What is Control Valve? Working Principle & Function | Linquip
Control Valve

What is Control Valve? Working Principle & Function

Control Valves respond to a controller's signal by changing the size of the flow passage. As a result, the flow rate may be regulated directly, and pressure, temperature, and liquid level can be controlled as well. Control valves are pieces of Industrial Equipment that employ a set point to adjust for load disturbances when controlling the flow of a fluid such as water, gas, steam, or chemical compounds. As a general rule, control valves should be set to function between 20 and 80 percent open at maximum flow rates and no less than 20 percent open at minimum flow rates. For the selection of the right control valve for a process, several factors must be considered, including the valve's flow characteristics, size, noise, cavitation potential, body material and trim, actuator size and type, and dynamic response to changes in control signals. The choice of pressure and temperature depends on the application and the pressure conditions.



Basics of Control Valves | Linquip

Control valve (Reference: www.vectorvalves.com)


What is Control Valve?

The operating principle of a control valve is to regulate the flow of fluid by changing the size of a flow passage in response to a control signal. In this way, the flow rate can be directly controlled as well as other process parameters such as pressure, temperature, and liquid level.

Basics of Control Valves

Control valves are power-operated devices that control the flow of gas, oil, water, or steam. It is a final control element and a crucial part of a control loop. Among the final control elements in industry today, the Control Valve is by far the most common.

A control loop consists of a valve, a transmitter that measures the controlled parameter (usually pressure, flow, level, or temperature), and a controller (a computer of sorts). An error in the variable to be controlled (such an error is detected by the transmitter) triggers a signal change by the controller sent to the control valve, which responds by altering the flow rate through the valve enough to restore the variable (such as pressure).

In a control valve assembly, there are four components: a valve body (either with a reciprocating or rotating stem), a valve positioner (either a controller or computer that translates electronic signals from the controller into air signals to control valve stem position), an airset (or regulator) to supply air pressure to the valve positioner, and an actuating device (usually a diaphragm spring).

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Working Principle of a Control Valve

There are three ways to operate control valves: electronically, pneumatically, and hydraulically.

Pneumatic Actuated

Pneumatic actuators modulate a control action using an air or gas signal from an external source. Pneumatic signals are sent to the actuator through a top port. It then distributes the signal across the diaphragm of the actuator. In consequence, the diaphragm exerts pressure on its plate. As a result, the valve stem moves downward, thereby stoking the control valve.

Electric Actuated

This device is motor-powered. The motor shaft rotates when an electrical signal is applied. Flow modulation is achieved by converting this movement into a linear motion, which drives the valve stem.

Hydraulic Actuated

The hydraulic actuator is similar to the pneumatic actuator but uses hydraulic oil as the signal fluid to control the valve's action. When the valve stems need to move with high force, they are used instead of pneumatic or electric actuated valves.

Components of a Control Valve

The parts of a control valve are as follows:

Body

This is a kind of pressure vessel that contains an opening or orifice. The controlled liquid passes through the valve body. This allows the flow regulation behavior to be monitored.

Trim

In addition to the body, the trim is another part of the valve that comes in direct contact with the fluid. The trim includes the seat, disc, plug, and stem.

Actuator

Actuators provide force to operate valves using electric or pneumatic media.

Bonnet

The bonnet provides a mounting for the guide and actuator and a path for the stem. The centerpiece, packing, packing nut, and guide make up this component. A packing connects the bonnet to the stem and acts as a fastener. Without it, leaks are more likely to occur.

Function of a Control Valve

Function of a Control Valve | Linquip

Arrangement of a control valve to modulate the flow rate in a line (Reference: www.steamline.com)

Pressure or flow rate can be controlled automatically with control valves, and they are available for any pressure. The "controller," as shown in the above figure, receives the pressure signals, compares them to the pressure drop of the desired flow, and adjusts the control valve to allow or prevent the flow depending on the difference between the actual and desired pressure drop.

When different plant systems operate at pressures and temperatures that call for Class 300 valves, sometimes (if the design allows) all control valves chosen will be Class 300 for interchangeability. If none of the systems exceed the ratings for Class 150 valves, then this is not necessary.

Various arrangements can be devised to control various process variables. The most common controlled variables are pressure, temperature, level, and flow rate.

A control loop would not work without control valves, which are often the most overlooked. It usually occurs because the instrument engineers lack familiarity with the various facets, terminologies, and engineering disciplines involved in various operating conditions, including fluid mechanics, metallurgy, noise reduction, and piping and vessel design.

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Usage of Control Valves

A process plant consists of a number of control loops networking together to produce a product. A control loop is designed to keep crucial process variables such as fluid levels, pressures, and temperatures within defined limits. By doing so, quality can be assured for the final product. There are internal disturbances in each of these loops that may affect the process variables.

To collect data about the process variables, sensors and transmitters are employed. Control valves are end devices used to control process flow based on this data. Using this instrument helps decide what needs to be done so that the process variable is returned to the set point.

In addition to reducing water losses, control valves can help manage water supplies by maintaining a certain level of pressure or flow regardless of fluctuations in the supply network. Pressure and flow are managed efficiently using automatic control valves, resulting in:

  • Less water loss due to leaks,
  • Reduced risk of pipe bursts and water hammer,
  • Fewer interruptions to consumers,
  • Fewer contamination risks,
  • Lower costs for water companies.

Common Applications of Control Valves

In most fluid handling methods, control valves are used as the final control element. The control of a process or process-related utility requires some elements of control in:

  • Process industries, whether oil and gas,
  • Refineries,
  • Petrochemical industry,
  • Chemical industry,
  • Specialty chemicals,
  • Pharmaceuticals,
  • Power plants.

FAQ about Control Valve

  1. What Are Control Valves Used for?

. Flow, pressure, temperature, or other variables are controlled by control valves in many processes. Variables such as pipe size, system pressure, system flow conditions, and process conditions will determine the type of valve that is used.

  1. How Do Control Valves Work?

A control valve regulates the flow or pressure of a fluid. In most cases, they are activated by signals generated by independent means such as flow meters or temperature gauges. They are equipped with actuators and positioners.