The union fitting is one of the most overlooked pieces of plumbing that aid in plumbing repair in certain situations.
It should be used in systems that are hard plumbed (no tubing) with components that are prone to breaking down or leaking, such as pumps, bulkheads and threaded fittings.
An example would be to use them on the ‘in’ and ‘out’ ports of a hard plumbed, external pump. To remove the pump for repair, all that has to be done is to loosen and unhook the union fittings so the pump can be easily removed. You can use these in conjunction with ball valves to cut the water flow off to the item to be repaired. Combination ball valves/unions are also available.