Plug Valves working principle and types

Plug valves (also known as cock valves) consist of a tapered or parallel side plug which can be rotated a quarter turn within a valve body. The quarter turn gives full closure or fully open operation of the valve.

Plug valve is a Quarter-turn rotary motion Valve that uses a tapered or cylindrical plug to stop or start the flow. The disk is in plug shape, which has a passage to pass the flow.

In the open position, this bored passage is in line with the flow. When the plug is turned 90° from the open position, the solid part of the plug blocks the flow.

It is used in place of a gate valve where the quick operation is required. It can be used in high-pressure temperature services.

There are two basic types of plug valves :

Lubricated plug valve

Non-lubricated plug valve

Lubricated plug valve

The plug inside a lubricated plug valve has a cavity in the middle along its axis. You can see this in the image. Lubricant chamber at the bottom and the sealant injection fitting at the top ensure the supply of lubricant.

The small check valve below the injection fitting prevents the sealant from flowing in the reverse direction once the sealant is injected into the cavity.

Plug surface gets constantly lubricated by the sealant that moves from the center cavity through radial holes into lubricant grooves on the plug surface. Now why we required all this? Many plug valves are of all-metal construction.

The narrow gap around the plug may allow leakage, and if you reduce the gap further, it will increase the friction and plug may get stuck inside the valve body.

The lubricant reduces the force required to open or close the valve and allows smooth movement of the plug. The lubricant also prevents corrosion of the plug.
The lubricant material must be compatible with the fluid of the pipeline. It should not dissolve or wash away by the flow medium as this could contaminate the fluid, and damage the seal between the plug and the body, resulting in leakage. Also, the sealant used must be able to withstand a temperature of the flow medium.
Lubricated plug valves are available in the large size range, and they are fit to work in high-pressure temperature services. These valves are subject to less wear and provide better corrosion resistance in some service environments.

Non-lubricated plug valve

A non-metallic elastomeric sleeve or liner is used in this type of plug valve. This sleeve is installed in the body cavity of the valve. The polished tapered plug acts as a wedge and presses the sleeve against the body. This nonmetallic sleeve reduces the friction between the plug and the valve body. Non-lubricating plug valves required minimum maintenance. Due to the non-metallic seat, these valves are not used in high-temperature services. Lubricating and non-lubricating plug valves are capable of providing a bubble-tight shutoff and are of compact size.

Multi-Port Plug Valves

Here you can see the 3-way multiport plug valve. The top image is of 3-way 3-port design and bottom is 3-way 2-port design. Multiport valves are used in transfer lines and for diverting services. A single multiport valve may serve the purpose of three or four gate valves or other types of the shutoff valve. However, sometime multiport valve does not completely shut off flow. Great care should be taken in specifying the particular port arrangement for proper operation.

Plug Valve Application

  • Plug valve used as on-off stop valves and capable of providing bubble-tight shutoff.
  • It can be used in different types of fluid services such as Air, gaseous, vapor, Hydrocarbon, slurries, mud, and sewage applications.
  • Plug valve can be used in a vacuum to high-pressure & temperature applications

Plug Valve Advantages

  • Simple design with few parts
  • Quick to open or close
  • inline maintenance possible
  • Offers minimal resistance to flow
  • Provides reliable leak-tight service
  • Multiple port design helps reduce the number of valves needed and permits a change in flow direction

Disadvantages of Plug Valve

  • It requires greater force to operate, due to high friction
  • Larger valves cannot be operated manually and required an actuator
  • Pressure drop due to reducing port
  • Cost of Plug valves may be more than ball valves for given size and class

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