OSCILLATIONS - CAUSES AND SOLUTIONS

摘要

Oscillations result in a loss in process efficiency and/or capacity. In the extreme, oscillations can cause loss of yield from off-spec product, cross neutralization of reagents, cyclic heating and cooling, and down time by activation of a Safety Instrumented Systems (SIS) or relief system. The more common scenario for continuous processes is a setpoint further away from a real or perceived optimum to prevent violating an equipment or process constraint. For batch processes, the need to reduce variability results in longer than necessary batch hold times to do analysis and correction or to wait for loops to stabilize or reactions to complete. The root causes of oscillations in control loops will be explored in this tutorial. The source of the problem will be located in terms of the automation system measurements, valves, control strategies, process design, and PID parameters. Simple troubleshooting techniques will be offered. A quick fix by changes in controller tuning and configuration will be given for each of the causes along with a long term solution in terms of system improvements to eliminate the source of the problem.