Hello ;
As a process engineer, you know how important it is to choose the proper control schemes for your plant equipment. Process control is essential to ensure that the operating parameters are within their acceptable limits and to optimize the system's performance.
But how do we choose the best control scheme for a specific application? What are the factors to consider? What are the common pitfalls to avoid?
In this email, I will share some tips for choosing the system control scheme. I will also give you an example of a common control scheme that is used in many industries: the split range control.
Tip 1: Define the parameter that needs to be controlled
The first step is to identify the parameter that has the most impact on the system performance.
For example, if you want to control the operation of a separator, the parameters most probably will be the separator pressure and the liquid level.
If it is a pump or a compressor, then it will be a flow controller. If it is an exchanger or an air cooler, then it will be the outlet temperature of the target stream.
For a reactor, you may need to control the flow ratio of the reactants, the pressure, or the temperature, depending on the type of the reaction. For a tower, you may need to control its pressure, the inlet flow, or the temperature of specific trays.
Tip 2: Choose the parameter that is easier to control and directly affects the system
Sometimes, we may have more than one parameter that can be controlled to achieve the same objective. In this case, we would choose the one that is easier to control and directly affects the system. For example, when controlling the performance of a pump, it is more common to use a flow controller rather than a pressure controller, although the pump is mainly used to raise the pressure of a fluid.
This is because the pump output is head not pressure, and the head depends on the fluid density, which may vary. So controlling the fluid flow would be much easier in this case.
Tip 3: Choose the control method that fits your system from the point of view of operational stability, equipment safety, and cost optimization
There are different methods to control a parameter, such as changing the flow of a fluid, adjusting the speed of a motor, bypassing a portion of the stream, or recycling the excess flow.
Each method has its advantages and disadvantages, and you need to consider them carefully before making a decision. For example, if we want to control the temperature of a fluid that is cooled by another process fluid, changing the flow rate of the cooling fluid may negatively affect the process.
In this case, we may use a bypass controller, where a portion of the fluid inlet bypasses the exchanger and mixes with the outlet fluid to reach the desired temperature.
Example: Split range control
Split range control is a common control scheme that is used when you need more than one source to control a parameter. In split range control, you have two final control elements, such as valves, that are controlled by the same controller. One of the valves is normally varied against the variable to be controlled up to a certain limit. After this limit, the other valve starts to adjust while the first valve is fully open or fully closed.
A famous example of split range control is the pressure control of a vessel. The pressure of the vessel is controlled by two valves: one that sends the gas to the downstream facility, and another that sends the excess gas to the flare.
When the pressure is low, the first valve starts to close to build up the pressure. When the pressure is high, the first valve starts to open to reduce the pressure. If the pressure is too high, the second valve starts to open and send the gas to the flare, while the first valve is fully open.
Split range control can be used for other parameters as well, such as temperature, level, or flow. The type and application of split range control depend on the criticality of the parameter to be controlled.
If you want to learn more about the tips to choose proper control schemes, and see more examples and applications, I invite you to check out this article:
This article will give you a comprehensive overview of the considerations that are usually taken to ensure you have chosen the correct control scheme. It will also help you to understand the dynamics of different control loops and how they affect the system response.
I hope you find this email useful and informative. If you have any questions or feedback, please feel free to reply to this email or leave a comment on the article.
Have a nice day!
Mohamad
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