Plug valves are valves with cylindrical or conically tapered "plugs" which can be rotated inside the valve body to control flow through the valve.

A plug valve is a simple type of valve that allows or blocks the flow of a liquid through a pipe. The plug itself is often shaped like an upside-down ice cream cone or a cylinder. A handle on top allows the user to turn the plug valve so it rotates and stops or starts the flow of liquid. A minimum of two holes, known as ports, must exist in the plug for liquid to flow. The ports are located on opposite sides of the plug, and when the plug is turned to the open position it creates a passage for the liquid to flow through.

This arrangement of two ports is known as a 2-port valve. The pipe in which the plug valve is installed has an area for liquid to flow in and another for liquid to flow out. When the 2-port plug valve is turned on, the ports line up with the holes in the pipe and the liquid flows straight through as if the plug valve was not there. Other, more complex plug valves known as 3-way valves allow liquid to flow to a combination of different ports. One port may connect to either of the opposite two ports, or all three may connect at once.

Two major categories of vales exist: stop valves and check valves. Stop valves follow a basic design structure to turn the flow of liquid on and off, or allow partial flow. Check valves, in contrast, allow liquid to travel only one way and often do not have a handle. Plug valves fall into the stop valve category.

Other stop valve types include gate valves and ball valves. Though similar to plug valves, the shape of the parts and how they work to stop the flow of liquid typically determines the specific group they belong to. Regardless of group, stop valves typically control liquid through the use of a handle.

Operation of a plug valve is simple, making it a common valve choice. The user twists the handle and the ports are moved away from the opening in the pipe. In certain valves, a full turn is not required. Quarter-turn valves, for example, only require the user to make a short, quarter turn to stop the flow of liquid. Often, plug valves can lessen the flow without turning off completely if the user turns the valve half-closed but leaves it open wide enough for some liquid to make it through the ports.

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