Socket Weld Flanges

Socket weld flanges have a socket in which a pipe is inserted; the pipe is secured by one fillet weld located on the exterior of the flange hub. A significant disadvantage with this type of flange is that it is not considered a high-integrity joint because the weld is difficult to prove; thus socket weld flanges are only suitable for low to medium classes (≤ ASME 600). Due to their lower integrity and unsuitability for use at higher pressures, socket weld flanges almost always have flat or raised faces.  

Socket Weld Flange Cross Section

Socket weld flanges are designed for small nominal pipe sizes (≤ 4 inches, ≤ 10cm) and are common for ½ to 2-inch pipe sizes (1.3 to 5cm pipe sizes). The mechanical strength of a socket weld flange is similar to that of a slip-on flange, but the slip-on flange may use two welds.

The ASME B31.3 standard states that an approximate gap of 1⁄16th of an inch (1.5mm) between the socket and pipe end is required; this gap allows for thermal expansion due to welding and reduces the likelihood that the weld will crack. In practice, this means that the pipe should be fully inserted into the flange, then retracted by the desired amount (1/16th of an inch, 1.5mm); this gap has been highlighted red on the above image.

Socket Weld Flange

The advantage of socket weld flanges is their simple design, they are well suited for small pipe size applications e.g. 2 inches (5cm) and below, and for non-critical applications e.g. non-hazardous systems; they are not suitable for highly erosive or corrosive systems.


Non-Destructive Testing (NDT) Techniques

Socket weld flanges have only one fillet weld and this can be subjected to radiography to prove the weld, although in practice it is impractical. Magnetic particle inspection (MPI) and dye penetrant inspection (DPI) can be used to prove the weld. 


ASME Standards

Socket weld flanges are not specified in ASME class 2500 rating as per the ASME B16.5 standard. The ASME B31.3 standard states that an approximate gap of 1⁄16th of an inch (1.5mm) between the socket and pipe end is required.


Flange Types, Faces, and Surfaces - Explained!

This video is part of our Piping Flange Fundamentals Video Course


Additional Resources