What is a Socket Weld?
Socket welding (SW) is used for welding pipes and fittings including reducers, tees and elbows.
Socket weld pipe fittings are used to permanently join pipes that are inserted into a recess in the fitting, flange or valve. Once correctly inserted, fillet type sealing welds are applied to join the pipe to the fitting.
These fittings are commonly known as SW pipe fittings, with SW couplings, SW elbows and SW tees being examples of different types.
Socket welds can be used to change the direction of travel for pipe networks as well as join pipes at different angles and of different diameters.
The high leakage integrity and structural strength of socket welds allow them to be used for a range of piping network applications.
What is a socket weld flange?
Socket-weld pipe flanges are typically useful on smaller sizes of high-pressure pipes. These pipe flanges are attached by inserting the pipe into the socket end and applying a fillet weld around the top. It allows for a smooth bore and better flow of the gas or fluid inside the pipe.
Slip-on pipe flange with a hub is a more common connection that comes with published specifications that range from ½” thru 24”.
Socket weld pipe flanges are usually provided with an RTJ facing, flat face, or raised face. When a raised face is necessary for socket weld pipe flanges, the standard height is 1/16” for socket weld pipe flanges under 400#. For socket weld pipe flanges of 400# and up, the standard socket weld pipe flange raised face height is ¼”.
In simple words, a socket weld is a pipe bridging component installed into a recessed area of fitting or flange. The socket can be considered a female member of the joint that harbors the pipe. It is also known as a fillet weld joint as it can be secured by only one fillet.
How Socket Welding Works
Socket welding is named for the fittings into which the pipes are placed for welding. These fittings include recessed sockets into which the pipes are inserted before being welded into place.
When inserting the pipe into the socket it is important to leave a gap between the end of the pipe and the bottom of the socket. This gap prevents a stress failure from occurring when the heat from the welding process causes the pipes to expand against the socket.
This gap can be manually measured and marked with a reference line on the pipe, or a permanent fitment tool can be inserted into the fitting to ensure the pipe does not bottom out into the socket.
Once placed in position, the pipe is fixed in place by fillet welding where the pipe diameter meets the socket.
Socket welds create sealed, leak-proof, high pressure pipeline configurations for the transporting of liquids or gases.
Unlike butt welding, socket welds don’t require any pre-weld machining, although the pipe ends should be clean in order to assure the integrity of the weld.
Types of Socket Weld Fittings
Similar to butt weld fittings, socket weld fittings are available in various material types, including stainless steel, carbon steel, and alloy steel.
These fittings are used in a wide range of applications and include socket weld elbows, socket weld tees, reducing tees, reducers, couplings, and socket weld flanges, among others.
Socket weld fittings differ according to the pipe diameters and orientation they allow, meaning wide pipes can be joined to narrower ones and a pipe network can change direction or include branches. Socket weld fittings also include different coupling thread arrangements, as shown below:
#1. Socket weld Couplings.
- Full-coupling: This pipe can join two pipes or to a nipple.
- Half-coupling: This can be directly welded to the run pipe in order to make a branch connection.
- Reducing coupling: It helps to join two different outside diameters of a pipe.
- Reducer insert: These have been manufactured according to MSS SP-79. It helps to enable economic and quick combinations of pipeline reductions. As such, these can be made by using standard socket weld fittings.
#2. Socket weld Union.
A union is a type of screwed joint consisting of three interconnected pieces. It comprises two internal threads and a centerpiece that draws the ends together when rotated.
Therefore, it is important to tightly screw the unions before welding the ends together. This helps minimize seat warping.
#3. Socket weld Elbow.
Socket weld elbows, also known as SW elbows, come in two types: 45 degrees and 90 degrees. They are manufactured through forging processes and are available in both long radius (LR) and short radius (SR) models.
The LR model has a radius of 1.5 times the outer diameter (OD), while the SR model has a radius of 1 time the OD. Socket weld elbows are commonly used in pipelines to change the direction of fluid flow.
#4. Socket weld Tee (SW Tee).
Tee Straight and Reducing Tee
A socket weld reducing tee is used to create a 90-degree branch from the main run of a pipe. It has a smaller diameter branch that connects to the smaller pipe branch.
#5. Sockolet.
A sockolet (a combination of “sock” and “olet”) is a type of socket weld pipe fitting that serves the same purpose as a weldolet or threadolet.
It is used to weld a smaller diameter pipe into a larger diameter pipe body. However, what sets sockolets apart is the unique socket they have, which is located inside the olet.
This socket provides a secure 90-degree turn and higher strength to handle high pressures.
The bore of the sockolet is connected to the outlet bore, while the counter bore spans the outer diameter (OD) of the outlet to support the pipe seated on the socket, ensuring stable installation and welding strength.
Like other socket pipe fittings, sockolets are available in pressure ranges of 3000#, 6000#, and 9000#.
#6. SW Cross.
The Cross can also help to make a 90-degree branch from the main run of the pipe.
#7. SW Cap.
Cap is the end cap. It is used to seal the end of the pipe.
Advantages of Socket Weld Fittings
- For socket weld joints, the pipe can be inserted into the recessed area of the fitting and both pipe and the fitting do not need bevelled preparation. This makes the installation and soldering process simple.
- Temporary tack welding is no needed for alignment because in principle the socket weld fitting ensures proper alignment.
- The weld metal cannot penetrate into the bore of the pipe.
- They can be used in place of forged threaded pipe fittings so the risk of leakage is much smaller.
- 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 Fittings
- The welder should ensure for an expansion gap of 1/16 inch (1.6 mm) between the pipe and the shoulder of the socket. ASME B31.1 para. 127.3 Preparation for Welding (E) Socket Weld Assembly says:
- In the 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.
- Socket Weld Pipe Fittings are usually resistant to about half the strength of the butt weld fittings. Therefore, SW fittings are mainly used for small pipes with a diameter of NPS 2 or smaller.
- For the socket weld pipe fittings, the price is much higher than with similar size of butt weld fittings.
- The expansion gap and internal crevices left in socket weld piping system promotes corrosion and make them less suitable for corrosive or radioactive applications where solids build-up at the joints may cause operating or maintenance problems.
- Socket welding is not acceptable for Ultrahigh Hydrostatic Pressure (UHP) in Food Industry application since they do not permit full penetration and often leave overlaps and crevices that are very difficult to clean, creating virtual leaks.
- The purpose for the bottom clearance in a Socket Weld Joint 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. This process takes a lot of time, precision and skills.
- 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.
Uses and Applications Socket Welding
As seen above in the ‘disadvantages’ section, there are some applications that are not suitable for socket weld use, but the high-pressure ratings associated with socket weld fittings mean that they are ideal for a wide range of other industrial applications.
SW pipe fittings can be used in pipelines to transport flammable, toxic or hazardous chemicals safely due to the lowered risk of leakage compared to other joining techniques.
Creating a leak-free permanent join, SW pipe fittings also allow for excellent flow characteristics. When manufactured to ASME and ASTM standard specifications, these fittings are shown to have met requirements for performance, including tolerances, pressure and temperature ratings, dimensions, materials and markings.
Socket weld pipe fittings are divided according to material type, such as alloy or carbon steel and stainless-steel pipes.
The different types of fitting lend themselves to different applications, whether couplings, reducers, reducing and regular socket weld tees, elbows, or flanges, with each available in different material types.