Union

A union is a pipe fitting that allows two pipes to be connected and disconnected without rotating either pipe. Unlike a coupling (which requires both pipes to be threaded), a union has three parts that let you join or separate a pipe run at any point.

How it works

A union consists of two threaded ends and a central nut (called the union nut). Each pipe end is threaded into one of the union’s halves. The union nut threads onto one half and pulls the two halves together, compressing a sealing surface (gasket, face seal, or metal-to-metal) between them. To disconnect, you simply unscrew the union nut — neither pipe needs to rotate.

Why unions exist

In a straight run of threaded pipe, you cannot disconnect the middle without unscrewing every section from one end. A union solves this by providing a disconnect point that can be opened without rotating the pipe. This is essential for:

• Pump and valve maintenance: You need to remove a pump or valve without dismantling the entire pipe run.
• Equipment servicing: Boilers, water heaters, and filters all need union connections for maintenance.
• Installation: Unions let you assemble sections on the bench and bring them together on-site.

Types

• Threaded union: The most common type in plumbing — two threaded halves joined by a union nut. Used with black iron, galvanised, and brass pipe.
• Compression union: Uses compression fittings instead of threads, suitable for copper and plastic tubing.
• Flanged union: Uses bolted flanges for large-diameter industrial piping.
• Socket-weld union: For high-pressure applications — the pipe ends are welded into the fitting.