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Plug valves are a form of quarter-turn valve that features a cylindrical shaped or tapered plug that is used to block or allow the flow of fluid through the valve. As the valve handle is rotated through 90o from open to closed, the internal plug rotates, and the solid portion of the plug is moved in line with the valve ports, causing the flow to be blocked. By rotating the handle to the open position, the internal plug is again rotated, but in this position the plug’s opening is in line with the valve ports, allowing the fluid to pass through the valve. In many respects, plug valves are similar in operation to ball valves, more about which can be found in our related guide All About Ball Valves.

This article will provide a review of plug valves, including the components & operation, the different types, key specifications, and advantages & limitations. To learn more about other types of valves, review our related guide Understanding Valves.

Plug Valve Components and Operation

Plug valves components are similar to those of other types of fluid handling valves – the primary ones being the:

• Valve body
• Valve plug
• Valve port
• Valve stem
• Valve actuator

Figure 1 below shows a simplified illustration of the cross-section of a plug valve, identifying these components.

Figure 1 - Simplified diagram showing the cross-section of a plug valve.

Image credit: Christian Cavallo, LLC

Valve body

The valve body or housing contains the inlet and outlet ports of the valve and holds the other components of the plug valve in an integrated assembly. Valve bodies come in different materials, including steel, brass, bronze, cast iron, stainless steel, aluminum, PVC, or PVDF. The selection of material depends on the operating conditions to which the valve will be exposed. For applications involving a corrosive chemical (sulfuric acid, for example), lined plug valves are utilized. These valves are fabricated with a protective coating of PTFE, PVDF, PVDC, polyethylene, polypropylene, or a similar polymer applied to the interior of the valve body to which exposure can occur. The lining or coating resists the corrosive effects of the chemical and protects the valve body from degradation. When sanitary conditions are essential, such as in food processing applications, sanitary stainless steel plug valves are employed.

Valve plug

The valve plug is the component that serves as the flow control mechanism for the plug valve – it is functionally similar to the ball in a ball valve and serves to restrict or pass fluid flow depending on how the valve port in the plug is aligned with respect to the channel in the valve body. Many plug valves have tapered plugs, some do not and are cylindrically shaped.

Valve port

 The valve port is the opening that is cut into the valve plug. The shape of the opening is typically rectangular but may also be diamond-shaped or round as shown in figure 2 below.

Figure 2 - Outline diagrams of plug valve port options

Image credit: https://hardhatengineer.com/what-is-plug-valve-a-complete-guide-for-engineer/

Valve stem

The valve stem attaches the valve plug to the handle of the valve (or actuator) and enables the plug to rotate as the handle is turned, which causes corresponding rotation of the valve stem. The stem connection to the exterior part of the valve body passes through a section called the bonnet or cover, which contains seals and packing to prevent fluid leaks. Most plug valves are what are called top-entry valves, meaning that the valve cover is located on the top of the valve body and the plug itself is tapered downwards, i.e. decreases in diameter from top to bottom. Some designs, known as inverted plug valves, have the plug tapered upwards and installed from the bottom of the valve body. Plug valves with metal valve bodies are usually constructed so that they can be serviced and repaired without the need to physically remove the valve from the piping system. Other lower-cost plug valves, such as plastic plug valves used for drinking water applications, ae generally non-repairable items.

Valve actuator

The valve actuator serves to change the valve position or state from open to closed and vice versa. Plug valves can be manually actuated by the use of a handle, lever, handwheel, or a powered actuator – sometimes referred to as operators. Because the flow control element (plug) has a larger surface area than the ball in a ball valve of the same size, the operating torque of a plug valve can be higher. This higher torque requirement means that the handles on plug valves tend to larger than those on ball valves, a fact that needs to be considered when planning for valve placement and installation.

For larger sized plug valves, a geared operator is used to provide mechanical advantage to keep operating torques manageable for manual actuation.

The basic operating principle of plug valves is simple – a quarter-turn movement of the valve handle or actuator changes the plug position so that the valve port moves from being parallel to the direction of flow to be perpendicular to the direction of flow. That motion changes the plug valve state from open to closed. Reversing the handle position changes the valve state back to open.

Types of Plug Valves

There are several types of plug valves available, which include:

• Lubricated plug valves
• Non-lubricated plug valves
• Multiport plug valves

Lubricated plug valves make use of a lubricant injected under pressure into the valve between the plug face and the valve seat. The addition of the lubricant provides reduced friction, lowers the operating torque of the valve, and provides port sealing. Lubricated plug valves are generally considered to be high maintenance, as additional lubricant needs to be added frequently to maintain the sealing capability of the valve, sometimes after just several or a single operation. This requirement makes them suitable only for applications where cycling the valve is usually not needed.

It is important to note that the selection of lubricants must be made based on the nature or properties of the media or fluid that is flowing through the valve. The lubricant acts as a renewable seal and must not be dissolved and carried away by the fluid passing through the valve. Should this occur, it would result in both the contamination of the fluid as well as the destruction of the seal that exists between the plug and the valve body, leading to leakage. The anticipated fluid temperature must also be taken into account when selecting a lubricant for use.

Non-lubricated plug valves do not make use of an injected lubricant for sealing but instead rely on an elastomeric liner which sits at the bottom of the plug between it and the valve body. This liner or sleeve serves to reduce friction between the plug and the valve body as well as providing for a bubble-tight seal in the valve. Non-lubricated plug valves are used in applications where maintenance would be difficult to perform, examples being with reactive fluids or hazardous chemicals.

While many plug valves contain a single inlet and outlet port, multiport plug valves provide additional numbers of ports for connections and allow plug valves to function as more than just simple shutoff valves. Multiport plug valves are available as 3-, 4-, and 5-way designs which can enable a single multiport valve to replace several conventional shutoff valves. The valve positions can serve to connect one inlet port with one or more outlet ports, thereby enabling the plug valve to function to divert flows or mix flows together. These valves are described as having an “L” or “T” configuration.

Plug Valve Key Specifications

Plug valve specifications that are typically listed by manufacturers or suppliers are summarized below. Note that this information is for general knowledge and will help to prepare for engagement with suppliers. Differences in the manner in which plug valves are specified can exist from supplier to supplier.

• Valve body material – the material from which the valve body or housing is fabricated, such as stainless steel, PVC, PVDF, brass, cast iron, or bronze.
• Number of ports – the porting design of the valve, such as 2-way, 3-way, or 4-way.
• Plug material ­– the material from which the flow control plug is constructed. Common material options include brass, stainless steel, or plastic.
• Valve shape – represents the valve shape as being inline, L, or T.
• Pipe size – the diameter of the pipe into which the valve is being fitted. Common plug valves sizes align with pipe sizes to facilitate ease in sizing and installation.
• Port shape – reflects the port design such as rectangular, diamond, or circular.
• Connection type – represents the desired port connection fitting style for the valve. Common options included threaded, compression, barbed, flanged, and socket weld.
• Gender – identifies whether the connection type is male or female.
• Thread type – for threaded valve connections, reflects the thread type on the valve port such as NPT, UNF, NPTF, BSPT, etc.
• Seat material – the material from which the valve seat is fabricated, such as metal or a soft seat such as a fluoropolymer.
• Seal material – the materials used in the valve seal.
• Flow rate or flow coefficient – a measure of the ability of the valve to pass fluid, in units of volume per unit time. Sometimes, the flow rate is expressed as a flow coefficient, Cv, which is defined as the number of U.S. gallons of water @ 60oF that can pass through the valve per minute at a pressure drop of 1 psi.
• Maximum rated pressure or pressure range – establishes the safe range of operating pressure for the valve. This value may also reflect operating temperature.
• Media or fluid application – indicates the type of liquid or gas to which the valve will be exposed, such as water, natural gas, fuel oil, air, hydraulic oil, food, solvents, etc. The nature of the media and its properties will determine the suitable materials for the valve body, seals, seats, ball, and other components.
• Actuation type – describes the means by which the plug valve will be actuated, such as manual (handle or lever), or electrical, for example.
• Operating torque – the amount of torque needed to turn the valve between the open and closed positions, measured in N-m or lbs-F.

Plug Valve Advantages & Limitations

Plug valves have several advantages, which are:

• They are simple in design
• They are fast to open and close given that they are quarter-turn valves
• For repairable models, maintenance can be accomplished while installed
• Models are available as multiport designs
• Flow resistance/pressure drop is minimal
• They have  a small package size

Some of the limitations of plug valves are that they are not able to precisely regulate flow, can have a higher operating torque due to friction, and may require an automatic actuator in larger sizes.

Summary

This article provided a summary of plug valves, including their components, the different types, key specifications, and advantages & limitations. For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories, including over 500 suppliers of plug valves.

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