Which type of fitting is commonly used to change the direction of piping?

The elbow fitting is the correct choice for changing the direction of piping. It is specifically designed to create a turn in the pipeline, allowing for a change in flow direction, which is often required in plumbing systems. Elbow fittings typically come in various angles, with 90-degree and 45-degree elbows being the most common, which allows for flexibility in designing the piping layout to navigate around obstacles or to align connections properly.

In contrast, a union fitting is used to connect two pipes together, allowing for easy disassembly; a coupling fitting is also meant to connect two pipes, but it does not provide the same level of convenience for disconnection as a union. An adapter fitting is used to connect two pipes of different sizes or styles, serving a different purpose than changing the direction of flow. Thus, the elbow fitting is uniquely suited for directional changes in piping systems.