A plug valve features a cylindrical or conic disc, with one or multiple horizontal hollow passageways, that can be rotated to open, close or control the flow of the fluid conveyed by the pipeline through the valve. The most used plug valve is the two-port lubricated type with an open and closed position (multi-port types are also available). The two ports are positioned at the sides of the valve and a stem is connected with a lever at its top to open and close the device.
A plug valve can be ordered with flanged, hub or butt weld ends and with a cast (> inches) or a forged body material (< 2 inches diameter). The two key specifications for plug valves are the API 6D and API 6A.
Plug valves can be used for liquids or gases such as:
- Neutral liquids, like oil and water
- Acidic and Alkaline (corrosive) liquids
- Neutral gases, such as steam and air
- Corrosive gases, including chlorine and acid vapors
TYPES OF PLUG VALVES
There are four main types of plug valve, each with specific features and benefits.
Lubricated Plug Valve
A lubricated valve uses a lubricant to reduce the friction and seal the ports tightly. The lubricant is typically a base oil that improves the viscosity of the moving parts, like amorphous or fumed silica, which is injected between the plug face and the body seat at high pressure.
The manufacturers of plug valves recommend a type of lubricant that matches the process fluid, and to reseal the valve after a certain number of cycles (or after every cycle for some applications).
Due to this maintenance requirements, lubricated plug valves are used in applications that have non-continuous operations. This type of plug valve suits processes with mildly abrasive particles suspended in the fluid such as upstream applications, gas pipelines that require bypass valves, and as blow-down valves on valve stations and kicker valves.
Lubricated plug valves are metal seated valves.
Non-Lubricated Plug Valve
This plug valve features a tapered or conical disc that acts as a wedge and presses a soft-material sleeve against the body cavity (the sleeve reduces the friction between the plug mechanism and body).
Non-lubricated plug valves are preferred to the lubricated type when low maintenance is required, like in services for sulfur, hydrogen fluoride, or in cases where the fluids could solidify and block the valve. The downside of non-lubricated plug valves is that they suit only specific applications in terms of temperature, pressure and type fo conveyed fluids (chemical resistance) due to the non-metallic seat they use.
The three main types of non-lubricated plug valves are: fully lined, elastomer sleeve and, lift-type plug valve.
Eccentric Plug Valve
The design of an eccentric plug valve features a half plug that is a key benefit for applications that require a stronger seating force and minimal friction from the open to the closed position. Eccentric plug valves show also improved shut off capabilities compared to the concentric design. The typical applications of this type of plug valve are dirty waters, sewage, sludge and slurries, air, and other similar services requiring tight shut off.
Double Expanding Plug Valve (Double Block and Bleed – DBB)
This is the most complex plug valve design, as it features multiple components that allow a mechanical expansion of the valve to give it a true double block and bleed function. They are often used to prevent product contamination in applications that do not require a double isolation.
The expanding plug valve uses a mechanism that rotates between the open and closed position and protects both seals from the flow path. During the rotation, the body and seals do not get in contact, and slips expand onto the body seat when the valve is closed to prevent wear or abrasion to the seals.
PLUG VALVE SHORT AND LONG PATTERN
There are three different designs of plug valves.
- The short pattern design type has a compact face to face dimensions and port areas accounting for 40% to 60% of a full-bore type
- The regular pattern has a longer face to face dimensions and a port area of 50-70% of the full-bore plug valves and is designed to provide minimal loss of flow.
- Full bore design has long face to face dimensions and a round port.
This last configuration provides unrestricted flow and allows pigging operations of the pipeline. Modern plug valves are an evolution of a basic design that was developed back in the Roman empire.
MANUAL AND ACTUATED PLUG VALVE
Plug valves may be manually operated, with a wrench or a gear or actuated with Electric, Electro-Hydraulic, Pneumatic-Hydraulic and, Gas Over Oil Actuators.
(Source: CTE Skills Youtube Channel)
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