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Feb
15

Actual Control Valve Scenarios – Part 3

(5) A column distillate receiver has a relatively constant amplitude level oscillation

(a) The amplitude of the oscillation is relatively constant for a given tuning

(b) The amplitude and period of the oscillation increases as the controller gain is decreased*

(c) Actual readback of level valve position indicates the minimum change in valve position routinely is 0.05% but the valve position stays at its previous position for a reversal of controller less than 0.5%

(d) Conclusion – the control valve has dead band which for an integrating loop causes a limit cycle

(6) A continuous evaporator has a constant amplitude temperature oscillation

(a) The amplitude of the oscillation stays the same when the controller gain is changed*

(b) Actual readback of steam valve position indicates the minimum change in valve position is 0.5%

(c) Conclusion – the steam valve has a resolution limit that for a self-regulating loop causes a limit cycle

(7) A process dead time varies

(a) The production rate is relatively constant

(b) The dead time increases as the controller gain is decreased *

(c) The dead time increases as the change in set point is decreased*

(d) Actual readback of level valve position indicates the minimum change in valve position routinely is 0.05% but the valve position stays at its previous position for a reversal of controller of less than 0.5%

(e) Conclusion – the changes in process dead time are caused by valve dead band (dead time is dead band divided by the rate of change of controller output for signal reversal)

(8) A process dead time varies

(a) The production rate is relatively constant

(b) The dead time increases as the controller gain is decreased *

(c) The dead time increases as the change in set point is decreased*

(d) Actual readback of level valve position indicates the minimum change in valve position routinely is 0.05% but the control valve takes from 5 to 50 seconds to catch up to the controller output for a reversal of controller (time to catch up increases as the change in controller output is decreased).

(e) Conclusion – the changes in process dead time are caused by a positioner with poor sensitivity that causes a slower exhaust or fill rate to actuator for smaller changes in controller output

* – these controller tuning or set point changes provide affirmation but are not required to diagnose the problem

** – valve diagnostics confirm it is not a valve problem