control valve sizing

ACTUATOR SELECTION AND SIZING

ACTUATOR SELECTION

The most important criteria for selecting an actuator are the shutoff differential pressure and the minimum available air pressure in the system. The shutoff differential pressure refers to the maximum pressure difference across the valve when it is fully closed.

The selection process begins with choosing the appropriate valve type for the specific application. Once the valve is selected, the actuator is sized by considering the unbalanced forces acting on the valve trim as well as the frictional forces within the valve assembly.

For accurate and compliant selection, the client’s control valve specification should always be used as the primary reference. Actuator selection ultimately depends on both the shutoff differential pressure and the process requirements.

  1. Pneumatic actuators

             Single acting diaphragm with spring return

Use single acting diaphragm actuators when the thrust is moderate. When the valve size is very large and the shutoff differential pressure is very high diaphragm actuator cannot be a good option . This significantly increases the size of the actuator. In this case we have to move to the piston actuators. The two different types of piston actuators are piston actuators with spring return and Double acting piston actuators. Double acting cylinder shall be used with air accumulator for the predetermined

  • Electric actuators and Hydraulic actuators.

Electric actuators and hydraulic actuators are used in areas where air supply is not available. In some remote areas example in onshore well heads, the provision of air supply is not available. In the case we have to go either with  electric of  hydraulic actuators. In on shore well head area all the valves are hydraulic  valves including SSV, wing valve, choke valve. If we use the hydraulic actuator the setup will be different a pressurized hydraulic system required with a reservoir. If the valve are part of the well head the hydraulic supply shall be from the well head control panel. Other the hydraulic valve require a separate hydraulic pressure  supply and return system.

ACTATOR SIZING

Step involed in the Actuator sizing.

  1. Select the valve based on the calculated Cv.
  2. Check the worst case maximum differential pressure and minimum air supply.
  3. Find out the port area of the valve.
  4. Calculate the all the forces acting on the port during the shutoff condition. This is the calculated torque.
  5. Find out the required torque by using a safty factor that add to the calculated torqure.
  6. This required torque valve may not be available on the vendor actuator table. So select a torque that is slightly higher than the required torque.
  7. NB: normally valve selection and sizing are performed by the selected vendor.

Unbalanced forces acting on the actuator is given below.

Frequired​=FΔP​+Fseat​+Fpacking​+Fsafety​

FΔP​

Is the shutoff pressure drop when the valve is fully closed.

Fseat​

This is friction required to avoid the seat leakge as per ANSI FCI 70-2.

Fpacking​

Force required to overcome the packing friction.

Fsafety​

This is the safety factor, The force we calculated the minimun for required , but in real some other forces also acting on the valve and this safetry factor is used to counter that additional force. The actuator force shall be greated than the required force.In calculating the actuator force minimum air supply shall be used .This is the worst case scenerio.

EXAMPLE

Here we will manually calculated the required force and  and select the actuator from the fisher manual. Here the friction values will be taken from the fisher valve specification after the valve selection.

PROCESS DATA

size the valve using the fisher valve specification manager based on the below process data and select the valve that best suited as per the Cv and application.

valve data from vsm

After the valve selection click the fisher actuator sizing tab, the sizing inputs are auto detected by the FVSM based on the selected valve. The friction values are highlighted in the green for the calculation.

calculate the required force

Unbalanced area= 0.2700 in2

Force acting on the unbalanced area- F= 19.6 brag X0.2700in2

=  284.27psi *0.2700in2

//* change the bar to psi unit*//

= 76lbf

Seat load force = 136.533lbf/in= 992.17

//* change the lbf/in to in*//

Valve friction = 180 lbf

Calculated force = 76 lbf+992.17 lbf/in+180

= 1248.7 lbf( 5554.49 newton)

Safety factor = 1248.7 lbf *1.3= 1623.31

//* add safety factor for the additional forces*//

comparsion with vsm calculated result

The calculated force is almost equal to the FVSM actuator sizing output

manual calculated output= 1248.7 lbf

FVSM output =1249.7 lbf

actuator size selection

Select an actuator that has a torque greater than the required torque from the fisher actuator manual. The selected actuator size is highlighted in green

The 45i actuator is selected for this application because it provides maximum allowable thrust greater than the required valve torque and has a stem diameter compatible with the valve stem size. Although the 34i actuator also delivers torque higher than the required valve torque, it is not selected due to a stem diameter mismatch with the valve stem, making it unsuitable for proper installation and operation.

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