Socket Weld Unions — MSS SP-83

What is a Socket Weld Union?

A union has a male end, a female end and a threaded nut that draws them together. Each end has a socket that is fillet welded to the pipe; the nut can be undone to separate the line for maintenance, equipment removal or cleaning. MSS SP-83 covers Class 3000 and 6000 pipe unions with socket welding and threaded ends in carbon steel, alloy steel, stainless steels and nickel alloys. The seat is usually a ground metal-to-metal joint.

Key features

• Dimensions and testing per MSS SP-83
• Class 3000 and 6000
• Metal-to-metal ground seat
• Allows removal of equipment without cutting the pipe

Specification

Applications

• Connections to instruments, pumps and small equipment
• Lines needing periodic dismantling
• Chemical dosing and sampling lines
• Skid-mounted packages

Union vs coupling

A coupling joins two pipes permanently with two fillet welds. A union also has two welded ends, but they are held together by a threaded nut on a ground seat, so the joint can be opened and closed without cutting the pipe. Use unions next to pumps, strainers, instruments or equipment that must be removed for maintenance; use couplings for permanent in-line joints. Check that the piping specification allows unions for the service.