Definition and Details of Socket Weld Fittings ASME B16.11

A Socket Weld is a pipe attachment detail in which a pipe is inserted into a recessed area of a Valve, fitting or flange. In contrast to buttweld fittings, Socket Weld fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.

To join pipe to Valves and fittings or to other sections of pipe, fillet-type seal welds be used. Socket Welded Joints construction is a good choice wherever the benefits of high leakage integrity and great structural strength are important design considerations.

Fatigue resistance is lower than that in butt-welded construction due to the use of fillet welds and abrupt fitting geometry, but it is still better than that of most mechanical joining methods.

Some details of Socket Weld fittings

SW Fittings are family of high pressure fittings are used in various industrial processes.

  • They are used for lines conveying flammable, toxic or expensive material where no leakage can be permitted, and for steam 300 to 600 PSI.
  • They are used only in conjunction with ASME Pipe and are available in the same size range.
  • They are used in areas where pipe-work is permanent and are designed to provide good flow characteristics.
  • They are are produced to several ASTM standards and are manufactured in accordance with ASME B16.11. The B16.11 standard covers pressure-temperature ratings, dimensions, tolerances, marking, and material requirements for forged carbon and alloy steel. Acceptable material forms are forgings, bars, seamless pipe, and seamless tubes which conform to the fittings chemical requirements, melting practices, and mechanical property requirements of ASTM A105, A182, or A350.
  • They are available in three pressure ratings.. Class 3000, 6000 and 9000.

Types of socket weld fittings by class, size and wall thickness

Socket Weld Fittings
Description Class Designation
3000 6000 9000
Elbows 45 and 90 degrees, Tees, Crosses, Couplings, Half-Couplings, End or Pipe Caps 1/2 - 4 1/2 - 2 1/2 - 2
1/2 - 4 1/2 - 2 1/2 - 2
1/2 - 4 1/2 - 2 1/2 - 2
Pipe Size
by Wall Thickness
SCH 80

Plugs and Bushings are not identified. They may be used up through Class 6000 NPS

Advantages and Disadvantages of Socket Weld fittings


  • The pipe need not be beveled for weld preparation.
  • Temporary tack welding is no needed for alignment, because in principle the fitting ensures proper alignment.
  • The weld metal can not penetrate into the bore of the pipe.
  • They can be used in place of threaded fittings, so the risk of leakage is much smaller.
  • Radiography is not practical on the fillet weld; therefore correct fitting and welding is crucial. The fillet weld may be inspected by surface examination, magnetic particle (MP), or liquid penetrant (PT) examination methods.
  • 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.

Typical Socket Weld joints

1..Socket weld flange  2..Pipe  3..Socket weld elbow  4..Fillet weld  5..Expansion gap


  • The welder should ensure for a expansion gap of 1/16 inch (1.6 mm) between de pipe and the shoulder of the socket.
    ASME B31.1 para. 127.3 Preparation for Welding (E) Socket Weld Assembly says..
    In assembly of the joint before welding, the pipe or tube shall be inserted into the socket to the maximum depth and then withdrawn approximately 1/16" (1.6 mm) away from contact between the end of the pipe and the shoulder of the socket.
  • The expansion gap and internal crevices left in socket welded systems promotes 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.
  • Socket welding are unacceptable for UltraHigh Hydrostatic Pressure (UHP) in Food Industry application since they do not permit full penetration and leave overlaps and crevices that are very difficult to clean, creating virtual leaks.

The purpose for the bottoming clearance in a Socket Weld is usually to reduce the residual stress at the root of the weld that could occur during solidification of the weld metal, and to allow for differential expansion of the mating elements.

Fittings for Socket Weld Pipe systems

Socket Weld Full Coupling

A Full Coupling, joins pipe two pipe or to a nipple etc..

Socket Weld Half Coupling

The Half Coupling can be directly welded to the run pipe, to make a branch connection.

Socket Weld Reducing Coupling

Reducing coupling
The Reducing Coupling joints two different outside diameters of pipe.

Socket Weld Reducing Insert

Reducer insert
Socket Weld Reducer Inserts are manufactured to MSS SP-79. They enable quick and economic combinations of pipeline reductions to be made using standard Socket Weld fittings.

Socket Weld Union

Union (MSS SP-83)
It is a screwed joint design and it consists of three interconnected pieces.. two internally threads and a centerpiece that draws the ends together when rotated. Unions should be screwed tight before the ends are welded to minimize warping of the seats.

Socket Weld Elbow 90 degrees

Elbow 90°
This Elbows make 90° changes of direction in the run of pipe.

Socket Weld Elbow 45 degrees

Elbow 45°
This Elbows make 45° changes of direction in the run of pipe.

Socket Weld Tee Straight

Tee Straight
This Tee makes 90° branch from the main run of pipe.

Socket Weld Cross

Crosses makes 90° branch from the main run of pipe.

Socket Weld Cap

Cap (End Cap)
Seals the end of pipe.

Remark(s) of the Author...

A simple manner to make a 1/16" Gap...

  • Have you ever seen a Socket Weld contraction ring?.
    It is a split ring that is engineered and designed to give a pre-measured 1/16" minimum gap for socket welds. Made from a certified stainless steel, and resists corrosion from chemicals, radioactive materials and water. Once inserted into the fitting the ring becomes a permanent part of the joint. It will not rattle or vibrate even under extreme pressure.
  • Another manner is the applying of in water-soluble board. Make rings with a hole punch with outside and inside diameter of the pipe. Insert the ring into the flange or fitting and after hydrotesting there is no ring anymore.

For both solutions, ask your customer for permission.

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