We have learned that valves are used to either shut-off flow, change the flow path or throttle flow, and that valves are characterized by how their flow coefficient varies with relation to closure member position, but valves are typed according to the motion of their closure member. The two types of control valves are linear motion and rotary motion.
Linear Motion Control valve
Linear motion valves have a closure member that moves with a linear motion to modify the rate of flow through the valve. Linear motion valves are generally named for the shape of their closure member. Common linear motion valves include globe, gate, diaphragm and pinch valves.
Globe Type Control Valve
Globe valves are so named for their globular shaped cavity around the valve seat area. The closure member of a globe valve is a plug with a flat or convex bottom that is lowered onto a matching horizontal seat located in the center of the valve. Raising the plug opens the valve, allowing fluid flow. Globe valves have good throttling characteristics but because the flow path is not linear they have a relatively high pressure drop across the valve. Globe valves are used in throttling and shut-off applications where this pressure drop is acceptable. The three primary body designs for globe valves are Z body, Y body and angle.
Key Features:
- Flow Control: Provides excellent throttling capabilities due to the disc’s controlled movement.
- Pressure Drop: High-pressure drop due to the non-linear flow path.
- Design: Available in various body designs such as Z-body, Y-body, and angle-body, depending on the application needs.
Applications:
- Used in systems where throttling is needed, like steam control, fuel oil systems, and water systems.
Advantages:
- Superior flow control precision.
- Well-suited for shut-off and regulating applications.
Disadvantages:
- High-pressure drop.
- Not ideal for applications requiring low resistance to flow.
Gate Type control Valve
The closure member of a gate valve is a flat face, vertical disc, or gate that slides down through the valve to block
the flow. Gate valves are designed to operate in their fully open or fully closed position and therefore are found only in
flow shut-off applications. When fully open the disc is removed completely from the flow stream. This offers
virtually no resistance to flow when the valve is fully open, therefore gate valves operate with little pressure drop across the valve. Gate valves have very poor flow throttling characteristics and are not used for throttling purposes.
Key Features:
- Linear Motion: The gate moves up or down perpendicular to the flow, providing full shut-off when down.
- Low Pressure Drop: When fully open, the gate valve offers minimal resistance to fluid flow, resulting in low pressure drop.
- Rising or Non-Rising Stem: Rising stem designs show the valve position, while non-rising stems are preferred in confined spaces.
- Bi-Directional Flow: The valve can control flow in both directions.
Applications:
Gate valves are best suited for applications where the valve remains fully open or fully closed, as they are not ideal for flow throttling. They are widely used in industries such as:
- Water Supply and Distribution: Often used for isolating flow in water pipelines.
- Oil & Gas Pipelines: Gate valves are used for shutting off flow in oil and gas lines due to their tight shutoff capability.
- Power Generation: Common in steam, feedwater, and cooling water systems.
- Chemical Processing: Useful in situations where leak-tight shut-off is required.
Advantages:
- Minimal Pressure Drop: When fully open, the gate valve offers an unobstructed flow path, resulting in a low-pressure drop.
- Tight Shut-off: Provides excellent shut-off capability, making it suitable for isolation purposes.
- Bi-Directional Flow: Can be used in either direction, which adds flexibility in installation.
- Cost-Effective for Large Diameters: Gate valves are more economical than other valve types for large pipe diameters.
Disadvantages:
- Not Suitable for Throttling: Gate valves are not designed for flow regulation and should not be used in partially open positions.
- Slow Operation: These valves require multiple turns to fully open or close, making them less suitable for applications requiring fast response times.
- Large Size and Weight: Due to their construction, gate valves are bulky, especially in larger sizes.
- Wear and Tear: The gate can become damaged over time if used for throttling, as the high velocity of flow across the partially open gate can cause erosion.
- Potential for Leakage: If debris gets caught between the gate and the seat, it can prevent a proper seal, causing leakage.
Diaphragm Type control Valve
The closure member of a diaphragm valve is a flexible surface (the diaphragm) that is deformed. The main advantage of a diaphragm valve is that the stem seal is eliminated. Diaphragm valves are used mostly for shut-off service of slurries, corrosive or viscous fluids but may also be used in flow throttling applications as well.
Diaphragm valves may be used in pumping applications with a set constant pressure on the diaphragm. This allows flow to be stopped in the absence of a motive force (pump), but when a sufficient pressure is generated in the pipe to overcome the force on the diaphragm flow is allowed.
Key Features:
- No Stem Seals: Since the diaphragm acts as the closure and sealing element, there is no need for stem packing, reducing the chances of leakage.
- Flow Path: Minimal internal obstruction, reducing the risk of clogging, especially in slurry or viscous fluid applications.
Applications:
- Ideal for food processing, pharmaceutical, and chemical industries.
Advantages:
- Well-suited for corrosive and abrasive fluids.
- Tight shutoff with minimal leakage.
Disadvantages:
- Limited to low-pressure applications.
- Not ideal for very high temperatures.
Pinch Type control Valve
A pinch valve is similar to a diaphragm valve, however in a pinch valve the entire valve body is flexible and the closure member pinches the valve shut closing off flow. As a pinch valve has no internal obstructions it has a very low pressure drop and is well suited for applications of slurries or liquids with large amounts of suspended solids
Key Features:
- Flexible Tube: The only part in contact with the media is the flexible sleeve or tube.
- Simple Operation: Pinching the tube either manually or via automation controls the flow of the media.
Applications:
- Widely used in the mining, wastewater treatment, and food processing industries, particularly for slurry and bulk solid handling.
Advantages:
- No internal obstructions, leading to very low-pressure drops.
- Ideal for slurries, corrosive, or abrasive fluids.
Disadvantages:
- Limited to low-pressure and low-temperature applications.
- The flexible sleeve may wear out and need regular replacement.
Rotary Motion Type
Rotary valves have a closure member that moves with a rotary motion to modify the rate of flow through the valve. Like linear motion valves, rotary motion valves are generally named for the shape of their closure member. Common rotary motion valves include ball, butterfly and plug valves.
Ball Type control Valve
The closure member of a ball valve is shaped like a ball with a port for fluid flow. A ball valve allows straight-through flow in the open position and shuts off flow when the ball is rotated 90 degrees. Because of their quarter turn
actuation and low pressure drop ball valves are commonly found in flow shut-off applications. Depending on the particular flow port configuration of the ball they may be used in flow throttling applications as well.
Key Features:
- Operation: Quick quarter-turn operation from fully open to fully closed.
- Flow Path: Offers minimal resistance to flow, resulting in a low-pressure drop.
- Seal: Tightly shut off, making it ideal for applications where tight sealing is critical.
Applications:
- Oil and gas, chemical processing, water treatment, and HVAC systems.
- Suitable for both high-pressure and high-temperature conditions.
Advantages:
- Fast actuation and easy operation.
- Tight sealing, with minimal leakage in closed positions.
- Low-pressure drop and compact design.
Disadvantages:
- Not ideal for throttling except for specialized ball valve designs.
- Seats can wear out over time due to erosion and frequent cycling.
Butterfly Type control Valve
The closure member of a butterfly valve is a circular disc or vane with its pivot axis at right angles to the direction of flow in the pipe. Like ball valves, a butterfly valve allows straight-through flow in the open position and shuts off flow when the ball is rotated 90 degrees. Because of their quarter turn actuation and low pressure drop butterfly valves are commonly found in flow shut-off applications. Unlike ball valves, butterfly valves are generally not used for flow
throttling applications. The advantage of a butterfly valve over a ball valve is its relative compactness
Key Features:
- Compact Design: The valve is lightweight and compact, requiring less space than other valve types like gate or globe valves.
- Quarter-turn Operation: Just like ball valves, butterfly valves only require a 90-degree turn to fully open or close.
- Flow Path: Allows high flow capacity, but typically has more pressure loss than a ball valve.
Applications:
- HVAC, water distribution, wastewater treatment, and food processing industries.
- Used in low-pressure applications and where tight space constraints exist.
Advantages:
- Simple, compact, and lightweight design.
- Fast operation due to quarter-turn actuation.
- Can handle large flows with relatively low pressure drop.
Disadvantages:
- Limited throttling ability; not ideal for fine control.
- Prone to cavitation in certain applications when throttled.
Plug Type control Valve
The closure member of a plug valve is a cylindrical or tapered cylindrical shaped plug with a flow port.
Like a ball valve, a plug valve allows straight-through flow in the open position and shuts off flow when the ball is rotated 90 degrees. Like ball valves, plug valves are found mostly in flow shut-off applications. However plug valves are available in much larger sizes that ball valves.
Key Features:
- Design: The plug has one or more hollow passageways allowing flow when aligned with the inlet and outlet.
- Flow Path: Provides straight-through flow with minimal pressure drop when fully open.
Applications:
- Oil pipelines, natural gas applications, and chemical processing.
Advantages:
- Simple design and operation.
- Can handle a variety of fluids and flow conditions.
Disadvantages:
- Plug valves tend to require more force to actuate than ball or butterfly valves.
- Not typically used for throttling applications.
Needle type Control Valve
Needle valves are linear motion valves that offer precise flow control. Their closure element is a tapered needle that can be adjusted to finely control fluid flow. This makes them ideal for applications that require precise regulation of flow rates, particularly in low flow or small systems.
Key Features:
- Fine Flow Control: The needle-shaped plunger allows for accurate and gradual control of flow.
- Design: Small valve size and slow actuation for fine adjustment.
Applications:
- Typically used in instrumentation, fuel lines, and other small systems requiring fine control.
Advantages:
- Excellent for applications requiring precise flow regulation.
- Can be used for both throttling and shut-off.
Disadvantages:
- Not suitable for high flow rates.
- Requires frequent adjustment to maintain accurate flow.
Also read
Control Valve Noise and Cavitation
Difference between single seated & double seated control valves
