NB: This article first appeared as part of the Talking the Torque series in Water Magazine
Martin Hardwidge sets foot upon the minefield of actuator batteries, backups and failsafes, and tries to answer questions around what customers can expect, and what is reasonable to achieve in a typical water and wastewater application.
I guess the first thing to make clear is that I’m not talking about SIL (Safety Integrity Level) applications. That’s an interesting discussion to have, but not our topic here. This piece is about the variety of options available that provide a ‘back-up’ for an electric valve actuator.
So – first question: What do you mean by ‘back-up’? Do you mean ‘back-up’, as in ‘save data and configuration parameters’, or do you actually mean ‘fail safe’ – if there’s an emergency situation or a loss of power, the valve does ‘something’ to make the system safe.
Let’s take the simple scenario first. If power fails on a non-critical system, and we are fortunate in the UK that we have a pretty robust power distribution system, what do you need to know when the lights come back on?
Well, first you might want to be confident that the actuator configuration parameters are safe, so valve automation can carry on as before. Any modern actuator will have either a non-volatile EPROM or small on-board battery backing up operating parameters and data in the same way that any measurement instrument would have.
However, there’s another important consideration – where did the valves stop? Are they half open, fully open, or closed? AUMA’s system includes a mechanical position indicator so you can see the valve position at a glance with or without power, and the potentiometer built into the mechanical indicator means that when the display returns, it shows the accurate position without needing a battery to back that part up.
So far, so good. The bigger challenge occurs when you are considering what needs to happen to fail safe in the event of a catastrophic event or a power failure. An entirely different kettle of fish. Straightforwardly, if there is a risk to life or significant damage, you should look to a SIL system, with safety instrumentation and a dedicated control system and, as I mentioned above, that is outside the remit of this article though very much within AUMA’s capabilities.
First off, most systems demand a hard-wired emergency function, so in response from an emergency signal from the plant control the valves will be either held or driven to a pre-programmed emergency position. In this regard an AUMA variable speed actuator provides a great combination of high accuracy control in normal operation with the added reassurance of programmable speed curve operation in an emergency.
When power fails, what needs to happen? Some of that is determined by the valve – a butterfly, ball or plug valve, for example, with a maximum quarter turn to fully open or close, is a different challenge from a big multi-turn gate valve or penstock. For absolute certainty, a quarter turn valve can include a mechanical, spring-loaded failsafe, but that would be of no help in a multi-turn application. For compact actuators such as AUMA’s Profox, a supercapacitor system can provide several actuations to keep the system operating for a short time.
AUMA believes that it is vital, where battery failsafe is needed, to provide a proper, purpose-built system to deliver certainty. Think of the challenge here – it might easily be fifteen or twenty years before a failsafe is used. If the battery system was an inbuilt one, how would you know it was still working? How can you test or maintain it while maintaining actuator operation?
At AUMA, we build our failsafe systems to suit the customer requirements, after a proper discussion with the end user. Usually, several actuators can be powered from the same panel, and proper displays and controls can be included that ensure that we can maintain power in the event of a failure, maintain site communications throughout and keep the valves working in difficult circumstances over decades of time. Because, after all, isn’t that certainty what we’re looking for?