Actuator Bench-set
An actuator bench-set is the instrument air pressure required to begin moving the stem against its closure spring and to fully stroke the actuator over its entire range. These settings are applied by adjusting the spring compression to compensate for the process pressures that will act on the plug area. This setting is typically performed on the actuator before it is mounted onto the valve. The graph below shows the bench-set response of an actuator.
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| Actuator Bench-set Response Graph |
The diagram above is for a direct acting, ATC – FO (air-to-close, fail open) control valve with a signal of 3psig signifying a fully open valve and 15psig a fully closed valve.
Valve Stroking
All control valves after being in service for some time requires stroking. This is because, the valve is a mechanical device with physical components that should be
in good working conditions always otherwise failure is inevitable. The procedure involved in checking out the operation of a control valve is called stroking the valve. This exercise provides for an operational check of the valve components such that problems can be identified easily and corrected early.
in good working conditions always otherwise failure is inevitable. The procedure involved in checking out the operation of a control valve is called stroking the valve. This exercise provides for an operational check of the valve components such that problems can be identified easily and corrected early.
In valve stroking, a pneumatic signal that represents the throttling range of the valve is applied to the actuator. Typically, this is an air pressure signal of 3 to 15psig. For sliding-stem control valves e.g Globe valve, the mechanical movement of the valve between when fully open and fully closed called stroke is adjusted based on the valve design. For example in the case of air-to-open, fail close valve, working with a 3 to 15psig signal:
(1) When 3psig is applied to the valve actuator, the valve should be fully closed. If not, a mechanical adjustment is made until the valve is fully seated and closed. This may involve adjusting the valve stem where it is mechanically coupled to the valve bonnet assembly.
(2) With a maximum signal of 15psig applied, the valve should be fully open. If not, the spring tension of the spring mounted within the actuator is adjusted (reduced) until the valve is fully open.
Control Valve Hysteresis
When stroking control valves, there will always be some difference in the actual physical position of the valve system versus the demand signal, depending on whether the signal was increasing from a lower to a higher signal or whether the signal was in a decreasing mode from a higher to a lower signal level. For example, as the signal increases to the actuator from 3 to 15psig, with a control signal of 9psig applied to the valve actuator, the valve stem position may be 50% of valve stroke, while, with a decreasing signal from 15 to 3psig, the same control signal of 9psig may result in a valve position of 58 – 60% of valve stroke. This occurrence is described as hysteresis. Hysteresis is a typical occurrence in control valves and is due to the fact that the valve is a mechanical device that is subject to friction and the forces of inertia. Most control valves exhibit a hysteresis band of 2 – 5% of valve stroke. Where a control valve has a large hysteresis dead band, the valve stroke versus controller signal will not be accurate and the use of a valve positioner is a must. The diagram below illustrates the concept of valve hysteresis:
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| Control Valve Hysteresis Graph |
