PICV Failsafe Actuators: Power Back-Up vs. Mechanical Failsafe

19 January

This article will look at what to consider when choosing a failsafe actuator for your Pressure Independent Control Valve (PICV) application. Failsafe actuators will basically ensure that the PICV can operate for one cycle when disconnected from power (either closing or opening of the water flow). Based on this application, project designers can predetermine which valves to open and which valves to close during power failure, so that they can operate critical areas by using a standby power source like a generator for the pumping system.

In general, there are two main categories of failsafe actuators. Firstly, there are mechanical failsafe actuators. These come in two versions – a thermal version with a sensitive material which changes physical size with the presence of electricity. And secondly, the spring-loaded version which, in case of power failure, is designed to apply a force in the opposite direction of the desired motion of the actuator. The power back-up actuators commonly come with either super-capacitors or a rechargeable battery pack. Power back-up solutions are generally recommended being used in the waterborne part of the HVAC systems as the spring return option holds restrictions in terms of performance, system accuracy and lifespan, whereas spring return options are more suitable for fast acting requirements, such as damper applications on the air side. On the waterborne side of a HVAC system, failsafe function is urged to operate gradually to avoid water hammering, pipe vibrations and noise. Due to this, we do not recommend installing spring return actuators on PICVs.

We tend to propose PICVs with electrical power back-up failsafe actuators. These are, depending on valve size, with super-capacitors (for the threaded valve sizes DN15-DN50, ½”-2”) or a rechargeable battery pack (found on our flanged valve sizes DN50-DN250, 2”-10”).

Thermal Failsafe Actuators

The thermal actuator is designed to convert thermal energy into mechanical motion, using the expansion and contraction of wax inside the element based on temperature variations. The integrated electrical resistor generates heat when electrical current flows through it, producing the required temperature change needed for operation.
With their low cost, these actuators are preferred in ON/OFF applications, but their use is limited in such analogue applications as they are also affected by ambient temperature.

Spring Return Failsafe Actuators

Spring return actuators are common in single turn actuators, including linear actuators utilizing a single turn gear, whereas many FlowCon PICV actuators are multiturn, up to 3,600° for superior rangeability. It is standard practice to use spring return actuators in damper applications, but it is not recommended using fast acting spring return actuators in chilled water systems due to a few reasons:

• The spring return actuators are generally fast acting, which may impose water hammering, which could lead to damage of the installed valves and other installed components.
• Spring tension will change depending on the spring elongation providing an uneven force
distribution, which may impact position accuracy.
• Spring return actuators are not very accurate in their positioning as the spring coefficient will change over the lifespan of the product. This will reduce both failsafe performance and accuracy.
• Force is inappropriately distributed as the spring is generally strongest in fully open position (fully elongated), where the spring force is not really needed and weakest in the failsafe position (fully compressed), where you in fact will require the force.
• The actuator motor needs to work against the spring stress, which would cause increased actuator torque. From this you would also see an unwelcome increase energy consumption.
• Large size spring return actuators are a little on the noisy side, and not recommended for comfort air-conditioning applications.

Based on all of the above, it is not our recommendation to use spring-return actuators in the waterborne side of a HVAC modulating system.

Super-capacitor Failsafe Actuators

When we look at durability, a super-capacitor actuator – such as the FlowCon FNJ – is a dependable and reliable electrical component, ensuring continued use for many years. The concept behind a super-capacitor failsafe actuator is to utilise the energy stored in the super-capacitors to perform one final critical function when there is a power failure.

Not only are they reliable, but additionally the super-capacitors recharge almost instantly, meaning that it will be practically impossible to discharge in a HVAC installation. Our super-capacitor failsafe actuators, sourced from our friends at FlowCon are used on our Green/GreEQ PICV valve range. The actuators are linear stroke actuators which provide full control and high resolution in combination with these small PICVs.

Battery Failsafe Actuators

The battery pack we use is designed and tested for minimum of 1000 failsafe cycles, so this also has proven and documented high reliability. In the ten years that FlowCon and FloControl’s battery failsafe actuators have been produced and dispatched, not a single battery pack has required replacement. A failsafe actuator with a rechargeable battery pack is designed to provide reliable operation in case of power failure to operate the valve to either fully open or fully closed position. It also has the option to leave the actuator in the current position if power is lost.

The battery failsafe actuators are used on our SM PICV valve range. Standard, failsafe and BACnet actuators are multi-rotation actuators and they hold up to 3,600° rotations. This results in substantially more position points than with a conventional 90° actuator and thereby also higher accuracy in performance.

It is worth noticing that the battery condition is available on both the actuator display and directly in the BMS system if BACnet versions are chosen. This way battery life can be monitored either during regular system service or automatically with built-in alarms to ensure that failsafe functionality is always optimal regardless of the actuator age and past operations.

Conclusion

Choosing the optimal actuator for the application in question is important. Having mentioned various aspects concerning the choice of failsafe actuator for any PICV application, we recommend using either linear or multiturn actuators with power back-up on PICVs for failsafe applications. This will ensure utilization of the full working potential of the PICV valves, which will obtain the best energy efficiency, reduce the building carbon footprint and provide optimal system protection.

Original document by Torben W. Ibsen, CEO, M.Sc. (Mech.Eng.), FlowCon International ApS, Denmark, re-transcribed by FloControl Ltd. (UK).