A split range control strategy uses software or hardware-based splitter and two or more final control elements to control a process variable. You can find split range strategies in different applications across process industries such as temperature control with heating/cooling media, large and small valves installed in parallel, pressure control with a vent valve, etc.
Usually, split range applications are tricky and time consuming, and you should know what I’m talking about as a process control engineer.
Oscillations over the entire range of operation and across the split point, typically improperly selected at 50%, are common in real operation.
This blog will discuss the usual problems you might face when configuring and tuning a split range strategy and the possible and practical solutions to overcome it.
Split range challenges
Very often, clients call us to check the performance of their split range strategies. As pointed out already, split range applications are challenging, so don’t feel bad and lonely if you are facing this in your plant. It is more common than you think!
Your split range strategy could be causing oscillations and process instability due to different factors:
- Sub-optimal PID tuning
- Wrong selection of the split point
- Poor control valve performance (control valve stiction, deadband, type, etc.)
- Non-linear installed control valve characteristic
- Poor actuator/positioner performance
- Process non-linearities (process dynamics differs over the range of operation, e.g. cooling/heating, gas/liquid)
“The good news is that if you apply a comprehensive approach and consider the split range strategy as a whole, then you will succeed and not die trying.“
How to succeed and not die trying?
Carefully read the following tips which will help you out:
- Use the right tools to identify the process dynamics for the entire range of operations and come up with optimal PID tuning parameters. If it is required, you should implement PID tuning scheduling based on the PID controller output (PID tuning parameters can be changed automatically based on the PID controller output). INCATools can help you out to achieve this.
- Select the split point based on the control valves and process gain. The most common practice is to select the split point as 50%. However, this only applies if the control valve sizes and operating conditions are the same for both control valves. If the process gain is unknown (i.e. in new applications) you can use the control valves flow coefficient (Cv/Kv) to develop an initial split point.
- Use a control valve designed for throttling with a smart positioner, the right actuator, excellent resolution, and response time. Control valves often suffer from mechanical problems (e.g. stiction, deadband) near to the closed position. To overcome this, an overlap in the split configuration can be useful (see image below).
4. Do not configure a deadband in the split range strategy. A deadband might only increase the oscillations around the split point.
5. Use a “software-based” signal splitter. This will make your life and job easier. The advantages are:
- Easy to maintain.
- Easy to understand.
- Flexibility to implement split range strategy variations (split point and overlapping configuration).
- Centralization of the split range logic in the control system.
- Reduction of physical failure points.
6. Last but not least, you must provide proper training and information to the operators
What benefits could you get from a well configured and tune split range PID control strategy?
Typical benefits for split range PID control include:
- Savings on control valve maintenance, wear and tear is reduced.
- Energy savings (e.g. steam, cooling water, gas, compressed air, etc.).
- Reduced process variability.
As a practical example, you can think of a batch reactor operation: it can waste energy from oscillations in the reactor temperature, especially around the split range point. Isn't this a powerful reason to pay more attention and improve your split range strategies?
So, if you want to learn more about how to go from this behaviour:
Talk to an expert
We are happy to advise you about how PID tuning, process knowledge, and the right tools can help you accomplish optimized plant performance. Talk to an expert to set up your split range strategy.