Why is seal-welding of screwed connections not allowed?? 4

[KernOily]

OK, here's a question for y'all. Most owners do not allow the back-welding, or seal-welding as it's sometimes called, of threaded (screwed) piping connections. I am trying to find out why. These are the reasons I've found so far, besides the obvious: (i.e. the threads decrease the amount of surface area for the weld, since a backweld is a fillet weld and not a groove weld, and the metal thickness is decreased substantially, which may be an issue for corrosion, pressure, or fluid velocity):

1) If a threaded fitting is fully made up tight, and the connection is then seal-welded, the differential thermal expansion of the pipe fitting relative to the pipe due to the heating from the weld can cause enough stress in the weld to actually cause a crack in the weld. These cracks are not usually detectable by eye and have to be found using liquid-penetrant or magnetic particle NDE methods. This cracking is also the reason why socketweld fittings are fully pressed together and then backed out 1/16" to allow for expansion of the fitting relative to the pipe.

2) At the Chevron refinery in LA, they found that many seal welds had been made on screwed connections that had pipe dope or Teflon tape in them. During welding, the tape or dope had melted, flowed out of the threads, and mixed with the molten weld metal thus contaminating the weld and reducing its strength and sealing properties.

3) At the ARCO refinery in Carson, they found that many existing screwed connections had been made up by screwing together just a few threads (i.e., just enough to hold the two pipes or fittings together in place) and then seal-welding the connection. This leaves a thin cross-section of the pipe, i.e. at the thread root diameter, outside of the weld (recall that pipe threads are tapered). Seal welds are not strength welds per se and the weld should not be considered as a part of the strength requirement of the connection. The ARCO refinery x-rayed all of their seal welds and if the thread engagement was not up to snuff, they took the fittings off and replaced them with socket weld fittings. It was amazing how many had to be removed.

OK, having said all that, it's my understanding that seal welding is acceptable under some conditions. For example, NFPA 58 - 1998 (the LPG gas code) and Cal OSHA allow seal welding for some product service.

OK, what do you all know? Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

 

[butelja]

All 3 of the reasons you have listed are indeed valid. A critical 4th reason is as follows. It is very easy to have some undercut in a fillet weld if a welder of moderate (OK - poor) skill does the work. The exposed threads already present a big stress raiser, but if they are partially reinforced by the fillet weld and then the remaining exposed threads are further undercut, the effect of the stress raiser is far worse than an ordinary screwed joint.

For what its worth, the stress raiser and weld contamination issues are the only 2 I've commonly heard in the past. Also, if you do not want the system to be disassembled, then why start with screwed joints? Use of socket weld fittings will save labor (cost) compared to back welded threaded connections.

 

[Hush]

One reason I've used (could be so much bull but there's a certain logic to it) On a tapered thread the seal is formed by yielding the flanks of the thread to form a metal to metal seal. If you heat the threads enough to relieve the stress you lose the seal.

 

[hazard]

Threaded connections are allowed on low pressure systems only, the main advantage over socket weld (another alternative to full penetration butt welded piping in low pressure service )is that the system can be demolished and rebuilt without adverse effect on the variuos piping components. Maybe not a significant consideration as far as pipe and fittings are concerned but when PCV´s and other threaded inline appendages are involved seal welding can cause considerable expence and excessive time delays because when components are seal welded after installation the only way to remove them is to cut them out.

 

[TD2K]

The reasons I've heard over the years why seal welding is typically frowned on matches Pete's and the other comments.

One other reason I've heard is that seal welding a threaded connection, versus using a SW connection initally, sets up a good crevice corrosion cell. I would have thought a similar, though lower, possibility exists with socket weld fittings also.

I'd disagree with Hazard's comment to the extent that many companies prohibit threaded connections in hydrocarbon service regardless of the operating pressure unless you have a welded or flanged upstream isolation valve and many frown on threaded connections even then unless used for instrumentation leads, etc (drain caps not withstanding). And, when I spent a few months with Total, they didn't even allow threaded drain valve caps, their central engineering group and safety people were pushing flanges if the piping spec didn't allow threaded connections (that was still a hot issue being debated when I left).

When I first got out of University, the plant I was working in had a big survey for any threaded connections in HC service and were seal welding them during the next S/D. This came from a couple of serious fires DOW had experienced due to leaking threaded connections in HC service.

 

[ralphsare]

can't get away with a threaded joint and the Code realizes this. why because its a standard in a lot of the mechanical equipment and instrumentation - pumps, vessels, reactors, thermowells, PI, TI, etc.

i dont follow the hazard previously stated either. if the plant is built to Code requirement then surely everyone one knows the limit of the threaded connection - the condition is spelled in 314 of B31.3. Seal welding frowned upon? its allowed under the rules of the Code - 311.2.6.

If youre using seal weld to add strenght to the joint then youre using it for the wrong reason. but if the intention is to provide tightness againts leakage - sure its allowed. most especially if your trying to mitigate leak of hazardous material.

stress raiser? not if you followed 314.
what about if you need to break the joint? no sweat, get a hot work permit and ground of the weld. replace damage threaded joint and re-weld. no big deal. Seal weld is not the same as doing a socket weld. different weld procedure and qualififcation.