Why is the Gap needed on Socket Welds

The socket weld is a pipe attachment detail in which a pipe is inserted into a recessed area of a valve, fitting, or flange. The joint construction is a good choice wherever the benefits of high leakage integrity and great structural strength are important design considerations.

Advantages of Socket Weld Pipe Fittings

  • The socket weld pipe fittings can be used in place of threaded fittings, so the risk of leakage is much smaller.
  • Socket weld pipe need not be beveled for weld preparation.
  • The weld metal can not penetrate into the bore of the pipe.
  • Construction costs are lower than with butt-welded joints due to the lack of exacting fit-up requirements and elimination of special machining for butt weld end preparation.

Disadvantages of Socket Weld Pipe Fittings

  • Socket weld pipe fittings are unacceptable for UHP pressure in food industry applications since they do not permit full penetration and leave overlaps and crevices that are very difficult to clean, creating virtual leaks.
  • The socket weld pipe fittings should ensure an expansion gap of 1/16 inch (1.6 mm) between the pipe and the shoulder of the socket.
  • The expansion gap and internal crevices left in socket welded systems promote corrosion and make them less suitable for corrosive or radioactive applications where solids buildup at the joints may cause operating or maintenance problems. Generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.

The Gap and WHY?

Thermal expansion, If there is not a GAP, and if the pipe grows more than the socket welded fitting, then the fillet weld may crack because of the below reasons:

  • During fabrication: If the Pipe wall thinner than the Socket Welded Fitting, then the pipe becomes hotter than the fitting during welding.
  • Hot service and Hot cyclic condition: If the line is uninsulated, the Socket will not heat up as much as the pipe.

During in-service inspection using Profile RT, and occasionally, SW fitting found without a “Gap”. If this pipe has withstood the long-time operation with no issues, usually there is no reason to repair or add the “gap”. However, if the system operating temperature or cyclic condition increased, then a discussion with the pressure equipment engineer.

During fabrication, NDT is not obligated to performed after welding to verified the Gap. See the interpretation:

Interpretation: 16-06
Subject: ASME 831.3-1996 Edition, Para. 328.5.2, Welding Requirements – Fillet and Socket Welds. Date Issued: May 20, 1997. File: 831-96-0588
Question: In accordance with ASME 831.3-1996 Edition, para. 328.5.2 and Fig. 328.5.2c, what is the minimum gap acceptable in a socket-welded joint after welding?
Reply: The ‘116 in. approximate gap shown in Fig. 328.5.2c is “before welding”. The Code does not provide a gap dimension after welding.

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