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Control Valves | Watson McDaniel

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Control Valves

What are Control Valves used for?

Control Valves are used to regulate the flow of steam, water or other fluids to maintain a process such as heating water with steam or cooling down a process using chilled water. They are only one component of a control loop and they rely on other components for proper function (i.e. controller, sensor, transducer, etc.). When properly installed and set-up with appropriate control loop components, these valves provide extremely accurate control.

Components of a Control Valve

Control Valves consist of 2 main components – a Body and an Actuator. The Body is the portion where fluid travels through, such as steam for heating. The Actuator is the component which takes an external signal such as air or an electrical signal and uses it to open or close the valve to maintain the set point.

Types of Control Valves

Control Valves are typically first described by which type of body they use: 2-way or 3-way. The 2-way body is the most common with single inlet and outlet ports for a fluid to pass though. A 3-way body has 3 ports for mixing 2 different fluids into a common line or diverting various flows.

Pneumatically Actuated

A Pneumatic Actuator is the industry-standard for fast, reliable, and accurate control, using air as the external control signal. The air signal is then balanced by springs to provide Fail Open (Normally Open or Air-to-Close) or Fail Closed (Normally Closed or Air-to-Open). If position feedback or higher close-off pressures are required, a Pneumatic or Electro-Pneumatic Positioner can be added. When an Electro-Pneumatic Positioner is selected, a separate I/P Transducer is not required as it is integral to the Positioner.

Electrically Actuated

For automation, increased accuracy, or use in facilities that do not utilize compressed air, Electric Actuators can be considered. These units offer a variety of additional options and benefits such as increased Control Valve close-off pressures, reliable supercapacitors for Fail-Safe positions, as well as field-configuration and set-up customization.

How Control Valves Work

A control loop is a process management system designed to maintain a process variable at a desired set point. Each step in the loop works in conjunction with the others to manage the system. Once the set point has been established, the control loop operates using a four-step process.

Sense

Measure the current condition of the process using a sensor, which can be a thermocouple or RTD transmitter.

Compare

Evaluate the measurement of the current condition against the set point using an electronic PID controller.

Respond

Reacts to any error that may exist between the measured temperature value and the temperature set point by generating a corrective pneumatic signal.

Affect

Actuate the control valve that will produce a change in the process variable.

The loop continually cycles through the steps, affecting the process variable (water temperature) in order to maintain the desired temperature set point.

2-Way Control Valves
2-Way valves are the most common type of control valves. These devices are capable of modulating flow at varying degrees between minimal flow and full capacity in response to a signal  from external control device. Available with Pneumatic or Electrical Actuators.
3-Way Control Valves
3-Way Control Valve is used for mixing two liquid flows, or for diverting the flow of liquid to or around a device (bypass). In order to produce a consistent flow for stable operation, the pressure drop across both flow paths (from inlet to outlet) must be nearly equal.
Control Loop Accessories
Controllers and sensors devices to help control valve function properly and more efficiently.