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.

Smart Electric Valve Actuators Operate as Fast as Pneumatic Actuators

Traditionally electric valve actuators could not be considered to replace pneumatic valve actuators where the fast opening and closing speeds that are standard in pneumatic actuators were required, resulting in the expense of installing an air compressor, air pipework, driers and filters to drive the air actuators. With far fewer moving parts air actuators can comfortably be used in applications where a high number of cycles are required, another ‘Achilles heel’ of electric valve actuators as the relevant European norm limits the performance of part-turn on-off electric actuators to just 10,000 cycles.

Some new smart electric valve actuators are now able to operate at speeds under 1 second making them contenders to replace costly compressed air systems and offering customers a real lifetime cost advantage. One application for fast-acting electric actuators is in filling systems, traditionally a stronghold of the pneumatic actuator.

J4CS recently tested an AVA S20 fast acting on-off smart actuator fitted to a 1” stainless steel ball valve from Actuated Valve Supplies in the UK and can report that the actuator operated the electric ball valve in 1 second effortlessly and faultlessly over a test of 30,000 cycles. It is clear from our test that the build quality of the AVA is high to withstand our test and we would not be surprised that the claim from the manufacturers’ UK agent that they have fast acting AVA modulating actuators in an application that have exceeded 6 million movements is true.

Modern manufacturing techniques have enabled the consistent production of highly accurate components which when assembled under quality-controlled conditions, produce products with a high build quality that result in accurate and reliable performance in service, and offer a long service life. Following our tests, our opinion is that the AVA electric actuator is an excellent example of the use of modern manufacturing techniques with an obvious and deep understanding of the benefits of strict attention to all aspects of quality control.

Improved Functionality from Smart Electric Valve Actuators

The design of smart electric valve actuators remained near stagnant for many years with most designs relying on the traditional cam and micro-switch solution to stop the motor. So-called design improvements often saw small changes to improve the design of the cams and switches, or the introduction of a brushless motor, for example, rather than a more radical change to the basic principals of operation.

Recent advances have seen a move towards digital control of the actuator’s position via digital magnetic positioning technology, which offers not only improved accuracy as it is digital but also a non-mechanical system which has obviously improved lifetime service implications compared to mechanical systems that are prone to ‘wear and tear’.

Whilst the elimination of mechanical components hints at improve total life cost as there as fewer parts that can fail, it is the switch to control of the functionality of the electrical actuator by software that is the more significant benefit. Employing brushless motors from the start, actuators like the AVA actuator available from Actuated Valve Supplies in the UK, recently tested by J4C-S have embraced modern technology and offer a dazzling list of standard and optional features, all controlled by software.

Combining a user interface comprising of a bright OLED screen and external push buttons, users not only have useful operational information displayed on the screen but have the ability to either control the electronic actuator locally, more commonly referred to a ‘local control’, but can make minor adjustments to several useful operational parameters that separates valve actuators like the AVA actuator from most rivals in its class, from a user perspective.

Features such as speed control, external adjustment of zero and span (open and closed positions) which in electric actuators with cams and switches requires the cover to be removed to access the switches, and external setting for 3 positions become standard features in software-driven actuators rather than ‘factory specials’ from manufacturers still using the original cam and switches design.

Copyright Journal for Control-Systems Engineering 2020