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Feb
08

Actual Control Valve Scenarios – Part 2

(1) A compressor shuts down

(a) The first out sequence indicates the compressor tripped on high speed

(b) A precipitous drop in suction flow followed by a rapid 1-2 second oscillation in suction flow preceded the speed deceleration from compressor shutdown

(c) The readback of actual surge butterfly position indicates the valve closed before the initial drop in flow.

(d) The surge set point flow controller set point was constant.

(e) The butterfly disc closed despite a controller output asking it to be open.

(f) The control valve is fail-open (inc-close) so a loss in signal or activation of the solenoid valve is not the cause

(g) Conclusion – the volume booster sensitivity and actuator size and type caused a butterfly disk instability at high flow

(2) A thermal oxidizer shuts down

(a) The first out sequence indicates the oxidizer tripped on high temperature

(b) A spike in natural gas flow occurred before the trip

(c) The natural gas set point was constant

(d) A readback of actual gas valve position indicates the gas valve position was relatively constant before the spike and started to closed after the spike

(e) The pressure transmitter upstream of the gas valve spiked high about the same time as the flow spike

(f) Conclusion – the natural gas pressure regulator upstream went open

(3) A pH tank has sustained nearly equal amplitude oscillations

(a) The pH oscillation amplitude stayed the same when the controller gain was increased or decreased*

(b) The pH amplitude changed for a different pH set point*

(c) A readback of actual valve position indicates the minimum change in valve position is 0.5% or alternately indicates a step in an actual valve position change always precedes the pH change.

(d) Conclusion – the oscillation is caused by the resolution limit (stick-lip) of the control valve which multiplied by the process gain is the amplitude of the pH limit cycle (a change in pH set point changes the process gain from the operating point nonlinearity associated with the titration curve)

(4) A column sump level has very slowly decaying oscillations

(a) The amplitude of the oscillation takes a day to decay

(b) Feed, steam, and reflux flows are relatively constant

(c) The oscillation is more persistent (decay and period slower) when the level controller gain is decreased

(d) Actual readback of level valve position matches the controller output within 0.05% a couple of seconds

(e) Conclusion – the controller gain is below the low gain limit (controller gain multiplied by controller reset time must be greater than 4 divided by the integrating process gain) A controller gain that is too high causes faster oscillations that would die out if the controller gain is decreased**

* – 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