caused by the welding, you are going to have the weld and its associated heat whether you have a pup or not. You will have the weld temp at the end of the elbow and whatever it is welded to anyway. Now, if I have misunderstood and you are speaking of bending pipe, it is still a different answer. When bending pipe you need to leave a straight piece for the machine to grab on to the pipe. In which case, yes, you would need to leave a straight piece per the fabricators recommendations. As there may be different requirements for different machines, I would check with the fab shop for their needs.

 

[BigInch]

On a pipe cold bent "fitting", leave at least D/2 straight length on each end to avoid welding into a possible ovaled area.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[ornerynorsk]

Avoid mating an elbow trimmed to less than 90 to a perfect 90 elbow, just trim both elbows so fit-up is equal. This will smooth out the flow, eliminate restriction and turbulence.