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.

FAQs

What is a fail-safe actuator in a PICV application?

A fail-safe actuator in a PICV (Pressure Independent Control Valve) application ensures that the valve completes one last operation—either opening or closing—during a power failure, maintaining essential HVAC functions.

How does a fail-safe electric actuator differ from a mechanical fail-safe?

A fail-safe electric actuator uses stored electrical energy (via battery or supercapacitor) to perform one final action during a power loss. In contrast, a mechanical failsafe actuator, such as a spring return type, depends on physical mechanisms that are often fast acting and may not be ideal for PICVs.

When should I use a fast-acting actuator?

 Fast-acting actuators are best suited for airside HVAC applications like dampers, where speed is essential. However, they are not ideal for chilled water systems or PICVs, as their rapid movement may cause water hammering and noise.

Why are spring return actuators not ideal for PICV valves?

Spring return actuators are typically fast acting and can cause unwanted pressure surges and mechanical wear. For PICV valves, a more stable and controlled response is needed, which is why a fail-safe electric actuator is generally preferred.

What type of fail-safe actuator is best for small PICV valves?

For smaller PICVs (DN15-DN50), supercapacitor-based fail-safe electric actuators are ideal. They recharge quickly, are reliable, and ensure smooth operation without adding stress to the system.

Are PICV actuators with battery failsafe reliable?

Yes, battery-powered PICV actuators, such as those used in the SM valve range, are highly reliable. With up to 1,000 guaranteed failsafe cycles, they ensure dependable performance even during unexpected power outages.

Can a fail-safe electric actuator prevent water hammering?

Yes. Unlike fast-acting actuators, fail-safe electric actuators operate gradually, which helps prevent water hammering, pipe vibration, and system noise in water-based HVAC applications.

What’s the benefit of using a PICV actuator with multiturn operation?

Multiturn PICV actuators offer high resolution and control accuracy, with up to 3,600° rotation. When paired with fail-safe features, they optimise energy use, system stability, and overall performance.

How does a thermal fail-safe actuator work?

A thermal fail-safe actuator uses heat-induced expansion and contraction of wax to create motion. While cost-effective for basic ON/OFF control, it’s less precise and influenced by ambient temperatures, making it unsuitable for most PICV systems.

What’s the best fail-safe actuator for chilled water systems?

For chilled water systems, a fail-safe electric actuator with a supercapacitor or battery backup is recommended. These actuators provide smooth, controlled movement and avoid the risks associated with fast-acting mechanical failsafes.

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