A plug valve is a quarter-turn rotary motion manual valve. It uses a cylindrical or tapered plug (plug shaped disk) to permit or prevent straight-through flow through the body. Plug valves offer a straightway passage through the ports so that fluid can flow through the opening plug with a minimum of turbulence. Flow can be in either direction with fully open or fully closed.
The plug valves have been used in many different fluid services. Their performance is good in slurry applications. They are used in bubble-tight services as an on-off stop valve. They are used in air, gaseous & vapor services, Natural gas & oil piping systems, and vacuum to high-pressure applications. They are good for on-off valves, diverting services, and moderate throttling. In this article, we will explore the parts, working principle, types, symbol, advantages, and disadvantages of Plug Valves.
Components of a Plug Valve
Plug valves have all the general parts such as stem, body, disc and the bonnet. It is different from other valves due to its plug disc design.
The Plug Valve Body:
The valve body has all the internal components of the valve. The body connects to the pipes through an end connection.
The Plug valve Stem:
The stem of the valve connects the disc (or disk) to the actuator. This helps in transferring the movement of the actuator to the plug disc thereby opening or closing the plug disc. The plug valve has a little stem that connects to the disc.
The Plug valve disc:
The plug disc which is known as the tapered disc acts as a plug hence differentiating the plug valve and the globe valve. Out of all the valves, the plug valve, ball valve, and globe valve have some similarities. The major difference is in the way that how the disc looks like. The Globe valve has a cylindrical disc. The plug valve has a solid metal piece with tapered ends.
Plug valves has the following port patterns:
- Round opening: It has round ports in both plug and body.
- Rectangular opening: It has rectangular ports of the full bore section.
- Standard opening: In this, the area through the valve is less than the area of the standard pipe.
- Diamond opening: It has a diamond-shaped opening through the valve.
- Multi-port: with three or more pipe connections used for transferring services.
- Venturi design: It has reduced area opening with Venturi flow through the body.
Plug Valve Working Principle
Plug valve comprises a body with a tapered or a parallel seating into which a plug fits. The plug is formed with a port, the position of the port controls the amount of opening through the valve. Ports are known as the openings in the valve body through which fluid can enter or leave. A 90 degree of the plug fully opens or closes the fluid flow. Plug valve is not as efficient as ball valves and can only operate fully open or closed.
The simple plug valve is suitable for low pressure, low-temperature applications and is made in large sizes of 250 to 300 mm. The main limitation of the plug valve is that if wide variations in fluid temperature are involved then differential expansion is inevitable, leading either to undue stiffness of operation or loss of pressure-tightness.
The plug may be tapered, parallel, and the movement may be plain or lubricated. Another variation is known as the ball plug valve, in which the plug is spherical, having circular ports rotating between circular seats of the concave section.
Types of Plug Valves / Plug Valve Types
Plug valves are of following types:
- Lubricated Plug Valves
- Non-lubricated Plug Valves
Lubricated Plug valves:
These valves use lubricants or sealants to ease their operation over a wide range of operating pressure. The lubrication film which is used between plug and body provides a seal and that requires periodic lubricant injection. The sealant must be like that it does not get washed away with the fluid as the fluid can get contaminated and it should be able to withstand line temperature. These lubricants in plug valves are mostly plastic sealants. The use of effective sealant with properties such as proper elasticity, resistance to chemicals, and its ability to form an impervious seal around each body part is essential. The lubricating film also protects the metal surfaces between the plug and body from corrosion.
Lubricated plug valves are manufactured in sizes ranging from DN 15 to DN 900. They are used in applications with pressures over 2500 psi.
Non-Lubricated plug valves:
These plug valves are usually used for lower pressure lines. These valves contain an elastomeric body liner or sleeve installed in the body cavity. The plug that is tapered and polished acts like a wedge and tends to press the sleeve against the body. Thus the sleeve reduces the friction between the plug and the body.
There are three types of non-lubricated plug valves:
- Lift-type plug valve
- Elastomer sleeved plug valve
- Fully lined plug valve
Lift type plug valve provides a mechanically lifting arrangement to disengage tapered plug from the seating surface to permit easy rotation. The lifting can be achieved by an external lever.
The elastomer sleeved plug valve has an elastomer sleeve. It is a TFFE sleeve which surrounds the plug completely. The elastomer sleeve is engaged and locked in place through a metal body. This sleeve has less coefficient of friction and is also self-lubricated.
Fully lined plug valve: These types of valves use inexpensive ductile iron or cast iron body. The body and plug of the valve are fully lined with Teflon in a way giving the valve the ability to resist corrosive fluids using iron as the body material.
Plug Valve Symbol
In P&ID the following plug valve symbol is used for identification and differentiating from other valves.
Advantages of plug valve
The main advantages of a plug valve are:
- It has a simple design and fewer parts
- It can be quickly open or close.
- This valve offers minimal resistance to flow.
- The use of multi-port designs helps in reducing the number of valves needed and permits a change in the flow direction.
- It provides a reliable leak-tight service.
- They are easy to clean which can be done without removing the body from the piping system.
Disadvantages of plug valve
The limitations or drawbacks of a plug valve are:
- As a large amount of friction is required to rotate the plug, which results in greater force to operate these valves.
- Actuators are required for larger valves.
- These plug valves cost more than ball valves which have a similar design.
- Pressure drop is more due to reducing port.
Codes and Standards for Plug Valve Design
The industry codes and standards governing the plug valve design, test and inspection are:
- API 599 – Metal Plug Valves – Flanged, Threaded, and Welding Ends
- API 6D – Specification for Pipeline valves
- API 598 – Valve Inspection and Testing
- API 6FA – Fire Test for Valves
- BS 5353 – Specification for steel plug valves
- BS 5188 – Specification for cast iron plug valves
- MSS SP 61 – Pressure testing of steel valves
- MSS SP 25 – Standard marking system for valves, fittings, flanges & unions
- MSS SP 78- Gray Iron Plug Valves, Flanged and Threaded Ends
Plug Valve vs Ball Valve
Even though both the Plug valve and the Ball valve are quarter-turn rotary valves, they have few dis-similarities. The major differences between a plug valve and ball valve; i.e, Plug valve vs Ball valve is listed below:
|Plug Valve||Ball Valve|
|The disk of a plug valve is cylindrical or conical||The disk shape in a ball valve is spherical|
|The disk size is comparatively larger.||Disk size is smaller.|
|The Sealing surface is larger in plug valves.||The Sealing surface is comparatively smaller in ball valves.|
|More torque requirement for plug valve operation. So, limited application of bigger size plug valves||Ball valves provide torque free operation and suitable for all sizes.|
|Plug valves are heavier as compared to ball valves of the same size and rating.||Lighter in weight.|
|Ease of maintenance.||Difficulty maintenance.|
|Lifespan of plug valves is normally less than ball valves||More lifespan.|
|Cost of plug valve is less.||Comparatively expensive as compared to plug valves.|
|Plug valves are full port, thus allow full flow.||Ball valves are available in full and reduced port.|
|Construction is simple.||Ball valve construction is complex.|
|Plug valves provide better flow control as compared to ball valves.|