The best PID solution is to eliminate the split range by the use of a valve position controller (VPC) that has directional velocity limiting (DVL) of the PID output and a dynamic reset limit (DRL) as a PID option. The enhanced PID (PIDPlus) with a sensitivity limit offers advantages for eliminating limit cycling and reducing interactions between the process PID and the VPC. The VPC solves both the rangeability and sensitivity requirements. For manipulated flows with opposite effects (coolant and steam, acid and base reagent, and nitrogen and vent valves), split range control insures the opposing valves are not open at the same time. However, 98% of split range loops are oscillating across the split range point resulting in unnecessary alternating use of heating and cooling, acids and bases, and nitrogen and venting putting additional loads on utility, waste treatment, and vent systems. The judicious use of VDL, RDL, and the PIDPlus with a sensitivity limit can stop unnecessary excursions across the split range point. Secondary flow loops can isolate the valve nonlinearity form the primary process loop. Precise valves with diaphragm actuators, digital valve controllers, and 2x actuator size can minimize stick-slip that may be an order of magnitude larger near the closed position. Finally, a smart split range point and adaptive control can compensate for process nonlinearities.
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