Union Article

Pipe Union

What Is a union pipe Fitting?

A union is a threaded fitting which allows the pipe work to be separated and reconnected without any horizontal movement in the pipe.

It can be a standalone pipe fitting connecting two pieces of pipe, or an integral part of another fitting (such as a ball valve) which allows it to be separated.

An O-ring seal sits within the union body, compressing when the union is tightened.

What Do "Single Union " and "Double Union" Mean in Relation to a Valve?

When a valve description is prefixed by the terms "single" and "double union", it is referring to the ends of the valve having a threaded nut which, when unscrewed, will separate the pipe from the main valve body.

A "double union" valve has a union nut at either end so that the entire body of the valve can be removed, whereas a "single union" valve only has a union nut at one end (the supply end), which allows the pipe to be split, but does not enable the removal of the valve body.

Why Are Unions Used in Pipe Systems?

Unions are useful in pipe systems as they allow equipment, such as filters etc., to be removed for servicing or replacement. They are also sometimes used in spools of pipe which, when connected, make up the system.

Do Unions Do the Same Job as Flanges?

Like unions, flanges can also separate pipe without any horizontal movement of the pipe. Unions are a much quicker way of separating the pipe, as flanges require numerous nuts and bolts to be removed and then reinstalled.

Due to the bulk and cost of unions, they are rarely used in sizes above 90mm or 3", but are available up to 110mm or 4". Flanges are usually used to perform the same job at sizes at and above 110mm or 4" pipe.

Why Does a Union Leak?

Unions work by compressng a hard plastic face onto a soft O-ring or gasket. The most common reason for a union not sealing is that the connecting pipe is not 100% parallel to the receiving pipe.

In this instance, the face will not be exerting even pressure on the seal, and so a leak occurs in the areas where there is the least, or even no, pressure.

Some fitters will seal a leaking union by adding a small amount of Vaseline to the O-ring, which sometimes works in lower pressure applications but not in higher ones. There is also the potential issue of the Vaseline contaminating the fluid in the pipe.

4 Places Where You Should Use Pipe Unions

1. Either side of a piece of equipment, such as pumps, sight-glasses, flowmeters etc., that may need to be removed for servicing or replacement.
2. At the start and end of a pipe spool.
3. At the end of a pipe where you wish to have some rotational movement in the pipe, such as a tank inlet.
4. Either side of valves that have a solvent weld or threaded connection without unions or flanges.