In this article, I will cover the bench ser range of the control valve actuator.
The pneumatic actuator operates on the principle of the Hooke's laws to applied air pressure to valve stem position.
Hooke's law directly gives a relation between the applied force and the elasticity of the spring. Given by below formula,
F = KX
Where F = force applied to the spring
K = constant of elasticity
X = displacement of the spring
Hooke's law is a linear, which means that spring motion will be linear related to an applied force.
the working area of the diaphragm and piston remains the same, so the relationship between actuating fluid pressure and force will be proportional.
Now we have a formula,
F = KX
due to applied force,
F = PA
put this equation into the above equation,
PA = KX
so, the displacement is,
X = PA/K
Displacement is a relation between pressure, area and of an elasticity constant.
You can see in the below figure of the valve's actuator which has defined above-stated parts.
For example, you have removed the valve from the process for the maintenance purpose. Now when assembling it again a valve body stem and actuator stem must be coupled properly to move in between fully open to fully close.
Keep in mind that there are two mechanical adjustment need to be done in order to function the valve properly for a sliding stem control valve.
Spring adjuster and stem connector. The false setting will damage the internal valve component.
If you couple stem connector little bit away it binds the motion for fully open it will go beyond its maximum limit and for fully close it will damage the seating area.
If you couple connector too close then neither it will fully close nor it will fully open. so care must be taken while coupling stem connector.
You can see all the above explanation in the below image.
To make sure that stem connector is properly coupled check it rated travel by measuring using tap, so based on that you can make a conclusion.
After coupling stem connector, Now the spring adjuster must be set for the proper bench set.
This is a pneumatic air supply which is used to lift or close the valve. To open the valve apply air pressure from 3 psi to 15 psi. Here, so at which valve starts to move is a bench set pressure, Here in our case bench set is 3 psi.
In many other cases where you can find bench set range like 5-15 psi, 6-15 psi means valve should start to move at lower bench set here it is 5 psi and 15 psi is a higher bench set at which valve should stay fully open or close.
In conclusion, we can say that bench set is a very important parameter, before carrying it follow the manufacturer's instruction.