Rugged instrumentation for reliable measurement and control

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Valve Hold in Place at Loss of Electric Signal

Many applications require a control valve to remain in the position it held prior to loss of power or loss of signal from the control system: Valve Hold in Place or Lock in Last is applied upon loss of the control signal. The Valve Hold in Place strategy uses solenoid valves in the Open/Close airlines between the positioner and actuator and it uses a trip amplifier, like the Moore Industries Model ECA, monitoring the 4-20mA control signal.

Alarms 2 Valve Hold in Place at Loss of Electric Signal

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The relays of the trip amplifier are controlling the Open/Close status of the solenoid valves.  In normal operation the two solenoid valves are energized and the valves are open allowing passage of the air from the positioner to the actuator. Upon loss of the 4-20mA control signal, the trip amplifier de-energizes its relays closing the solenoid valves. With the solenoids closed the air is trapped within the actuator’s loading chamber, locking and holding the control valve at the last position. Because the trip amplifier’s relays are failsafe, the same solenoid closures would occur upon power failure to the trip amplifier. When loss of signal or failure occurs a secondary contact alerts the controller that the valve is in a Hold in Place position.

The trip amplifier can switch solenoids with a coil voltage up to 250 VAC/5A or  30VDC/5A. The response time of the trip amplifier to a loss of the control loop is <50mS.

Key attributes:

  • Fast speed of response for the failsafe relays
  • Contacts ratings for both DC and AC solenoids
  • Fully-adjustable trip point