Putting the ‘manual’ into automatic control
“WHO looks after PID tuning at your site?” This is a question I often ask many professionals. The answers vary from a definitive department, to “maintenance department”, “instrumentation department”, or “don’t know”. The replies almost always suggest to me that there is not enough attention paid to this important but generally forgotten layer of automation.
PID (proportional-integral-derivative) controllers still form the backbone of automatic control systems in refineries, (petro)chemical installations, power plants and numerous other processes. The success of PID schemes lies in the simplicity of their implementation, reliability of the hardware, and the stabilising effect on processes. It is, however, necessary to enter values for P, I and D – referred to as tuning parameters – to make PID controllers function in a stable and efficient manner.
Proportional integral and derivative (PID) loops are a basic layer of process automation in the form of an equation, implemented normally in a distributed control system (DCS). The flow through a pipe with a valve can be set to a desired value because a PID loop is working in the background to measure the actual flow (process value, or PV) and compare it to the desired value (setpoint or SP) and automatically adjust the valve opening (output or OP) to maintain the flow at the desired value. This configuration is called a “flow loop”. Other configurations that set the temperature and pressure would be referred to as “temperature loop” or “pressure loop”. The P, I and D parameters can be adjusted or “tuned” in the DCS to obtain a stable and fast response to a change in the desired value or the impact of disturbances which may impact the PV. For the PID loop to behave in a stable manner, it is essential to set the correct P, I, and D parameters and calculating them is not always straightforward. The PID loop is essential to set desired values for various process parameters such as flows, temperatures and pressures, so the process may operate at target conditions.
When a new plant is commissioned, it’s quite common to see default PID tuning parameters implemented in several of the PID loops. I once visited a newly-commissioned plant and was told that the engineering contractor had only tuned 10% of the PID controllers, the rest had default parameters that came with the distributed control system (DCS). I have since learned that this is quite a common occurrence. Whilst these defaults may work in many loops, they are not optimised for best performance. Over the years, the poor tuning will manifest in the form of unstable operation, at which time the PID tuning may get rectified. Good practice would be to ensure that while a new plant is being commissioned, the PID tuning is reviewed and optimised for best performance. This action early in the life cycle of the plant will ensure stable performance of PID loops at a variety of operating conditions as well as reduced wear and tear on valves, actuators and other equipment.
Catch up on the latest news, views and jobs from The Chemical Engineer. Below are the four latest issues. View a wider selection of the archive from within the Magazine section of this site.