Articles, references and workshops abound on the subject of proper control valve sizing. But not so many are the opportunities to learn about guidelines for proper sizing of the control-valve actuators.
Any piece of equipment is only as good as its weakest component.
Even a properly sized control valve won’t do its job if the actuator moving it isn’t properly sized as well. Let’s look at the issue of proper actuator sizing for sliding-stem globe valves.
There’s more to the actuator sizing than determining what physically fits on the control valve. The responsiveness and shutoff capability of the valve are a product of actuator performance. An undersized actuator can be sluggish to respond to instrument signal changes, affecting the entire control loop. It may not have enough force to adequately seat the valve, causing objectionable leakage when closed.
Likewise, an oversized actuator can cause responsiveness problems and put excessive force on trim components that lead to premature valve wear. To operate the valve properly, the actuator has to overcome and balance "the Force" working against the movement of the valve.
Time to explain
So what is "the Force"? It’s made up of several different forces all working together. The three most common are the:
- Static unbalance of the valve plug
- Packing friction
- Seat loading
The static unbalance of the valve plug refers to the force acting on the plug by the flowing process fluid. In considering this force, one must know what kind of trim (plug, seat, and cage) the valve contains; and whether it is pressure-balanced trim or unbalanced trim.
A pressure-balanced trim uses the process pressure to balance the static pressure acting on it, while unbalanced trim takes the full force of the process fluid.
Additionally, some valves designs direct process flows up through the seat and past the plug, while others are to flow down past the plug and then through the seat. For pressure-balanced trim, consult with the OEM, or the sizing software for a specific brand, to find the actual listed unbalanced area of a given trim.
The following example uses the unbalanced area of a 2-inch Fisher ET (Pressure Balanced Trim) with a full-size port, nominal port diameter is 2.3125 inches, with an unbalanced area of 0.27 inches square. The upstream Static Process Pressure (P1), and the Shutoff Differential Pressure (dP) is 200psig.
Static Unbalance = Unbalanced Area of Plug X P1
Static Unbalance = 0.27in2 X 200psig
Static Unbalance = 54 pounds of force (lbf)
This next example will show the difference in static unbalance between pressure-balanced and unbalanced trim, by using an unbalanced trim of a 2-inch Fisher ES (Unbalanced Trim) with a full-sized port, nominal port diameter is 2.3125 inches, but we must first calculate the unbalanced area which is equal to the area of the port.
Unbalanced Area = (Port Dia./2)2 X Pi
Unbalanced Area = (2.3125 inches/2)2 X 3.14
Unbalanced Area = 4.20 inches square
Using the same Static Process Pressure of 200psig, now we can calculate the Static Unbalance.
Static Unbalance = Unbalanced Area of Plug X P1
Static Unbalance = 4.20 inches square X 200psig
Static Unbalance = 840 pounds of force (lbf)
Note the difference in static unbalance between a valve with pressure balanced trim and a valve with unbalanced trim.
Additional forces accounted
The next force that must be accounted for is the packing friction. Valve packing seals the process from leaking around the valve stem that moves up and down as the valve opens and closes. Different sizes and styles of valves can have different stem diameters and different sized packing rings. Additionally, there are different packing styles and materials. Some use PTFE, some use graphite for high temperature applications, and others will use other engineered materials.
While some use single sets of packing rings, others use double sets. Each packing size, style and material will have different friction forces. Consult with the valve manufacturer or its sizing software to determine the friction for each size and type of packing. For our example, using the 2-inch valves, which have a 0.5-inch diameter valve stem, we will use standard PTFE V-Ring packing having a friction of 50lbf.
Another force the actuator must compensate for is seat loading. This is the pressure the actuator applies between the plug and the seat. Seat Loading is related to the Shut-Off Classification and control valve size. If an actuator doesn’t apply enough seat loading, the valve will not shutoff adequately when closed. Different valve types, trim materials and sizes will have different seat loading requirements. This can be found in a valve’s technical specifications from the original manufacturer and is expressed in pounds of force per lineal inch, or lbf/in.
Continuing the example from above, using the 2-inch Fisher ET and ES, let’s consider the use of a Metal Seat and the same 2.3125 inch port diameter. These type/size of valves have an industry standard Class IV shutoff, requiring 40lbf/in of seat loading.
Seat Loading = Port Circumference X Class IV Seat Load Force
Seat Loading = (2.3125 inches X 3.14159) X 40lbf/in
Seat Loading = 290.6 lbf
Thrust involved
We can now add up all these factors to determine the required thrust the actuator must apply to properly operate and shutoff its control valve. First, using the example of the Pressure Balanced Trim of the 2-inch Fisher ET:
Required Thrust = Static Unbalance + Packing Friction + Seat Loading
Required Thrust = 54lbf + 50lbf + 290.6lbf
Required Thrust = 394.6lbf
Example using the Unbalanced Trim of a 2" Fisher® ES
Required Thrust = Static Unbalance + Packing Friction + Seat Loading
Required Thrust = 840bf + 50lbf + 290.6lbf
Required Thrust = 1180.6lbf
The next step is to select a type and size of actuator that can deliver the required thrust or greater. This is done through the brand specific sizing software or by consulting the OEM documentation. As long as the actuator delivers enough and not "too much" thrust, the valve should respond satisfactorily to signal changes and shutoff according to its classification when the valve is completely closed.
Additional aspects to consider in actuator sizing, but that are outside the purview of this article, include 1) type of actuator (e.g., spring & diaphragm, single- or double-acting piston); 2) actuator fail action on loss of instrument signal; and 3) actuator spring rate and bench set,.
Jerry Butz, BSEE, CMRP is a process control engineer and director of customer engineering at Automation Service. Automation Service is a global leader in remanufactured process controls including control valves and actuators. Jerry has 29 years of hands-on experience in process control and heavy industrial processes, including petrochemical, extrusion/web handling, and bio-fuels. He is a Certified Maintenance & Reliability Professional, as well as a certified Six Sigma Green Belt. His background includes troubleshooting and root cause failure analysis in addition to sizing and specifying automatic control valves. Jerry can be contacted via email at [email protected].
