Thursday, September 14, 2017

Electric Actuator for Linear and Quarter Turn Control Valves



Many process control valve installations present the option of selecting either electric or pneumatic actuators as part of the control component train. Pneumatic actuators have been in use for many years, but advances in electric motor design that delivered greater torque and more precise operation have brought electric valve actuators into a prominent market position.

Electric actuators are compact and comparatively self contained, requiring only cable connections and none of the additional devices sometimes needed for a pneumatic installation. There are some points of advantage to consider with electric actuators. Rotork introduced their CVA line of electric actuators almost ten years ago, making it something of a mature product now. Here are some advantageous points about the CVA actuators that likely apply generically as well.

  • Setup is accomplished with a Bluetooth enabled device which provides quick calibration of open and closed positions, as well as establishment of valve setup parameters.
  • A separately sealed electrical connection compartment keeps motor and mechanical compartment isolated from the environment while electrical connection section cover is removed.
  • An on board datalogger records thrust and position data over time for use in asset management and service functions. Data can be downloaded by Bluetooth or transmitted by common protocol to another station.
  • Change in setpoint produces a rapid and precise change in valve position with high resolution accuracy and repeatability.
  • Actuator can be programmed to move to a preset condition in the event of a loss of electric power. The energy to achieve the failsafe position is stored in the actuator.
  • Force balance positioning used in pneumatic valves, with spring force vs. air pressure, has resilience that can result in a change in position of the valve trim in response to a bump in system pressure. Resistance from the gear train on electric drives prevents this movement.
  • Static friction of the valve packing and other parts increases the amount of force to intially get the valve moving toward a new position. The additional time required to build air pressure and force to overcome static friction results in delayed valve response, then overshoot of the new setpoint. A combination of a sensor system and the mechanical drive section of an electric actuator eliminates overshoot and delayed response.

Electric actuators can be had in quarter turn and linear versions, with torque ranges suitable for a broad range of process control applications. The datasheet below, from Rotork, provides useful illustrations of the actuator interior, along with additional detail about electric actuators. Share your process control valve requirements and challenges with product application specialists, combining your own process knowledge and experience with their product application expertise to develop the best solutions.