Control Valve Operation

The opening or closing of automatic control valves is usually done by electrical, hydraulic or pneumatic actuators. Normally with a modulating valve, which can be set to any position between fully open and fully closed, valve positioners are used to ensure the valve attains the desired degree of opening.

Air-actuated valves are commonly used because of their simplicity, as they only require a compressed air supply, whereas electrically-operated valves require additional cabling and switch gear, and hydraulically-actuated valves required high pressure supply and return lines for the hydraulic fluid.

The pneumatic control signals are traditionally based on a pressure range of 3-15psi (0.2-1.0 bar), or more commonly now, an electrical signal of 4-20mA for industry, or 0-10V for HVAC systems. Electrical control now often includes a “Smart” communication signal superimposed on the 4-20mA control current, such that the health and verification of the valve position can be signalled back to the controller. The HART, Fieldbus Foundation, and Profibus are the most common protocols.

An automatic control valve consists of three main parts in which each part exist in several types and designs:

  • Valve actuator - which moves the valve’s modulating element, such as ball or butterfly.
  • Valve positioner - Which ensures the valve has reached the desired degree of opening. This overcomes the problems of friction and wear.
  • Valve body - in which the modulating element, a plug, globe, ball or butterfly, is contained.

Control valves are devices used to regulate and control the flow, pressure, level, or temperature of a fluid or gas in a system. They operate by modulating the flow through an orifice or opening, which can be adjusted to achieve the desired control parameters.

Here’s a general overview of control valve operation:

Valve Actuation

Control valves are typically equipped with an actuator that provides the necessary force to move the valve mechanism. Actuators can be pneumatic, hydraulic, electric, or a combination of these, depending on the specific application.

The actuator receives a control signal from a controller, which determines the desired valve position based on the process conditions.

Valve Positioning

The control signal from the controller adjusts the position of the valve by actuating the actuator. The actuator translates this signal into mechanical motion to open, close, or position the valve at a specific point between fully open and fully closed.

The valve position can be controlled manually or automatically, depending on the control system in place.

Flow Control

As the valve position changes, the size of the flow passage through the valve (orifice) varies. This, in turn, affects the flow rate of the fluid or gas passing through the valve.

By adjusting the valve position, the control system can regulate the flow rate to maintain the desired setpoint or control parameter. Different types of control valve designs, such as globe, butterfly, or ball valves, offer different flow control characteristics, such as linear, equal percentage, or quick opening.

Feedback and Control Loop

Control valves are typically part of a control loop that includes sensors, a controller, and the actuator. Sensors, such as flow, pressure, level, or temperature sensors, measure the process variable.

The controller compares the measured value to the desired setpoint and sends a control signal to the actuator to adjust the valve position accordingly. This closed-loop feedback system continuously monitors and adjusts the valve position to maintain the desired control parameter within a specified range.

Ancillary Components

Control valve operation may also involve additional components, such as positioners, which enhance the valve’s accuracy and responsiveness by providing precise control over the valve position.

Positioners receive the control signal from the controller and adjust the actuator’s position to ensure that the valve is positioned correctly. Other accessories, such as limit switches, solenoid valves, or transmitters, can be added to provide additional functionality or feedback signals for monitoring and safety purposes.

control valve operation involves the precise positioning and modulation of the valve to achieve the desired control objectives, such as flow regulation, pressure control, or temperature management in a wide range of industrial processes and systems.