Pipe Unions: An overview

Pipe unions are threaded fittings that allow for the easy separation and reconnection of pipe work without any horizontal movement. In a system that may otherwise be welded an inseparable, this function is useful in many instances. All unions have an 'O' ring seal that sits within the union body, that becomes compressed when the union is tightened. Unions can be standalone fittings (as shown in the main image) or they can be an integral part of another fitting, such as on the ends of a ball valve.

'O' Ring Seal Materials

The 'o' ring seals in a union are made from a 'rubber like' material that can be compressed between rigid separable components, to keep the piped media from leaking out from between the rigid components. There are two main materials used for union seals. The 'standard' seal material in many plastic unions is EPDM (Ethylene Propylene Diene Monomer) which is perfectly suitable for water and many other commonly piped media. The second material is FPM (FKM), which has completely different resistance profile to EPDM. Aggressive medias can attack the 'o' ring seal material and cause the seal to fail; If piping any media other than water, you may want to double check compatibility.

Double union and Single union

When a valve is described as "single union" or "double union," it refers to the ends of the valve having integrated unions, which consists of a threaded nut that separates the ends from the main valve body. A "double union" valve has a union nut at either end, allowing for the removal of the entire valve body, while a "single union" valve only has a union nut at one end. More information on ball valves

Unions are useful in pipe systems as they allow for easy removal of equipment, such as filters, for servicing or replacement. They are also used in spools of pipe that make up the system.

Union Joints & Flanged Joints Compared

While flanges can also separate pipe without any horizontal movement, unions are a quicker option as they require no nuts and bolts to be removed and reinstalled. However, unions are rarely used in sizes above 90mm or 3" due to their bulk and cost, with flanges being the preferred option for larger pipe diameters.

Union Leaks

The most common reason for a union leak is that the connecting pipe is not 100% parallel to the receiving pipe, causing uneven pressure on the seal. Some fitters may use Vaseline to seal a leaking union, but this is not recommended for higher pressure applications and it can contaminate the fluid in the pipe.

Where to use union joints

Pipe unions should be used at either side of equipment that may need to be removed for servicing or replacement, such as pumps, sight-glasses, flowmeters etc. They should be used at the start and end of a pipe spool, at the end of a pipe where rotational movement is required, and either side of valves that have a solvent weld or threaded connection without unions or flanges.