What is the bench set of the control valve actuator?
- The bench set of a control valve is defined as the pressure range required for stroking the actuator valves to the rated valve travel from a fully closed 0% to a fully open 100% position with no valve forces present.
- The term bench set originates from the practice of placing the actuator assembly on the bench to adjust the initial compression of the actuator spring along with the valve assembly “on the bench” is adjusted by bench set pressure for the proper operation of control valve actuator assembly.
- An initial compression is important when the valve actuator is engaged in service after the diaphragm operating pressure is applied.
- By uncoupling the actuator from the valve we make sure it’s isolated from the valve stem forces.
- Here on the bench, we have a fisher 657 spring and diaphragm actuator and it is mounted to a fisher easy e valve body.
- During mounting of the actuator to the valve assembly, and accurate adjustment to the bench set is easily made.
- During spring adjustment we must ensure that there must not be any friction applied by loose packing.
- Considering the nameplate on the actuator we can see that the bench set for this actuator is specified as 3 PSI to 11 PSI.
- With this valve assembly, we must observe that the rated valve travel is of three quarters which is 75% of the inch, which occurs as we increase the input pressure from 3 PSI to 11 PSI. Because this is a direct-acting actuator. Because this is a direct-acting actuator.
- The bench set range of the valve actuator establishes the assumption that there is no packing friction.
- The packing friction for correct adjustment to be made is required to determine the bench set range after the actuator is connected to the valve and the packing tightened.
- The bench set adjustment for the new control valve and actuator systems can be easily performed on the bench itself.
- The bench set for the actuator can also be performed in the process field in case of packing friction.
- If the process is not critical the actuator can be removed from the process for the bench set.
- The air connection is on the top of the diaphragm casing. And we have the ability to change the pressure signal through the hand-operated regulator.
- Notice that the valve stem and valve actuator stem are not coupled together.
- At zero PSI the spring pushes the actuator to the upper travel stop.
- This is important because the bench set is always calibrated from the upper travel stop.
- The stem does not move as we increase the pressure from 0 PSI to 3 PSI due to the initial compression of the spring.
- At 3 PSI we see the first movement of the actuator the stem is detected at the lower bench set value.
- Increasing the pressure in the actuator, the stem continues to stroke down as it approaches 11 PSI.
- Now the rated travel of the valve has reached three-quarters of an inch which is the upper bench set value.
- Finally, the bench set on this actuator is calibrated correctly from 3 PSI to 11 PSI
How is the bench set adjusted?
- The bench set is an important concept to understand because improperly adjusting the bench set can result in the valve short stroking and not achieving the rated capacity or inadequate seating force to shut off the valve.
- Technicians and Engineers get into trouble by trying to make the actuator spring meet those same bench set numbers after it is connected to the valve.
- A bench set is an easy way to verify that the correct spring is installed in the actuator and that it’s in good operational condition.
- The bench set is adjusted by changing the compression wound into the spring adjuster and it’s a best practice to uncouple the valve from the actuator when adjusting the bench set.
- By turning the spring adjuster we can change the response of the actuator.
- If we turn the spring adjuster into the actuator to increase the compression on the spring, it will take a higher pressure to see the initial movement of the actuator.
- Now we see that the movement starts at about 4 PSI and it reaches the rated travel at 12 PSI.
- The span of pressure stays the same at 8 PSI but has slid to a higher pressure range.
- If we turn the spring adjuster out of the actuator, I am decreasing the compression on the spring. This means that the actuator will start moving at a lower pressure.
- Now we see that the initial movement at about 2 PSI reaches the rated travel at 10 PSI.
The key thing to remember is that the bench set is adjusted with the actuator disconnected from the valve.
Turning the spring adjuster is the best way to fine-tune where our bench set values fall within the operating range of the actuator but with the same spring, the span stays the same.
In order to change the bench set span, we must change the spring. Here we have one spring that has a lighter spring constant and another spring that has a heavier spring constant.
If we apply the same force to both springs, the lighter spring gets more compressed by that applied force.
This means that the lighter spring takes less force compared to the heavier spring from the actuator to compress to the rated valve travel. Changing to a lighter spring will shorten the bench set span whereas changing to a heavier spring will lengthen the bench set span.