The control valve and variable speed drive are the final control elements that directly affect the process by manipulating a flow. The expectation is that these elements do their job and do not adversely affect the tuning and performance of the loop.
Deadband and resolution cause sustained equal amplitude oscillations called limit cycles that cannot be eliminated by controller tuning. A resolution limit causes limit cycles if there is integrating action at just one point in the control loop either in the process (e.g. level), positioner, or the process controller (e.g. PID).
Variable speed pumps can save energy when the flow is reduced. However, the turndown of the pump speed may not achievable if there is overheating at low flows, excessive sensitivity at low rpm, and cycling at high destination pressures.
The threshold sensitivity and resolution of measurements (smallest incremental change) has improved dramatically to better than 0.05% with new sensor technology and smart compensation of extraneous effects of operating conditions. The response of the air-actuated control valve has improved, but the smallest incremental change in position varies widely depending upon...