What is a plug valve ?
Plug valves are valves with permanent cylindrical "plugs" which can be rotated inside the valve body to control flow through the valve. This method of operation makes them very similar to ball valves. The plugs in plug valves have one or more holes straight through the plug. These holes are for fluid to flow through the plug when the valve is open. Plug valves are simple and often economical.
Plug valves usually do not have bonnets but often have the end of the plug with the handle exposed to the outside. In such cases, there is usually not much of a stem. The stem and handle are often joint in one piece, often the handle is a simple L-shaped handle attached to the end of the plug. The other end of the plug is often exposed to the outside of the valve too, but with a mechanism that holds the plug.
The simplest and most common type of plug valve is a 2-port valve with two positions: open to allow flow, and shut (closed) to stop flow. Ports are openings in the valve body through which fluid can enter or leave. The plug in this kind of valve has one passageway going through it. The ports are typically at opposite ends of the body therefore, the plug is rotated a quarter turn to open or shut positions, making this valve a quarter-turn valve. There is often a mechanism limiting motion of the handle to a quarter turn.