What is Flow Control Valve?
A flow control valve regulates the flow or pressure of a fluid. Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges.
Flow control valves are normally fitted with actuators and positioners. Pneumatically-actuated globe valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball and butterfly valves are also used.
Globe control valve with the pneumatic actuator and smart positioner. The loop current and stem travel are displayed.
Control valves are normally fitted with actuators and positioners. Pneumatically-actuated globe valves and diaphragm valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball and butterfly valves are also used.
Control valves can also work with hydraulic actuators (also known as hydraulic pilots). These types of valves are also known as automatic control valves. The hydraulic actuators respond to changes of pressure or flow and will open/close the valve. Automatic control valves do not require an external power source, meaning that the fluid pressure is enough to open and close them.
Automatic control valves include pressure-reducing valves, flow control valves, back-pressure sustaining valves, altitude valves, and relief valves.
Types of Valves
Common types of valves include:
1. Ball Valve
Predominantly equipped with quick-acting 90-degree turn handles, these valves use a ball to control flow to provide easy on-off control. Generally accepted by operators to be faster and easier to operate than gate valves.
2. Butterfly Valve
Using a compact design, the butterfly valve is a quick-acting rotary motion valve ideal for tight spaces thanks to its wafer-type design. Butterfly valve bodies are offered in many different configurations.
3. Check Valve
Used to prevent backflow, these valves are typically self-activated allowing the valve to automatically opens when media passes through the valve in the intended direction and close should flow reverse.
4. Gate Valve
As one of the most common valve types, gate valves use linear motion to start and stop the flow. These are typically not used for flow regulation. Instead, they are used in fully open or closed positions.
5. Knife Gate Valve
Typically used for controlling the flow of media containing solids, the knife gate valve features a thin gate-controlled through linear action which can cut through materials and create a seal.
While not suited for high-pressure implementations, these valves are ideal for use with grease, oils, paper pulp, slurry, wastewater, and other media which might obstruct the operation of other valve types.
6. Globe Valve
Globe valves are typically applied in modulating control operations. Typically, available in three body types, T-body (as shown above), Y-Pattern, and Angle body.
7. Needle Valve
Typically used in small diameter piping systems when fine, accurate flow control is needed, Needle valves get their name from the point on a conical disc used within.
8. Pinch Valve
Often used for handling solid materials, slurries, and liquids with suspended solids, pinch valves use a linear motion. Typically, Pinch Valves feature an internal sleeve to isolate the media.
9. Plug Valve
Using a quick-acting quarter-turn valve handle, these valves control flow using tapered or cylindrical plugs. They provide some of the best ratings when tight shutoff is essential and are reliable in high-pressure or high-temperature environments.
10. Pressure Relief Valve
Used to help improve safety, these valves are spring-automated and will help to return a system to the desired pressure during over-pressure events.
Application of Flow Control Valve
Process plants consist of hundreds, or even thousands, of control, loops all networked together to produce a product to be offered for sale. Each of these control loops is designed to keep some important process variable, such as pressure, flow, level, or temperature, within a required operating range to ensure the quality of the end product.
Each loop receives and internally creates disturbances that detrimentally affect the process variable, and interaction from other loops in the network provides disturbances that influence the process variable.
To reduce the effect of these load disturbances, sensors and transmitters collect information about the process variable and its relationship to some desired set point. A controller then processes this information and decides what must be done to get the process variable back to where it should be after a load disturbance occurs.
When all the measuring, comparing, and calculating are done, some type of final control element must implement the strategy selected by the controller. The most common final control element in the process control industries is the control valve.
The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point.