Engineers are often tasked with troubleshooting and resolving process performance issues. The feedback from operators could be statements such as “this PID loop does not work in Automatic” or “I am always getting alarms in this variable” or “this PID loop has never worked in Automatic”. Understanding how to tackle these issues efficiently and effectively requires a systematic approach along with a good PID tuning tool. This blog explains how to go about achieving success in resolving PID tuning issues.
How well is the process operating?
It’s always good to chat with process operators. They provide valuable insight on why the process and instrumentation are operating a certain way. The operators may inform you that the process sometimes goes outside the operating envelope, frequently causing certain variables to go into alarm. Some PID loops may be operating in manual mode, making it challenging to control key product qualities. Operators may see process oscillations in many variables across the plant. These oscillations prevent them from pushing the plant close to the true optimum and cause operators to give themselves a safety margin to allow on-spec operation. These small issues mean that operators need to spend more of their time adjusting variables to maintain the process within specified constraints, adding to their stress levels. The process control engineer often needs to fix these problems.
“This PID loop has never worked in Automatic”
means that engineers need to investigate the configuration and check.
Investigating the root cause
The first step in an investigation into process issues is to validate the information provided by operators. Engineers could inspect process data and trends of key variables to see if there are violations of the operating envelope, oscillatory behaviour of variables, and alarm limit violations. Trending the Setpoint, Process value, and output of PID loops is a good way of checking the current and historic performance of PID loops, and whether PID tuning may be required. If several PID loops are observed to be in Manual, then a good strategy is to start with the most upstream PID loop, insert it in Automatic, use PID tuning software to tune it correctly, and then move on to the next PID loop downstream and so on. You may observe that one “rogue” PID loop may be causing several others downstream to become unstable. In this scenario, PID tuning of an upstream loop may resolve many issues experienced downstream.
In many cases, the control valve may not be moving smoothly. You can easily check this by making small changes of 1 to 2% in the valve output and observing the change in the process variables. If the process variable responds to small valve output changes, then it is safe to assume that the valve performance is adequate. If not, then you should service the valve, and you should perform the test again. Once the valve performance is satisfactory, the loop can be inserted in automatic and checked by making a setpoint change.
A comment from the operators such as “this PID loop has never worked in Automatic” means that engineers need to investigate the configuration and check. For example, there is some complicated logic preventing operators from using it, or perhaps the PID loop needs to be configured with a different process variable. Poor PID tuning can also be a cause of this, in which case using PID tuning software will help you resolve this issue. Sometimes transmitters may not be working well. This could be checked by changing the valve output and observing whether the process variables change as expected, in direction, and value.
Troubleshoot using tools and PID tuning software to help resolve issues!
Engineers can significantly improve process performance and reduce operator stress by systematically troubleshooting typical process problems. Using a PID tuning tool will help you reduce the time taken to calculate and implement robust tuning parameters. Investigating instrument faults such as valve or transmitter problems and poor PID loop configuration may help resolve long-standing issues resulting in better plant performance, reduced operator alarms, and ultimately better asset integrity and utilisation.
Do you want to make a significant improvement too? Understanding your PID tuning process is essential to provide more added value to your plant operation. Read the XXL Ebook: “The guide to PID tuning” and become an expert and accomplish optimal plant performance.