What are Control Valves?

By David Clayton

Industry Trends

Control Valves Overview

A control valve regulates the rate of fluid flow as the position of the valve plug or disk is changed by an actuator. Control valves are used to maintain a process variable as close as possible to the desired set point. Controller set points are typically flow rate, pressure, and temperature. Product parameters, such as density, concentration, liquid level and others can also be controlled with control valves.

A control valve installation consists of a valve body, actuator, positioner, and accessories. The body includes a bonnet assembly and trim parts. Its design withstands fluid static pressure and differential pressure, allows fluid flow, provides pipe-connecting ends, and supports seating surfaces and a valve closure member. Actuators are pneumatic, hydraulic, or electrically powered devices that provide the force to open and close the valve. Positioners monitor and control true actuator movement to maintain a desired set-point. Accessories include electro-pneumatic transducers, pressure regulators, hand wheels, position indicators, and limit switches.Control Valve Componenets

Control Valves Classification by Hardware, Software, and Services

ARC segments the Control Valves market by the nature of supply in terms of hardware, software, and services. Hardware revenues include the Control Valves assembly as a whole; most notably valve bodies, valve actuators, valve positioners, and accessories. Hardware revenues also include valve actuators or positioners shipped separately. Software refers to sizing, control, programming, supervisory software, and custom software related to control valve sales.

Suppliers provide project and maintenance services to manufacturers. This allows manufacturers to focus on core competencies and look to suppliers to assume single-point accountability to suit their needs and provide supplier-initiated maintenance and upgrade programs to preserve their investments. Project services include project engineering, application engineering, application software development, and project management. Maintenance services include installation, startup, and commissioning. Maintenance services also include non-contract maintenance and operations services. Non-contract maintenance services include engineering, programming, training, and network management. Operations services include performance enhancement services.

Control Valves Classification by Actuator Type

ARC has divided actuators into three main types: pneumatic, electric, and other actuators. Pneumatic actuators are diaphragm or piston-based. Diaphragm-based actuators use air supply from controller or positioner, and include various styles, such as direct acting, reverse acting, and reversible. Piston-based actuators use high-pressure plant air, and are double acting, to give maximum force in both directions. Electric actuators use an electric motor and some form of gear reduction to move the valve. Other types of actuators include hydraulic and electro hydraulic actuators.

Control Valves Classification by Positioner Type

ARC has segmented valve positioners into three main types: pneumatic, electro pneumatic, and digital. Pneumatic positioners receive a pneumatic signal and supply the valve actuator with the air pressure required to move the valve to the desired position. Electro pneumatic positioners accept 4-20 mA as input signal and convert it to the required air pressure. Digital positioners have microprocessors that provide expanded functionality. Digital positioners feature two-way digital communication that provides feedback to a control or monitoring system on issues such as stem wear and other diagnostic tasks. They can also have advanced capabilities, such as embedded PID control.

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