Identifying Fail Mode Specifications
Today as part three in my four part series, I’ll explain the next step in properly sizing an actuator: identifying fail mode specifications. For more information about sizing an actuator you can read my previous posts, Understanding Minimum and Maximum Supply Pressure and Choosing the Right Actuator Type
Close, Open, Last
Actuator fail mode is the position that the actuator takes after the control signal or the supply pressure is lost. This step is vital to the proper sizing of an actuator because if the wrong fail mode is specified, the actuator will not fail in the correct position when, often, people’s safety or the integrity of the system is in jeopardy. For example, if you have an over-pressure protection (OPP) application where the valve needs to close in the event of an over-pressure downstream of the valve, but a double acting actuator (which has a fail last mode) is specified and sized, the valve will most likely stay in place instead of closing automatically. I say “most likely” because even if you have a double acting actuator, you can still arrange it to close or open the valve on loss of controls signal if necessary.
In general terms, a single acting actuator should be used when a fail open or fail close position is needed, and a double acting actuator should be used when a fail last position is needed.
In specific terms, we recommend that end users specify if the fail mode is on loss of control signal only or on loss of supply pressure also. If the fail mode is not specified on both loss of control signal and on loss of supply pressure, you can get a different fail mode than required for your application once the controls are installed. For example, a double acting actuator can be fail last on loss of supply pressure, but can be fail close or open on loss of electrical signal. Many end users specify that they want a double acting actuator but then when the controls are put on the actuator (such as the 4-way single coil solenoid valve on the schematic below), they realize that the actual system is not fail last on loss of electrical signal. So the end user gets a fail last actuator on loss of supply pressure but a fail closed (or open) on loss of electrical signal, not a fail last on both.
Choosing the correct fail mode is a vital step when sizing an actuator for ALL applications. Do you have a complex valve automation application and need the opinion of an expert? We would be happy to help, contact us today.
How to Properly Size an Actuator
Part One: Understanding Minimum and Maximum Supply Pressure
Part Two: Choosing the Right Actuator Type
About the Author
Carlos Gamero holds a Mechanical Engineering degree from the University of Houston and is a licensed professional engineer on control systems. He joined MRC Global in 2006, where a mentor introduced him to valve automation. Carlos immediately fell in love with the opportunity to solve new problems every day. That love has not changed. In his current role, Carlos enjoys mentoring other valve automation engineers and working with his team to create valve automation solutions for our customers on a daily basis.