When both a variable frequency drive (VFD) and a control valve are available as operational tools, liquid level control systems are often designed with two single-loop controllers, LIC1 and LIC2, as shown in the diagram above. As many of us know, when there is one controlled variable and two manipulated variables, several control strategies are possible, such as cascade control, split-range control, ratio control, and others. However, to make a scientifically sound decision and solve the problem within the given constraints, more thought and analysis are required.
Understanding the Relationship Between Manipulated Variables and Liquid Level Control
Both manipulated variables, VFD and control valve, reduce the liquid level when increased, which means they work in the same direction. However, liquid level control requires both manipulated variables to work together effectively. In this case, designing a split-range control system is not appropriate. Why is this the case? The key to solving this problem is to consider how to combine these control methods to achieve optimal liquid level control in a cost-effective and efficient manner.
Depending on boundary conditions and system requirements, multiple solutions may be possible. Furthermore, as constraints and demands change, the feasible solutions are not limited to the ones mentioned in the article.
Solution 1: Direct VFD Control for Liquid Level
Many field systems currently use direct VFD control for liquid level management. This method has been proven to meet speed and failure rate requirements. If the control valve can be fully opened and the VFD controls the liquid level, this transforms the system from a multi-variable control problem into a single-variable control problem. This is an elegant and optimal solution—keeping the system simple yet effective.
However, if the VFD opening is too small, there could be equipment safety risks, or the VFD may not provide enough head for the system. In such cases, Solution 1 runs the risk of control failure. To mitigate this, the VFD can be set to a safe lower limit, and the control valve can take over the liquid level control. This approach simplifies the system further while still ensuring safety and stability.
Solution 2: Cascade Control for Liquid Level
Another option is to implement cascade control. Setting the VFD to a safe lower limit ensures the equipment remains safe and provides sufficient head for the system. However, if the control valve cannot meet the required liquid level control with its maximum opening, it will not be possible to keep the VFD running at its safe lower limit continuously. In such cases, cascade control is a viable solution.
Under this scheme, LIC1 can be set to 45%, and LIC2 to 55%. When the control valve is capable of maintaining the required liquid level, LIC1 is active, and the VFD is controlled to its safe lower limit. When the control valve can no longer handle the level control, LIC2 becomes active, and the VFD is adjusted accordingly to increase its output. By modifying only the setpoints, without changing the original design, this solution is both simple and efficient.
Solution 3: Valve Position Control for Liquid Level
In this case, it is crucial to determine the VFD’s safe lower limit. If the VFD’s lower limit is variable, safety considerations might reduce the system’s performance. An alternative solution is to use the control valve for liquid level regulation and treat the valve position as the controlled variable. The VFD would then adjust the valve position dynamically to maintain the desired liquid level.
This method allows for real-time optimization of the valve position, ensuring it remains at the optimal setting for best performance. By controlling the valve position instead of the VFD output directly, this approach addresses the dynamic safety constraints of the VFD. This solution effectively achieves the best performance with minimal complexity.
Solution 4: Advanced Control Methods for Multivariable Systems
In reality, when there is one controlled variable and two manipulated variables, there are many possible control strategies. The choice of the most appropriate method depends on the specific operating conditions and system requirements. Advanced control systems, which use models and parameters to optimize control strategies, can offer flexible, standardized, and efficient multivariable solutions. This represents the simplest yet most optimal form of multivariable control.
In cases where the two manipulated variables are both control valves, the control approach will vary depending on the purpose of each valve. Similarly, when the manipulated variables are flow rates, the optimal solution will change depending on specific conditions. Advanced control systems are well-suited to address these complex scenarios and provide robust solutions.
Mastering Control Strategy Design
To design effective control strategies, it is important first to master common complex control schemes. Then, practicing control design for common scenarios, such as one controlled variable and two manipulated variables, or two process variables and one manipulated variable, will help develop the necessary skills. As one gains experience, this knowledge can be expanded to more complex situations.
Understanding the core principles—adaptability, simplicity, and optimization—is key to designing control solutions. This approach not only applies to traditional control schemes but also extends to advanced control techniques, ensuring that solutions remain flexible, efficient, and adaptable to changing needs.