How to Adjust the Bench Set of a Control Valve Actuator ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How to Adjust the Bench Set of a Control Valve Actuator

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As already explained in Control Valve Actuator Bench Set and Valve Stroking, the bench set of a control valve is the pressure range required to stroke the valve from a fully closed to a fully open position.
The bench set pressure range is used to adjust the initial compression of an actuator spring with the valve assembly “on the bench” The correct initial compression is important for the proper operation of the valve-actuator assembly when it is put into service and the proper actuator diaphragm operating pressure is applied.

The bench set range is established with the assumption that there is no packing friction. Accurate adjustment to the bench set range can easily be made during actuator mounting process by making the adjustment before the actuator is connected to the valve assembly.

When attempting to adjust the spring in the field, it is very difficult
to ensure that there is no friction being applied by loose packing. Field adjustment of the bench set range after the actuator is connected to the valve and the packing tightened, will require the determination of the packing friction for correct adjustment to be made.

For a new control valve and actuator system, bench set adjustment is easily done on the bench. If you have an actuator in the field whose bench set needs to be adjusted, you either remove the valve if the process is not that critical and do a bench adjustment or you do an in-field adjustment in which case you have to consider packing friction.

The steps for Control Valve Actuator Bench Set adjustment enumerated below are for those done on the BENCH:
Control Valve Bench Set Adjustment

Step 1
Provide a means of anchoring the actuator e.g a vise and also provide means of temporary loading pressure for the actuator diaphragm. Also provide a certified pressure gauge (PG) suitable to accurately read the diaphragm pressure from zero through the upper bench set pressure marked on the actuator name plate.

Provide temporary means of applying and adjustable loading pressure to the actuator during the adjustment e.g Pressure regulator

Step 2
Ensure that the actuator stem is at the top of its travel as shown in the diagram above. Set the diaphragm loading pressure at 0psig. Then slowly raise the pressure from 0psig towards the lower bench set pressure while checking for the first movement of the actuator stem. The actuator stem should show movement at the lower bench set pressure. If movement occurs before or after the lower pressure is reached, adjust the spring adjuster into or out of the yoke until the actuator stem’s movement is first detected at the lower bench set pressure

Step 3
Be sure the spring adjuster is adjusted to meet the requirements of step 2 above

Step 4
Apply the upper bench set loading pressure to the diaphragm. This pressure extends the actuator stem towards the valve. Note, the actuator stem may slide over the valve stem as shown above. At the end of the actuator stem, use a marker or a cello tape to mark the valve stem. Note also that if the actuator stem does not pass over the valve stem, you will need to provide a method to mark this point of stem travel.

Step 5
Slowly decrease the diaphragm loading pressure until the lower bench set pressure is applied. Measure the distance between the marker or tape on the valve stem to the end of the actuator stem. The distance should match the travel span shown on the travel indicator scale. If the span of the travel is correct, bench set adjustment is complete. You can then proceed to connect the valve and actuator together.

Step 6
If the span is not correct, then the spring may have been damaged or you have the wrong spring installed on the valve actuator.