Socket Welded and Butt Welded Piping

http://cr4.globalspec.com/thread/27366

Socket or fillet welded piping leaves pockets at the joint that can accumulate debris.

This is unacceptable for some types of ultra-pure or hard to clean piping systems

I would add to "GUEST'S" comment some additional practical issues.

Socket Weld: Requires less skill to weld, fittings are bulkier, contamination possibility is higher, fittings can trap contamination from previous fluids if fluids change, more expensive fittings, usually are much heavier ( 3000 & 6000# ratings) and hence more expensive than butt weld fittings.

Butt weld: Takes more skill to weld full penetration, in or out of position etc, much cleaner interior surfaces after welding, fewer crevices to capture contamination, smaller fittings, fittings have same rating as pipe, more compact, easier to nest.

Socket weld fittings are usually higher priced than butt weld but you make up for the extra cost by spending a great deal less labor to install. When someone says that it takes less skilled welders to weld socket weld fittings they are technically correct but you are asking for trouble if you are thinking of doing this. Most companies give a welder qualification test on a butt weld joint in the 6G position so they hire welders that are capable to do it all in all positions. In my 30 years of pipe welding I have never seen a company actually give a welder qualification test on a socket weld joint. I mean who the heck would want to hire a welder so inexperienced that he could only be qualified to weld socket weld fittings? I would agree however that somewhat less skilled pipe fitters could be used to fit and install the S/W fittings than the butt weld fittings although I would not recommend doing this. This is due to the fact that socket weld fittings are more forgiving for out of square pipe cuts, no need to properly bevel the pipe ends and the fact that a really sorry pipe fitter can get by with inaccurate measurements to some extent. There are some services that socket weld fittings should never be used on any size pipe systems IMO. I am talking about services such as high wear slurry lines, Sulfuric and other acids and products where the extra turbulence created by using socket weld fittings would cause the pipe and fitting to wear or corrode and thus fail prematurely. These are the only type systems that I would think one would do better to use butt weld fittings on small bore (less than 2.5" OD) pipe.

The Logic behind this, as I experienced, can be classified into two categories.

One is Strength requirement and the other is smoothness requirement (specially by the process fluid characteristics).

As you know well already, the welding strength of socket welding can not be as it of butt weldng.

Furthermore, because of its shape, when the fluid is erosive or galling, for example slurry fluid or fluid with high hard particles, socket welding can not be permitted irrespective of size.

In practice, nowdays piping bending is getting popular more and more than welding (either socket or butt) since it is strong same with butt welding (but without any HAZ and no PWHT) with really continuous clean-smooth surface.