used for this purpose, particularly for gas-flow throttling. Plug valves generally can be readily repaired or cleaned without necessitating removal of the body from the piping system. They are available for pressure service from vacuum to 10000 psi (69000 kPa) and temperatures from 50 to 1500 F ( 46 to 816 C). Also, plug valves are available with a wide variety of linings suitable for many chemical service applications.
Construction of a Plug Valve
|Fig. 1: Plug valve with lubricant system, as specified in API Standard 600|
Plugs are usually tapered downward, while in some cases they are tapered upward. Most of the plug valves are top entry. In top-entry plug valves, the tapered plug is installed from the top of the valve. In some cases the plug is tapered upwards and is installed from the bottom opening in the valve body. Such a plug is called bottom entry or inverted plug valve.
Plug valves are also available with cylindrical plugs. The cylindrical plugs provide for larger port openings equal to or greater than the pipe flow area.
The regular pattern employs the tapered form of port openings, the area of which is from 70 to 100 percent of the internal pipe area. In some cases, the face-to-face lengths are greater than those of standard gate valves. The venturi pattern provides streamlined flow and thus permits reduction in the port size. The port opening area is approximately 40 to 50 percent of the internal pipe area. In most of the plug valves, the port opening varies from 60 to 70 percent of the pipe area.
|Fig. 2: Multi-port valves: (a) three-way, two-port; (b) three-way, three-port; (c) four-way, four-port;|
Types of Plug Valves
Lubricated Plug Valves
1. Establishes a renewable seal between the plug and the body. As a result, internal leakage is prevented or minimized.
2. Protects the seating surfaces against corrosion.
3. Acts as a lubricant, thus reducing the force required to open or close the valve. The lubricant pressure developed by a turn of the lubricant screw or injection of lubricant with a pressure gun exerts a powerful hydraulic jacking action on the plug, momentarily lifting it from the seat and making it easy to turn. Since the lubricant pressure is greater than the line pressure, it is virtually impossible for solids to lodge between the valve body and plug.
The type of sealant to be used must be compatible with the flow medium in the pipe. The sealant must not dissolve or be washed away by the flow medium. The washed-away or dissolved sealant could contaminate the fluid, and the seal between the plug and the body would be destroyed, resulting in leakage. In addition, the sealant used must be able to withstand the flow-medium temperature.
Lubricated plug valves are normally manufactured in sizes ranging from NPS ¹⁄₂ through 36 (DN 15 through 900). They can be used in applications with pressures over 2500 psi (17250 kPa). They have been used in services involving air, gases, acids, alkalis, water, steam, oils, fuels, among others. Lubricated plug valves are less subject to seizing or wear and may exhibit somewhat greater resistance to corrosion in some service environments.
Non-lubricated Plug Valves
The lubricant in effect becomes a structural part of the valve, as it provides a flexible and renewable seat. This eliminates the necessity of force fits and metalto-metal distortable-seat contacts to effect a seal. For this purpose, the lubricant must exhibit proper elasticity as well as resistance to solvents and chemicals to avoid the destructive action of the line fluid and to form an impervious seal around each body port, even under pressure. The film of lubricant also protects the metal surfaces between the plug and body from corrosion. The seal formed by the lubricant transmitted in a system of lubricant grooves circuiting each port aids in maintaining the essential film on the metal, closure surfaces.
Advantages of Plug Valves
1. Simple design with few parts.
2. Quick to open or close.
3. Can be serviced in place.
4. Offers minimal resistance to flow.
5. Provides reliable leaktight service. Seal can be maintained by injection of sealant or by replacement of sleeve, in addition to utilizing the wedging action of a tapered plug.
6. Multiple port design helps reduce number of valves needed and permits change in flow direction.
Disadvantages of Plug Valves
1. Requires greater force to actuate, due to high friction.
2. NPS 4 (DN 100) and larger valves require use of actuators.
3. Reduced port, due to tapered plug.
4. Typically, plug valves may cost more than ball valves.
Typical Applications of Plug Valves
1. Air, gaseous, and vapor services
2. Natural gas piping systems
3. Coal slurries, mineral ores, mud, and sewage applications
4. Oil piping systems
5. Vacuum to high-pressure applications