A plug valve is a valve designed with a cylindrical or conically tapered plug used to regulate, open or close flow of fluid. This valve uses a plug to control fluid flow. The plug rotates in the valve body. The plug is designed with one or more ports. When the plug valve is opened, fluid flows through the ports of the plug. Plug valve employ a simple design which makes them relatively cheaper. Plug valves do not have bonnets like other valves such as globe valves and gate valves.
The simplest design of a plug valve is a bi-directional fluid flow valve in which the open position allows fluid to flow while the closed position stops the fluid flow. The fluid flows through the ports which are the inlet and outlet valve ports. The simple design of a plug valve employs a quarter turn to open and close the fluid flow.
How does a plug valve work?
Fluid flows through a plug valve via the inlet and outlet ports. It is the position of the ports that determine the open or closed state of the valve. To open a plug valve, the handle is rotated for a quarter turn. This makes the plug to rotate in the anticlockwise direction to allow fluid flow. Fluid starts flowing through the valve when the inlet and outlet ports are aligned with the respective plug ports. Pressure drop occurs across the reduced area of the plug port.
To reduce pressure loss, it is recommended to use a full-area plug. To close the plug valve, the handle is rotated for a 90o degree turn which makes the plug ports rotate and become perpendicular to the valve ports which then blocks fluid flow from the pipeline through the valve. To regulate fluid flow, the plug is rotated for an angle of around 45o degrees. While at this position, the plug causes pressure drop twice relative to when the valve is fully open.
Types of plug valves
1. Lubricated plug valve
This is the plug valve that uses lubricant for easy opening and closing. A layer of lubricant is placed between the plug and the valve body. The lubricant layer helps to prevent fluid leakage by acting as a seal. Plug valve manufacturer recommend lubrication of this valve be done regularly to enhance long service life. The seal used should be able to withstand working fluid temperature as well as debris contained in the fluid. The lubricant also helps to prevent the formation of corrosion on the plug and valve body.
2. Non-lubricated plug valve
This is a plug valve that does not have lubricant film. This type of valve is used mostly in low-pressure lines. The valve is designed with an elastomer body liner or sleeve. The liner or sleeve is installed in the body cavity. The sleeve is compressed against the valve body using the sleeve. As such, friction between the plug and body is reduced. This type of valve needs less maintenance relative to the lubricated one.
3. Multi-port plug valve
This is a plug valve used in diversion and transport lines. The use of one multiport plug valve can provide services that will need the use of three or four gate valves or other types of shut-off valves.
4. Eccentric plug valve
This is a plug valve that uses a half plug. This valve is useful where there is a need for a high amount of seat force and low friction from the opening position to the closing position. This type of plug valve has superb closing ability free from fluid leakage.
5. Expanding plug valve
This is the type of plug valve which has a complicated design. This type of valve employs a design that allows open and closing positions of the valve to protect sealing materials from the flow path. The sealing elements and body do not touch each other which helps to prevent abrasion on the seal. This valve is suitable for use where double insulation is not required.