Plug Valves Types, Construction, Applications and Advantages

Plug valves, also called cocks, generally are used for the same full-flow service as gate valves, where quick shutoff is required. They are used for steam, water, oil, gas, and chemical liquid service. Plug valves are not generally designed for the regulation of flow. Nevertheless, in some applications, specially designed plugs are
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

The basic design of plug valves is illustrated in Fig. 1. Full flow is obtained when the opening in the tapered plug is aligned in the direction of flow. When the plug is rotated a quarter turn, flow is terminated. The body and tapered plug represent the essential features in plug valves. Careful design of the internal contours of the valve produces maximum flow efficiency. The port in the tapered plug is generally rectangular. However, valves are also available with round ports. Major valve patterns or types are identified as regular, venturi, short, round-port, and multi-port.

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;

The round-port full-bore pattern has a circular port through the plug and body equal to or greater than the inside diameter of the pipe or fitting. Operating efficiency is equal to or greater than that of gate valves of the same size. Use of multi-port valves is advantageous in many installations because it provides simplification of piping and convenience in operation. One three-way or four-way multi-port valve may be used in place of two, three, or four straightway valves. Refer to Fig. 2.

Types of Plug Valves

The following describes salient features of different types of plug valves:

Lubricated Plug Valves

The plug in a lubricated plug valve is provided with a cavity in the middle along its axis. This cavity is closed at the bottom and fitted with a sealant-injection fitting at the top. The sealant is injected into the cavity, and a check valve below the injection fitting prevents the sealant from flowing in the reverse direction. The sealant oozes out from the center cavity through radial holes into lubricant grooves that extend along the length of the seating surface of the plug. The sealant or the lubricant performs the following functions:

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

Non-lubricated plug valves contain an elastomeric body liner or a sleeve, which is installed in the body cavity. The tapered and polished plug acts like a wedge and presses the sleeve against the body. Thus, the nonmetallic sleeve reduces the friction between the plug and the body. Non-lubricating plug valves are used where maintenance must be kept to a minimum. Like lubricating plug valves, these valves also provide a bubble-tight closure and are of compact size.

Lubricants

The word lubricant does not precisely define the part this material plays in the efficient functioning of lubricated plug valves. More properly such valves might be called plastic sealed valves, and the lubricant could better be designated plastic sealant. The use of an effective lubricant is important, as, in operation, the valve structure and plastic scaling film are an integral unit, and each component is dependent on the other for ultimate performance.

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

The following summarizes the 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

The disadvantages include:

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

As indicated earlier, the plug valves can be and have been used in many different fluid services. They perform well in slurry applications. They are primarily used in bubble-tight services as on-off stop valves. The wiping-off action of a plug does not permit suspended particles to accumulate and form crud. The following are some 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

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