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.
Operation
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.
What’s the Function of a Flow Control Valve?
Flow control valves can serve a number of different functions within a hydraulic flow system depending on the specific type that is used. One of the most common uses of a flow control valve is to regulate the speed of motors or cylinders within the system.
This function is possible due to the capability of a flow control valve to affect the rate of energy transfer at any given point in a system by impacting the flow rate. The ability to reduce or increase pressure in a system has a number of benefits. System operators can use a flow control valve to rapidly depressurize a serviceable hose and change fittings quickly.
They are also used in many consumer applications such as showers, faucets, and lawn watering systems to easily reduce the amount of water consumed without impacting the overall system performance. Flow control valves are also known for their reliability and typically have a long operating lifetime as they are not prone to clogging due to their design.
Due to these flexible performance parameters, flow control valves have found wide use in applications across materials handling, food processing, and automated factory and warehouse equipment.
Types of Valves
common functional designations and their common design types include:
- Isolation Valves: Ball, butterfly, diaphragm, gate, pinch, piston, and plug valves
- Regulation Valves: Ball, butterfly, diaphragm, globe, needle, pinch, and plug valves
- Safety Relief Valves: Pressure release and vacuum relief valves
- Non-Return Valves: Swing check and lift check valves
- Special Purpose Valves: Multi-port, float, foot, knife gate, and line blind valves
Common types of valves include:
1. Ball Valve
A ball valve is a shut-off valve that controls the flow of a liquid or gas by means of a rotary ball having a bore. By rotating the ball a quarter-turn (90 degrees) around its axis, the medium can flow through or is blocked. They are characterized by a long service life and provide reliable sealing over the life span, even when the valve is not in use for a long time.
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
Butterfly valves are among the family of quarter-turn valves and work very similar to ball valves. The “butterfly is a disk connected to a rod. It closes when the rod rotates the disc by a quarter turn to a position perpendicular to the flow direction. When the valve opens, the disk is rotated back to allow the flow.
Butterfly valves are used for on-off or modulating services and are popular due to their lightweight, small installation footprint, lower costs, quick operation, and availability in very large sizes. These valves can be operated by handles, gears, or automatic actuators.
3. Check Valve
A check valve is a device that only allows the flow of fluids in one direction. They have two ports, one as an inlet for the media and one as the output for the media. Since they only allow media flow in one direction, they are commonly referred to as ‘one-way valves or ‘nonreturn valves.’ The main purpose of a check valve is to prevent backflow in the system.
A check valve relies on a pressure differential to work. They require a higher pressure on the input side of the valve than the output side to open the valve. When the pressure is higher on the outlet side (or the input side pressure is not high enough), the valve will close. Depending on the valve type, the closure mechanism is different. Unlike other valves, they do not need a handle, lever, actuator, or human to correctly work.
4. Gate Valve
A gate valve controls the media’s flow by lifting the gate (open) and lowering the gate (closed). A gate valve’s distinct feature is the straight-through unobstructed passageway, which induces minimal pressure loss over the valve.
The unobstructed bore of a gate valve also allows for a pig’s passage in cleaning pipe procedures, unlike butterfly valves. Gate valves are available in many options, including various sizes, materials, temperature and pressure ratings, and gate and bonnet designs.
5. Knife Gate Valve
Knife gate valves are designed mainly for on-off and isolation services in systems with high content of suspended solids. Knife gate valves are especially beneficial for handling slurry, viscous, corrosive, and abrasive media. The valves have a minimized pressure drop in the fully open position, they are easy to actuate, they have a relatively low weight, and are cost-effective.
6. Globe Valve
A globe valve is comprised of two halves, separated by an opening with a moveable plug or disc that can be screwed in, or unscrewed to open and close. They’re most commonly used to open, close, or throttle flow in a system.
All globe valves are designed with a stem that moves up and down to regulate flow inside the valve, contains a disc (ball, composition, or plug), and seat which is generally screwed into the valve body. Seats are designed in-plane parallel or inclined to the line of the flow.
There are three basic globe valve body designs: Tee, Angle, and Wye.
7. Needle Valve
A needle valve is used to accurately control flow rates of clean gasses or fluids. The adjustments are gradual and smooth for controlling the flow rate; however, they can also be used as a reliable shut-off valve.
However, they are generally only used for low flow rates and have a relatively large pressure drop from the inlet to the outlet. Common port sizes for needle valves range from 1/8” up to 2”. The most common application is to control the flow of gas like a propane needle valve does.
8. Pinch Valve
A pinch valve is a 2/2-way valve designed to shut off or control the flow of corrosive, abrasive, or granular media. It utilizes pressurized air to open or close the valve. In the open position, the valve has no restrictions and allows a wide range of media to pass through the bore. The flexible internal rubber sleeve in the valve keeps the media isolated preventing the risk of contamination.
Pinch valves are ideally suited for use with slurries and granular products like sand, cement, gravel, textile fiber, carbon, powder, pellets, chipping, glass fragments, etc. These valves are cost-effective, reliable, and easy to use, making them desirable in a wide range of industrial applications.
9. Plug Valve
A plug valve is shaped like a cylinder or cone and can be rotated inside the valve body to control the flow of fluids. Plug valves have one or more hollow passageways often placed horizontally to allow ease of flow through the valve when open.
The most common type of plug valve is the 2 port model with an open and closed position. The two ports are usually located on opposite sides of the valve with one passageway leading from inbound to outbound and the stem and handle located on the top. A plug valve also uses a quarter-turn valve, which is useful where quick and frequent operation is essential. The valve ends can be flanged, hub type, or butt weld.
10. Pressure Relief Valve
Pressure relief valves are vital safety devices used in a variety of machines. These devices depend on pressurized vessels or are themselves pressurized systems used to protect during times of overpressure, especially within hydraulics and liquid service machines.
These instruments perform this mighty task by automatically opening further when the pressure increases and closing when the pressure returns below its opening pressure.
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.