A introduction to plug valve
2008-08-15

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° to connect the port on the one side to either of the two ports on the other side. 

Early models of plug valves used metal-to-metal seals that were nonlubricated. This design is still used, but problems with galling and sticking limit their usefulness. The use of lubricant between the plug face and the seat eliminates most of these problems. The lubricant helps to control leakage around the plug, reduces wear between the valve contact surfaces, and slightly lifts the plug to reduce the operating torque required to operate the valve.

Because of its compactness and simplicity, the lubricated plug valve is gaining fast acceptance by power plant engineers. It is easy to operate, offers positive closure, and lends itself to automatic control - a popular trend in power plant operation.

Already well established in petroleum, chemical process and related fields, the lubricated plug valve now is proving its worth in power plants, within a wide range of temperatures and pressures. It can safely and efficiently handle gas and liquid fuel, boiler feed water, condensate, and similar elements.

Several basic features make the lubricated plug valve ideal for power plant application. Unlike many common types of valves, it has no high projecting yokes or bonnets, no exposed threads, no under hanging body to waste vital space. In complex piping systems, valves may be easily operated by centralized control. And there's no danger of leaks, even in gas piping systems, because of the valve's unique method of sealing.

Manufacturers continue to work toward increasing the scope of the lubricated plug valve. Not only are new alloys under test, but improved coatings may soon be available to help extend the life of the plug valve.

Probably the greatest challenge to today's manufacturer is the field of nuclear power plants. Radiation introduces unique factors in the design of piping systems, especially in the primary loops. Here, new types of corrosion resistant cladding will have to be developed before the plug valve can be applied. But outside the primary loops, piping in a nuclear power plant probably will not differ much from that in conventional plants. And the plug valve's inherent characteristics of safety, simplicity, and positive closure should make it an ideal component.