Plug Valves

Plug valves, similar to ball valves, are quarter-turn valves that provide positive shut-off. There are two primary types of plug valves: lubricated metal-seated plug valves and Teflon-sleeved plug valves. These valves can come with various end connections, including flanged, butt-welded, screwed, or socket-weld ends. The pressure classification for these valves is consistent with that of gate valves. The range of pressures at which these valves can be used depends on the seat material, seals, and lubricant. Plug valves with shaped ports are often utilized in flow control applications.

1. Metal-Seated Plug Valves

Lubricated plug valves feature lubrication of the seating surface, typically accomplished by injecting lubricant through the valve's operating surface, either via mastic sticks or a grease gun. The type of lubricant selected depends on the service conditions the valve is exposed to. Some designs incorporate a low-friction Poly Tetra Fluoro Ethylene (PTFE) coating on the valve plug's surface, a treatment known as "LOMU." This treatment reduces the frequency of lubrication required.

The design of plug valves is categorized into three patterns based on the port shape and the valve's overall length:

• Regular Pattern Valves: These valves have rectangular plug ports with an area nearly equal to the full bore of the pipe. The transition from the round body to the rectangular seat ports is smooth, minimizing turbulence. They are used where minimizing pipeline losses is critical.
• Short Pattern Valves: These valves have face-to-face dimensions similar to wedge gate valves, making them a suitable alternative to gate valves.
• Venturi Pattern Valves: These valves have a reduced port area with a throat designed to create a Venturi effect, helping to restore velocity head loss and minimizing total pressure drop.

The plug in these valves can be installed with the taper facing either the bottom or the top of the valve body. When the taper is positioned at the top, it is referred to as an inverted plug design, typically used in larger diameter valves (8 inches nominal bore and above).

2. Pressure-Balanced Plug Valves

Another design variant is the Pressure-Balanced Plug Valve, which eliminates the risk of taper locking caused by unbalanced forces. This is achieved by using line pressure to replace the sealant pressure, removing the need for regular sealant injection to maintain the valve's operability.

The pressure-balance system includes two holes in the plug that connect chambers at each end of the plug with the port, ensuring line pressure is maintained.

Valves featuring this pressure-balanced design are known as dynamically balanced plug valves. A notable example is the "Super-H" pressure-balanced valve by Audco, which requires lower breakaway torque for operation compared to reduced port ball valves—nearly half as much.

3. Teflon-Sleeved Plug Valves

Teflon-sleeved plug valves are designed to handle specific temperatures that the sleeve can withstand. These valves are available with a fire-safe atmospheric seal but do not have a fire-safe design through the seat. They can also be designed with an anti-static feature, similar to that used in ball valves.

To control emission rates, sleeved plug valves can be equipped with bellows seals. There are two main types of bellows seal designs for quarter-turn valves:

• Gooseneck or Bent-Straw Design
• Rack and Pinion Design: This design maintains a linear bellows, reducing stress and eliminating the need for forging.

An alternative design is the "caged" plug valve, where the plug is inserted into another plug, enhancing emission control and improving throttling capabilities while reducing wear. Additionally, plug valves are available with a fluoropolymer-lined metal body and plug.

4. Permaseal Plug Valves

Permaseal plug valves are similar to sleeved plug valves but use Teflon seats instead of sleeves, similar to the seats used in ball valves. These valves are designed for on-off applications and can handle clean, viscous, and corrosive liquids. The construction and operation of these valves are identical to sleeved plug valves.

Graphite seats can also be provided for high-temperature service, but this design may not offer drip-tight shutoff.

5. Eccentric Plug Valves

Eccentric plug valves feature plugs that are mounted off-center. These valves are used in corrosive and abrasive services for on-off operations. The eccentric action of the plug allows it to move into and away from the seat, reducing abrasive wear.

Eccentric plug valves are governed by the MSS-SP standards.