The Bench set is determined by the spring rate of the actuator failsafe range spring(s). It is expressed for a given actuator (with its particular diaphragm area) by the actuator’s pneumatic “LOAD” pressure that opposes the range spring and gives the theoretical pressures at beginning of stroke and completion of the full stroke.
This is typically performed on the actuator before it is mounted onto the valve.
The bench set is expressed as the pressure range from the start of the actuator stroke to the nominal travel of the valve. Because the spring rates of the actuator are not very consistent, it is reasonable to assume that only one of the bench set points can be met and that the critical value should be the one adjusted.
For air-to-open valves, the start pressure is critical for a valve that requires positive shutoff by the spring. On air-to-close valves, the end pressure value is critical to have enough force to overcome the spring force and the valve friction and to seat the valve.
For an air-to-open control valve with a 3 to 15 PSI signal range, the “bench set” pressure would be 3 PSI.
Bench set is an important parameter to the control valve because it establishes the seating load of the valve plug against the seat when the valve is fully closed.
- If the stem connector is set with the actuator and valve stems spaced too far apart, which means the total stem length is too long. the actuator diaphragm will bind travel at the upper end and the valve plug will bind travel at the lower end. The result is a valve that cannot ever fully open:
Look at the figure the valve can fully close but cannot be fully open.
- If the stem connector is set with the actuator and valve stems too closely coupled (i.e. the total stem length is too short), the actuator diaphragm will bind travel at the lower end and the valve plug will bind travel at the upper end. The result is a valve that cannot ever fully close: Here the valve can let full flow bust cannot let the valve fully shut, which is a serious problem.