What is A Control Valve?

How Does A Control Valve Work?

 

Control Valve Action

Using an air-operated control valve as an example, there are two control actions available:

“Air or current to open”: As the control signal value increases, the flow restriction lowers.

“Air or current to close”: The flow restriction rises as the control signal value increases.

Failure to safety modes can also occur:

Failure of an air or control signal to close “- If the compressed air supply to the actuator fails, the valve closes under spring pressure or with backup power.

Failure of an air or control signal to open “- If the compressed air to the actuator fails, the valve opens by spring pressure or by backup power.

The types of failure operation are required by the plant’s failure to safety process control specification. In the case of the cooling water, it may fail open, whereas it may fail closed in the event of chemical delivery.

Control Valve Action Parts

 Body 

Control valve body contains the modulating element, such as a plug, globe, ball, or butterfly. There are two basic types of valves; plug and seat valves, where the plug is closed against the seat, or quarter-turn valves that have a disc, ball, or cone turning against the seat.

Trim 

It is the valve trim that is responsible for regulating the flow of water. There is a fixed-in-place seat along with a moveable plug, disc, ball, or cone. Various passage shapes can be created using the trim, which manages the flow in a deliberate manner.

There are three main types of control valve trim:

Snap Trim (also known as Quick-Opening Trim) 

Snap Valve Trims open quickly and are used for on/off operation. Among the common applications are liquid dumps, pressure reliefs, and metering. Zirconia seats are also available for erosive applications.

Nominal Trim (also known as Linear Trim) 

Typically, nominal valve trim is used to throttle liquids, control liquid level, and prevent water hammering.

Equal Percentage Trim 

The equal percentage valve trim is used in throttling applications to control the flow or the pressure of gas and vapor.

Positioner 

This device ensures that the valve has been opened to the desired degree. Friction and wear problems are thus eliminated. Positioners have the function of delivering pressurized air to the valve actuator to set the valve’s stem or shaft corresponding to a set point in the control system. A positioner is typically used when throttling is required. 

Positioners are designed to take position feedback from the stem or shaft of a valve and deliver the pneumatic pressure to an actuator to open and close the valve. This positioner must be attached to or close to the control valve assembly. 

Positioners are categorized according to types of control signals, diagnostic capabilities, and communication protocols, including pneumatic, electric, electro-pneumatic, and digital.

Pneumatic Valve Positioner 

A pneumatic device sends and receives pneumatic signals. It is intrinsically safe and can close valves with a lot of force.

Using a single-acting valve actuator with range springs, a single-action (or a three-way) pneumatic positioner sends air to and exhausts air from only one side of the valve. However, double-acting or four-way pneumatic positioners can move air on both sides of the actuator.

Electric Valve Positioner 

Electrical signals are sent and received by electric valve positioners. Electric motors can be either single-phase or three-phase run on alternating current (AC) or direct current (DC).

Electro-pneumatic Valve Positioner 

Current control signals are converted into pneumatic equivalents by electro-pneumatic valve positioners.
 

Control Valve Action Arrangement

The following image illustrates how the flow rate in a line can be controlled using a control valve. A “controller” receives pressure signals and compares them with the desired flow. If the actual flow varies from the desired flow, the control valve adjusts to overcome the difference. It is possible to control any one of a number of process variables in a similar manner. The most commonly controlled variables are temperature, pressure, level, and flow rate.