(2) KernOily (Petroleum)
26 Feb 02 12:42
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?
P. J. (Pete) Chandler, PE
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA