Split range control is an advanced process control strategy where a single controller's output is divided to operate multiple final control elements, such as valves, in distinct operating ranges. Instead of one controller acting on one device, this method allows a single controller to manage several devices sequentially or in opposition, optimizing process efficiency and stability.
This approach is particularly useful in processes requiring coordinated action from different actuators to achieve a single control objective.
How Split Range Control Works
At its core, a split range controller takes a single control signal (typically 0-100%) and allocates different segments of this signal to manipulate different actuators. For instance, the first half of the controller's output range (e.g., 0-50%) might control one valve, while the second half (e.g., 50-100%) controls another.
Consider a liquid level control system in a tank:
- A single controller monitors the liquid level.
- As its output signal varies, in the initial portion of its range (e.g., 0-50%), it could precisely control the outflow of excess liquid by modulating a discharge valve to maintain the desired level.
- If the process demands change, and the controller's output moves into a different range (e.g., 50-100%), it would then begin to manipulate a second valve, perhaps an inlet valve to add more liquid or another outlet for emergency diversion, to further adjust the level or respond to other process requirements. This ensures precise control over the tank's level by orchestrating multiple outputs from a single control input.
Key Characteristics
- Single Input, Multiple Outputs: A single Process Variable (PV) and a single controller output but multiple manipulated variables (MVs).
- Sequential or Opposing Action: Actuators typically operate in sequence or opposition to each other. For example, one might open while the other closes, or they might operate independently in different output ranges.
- Enhanced Control: Allows for finer tuning and more precise control over processes that have varied operating conditions or require a wide range of corrective actions.
Common Applications of Split Range Control
Split range control finds extensive use in various industrial processes to manage diverse scenarios effectively.
1. Temperature Control (Heating and Cooling)
This is a classic example. A single temperature controller manages both a heating medium valve and a cooling medium valve.
- If the temperature is too high, the controller's output in one range (e.g., 0-50%) will open the cooling valve.
- If the temperature is too low, the controller's output in another range (e.g., 50-100%) will open the heating valve.
- At the setpoint, both valves are closed or minimally open to maintain stability.
2. pH Control (Acid and Base Addition)
In chemical processes, maintaining a precise pH often requires adding either an acid or a base.
- A pH controller's output in one range (e.g., 0-45%) opens the acid valve.
- Its output in another range (e.g., 55-100%) opens the base valve.
- A dead band (e.g., 45-55%) can be introduced where both valves are closed, preventing wasteful simultaneous addition and ensuring the process remains stable at the setpoint.
3. Pressure Control (Vent and Purge)
For reactors or storage tanks, a pressure controller might operate both a vent valve (to release excess pressure) and a purge gas valve (to add inert gas for pressure build-up or blanketing).
- High pressure opens the vent valve.
- Low pressure opens the purge gas valve.
4. Flow Control (Multiple Pumps or Valves)
Sometimes, very wide flow ranges require multiple pumps or valves. A split range controller can bring on additional pumps as flow demand increases or open larger bypass valves.
Benefits of Implementing Split Range Control
Implementing split range control offers several advantages for process optimization:
- Improved Control Precision: Allows for more accurate control across a wider operating range by utilizing the optimal actuator for a specific condition.
- Enhanced Efficiency: Prevents wasteful actions, such as simultaneously heating and cooling, or adding acid and base, leading to energy and material savings.
- Simplified Operator Interface: A single setpoint and tuning for one controller, rather than managing multiple independent controllers.
- Robustness: Provides flexibility to handle process disturbances more effectively.
- Optimized Equipment Utilization: Ensures that each final control element is used within its most effective operating range.
Split Range vs. Other Control Strategies
| Feature | Split Range Control | Cascade Control | Feedforward Control |
|---|---|---|---|
| Controllers | One primary controller | Two (or more) controllers, master/slave | One primary controller, one feedforward signal |
| Final Control Elements | Multiple (e.g., two valves) | One (slave controller acts on it) | One |
| Purpose | Manage different actions from a single PV | Improve disturbance rejection, faster response | Anticipate and compensate for disturbances |
| Example | Heating/cooling, acid/base addition | Tank level control with flow as slave PV | Furnace temperature with fuel flow as disturbance |
| Complexity | Moderate | High | High |
Design and Tuning Considerations
- Output Range Allocation: Carefully define the output range for each control element. A common practice is 0-50% for one and 50-100% for the other, often with a small "dead band" in the middle (e.g., 45-55%) where neither actuator is active to prevent chattering or oscillating actions near the setpoint.
- Valve Sizing and Characteristics: Ensure that the control valves are properly sized and have appropriate flow characteristics (e.g., linear, equal percentage) for their respective operating ranges.
- Tuning: Tuning a split range controller can be more complex than a single-loop controller. Each range might require slightly different responses from the final elements. It's crucial to tune for stability across the entire operating spectrum, especially at the transition points.
- Safety Interlocks: Incorporate safety interlocks to prevent unintended operation, such as both heating and cooling being fully open simultaneously.
Split range control is a powerful tool in process automation, allowing for a more sophisticated and efficient management of complex industrial operations by orchestrating multiple control actions from a unified command.
