Plug valves, also called cock or stop-cock Valves; date back to ancient times, where they were developed for use in citywide Roman plumbing systems. Today, they remain one of the most widely used valves for both on/off and throttling services. A plug valve is like a ball valve except that it has a tapered conical plug instead of a ball.
Plug valve are valves with cylindrical or conically-tapered "plugs" which can be rotated inside the valve body to control flow through the valve. The plugs in plug valves have one or more hollow passageways going sideways through the plug, so that fluid can flow through the plug when the valve is open. Plug Valves are simple and often economical.
The design of plug valves is fairly simple; the body is comprised of three main parts: body, cover and plug. The plug is a cylindrical, tapered, or generally cone-shaped device that can be raised or lowered within the seat to maintain, restrict or completely shut off flow. The Valve is opened by rotation, with the plug itself being the only element that is capable of movement.
Plug valves come in a range of sizes from 1/2" to 16" and can be manual or automated. The materials of construction range from carbon steel to titanium. The simplest and most common general type of plug valve is a 2-port valve, which has two positions, opens 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 fourth of a full turn to change from open to shut positions. This makes this kind of plug valve a quarter-turn valve. There is often a mechanism limiting motion of the handle to a quarter turn, but not in glass stopcocks.
Slightly conically-tapered metal (often brass) plug valves are often used as simple shut-off valves in household natural gas lines.
It is also possible for a plug valve to have more than two ports. In a 3-way plug valve, flow from one port could be directed to either the second or third port. A 3-way plug valve could also be designed to shift flow between ports 1 and 2, 2 and 3, or 1 and 3, and possibly even connect all three ports together. The flow-directing possibilities in multi-port plug valves are similar to the possibilities in corresponding multi-port ball valves or corresponding multi-port valves with a rotor. An additional possibility in plug valves is the have one port on one side of the plug valve and two ports on the other side, with two diagonal and parallel fluid pathways inside the plug. In this case the plug can be rotated 180