Bench set is the nameplate specification that is used to verify proper actuator operation. Bench set is expressed as the pressure range from the start of the actuator stroke to the valve’s rated travel.
When conducting the bench set test, the configuration of the actuator and valve is defined by the valve manufacturer. The most common configuration is to have the actuator disconnected from the valve stem. Some manufacturers require that the packing load be removed but that the valve plug and stem remain attached to the actuator.
This method accounts for the weight of the stem and valve plug in the proper operation and sizing of the valve and actuator assembly. On valve actuators that have bellow seals instead of conventional packing, the valve stem is left connected to the actuator.
If there is also conventional packing, the packing load must be relieved. The force of the bellows seal compression is significant and must be factored into the bench set of the actuator.
Because the valve forces are not present in many cases, the bench set pressure range is not the same as the pressure required to stroke the valve in actual service.
The steps for control valve actuator bench set adjustment is described below:
Provide a temporary pressure for diaphragm and a standard pressure gauge.
Ensure the actuator stem is at the top of its travel. Initially set the diaphragm with 0 psi. Then raise the bench set pressure to lower bench set pressure, check for the movement of the stem movement. The actuator stem movement should occur at the lower bench set pressure and if it occurs before the bench set pressure adjust the spring adjuster into or out of the yoke until the first movement happens at the lower bench set pressure.
Now apply the upper bench set pressure to the diaphragm, this makes the valve stem travel towards the valve. Mark at the end of the valve stem with marker or cello tape.
Decrease the pressure get the actuator to the lower bench set. Measure the length between lower bench set point and the marked area. If the length is equal to the span shown in the travel indicator then bench set adjustment is complete. Connect back the valve and actuator.
If the spans are not correct, then spring may be damaged or wrong spring is installed.