PICV Failsafe Actuators
Power Back-Up vs. Mechanical Failsafe
By Torben W. Ibsen, CEO, M.Sc. (Mech.Eng.), FlowCon International ApS, Denmark
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 op to 3.600° for superior rangeability. It is standard practice to use springreturn actuators in damper applications, but it is not recommended using fast acting spring return actuators in chilled water systems due to several reasons mentioned below:
- The spring return actuators are generally fast acting, which may impose water hammering, which again could lead to damage of the installed valves and other installed components and may also enforce pipe vibrations.
- Spring tension will change depending on the spring elongation providing an uneven force distribution, which may impact position accuracy.
- Furthermore, spring return actuators are not very accurate in their positioning as the spring coeffi-cient will change over the lifespan of the product, reducing both accuracy and failsafe performance.
- 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 and thereby also increased energy consumption.
- Large size spring return actuators are generally noisy and not recommended for comfort air-conditioning applications.
Based on 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
In terms of durability, a super-capacitor is a highly reliable electrical component, which would continue to work for many years.
The basic idea behind a super-capacitor failsafe actuator is to use the energy stored in the super-capacitors to perform a ‘last’ critical function when there is a power failure.
Besides being reliable, the super-capacitors recharge almost instantly, meaning that it will be practically impossible to discharge in a HVAC installation.
The super-capacitor failsafe actuators from FlowCon are used on the FlowCon Green/GreEQ PICV valve range. The actuators (both standard, failsafe and BACnet / Modbus versions) are linear stroke actuators which provide full control and high resolution in combination with these small PICVs.
Battery Failsafe Actuators
The battery pack used by FlowCon is designed and tested for minimum of 1000 failsafe cycles, so this also has proven and documented high reliability. In fact, for more than 10 years FlowCon has not replaced a single battery pack on any of our battery failsafe actuators supplied to projects around the world.
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. Actually, it also has the option to leave the actuator in the current position if power is lost.
The Battery failsafe actuators from FlowCon are used on the FlowCon SM PICV valve range. The actu-ators (both standard, failsafe and BACnet versions) 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.
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 it is FlowCon Interna-tional’s clear recommendation to use 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 and building carbon footprint, while providing optimal system protection.
FlowCon hopes that this Topic Letter has given insight to the topic. Please feel free to contact FlowCon for further assistance or any clarification.