Control Valve Actuators

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Valves are used to control the flow of fluids in process control applications by opening, closing, or partially obstructing the fluid flow and controlling the line pressure. Depending on the flow requirements of the industrial process system, some method of changing the positions of valves is needed. Some valves are operated with hand wheels or levers. Other valves must be open, closed, or throttled frequently. Manually positioning valves in these types of situations is not practical and instead of using hand wheels or levers to control actuation, automated valve solutions such as using a valve actuator for positioning the valves.

What is a valve actuator?

valve actuator is defined as a mechanical device that moves or controls a device such as a control valve. Actuators reduce the need for operators to go to every valve that needs repositioning and operate the valve by hand.

When valve actuators are used for valve control, the actuator can be operated from a remote location such as the control room. Valve actuators can easily operate the valves accurately and quickly.

Control valve actuators are used with valves that are designed for throttling or regulating flow. Valves that are used to automatically throttle flow are generally referred to as control valves.

Control Valve Actuators

The actuator of a control valve is the assembly that provides power for moving the control valve mechanisms. In a control valve loop, actuators move the plug, ball, or vane upon receipt of a signal from the control system to allow or disallow full or partial flow. There are three types of actuators in a Control Valve. They are

  • Pneumatic Actuators
  • Electric Actuators
  • Hydraulic Actuators

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Pneumatic Actuators:

Pneumatic Actuators are the most basic and widely used control valve actuators that use an air or gas signal from an external source to produce a modulating control action. The top port sends the pneumatic signal to the actuator that exerts pressure on the diaphragm plate to move the valve stem. On loss of driver power, pneumatic actuators provide a fail-safe response.

Pros

  • Lowest cost
  • Most cost-effective actuator from a purchase point of view
  • Fast-acting, high speed
  • 100% duty cycle
  • Simple to repair
  • Long-lasting
  • Simple to operate

Cons

  • Large & heavy – difficult to install in tight spaces
  • Requires expensive pneumatic air supply system that must be maintained
  • Not as accurate due to the compressibility of air

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Electric Actuators:

Electric Actuators of a control valve are motor-driven devices. A motor rotates when an electrical signal is received. A gear reduction drive converts this rotating motion into a linear motion to drive the control valve stem for flow modulation. They are used for On-OFF applications in isolation services and for continuous positioning control.

Pros

  • Simple installation
  • Data logging options available

Cons

  • Slow
  • Limited duty cycle (typically less than 30%)
  • Not available for fail-safe applications
  • Difficult to repair
  • Fragile

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Hydraulic Actuators:

Control Valve Hydraulic actuators use a hydraulic oil as the signal fluid. When the force required to move the valve stem is high, hydraulic actuators are used. Due to non-compressibility of the liquid they exhibit stable positioning.

Pros

  • Simple Installation – complex hydraulic supply system not required
  • 100% Duty Cycle
  • Fail-safe capability
  • Remote Diagnostics/Data logging
  • Fast-acting, high speed
  • Highly accurate
  • Variable speed motor allows for custom open & closing times
  • Custom open & closing times can easily be set in the field

Cons

  • Most expensive option
  • Difficult to repair

Specifying Control Valve

The first step in specifying a control valve is to define its function in the given application. In some, it will operate as an on-off valve that opens or closes following the commands of a programmable controller on, say, a batch process. In others, it will be used to remotely set a flow rate in a process—that is, it will be used as a manually controlled variable orifice in a pipe (an open-loop application).

Finally, in more sophisticated applications, the control valve will serve as the final control element in a process control loop and respond to the sometimes infinitely small variations of a signal coming from a controller (typically a computer). The signal will be generated in response to a deviation in the desired temperature, pressure, or level of a process fluid as measured by a transmitter.

More than 90 percent of all control valves use pneumatic actuating devices—either spring-opposed diaphragm types or piston actuated.

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