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Socket Weld (SW) fittings
A socket weld fittings involves two different sized pieces of pipe. The smaller one is inside the larger pipe. The
weld is completely around the outside circumference of the larger pipe.
Butt weld (BW) fittings
A butt weld fittings is two pieces of materials butted against each other and welded. Several different methods
exist for beveling the two pieces for good weld penetration. Both flat stock and pipe or tubing can be
butt welded. Generally, the weld is ground flush with the material surface.
Normally, the price of Socket weld fittings is higher than Butt weld fittings in similar size. But the extra cost of butt weld is higher for installation, considering labor cost and skilled welder.
Socket weld is easy to install. But it requires approx. 1/16″ of gap at the bottom of the pipe to socket into, to allow thermal expansion. This gap can lead to over stress problem that cracks the fillet weld of the fitting. Socket weld is troublesome in corrosive fluid service due to crevice corrosion. And it is easy to corrode due to the discontinuity of the smooth pipe internals.
Butt weld is the “best” in terms of strength, fatigue, and corrosion resistance, and temperatures compliance. However, it is more difficult to fit-up and weld properly, needing skilled welder and much time.
Socket weld (the fillet weld) resists around 1/2 the strength of butt welding. So it is mainly used for small pipeline (Small Bore Piping) with diameters NPS 2 or smaller.
Butt weld keeps better strength, good for high pressure or high temperature pipelines. So it is used when a welded joint with strength not lower than that of the base metal itself is needed.
Šaltinis: Kinijos vamzdžių jungiamųjų detalių gamintojas - Yaang Pipe Industry (www.ugsteelmill.com)