A plug valve is a type of valve that operates through a quarter-turn rotary motion. It is designed to provide a straightway passage through its ports, allowing liquid to pass smoothly with slight turbulence when the opening plug is in use. The valve can be fully closed or fully open, enabling flow in either direction. This article explains the plug valve working, types, and applications.
What is a Plug Valve?
A plug valve is a type of valve that uses a conically tapered or cylindrical plug to regulate the fluid flow through a system. The plug of the valve rotates inside the valve body. This plug has one or more hollow passages or ports.
When the valve opens, the fluid flows through these hollow passages of the plug. The plug valves have a simple design and are generally very inexpensive. If the plug is conically tapered, a handle or stem will normally be connected to the plug’s larger diameter end.
This valve doesn’t contain a bonnet but generally has a handle attached to the plug end. You can see this handle outside the valve body. The valve handle and stem are usually integrated into a single piece with a simple design. The handle typically has an L-shaped design.
The other end of the plug is also extended up to the external side, but there is a mechanism to secure the plug inside the valve body.
A bi-directional valve with two positions is the simplest and most famous type of plug valve:
- Open position to allow fluid flow
- Closed position to stop the flow
The ports act as the opening in the valve body through which liquid can flow in and out. Since the ports generally install on the opposite ends of the valve body. Due to this, the plug takes a 1/4 turn to change from the open position to the closed position. Therefore, this valve is also known as a quarter-turn valve. Usually, it has a mechanism that limits the movement of the handle to 1/4 turn.
A slightly conical metal plug valve also uses as a simple shut-off valve in domestic natural gas pipelines.
Working of Plug Valve
The body of the plug valve has a parallel or conical seat in which a plug is installed. This plug has one or more ports. The position of these ports decides the opening or closing position of the valve. The ports of the valve are also called passageways or openings through which media enters and exits.
The 90-degree rotation of the plug completely closes or opens the flow of fluid. The efficiency of these valves is not as good as that of the ball valves. They can only be operated as fully closed or fully open.
As the plug ports are aligned with the suction and discharge ports, fluid starts to flow inside the valve. The pressure loss may also occur across the decreased area of the port. This pressure loss may be reduced by using a full-area cylindrical plug.
As the operator turns the handle to a full 90° turn (or quarter-turn), the ports of the plug make a 90° angle to the flowing fluid, and the ends of the plug rotate through a sealing device (lubricant, sleeve, etc.).
As the 90° turn of the plug is completed, the plug port becomes perpendicular to the flow, completely stops the fluid flow, and closes the valve.
In the case of throttling, the plug is rotated to a 45° turn. In such a position, the plug can drop the pressure two times more than normal.
As you move the plug away from the full port position, the inlet port flow area decreases at the pressure drop point. After this, the flow enters the plug’s full-port area, where the pressure recovery process occurs.
The simple plug valves are ideal for both low-temperature and low-pressure services, which can be made in various sizes from 240 mm to 310 mm.
In the case of numerous changes in the fluid temperatures, the differential expansions of the plug valves are unavoidable, resulting in loss of tightness or excessive operational stiffness.
Read More: Working of Check Valve
Types of Plug Valves
The plug valve has the following major types:
- Lubricated valve
- Non-Lubricated valve
- Multi-Port plug valve
- Eccentric valve
- Expanding valve
1) Lubricated Plug Valve
As the name of the lubricated plug valve represents that it uses lubricant for its easy operation over a wide range of working pressures. It uses a layer of lubrication in between the body and the plug. This lubrication layer provides a seal and prevents the leakage of fluid.
The valve must be lubricated regularly. The proper lubrication of the valve also extends its service life.
The sealant must have the capability to withstand the temperature of the fluid, even the debris or solid mix with the fluid. The maximum lubricants of the plug valves have plastic sealants.
It is important to use an effective sealant with sufficient resilience, chemical resistance, and the capability to make a waterproof seal around any part of the body. The lubricating layer also stops the corrosion formation on the metal surface between the body and plug.
2) Non-Lubricated Plug Valves
As the name of the non-lubricating plug valve represents that it doesn’t need lubricant film for operation. These types of plug valves are most commonly used in low-pressure lines. This valve has an elastomeric body sleeve or liner. This sleeve or liner installs in the cavity of the valve body.
The polished, tapered plug works as a wedge and compresses the sleeve against the body. In this way, the sleeve decreases the friction between the body and the plug.
Non-lubricated plug valves need less maintenance than lubricated valves. Therefore, these valves are also employed in place of lubricated plug valves for applications that require minimum maintenance or for applications in which fluid may be solidified or trapped and clog the valve.
The non-lubricated plug valves have the following major types:
- Fully lined plug valves
- Elastomer-sleeved plug valves
- Lift-type valves
Read Also: Different Types of Valves
3) Multi-Port Plug Valves
These types of plug valves are used for transport lines and diversion applications. A single multiport valve has the capability to provide the services of 3 or 4 gate valves or other shut-off valves.
In some cases, these valves can’t close the fluid flow fully. They need proper safety and care for the proper operation.
4) Eccentric Plug Valve
This type of valve contains a half plug. Therefore, these valves are beneficial for applications that need high seat forces and minimum friction from the open position to the shut-off position. The torque seated valve also has an excellent closing ability.
These valves are used in a variety of flow control and shut-off applications, such as air, sludge, slurries, sewage, dirty and clean water, and many other applications.
5) Expanding Plug Valves
This valve has complicated construction. It has many components to mechanically expand the valve to provide true double block and drain functions with a single valve.
This type of plug valve uses a mechanism that revolves between open and shutoff positions to protect the seals from the flow path. Since the body and the sealing element don’t touch each other during the turning, which stops the abrasion or wear of the seal.
Expanding plug valves are also used in applications that don’t require double insulation to avoid media contamination.
Read Also: Working of EGR Valve
Components of the Plug Valve
The plug valve has the following major parts:
- Valve Body
- Stem
- Handle
- Plug Ports
- Plug disc
1) Valve Body
The body of the valve is also known as a casing or housing. It contains all the internal parts of the valve.
It prevents the internal parts from damage due to an external falling object or any other external object. The valve body attaches to the pipe via an end connection.
2) Stem
The stem uses to connect the actuator with the plug disc. The stem is connected to the handle or actuator. This part of the plug valve transfers the motion of the handle to the disc.
As the disc receives motion, it opens or closes the valve according to the requirements. The collar–diaphragm assembly or packing helps to stop stem leakage.
The leakage of the valve seat can be stopped by pressing the plug into the sleeve or other sealing processes.
3) Handle
It is an external part of the valve. The handle plays a big role in the valve opening or closing. An operator is required to operate the handle.
As the operator turns the handle, it transfers its motion to the stem, which further transfers motion to the plug disc.
4) Plug Ports
The plug disc has one or more ports. These ports are also known as passageways or passages. As the fluid starts flowing through the system, it passes through these ports.
5) Plug Disc
The plug disc has a tapered design. This disc contains one or more ports. It connects with the stem. It receives the motion of the valve handle through the stem.
The plug valve contains a solid metal disc with tapered ends. The design of the valve disc makes it different from other valves such as the globe valve and ball valve.
Read Also: Working of Gate Valve
Plug Valve Construction Materials
There are various materials used for the construction of the plug valves. The following are the most common materials that are used for the construction of the plug valve:
- Plastic
- Brass
- Bronze
- Stainless Steel
- Steel
Plug Valve Symbol
The plug valve symbol is given in below given diagram. This symbol uses to differentiate the plug valve from other types of valves.
Advantages and Disadvantages of Plug Valves
Advantages of Plug Valve
- The plug valve has simple construction and operation.
- It has fewer rotating parts.
- This valve opens and closes the fluid flow very quickly.
- It requires very low maintenance.
- The plug valves offer very low resistance to the flow.
- The plug valves provide more tight sealing than the ball valves.
- It has a low initial cost.
- These valves have easy maintenance.
- It can be removed easily for maintaining and cleaning.
Disadvantages of Plug Valve
- The plug valve has more weight than the ball valve.
- You can’t operate the large plug valves manually. Therefore, you will need an actuator to operate them.
- These are not ideal for throttling services.
- This valve needs high operational force because of its high frictional force.
- There is pressure loss due to decreasing ports.
Application of Plug Valves
- The plug valve is used to open and close the fluid flow.
- The plug valves are used for sewage, mud, slurries, hydrocarbon, vapor, and gaseous applications.
- They are also employed in different compressors and pumps.
- They are employed in low-pressure, low-temperature, high-pressure, and high-temperature applications.
Plug Valve vs Ball Valve
The main difference between the ball valve and the plug valve is given below:
| Plug Valve | Ball Valve |
|---|---|
| The plug valve uses a plug disc to control the fluid flow. | The ball valve uses a ball disc to control the fluid flow. |
| It has a conical or cylindrical disc. | It has a spherical disc. |
| The plug valves have easy cleaning. | The ball valves have tough cleaning. |
| This valve has a large disc. | It has a small disc. |
| The plug valve has a large sealing surface. | The plug valve has a small sealing surface. |
| It requires more torque for operation. | It doesn’t need operating torque. |
| The plug valves have more weight than the same size ball valves. | It has lightweight. |
| They have easy maintenance. | They have difficult maintenance. |
| The plug valve has less service life than the ball valve. | The ball valve has a long service life. |
| This valve doesn’t have many rotating parts as a ball valve. | It has more rotating parts. |
| This valve has less cost. | It is expensive. |
| It is only available in full port design. Therefore, it only allows full flow. | These valves are accessible in both reduced and full port designs. |
| It has an easy and simple construction. | It has complex construction. |
| This valve provides a full tight sealing. | It can’t provide a tight sealing as a plug valve. |
| The plug valve provides excellent flow control. | The ball valves can’t provide excellent flow control as plug valves. |
Read Also: Working of Ball Valve
FAQ Section
How many turns on a plug valve?
A plug valve has a conical plug disc. This plug takes a 90° turn to close the valve and stops the fluid flow.
What is a Plug Valve Used for?
The plug valves are most commonly used for low-temperature and low-pressure applications. These valves are best suitable for light industrial and commercial services such as directional flow control, and handling of liquid or gas fuels. The plug valves use in many applications because of their multiple port design options, minimal flow resistance, tight sealing, in-line maintenance capabilities, and smooth design.
Can plug valves be used for isolation?
Plug, ball, and gate valves are generally employed to deliver an excellent and tight shut-off. Butterfly valves and gate valves can’t provide tight shut-off as plug valves due to wear on their seat or plug.
What are the types of plug valves?
- Lubricated valve
- Non-Lubricated valve
- Multi-Port plug valve
- Eccentric valve
- Expanding valve
What are the components of a plug valve?
The main components of the plug valve are the body, stem, handle, plug, and ports. The handle is used to connect the stem that attaches it to the main plug.