There are many critical PID loops in an operating plant. Operations managers typically like to minimise changes on tuning parameters on such PID loops, quite often maintaining the status quo, even if improvements are possible or desirable. The performance of such PID loops could be optimised if they could be inserted in manual mode followed by moving the output a few times, collecting data, and calculating PID tuning using PID tuning software. Unfortunately, operations managers may not allow you to insert this critical loop in manual, so how can you calculate PID tuning parameters for such a loop while maintaining it in closed-loop mode? In this PID tuning example, we’ll show the best practices and pitfalls.
Is it possible to tune PID loops in closed loop mode?
In general terms, most PID loops cannot be properly tuned in closed-loop mode. However, if the loop cannot be put in manual mode, engineers must use their judgement based on the type of PID loop and process data to make an informed decision. Broadly speaking, there are 3 categories of PID loops that you may consider tuning in closed loop mode.
- The first is a PID loop that cannot be inserted in manual because the process rapidly becomes unstable, such as a quench flow to an exothermic reactor.
- The second is a critical PID loop that plant operators would prefer not to insert in manual, such as a compressor discharge pressure.
- The third is a very simple flow loop. Although this type of loop can easily be put in manual, it is often thought that using PID tuning software for these type of “simple” loops is not necessary.
How would you go about tackling these PID tuning problems if you were faced with them?
3 PID tuning examples for tuning in closed loop using PID tuning software
The example of a simple flow loop is perhaps best tackled first. In this case, making a step change will show you the response of the PV. If the response is slow, the gain and/or the integral time can be made faster (and opposite if the PV response is too fast). Changes should be made in small steps to ensure instability is not reached. This method however does not give good insight in the robustness of the tuning. Though good performance might be achieved after the tuning, there’s no guarantee that in a few days, under slightly different operating conditions, the tuning is still robust and providing acceptable closed loop behaviour. The better method is to insert the loop in manual, make steps in the output and then calculate the PID tuning parameters using PID tuning software.
In the case of an unstable loop such as a pressure or a critical PID loop such as a quench flow, you should check for valve output and process variable data at periods when there are no setpoint changes. See if you can obtain a model between the output and the PV. The
most likely result is that you will not get a correct model. Another option is to make a setpoint change, check the response of the output and then calculate the gain and dynamics manually by trending the output and PV responses. A third option would be to insert the loop in manual and conduct a test over a very short period of time (the duration of the test depends on the process dynamics). This open loop data may be used to calculate PID tuning parameters using PID tuning software. It is also possible to mix some of the open loop data with closed loop data to potentially improve the models and PID tuning. In all these cases, engineering judgement is required and using PID tuning software as a tool will help you get the best results. Even in the case the model you’ve obtained is not very accurate, PID tuning software will provide you with the possibility of explicitly taking the model uncertainty into account, so that the robustness of the PID tuning is guaranteed.
Tune with confidence!
Engineers using a PID tuning tool can tackle difficult PID tuning problems that may require closed loop tuning. In these scenarios, approach the problem with an open mind and work with your operations department to ensure you optimise the tuning with confidence.
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