NONIDEAL VALVES IN CONTROL - TUNING OF LOOPS AND SELECTION OF VALVES - EFFECTS WERE STUDIED AND SIMULATED

The discontinuous nonlinearities of control valves limit the control l oop performance. The effects of nonlinearities were studied by simulat ion and the results confirmed by tests with real equipment. The size o f the stick-slip step determines in most cases the minimum level of va riability achievable with a control loop. It is the most important non linearity parameter in selecting valves for applications where the var iability is critical. Optimal controller tuning considering valve nonl inearities has higher proportional and lower integral action. Higher g ain is needed to overcome the effect of backlash and stiction. Limit c ycle which increases variability is due to combined effect of nonlinea rities and integral action.