Control Valve Bench Set

Bench Set: The calibration of the actuator spring range of a control valve to account for the in-service process forces.

Bench set is initial compression placed on the actuator spring with a spring adjuster. For air to open valves, the lower bench set determines the amount of the seat load force available and the pressure require to begin valve opening travel.

For air to close valves, the lower bench set determines the pressure required to begin valve- closing travel. Seating force is determined by pressure applied minus bench set minus spring compression due to travel.

Because of spring tolerance, there might be some variation in the spring angle. the bench set, when the valve is seated, requires the greatest accuracy.


Study case at Auxiliary Steam supply form Boiler super-heater got passing problem. The valve specification as below
Valve manufacture : Copes - Vulcan.
Fluid : Steam
Max/Normal/Min Flow : 45,000/36,281/5,000 Kg/hr
P1 : 18,373/9,559/1,560 kPa
P2 : 1,501 kPa

Actuator
Diaphragm size : 1050 cm^2
Action : Spring close (reverse)
Air pressure available: 4.1 bar = 410 kPa
Air pressure require : 3.8 bar = 380 kPa
Spring rate : 28 kg/mm
Max stem thrust require : 1268 kg = 11,412 N
Spring previous compression : 1.2 bar = 120 kPa
Pressure full travel : 2.6 bar = 260 kPa
Full travel : 50 mm

Actuator Calculation:
Thrust Pressure : 11,412 N / 1050 cm^2 = 108,685.7 N/m^2 = 108.68 kPa
Spring constant : 28 kg/mm = 274,400 N/m
Spring pressure : 274,400 N.m / 50 mm = 5,488 kPa

This control valve is using reverse action meaning the valve close position with spring pressure 5,488 kPa against steam pressure 18,373 kpa. The control valve begin to open by add air pressure through actuator against spring pressure 120 kPa and maximum open by air pressure 260 kpa.

If the spring pressure goes to weak, the control valve begin to passing even though no air pressure.

These two picture below are describing how to single acting valve actuator tubing install. This auxiliary steam control valve use reverse acting.

Positioner operating and block diagram.

Zero Calibration.
1. Apply 4mA input signal to positioner, actuator should be in full close position.
2. Loosen zero adjustment set screw by alley wrench key.
3. Turn zero adjustment nut (clockwise move actuator down range) until 0%.
4. Tighten zero adjustment screw.

The cam, camshaft and drive arm rotate as an assembly. Cam motion is 90 degree or 45 degree depend on position-er type. Each cam shape (A, B ad C)has its own eight point center hole for mounting on the cam shaft.

Cam A is for a square root function, Cam B is for linear motion and Cam C is for square function.

Span Calibration.
1. Apply 20mA input signal to positioner, actuator should be in full open position.
2. Loosen span adjustment set screw by alley wrench key.
3. Turn span adjustment nut in appropriate direction until 100%.
4. Tighten span adjustment screw.

Zero and span adjustment can be see picture above.