Addressing Incorrect Tuning

A Loop tuning has a significant impact on control loop utilization and performance. Controller tuning parameters can be easily changed over a wide range to adjust loop operation for various process requirements. However, some expertise in tuning is required even when advanced control tools are used, and there is a chance that the behavior of a particular process will be unaccounted for in the tuning that was selected for the control loop. These are important reasons for considering loop tuning as a primary suspect in degraded performance.

During the design of a new process area or plant, initial loop tuning can be specified based on the type of measurement associated with the control loop, that is, whether pressure, level, temperature, flow, or some other process variable to be controlled. In most cases, these default settings may be close enough to support the initial startup of a new process area or plant. However, to minimize process variations from setpoint and to minimize the response time to setpoint and disturbance input changes while providing stable operation over a variety of operating conditions during or after startup, it is necessary to set the PID tuning based on the observed process gain and process dynamics for each control loop. Unfortunately, the instrument engineer involved in plant commissioning or plant operations may not have the opportunity to tune loops and, as a result, plant operation does not achieve maximum efficiency.

In an operating plant, various approaches can be taken by plant personnel to modify control loop tuning to improve control performance. The most direct and reliable way to manually or automatically establish PID block tuning is to calculate the tuning based on the identified process gain and dynamics as described in the following sections. With older control systems, the analog single loop controllers and programmable logic controllers (PLCs) used may not include tools that allow the PID tuning to be automatically established based on the observed process response to input changes. As is address in Chapter 4 and 5 of Advanced Control Foundation – Tools, Techniques, and Applications, most modern control system provide some to automatically establish loop tuning, as illustrated below.

When selecting the PID gain, it is important to consider that the process gain can change with the operating conditions. To ensure stable operation over the entire operating range of a control loop, the normal practice is to establish the PID tuning when the process is operating in the region with the highest process gain. As a result, higher variation in the controlled parameter can be seen when the process transitions to the low gain region of operation.