Capacitor Vs Batteries in Failsafe Electric Valve Actuators

Battery back-up systems or universal power supplies have been used as a safety system for many decades to provide an alternate power source to drive electric valve actuators to a pre-set failsafe position when external power fails. Nickel Cadmium batteries were generally replaced by Lithium Ion to improve battery performance and charge retention. However, all batteries require re-charging and have a limit to the number of times that the charge can be used before a re-charge. Failure of a battery trickle-charge system could result in a battery not recharging and therefore the safety failsafe system not be available when needed. A more extreme fault is that batteries can catch fire.

Until recently the size and weight of the battery back-up or UPS has not been a consideration and many manufacturers would attach an additional housing containing the failsafe system, adding considerable bulk and weight. And cost. A move towards more compact electric actuators, and a need to eliminate as many possible failure scenarios as possible cause a re-think and the use of capacitors was developed as an alternative to battery failsafe systems.

Capacitors are an electronic component that are quick to charge and then hold that charge until needed, and although single use per charge, can be charged and discharged millions of times. Modern high-performance capacitors are inexpensive and physically small, and battery back-up systems or UPS solutions typically can’t compete either on performance, size or cost.

Keen to test the newer capacitor failsafe electric actuator technology, J4CS recently tested the failsafe version of the AVA electric actuator from Actuated Valve Supplies and were surprised at all aspects of it.

1 The AVA Model 20 actuator is incredibly compact and sat comfortably in our hand, and as its failsafe system is by a capacitor, sees no housing size change when supplied as a failsafe actuator.

2 Fitted to a 2” PVC ball valve, J4CS opened the actuator using a 24VDC supply, then disconnected the 24VDC after 1 minute, causing the capacitor to discharge and close the actuator. After 1 minute the cycle repeated. The test was left to run on a continuous loop, thereby not restricting the number of operations.

3 J4CS stopped the continuous test without observing a single issue after 200,000 cycles, and with the ease in which the 200,000 cycles were achieved by the AVA actuator felt that the test could easily have continued to double the number observed.

4 J4CS were given an indication of cost which was compared with equivalent products online, whilst normally not commenting on commercial aspects, noted that the AVA capacitor is driven failsafe actuator was very competitively priced.

The conclusion arrived at was that where capacitors can be used to provide an alternative power source in failsafe systems in electric valve actuators, they offer significant benefits in terms of performance, functionality, size and apparently cost when compared to equivalent battery operated failsafe systems.

Footnote: Safeguards and restrictions introduced on flying batteries around the world following instances of batteries catching fire on aircraft do not affect or apply to capacitor based failsafe electric actuators.

Copyright Journal for Control-Systems Engineering 2019
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