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Definition and Details of Socket Weld Fittings |
updated October 2025
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
| 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 XS |
SCH 160 | XXS |
Advantages and Disadvantages of Socket Weld fittings
Advantages
- 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.
1..Socket weld flange 2..Pipe 3..Socket weld elbow 4..Fillet weld 5..Expansion gap
Disadvantages
- 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
Full-coupling
A socket weld coupling is used to connect two pipes of the same small diameter. It has two socket
ends into which the pipes can be inserted halfway before a fillet weld is applied to create a
tight connection.
Half-coupling
A socket weld half coupling is used to create a branch connection by welding one end directly to
a larger pipe and connecting a smaller pipe to the socket end.
Reducing coupling
A socket weld reducing coupling is used to connect two pipes with different diameters. It allows
a smaller pipe to be inserted into a larger socket, which is then welded with a fillet weld.
Reducer insert
A socket weld reducing insert connects two pipes of different sizes and is used together with a
standard straight socket weld fitting to reduce the diameter of a pipe. It enables a quick and
cost-effective reduction in pipe diameter by using the reducing insert in a standard socket weld
connection.
Union
A socket weld union is used to connect and disconnect pipes and combines a three-part threaded
connection design with rigid socket weld connections. It consists of two parts that are inserted
into a fitting and a third part that pulls them together when turned.
Elbow 90°
A 90-degree socket weld elbow is used to change the direction of a pipe, whereby the pipe is inserted
into a recess or “socket” in the fitting and then joined by a fillet weld.
Elbow 45°
A 45-degree socket weld elbow is used to change the direction of a pipe, whereby the pipe is inserted
into a recess or “socket” in the fitting and then joined by a fillet weld.
Tee Straight
A straight socket weld tee is a three-way pipe fitting that forms a 90-degree branch from a main
pipe. It has three sockets into which a pipe is inserted and then welded with a fillet weld.
Cross
A socket weld cross is a four-way pipe connection that forms a 90-degree branch from a main pipe.
It has four sockets into which a pipe is inserted and then welded with a fillet weld.
Cap (End Cap)
A socket weld end cap is used to permanently seal the end of a pipe. The pipe is inserted into
the socket on the cap and then welded with a fillet weld.
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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