Modern plumbing brings needed water into a home through intricately designed pipes and connectors. When remodeling, or building, a home, specific connectors should be used for different plumbing areas on welded and threaded pipe.
Couplings are plumbing connections between welded and threaded piping sections. They resemble threaded cylinders. In contrast, a union appears the same as a coupling, but has a nut surrounding the middle of the union's assembly body.
When a coupling needs replacement, the entire coupling needs to be able to spin free of the adjoining fixed piping threads. It's difficult to remove couplings, so they're typically used in areas that do not require periodic maintenance.Unions are commonly used in areas that need periodic repair. When removing a union, only the union's nut needs to spin free -- not the entire connector and piping.
You should use a union when connecting dissimilar pipe materials, such as steel and copper. The union provides a barrier between the two metals, preventing galvanic corrosion.