Which of the following fittings is NOT commonly used in piping systems?

In piping systems, various fittings are essential for directing flow, connecting different sections of pipe, or closing off sections of pipe. Elbows, caps, and tees are all integral components used frequently in various systems.

Elbows are utilized to change the direction of the flow in piping, allowing for bends and turns. Caps are used to seal the ends of pipes, preventing any fluids from escaping. Tees offer branch connections, enabling the flow from one pipe to split into two.

On the other hand, screws are not classified as fittings in the context of piping systems. While screws may be used in the assembly and securing of piping systems (for example, to attach flanges or to hold components together), they are not fittings designed to modify the flow path of the fluids in the system. Consequently, when considering fittings that facilitate the flow and management of liquids or gases in piping, screws do not fall into that category. This distinction solidifies the word "not" in the question regarding the commonly used fittings in piping.