By Jerry Butz, BSEE, CMRP
In the world of process control engineering, there’s so much to understand about automatic control valves. But one factor that probably doesn’t make the Top 10 List is control valve actuator bench set, as it applies to spring- and diaphragm-type actuators. This factor is an often-misunderstood point of confusion, and sometimes incorrectly described part of a control valve’s actuator specifications. But not understanding it can set one up for a failure in the form of a mis-sized actuator and spring. Maybe this information can help to clear the cloud of confusion and make it easier for engineers, technicians, and operators to understand.
The Importance of Control Valve Actuator Bench Set
Without getting too deep into it, we need to understand the factors that go into the sizing and selection of an actuator that provide the moving force to an automatic control valve. In order to adequately operate a control valve, the actuator must deliver enough thrust (for sliding stem globe valves) or torque (for ball and butterfly valves) to counter the forces acting upon the valve and its components. Those forces include:
- The force to overcome static unbalance of the valve plug/ball, or simply the force the process fluid pressure imparts and the plug/ball;
- The force to provide seat loading for proper shutoff tightness; and
- The force to overcome packing friction.
Those forces combined make up the minimum required force to move the control and provide the seating force to adequately shut off the process flow. (EDITOR’S NOTE: This topic was covered in detail in “Control Valve Shutoff Classification and ‘Allowable Leakage Rates’,” in the May 2015 issue of Flow Control, pages 31-32).
It’s the first two forces — static unbalance and seat loading — that are the factors that an actuator’s bench set or initial set are dealing with. We can think of bench set as a factory setting or an adjustment made to the actuator “on the bench” during the assembly of the actuator that preloads the actuator spring to counter the forces of static unbalance and seat loading.
Typical diaphragm loading pressure, or pneumatic instrument signal range, is 3-15 PSIG (sometimes 6-30 PSIG is used). Typical signal outputs of pneumatic controllers, electro-pneumatic transducers, and valve positioners are also 3-15 PSIG. So it would seem logical that a signal slightly higher than 3 PSIG applied to a spring and diaphragm actuator would begin to move the actuator and an increasing signal up to 15 PSIG would move the actuator through its full range of travel. And that’s the way it is supposed to work. But then introduce the forces of “A” and “B” previously mentioned, and an adjustment needs to be made; a factory adjustment called “bench set.”
The control valve actuator bench set adjustment is setting the actuator spring loading “on the bench” before the actuator is coupled to the control valve, before any of the forces mentioned above are introduced. For example, if we have an “Air To Open” actuator with a bench set spec of 7-15 PSI, the spring-loading adjustment and the travel stop is set so that the actuator stem begins to move up, or retract, when the air signal applied to the actuator diaphragm increases from 7 PSI, not 3 PSI, and reaches its full travel distance when the air signal reaches 15 PSI. By setting the actuator spring loading in this way, when the actuator is coupled to the control valve and the control valve is in the pipeline with process pressure applied, there is enough force applied by the actuator to the valve stem, plug, and seat ring to counter the process pressure and hold the valve plug against the seat ring with enough seat loading to provide the specified shutoff tightness. The valve and actuator are still going to operate as expected at 3-15 PSI when exposed to process pressures; that’s where the misunderstanding begins. The proper initial spring loading during actuator assembly provides for proper operation of the valve/actuator assembly when in the pipeline and in service. When 3 PSI is applied to the actuator diaphragm, the valve is still closed, and as the air signal begins to increase the actuator begins to move the valve stem up and reaches full travel at 15 PSI.
The same effects are in play with an “Air To Close” actuator, for example, using a 3-12 PSI bench set. The actuator spring loading and travel are set to begin to extend the actuator stem at 3 PSI at the top of its stroke, and reach the full range of travel at 12 PSI air loading. By setting the actuator spring loading in this way, when the full 15 PSI air signal from the controller/transducer/positioner is applied there will be that extra thrust from the actuator to counter the unbalance force of the process pressure acting on the plug and stem assembly.
If no bench set or spring loading is considered and the actuator is adjusted to move over the full range of its travel with a 3-15 PSI air signal, the process pressure can work against the actuator spring and actually force the valve plug to lift off of the seating surface, allowing process fluid to pass through the valve instead of being closed.
This doesn’t mean that a 3-15 PSI bench set is not a valid specification for an actuator. If the process pressure is low enough or the port size is small enough that the total of the three A, B, and C forces is less than the thrust the actuator can provide with 3-15 PSI bench set, then everything works. It’s all about delivering more output thrust from the actuator than what’s required to move the valve.
Final Thoughts On Control Valve Actuator Bench Set
One way to avoid the confusion and help give the control valve/actuator assembly better responsiveness is to add a valve positioner, or valve controller, to your assembly. A valve positioner receives an input signal (can be electronic or pneumatic) from the process controller and outputs a pneumatic signal to the actuator. The positioner is connected to the valve stem and senses the exact valve position. If you think of the positioner as a “valve position controller,” with the signal from the process controller serving as the valve position setpoint, then the positioner’s output is manipulated to drive the actuator and valve to the position desired by the process controller. Therefore, in respect to Bench Set, when you input a 3 PSI or 4 millamp signal to the valve positioner indicating you want 0 percent valve position, the valve positioner will apply whatever pneumatic signal it needs to the actuator to drive it to 0 percent valve position. Likewise, if the controller inputs a 9 PSI or 12 milliamp signal to the positioner indicating a 50 percent valve position, the positioner applies whatever signal is needed until the valve is at 50 percent valve position.
Another aspect with actuators that is sometimes misunderstood is the difference between the terms “Air Operating Range” and “Bench Set.” Air Operating Range refers to the low and high range of air pressure used in controlling the actuator position, the air control signal. This is typically going to be an industry standard 3-15 PSI or 6-30 PSI, for instance. The “Bench Set” is the lower and upper air signal pressures used to set the factory preloading of the actuator spring. They are not the same thing, although sometimes may have the same values; i.e., 3-15 PSI Air Operating Range/3-15 PSI Bench Set.
When it comes to rotary valve actuators, the forces are different, but in the same line of thought; a preloading of the actuator spring is set at the factory when assembling the actuator to offset the effects of the process pressure acting on the valve when inline and in service. Different manufacturers refer to the rotary actuator spring preloading with different terms, but “Initial Set,” “Initial Compression,” and “Bench Set” are the most commonly used.
Simply put, the actuator bench set, or initial compression, is a factory setting so that the actuator and valve assembly will respond to the air signal properly when under normal process pressure conditions. Understanding the theory behind actuator bench set and the difference between it and operating range will go a long way to properly sizing, specifying, and ordering automatic control valves and actuators.
Jerry Butz, BSEE, CMRP, joined Automation Service in 2010 and serves as the company’s Director of Customer Engineering. He previoulsy worked at DuPont for 16 years. Butz also has a background in electrical, instrument and reliability engineering, as well as construction, commissioning/startups, predictive/preventive maintenance, troubleshooting, root cause failure analysis and process control engineering. He has 29 years of field experience in the petrochemical, biofuels, and extrusion/web handling industries. Butz received his B.S. in electrical/electronic engineering from Hamilton Technical College, is a Certified Maintenance & Reliability Professional, and is ANSI registered and Six Sigma certified. Contact him directly at firstname.lastname@example.org.