Uses of Full Coupling and Union

Coupling for long runs and where you don't need to take things apart, but you can also use a union. The unions are more expensive and are bulkier.

I prefer to see a union just before a piece of equipment. This will allow for easier removal and replacement if maintenance is required on that piece of equipment. A couple of unions in a complicated web of pipe, really don't need a Chinese puzzle just to replace a valve or what ever. Unions can make your life easier, but be careful with them. They may look alike, thread on, but the seats may be different.

A coupling is an operator-usible connection, as one usually needs no tools to perform the connection. So where a connect/disconnect is required as part of everyday operations, then a coupling is needed.

A union is a spanner-operated device, therefore its use will be by a mechanical fitter, on an occasional basis for maintenance.

Some screwed pipe configurations would be impossible to assemble without introducing a union.

As no one has mentioned it so far, a common method of joining two pipes is by screwed joints. Where provision has to be made to dissemble a section of the pipework, a 'Running Coupling' is used consisting of a standard threaded coupler with one piece of pipe threaded long enough to allow the coupler to be screwed fully on, then with the other pipe aligned the coupler is screwed back on to the aligned pipe. Not as convenient as the other types already mention if it is required to regularly take apart.

Threaded pipe has a taper so that it tightens as you screw the coupling or other fitting on. threading the pipe and coupling so that you can screw the coupling completly on to the pipe will be sure to cause leaks. Make it hard to use pipe copound or teflon tape to help with the seal.

Coupling need tools to be corrrecly tightened though one post enplied otherwise.

Unions should be used where it is not possible to make up the pipe in position - double 45 degree off sets for an example.

I changed out my water heater and had to use a hacksaw to remove the old one, but I instaflled unions on the new water heater.