The design provides a means of linear actuation in which the gate moves into the flow path and seats into a recess opposite the stem, creating a strong seal. When opened, it is completely out of the flow path, resulting in zero flow obstruction or pressure drop.
For processes involving slurries, suspended solids, or grit, knife gate valves are a specialized form of gate valve that features a bevelled edge on the gate. This shape helps the gate cut through any build-up that might accumulate in the recess and reduce the amount of torque needed to fully close the gate.
Because the valve uses a linear actuation and threaded stem, moving from full-open to full-close states takes longer than most degree actuation valve types, such as butterfly valves or plug valves. However, this comes at the cost of requiring more space above the valve to accommodate the rising stem. Extended throttling use can create vibrations and resonance, leading to damage to the valve internals, seals, gaskets, and other critical elements. Gate valves also work best in high-pressure systems.
Use in low-pressure systems where a tight seal is critical might require special accommodations as low-pressure operation might lead to seeping at the valve while closed. The center of the plug features a flow channel of varying size and shape that ultimately determines the pressure drop and throughput characteristics of the valve.
Also, since the stem is not in the flow path, there are fewer worries of corrosion or wear as you might encounter with gate valves depending on their design.
Plug valves are also quick actuating, requiring only a degree throw and providing the same easy visual status checking as butterfly valves thanks to their valve handles. However, these handles can take up space, so be sure to account for the full range of motion required when implementing a plug valve in your system designs.
Because the plug sits across the entire flow path in the closed state, this means it can require considerable effort and place torque stress upon the valve to move the plug while under higher pressures. Lubricated plug valves work to mitigate these concerns by continually lubricating the metal plug, reducing the operating torque required while also providing excellent temperature resistance.
However, for lower temperature processes, there are also options with elastomeric coatings, optimizing the seal and torque requirements at a lower price point. They operate in a similar manner, providing most of the benefits and eliminating the concern of the valve requiring excessive torque to adjust when under higher pressures. However, their theoretical seal strength is lower. Reduced bore or reduced port valves restrict the flow of medium as the size of the bore is less than the diameter of the pipeline.
Ball valves can also be floating or trunnion mounted. Floating ball valves are used in applications where a bi-directional shut-off is required. Trunnion mounted ball valves are used in high-pressure, high-temperature operations. Most plug valves are full bore, especially rectangular port valves.
Round port and diamond port valves can have reduced bores as well but they can only be used in low-pressure applications. There are mainly two types of plug valves, lubricated plug valves and non-lubricated plug valves. Lubricated plug valves are usually made of metal and they feature a lubricant chamber that constantly lubricates the plug of the valve. The lubricated plug is easier to move, has low friction, and is corrosion-resistant.
Lubricated plug valves can be used in larger applications and handle high-temperature services. Non-lubricated plug valves feature a non-metallic elastomeric sleeve that is installed between the plug and the body of the valve. This sleeve reduces any friction between the plug and the body. However, the non-metallic seat means this type of valve cannot be used in high-temperature services. Generally, ball valves have a greater lifespan than plug valves.
A plug valve has to deal with greater torque and more of its surface area comes in contact with the medium. This means the valve has more wear and tear and is more prone to corrosion. Ball valves have special mechanisms in place that prevents the medium from being in constant contact with the disc. Though it has more moving parts, the low torque means less wear and tear. Ball valves can be controlled using manual, electric , hydraulic, pneumatic , and many other kinds of actuators.
They are easy to open and close and require very little force even when operating under high-pressure conditions. Plug valves can be operated using manual and electric actuators. They can be operated through pneumatic actuators as well but they are very expensive to install. Plug valves can be difficult to open and close in high-pressure operations due to the high amount of torque. This is the reason why plug valves are usually not used for larger applications.
Both ball and plug valves can be customized to suit specific applications. However, ball valves have a greater margin for customization than plug valves.
The sheer variety of ball valves available in the market means you have a lot of features to choose from. Ball valves can be unidirectional, bidirectional, or multidirectional. They can have one , two , or three-piece body as well as multiple ports. You can find top-entry, end-entry, side-entry, and split-body ball valves. Ball valves can also be customized to have double block and bleed capabilities.
Plug valves can also be customized but in a limited capacity. This is because they feature a simple structure with little room for improvement. You can find multi-port plug valves in the market that start from 2 ports and go all the way up to 5 ports.
Though care must be taken while operating multiport valves as they might not provide a very tight shut-off. All other kinds of customizations on a plug valve like installing electric or pneumatic actuators or adding anti-friction components are very expensive.
Both ball and plug valves are used extensively in different piping applications. Typically, valve selection boils down to what task it will carry out and in what space it will operate. Ball valves are a type of control valve with a spherical disc, which controls the flow through it.
The sphere has a hole, or port, through its middle. This is where flow occurs, when that port is in line with both ends of the valve. When the hole is perpendicular to the ends of the valve the valve is closed and flow is therefore blocked. There are four types of ball valves:. A full port or full bore ball valve has an over-sized ball, so that the hole in the ball is the same size as the pipeline, resulting in lower friction loss.
Flow is unrestricted but the valve is larger and more expensive. This is only used where free flow is required, for example in pipelines requiring pigging. This results in the flow area being smaller than pipe.
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