A split-range loop is a control strategy used to control multiple control devices with a single output signal. In a split-range control system, one controller’s output is split into two or more ranges, each of which controls a different device (or control valve) depending on the output value. This method is typically used when a process requires different control actions for different ranges of a process variable.

How a Split-Range Loop Works:

1. Single Controller Output:
   • A single controller generates an output signal (often in the form of a control signal, typically 4-20 mA or 0-100%).
2. Output Split:
   • This single output is then split into different ranges, each of which corresponds to a specific control device. Each device (such as a control valve or pump) is activated in a different range of the controller’s output.
3. Multiple Control Devices:
   • The output signal is used to control multiple devices, such as two control valves, each working within a different portion of the output range. One device might be activated for lower values of the control signal, and the other for higher values.
4. Range Allocation:
   • The range of the controller’s output is divided into sections, each corresponding to a specific action. For example, a range from 0-50% could control one valve, and a range from 51-100% could control another valve.

Example of Split-Range Control:

Temperature Control with Heating and Cooling: Imagine a process where both heating and cooling are required to maintain a constant temperature in a system.

• The controller measures the temperature of the system.
• For temperatures below the setpoint, the controller output might be directed to a heating valve (to add heat).
• For temperatures above the setpoint, the output is directed to a cooling valve (to remove heat).

If the temperature falls below the setpoint, the control signal might output from 0-50%, activating the heating system. If the temperature goes above the setpoint, the control signal might output from 51-100%, activating the cooling system.

Key Points of Split-Range Control:

1. Single Controller: A single control loop or controller regulates multiple control devices.
2. Range Division: The controller’s output range is divided into multiple segments, each corresponding to a specific range for different devices.
3. Multiple Devices: Typically used when two or more devices (e.g., control valves) are needed to control a process variable in different conditions.

Benefits of Split-Range Control:

1. Cost-Efficiency: You only need one controller to manage multiple control devices, which simplifies the system and reduces the need for multiple control loops.
2. Flexible Control: Split-range control allows you to manage multiple devices with different actions based on process conditions, such as heating and cooling in temperature control.
3. Efficient Process Control: It’s ideal for processes where a single variable must be controlled in different ways, depending on its value. It ensures that actions are taken appropriately based on the specific conditions.

Example Applications of Split-Range Control:

1. Temperature Control:
   • In systems requiring both heating and cooling, a split-range loop can control two control valves: one for heating and one for cooling, ensuring that the temperature is maintained precisely at the desired setpoint.
2. Flow Control:
   • A split-range control can be used in systems where the flow needs to be controlled by two devices (e.g., pumps or valves). For instance, one pump might be used for low flow rates, and another for higher flow rates, depending on the flow demand.
3. Level Control:
   • In tanks or vessels, a split-range system might be used to control both the inflow and outflow valves based on the liquid level. For example, if the level is too high, the outflow valve might open, and if the level is too low, the inflow valve might open.

Split-Range Control Setup:

A split-range loop can be configured with two control valves. Each valve is used to control a different part of the process:

• Valve 1: Controls from 0-50% of the output.
• Valve 2: Controls from 51-100% of the output.

The controller’s output (e.g., 4-20 mA) is divided into two ranges, with each range linked to a specific valve’s control action.

When to Use Split-Range Control:

• When a process requires multiple devices that act differently depending on the setpoint.
• In systems where heating and cooling, or two different actions (like filling and draining), need to be controlled simultaneously based on the same variable.
• When trying to optimize cost by reducing the number of control loops.