Plug Valves Information
Plug valves, also called cock or stop-cock valves or quarter-turn 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.
Plug valve design is fairly simple; the body is comprised of three main parts: body, cover, and plug. The body usually does not include a bonnet. 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.
Plug valves are simple and economical. The ports are typically at the opposite ends of the body and therefore require a quarter turn to open and close the system. The plug itself is the only element that is capable of movement.
There are several specifications to consider when selecting a plug valve. For more information on valves, including different types of actuation and flow coefficient, please see Industrial Valves Information.
Plug valves connect into a variety of systems. The connection used depends on the pressure and temperature of the working fluid as well as the frequency of the pipeline will be dismantled (valve removed from the line). Types of connections include:
- Compression fittings use a socket to receive the pipe and are fitted with a screwed union nut. The joint is made by the compression of a ring or sleeve on the outside of a plain-end pipe. These fittings are best used on copper pipes for low pressure applications. They can be easily dismantled.
- Clamp or bolt flange connections are easy to install or remove from the pipeline. To ensure a tight seal, a gasket is usually fitted between the machined facing of the flanges.
- Socket weld connections are used only on steel valves. A circumferential weld is made on the outside of the pipe so that 'icicles' and weld spatter are unable to enter the pipeline. The ends of the valve are socketed to receive plain-end pipe.
- Threaded connections are used for quick install/dismantle applications. They have internal or external threads for inlet or outlet connections which allow for quick and easy changes.
- Union connections are usually also threaded. They are secure and semi-permanent connections. They are easy to take apart and allow pipes of different metals to come together safely.
- Butt weld valve end connections are beveled to match wall thickness and machined at the end of a mating pipe. A circumferential weld is made the abutted mating bevels. Butt weld connections are only used on steel valves and for higher pressure/temperature applications which do not require frequent dismantling.
- Metal face seal connections use a gasket to form a face seal on each side of a fitting. The metal gasket is sandwiched between two fitting parts.
Non-lubricated vs. Lubricated
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.
Nonlubricated-plug valves may use a tapered plug with a mechanical lifting device that unseats the plug before it is turned to reduce the operating torque required. Or it may have an elastomeric sleeve or plug coating with a low coefficient of rubbing friction.
Plug valves are extremely versatile valves that are found widely in low-pressure sanitary and industrial applications, especially petroleum pipelines, chemical processing and related fields, and power plants. They are high capacity valves that can be used for directional flow control, even in moderate vacuum systems. They can safely and efficiently handle gas and liquid fuel, and extreme temperature flow, such as boiler feed water, condensate, and similar media. Plug valves can also be used to regulate the flow of liquids containing suspended solids (slurries).
Plug valves must adhere to certain specifications and standards. Standards such as API STD 599 covers design, materials, face-to-face dimensions, pressure-temperature ratings, and test requirements for metallic plug valves. There are application-specific standards such as API STD 593 for ductile iron plug valves.