Split Range Control in Control Valve Applications

What Is Split Range Control?

Split range control is a control strategy used in process industries where a single controller output signal operates two or more control valves over different portions of the signal range. Instead of using one valve for the entire process, the controller divides its output so that each valve responds within a specific operating range to achieve better process control and wider operating flexibility.

In a typical split range control system, one valve may operate from 0% to 50% of the controller output while another valve operates from 50% to 100%. This method is widely used in temperature control, pressure control, pH neutralization, and flow control applications where multiple final control elements are required for efficient and stable operation.

Why Split Range Control Is Used

Split range control is used when a single control valve cannot effectively handle the complete operating range of a process. By dividing the controller output between multiple valves, industries can achieve smoother control, improved process stability, and better response under varying operating conditions.

This control strategy is commonly applied in heating and cooling systems, reactor pressure control, flare systems, and pH control applications. For example, one valve may control heating while another controls cooling, allowing the same controller to manage opposite process actions efficiently and maintain the desired set point accurately.

For most control applications in process industries, it is required to have more than one control valve to respond to the single controller output.  The control valves must be configured to follow the control instructions issued by a particular controller.

Control valves are said to be split ranged or sequenced and configured to follow the command of a single controller.

The various modes of sequencing are employed in Split-ranged control valves.

What are the common modes of control valve sequencing?

In the process industry, this involves some modes of control valve sequencing are

Complementary Split-Range control:

In complementary split-ranging control mode for a particular controller output range both the control valve cannot be in a single state at a time. These control valves must be in complement with each other which means if one control valve is in an open state and the other must be in a close state and vice versa.

This split range control can be observed in a standby situation, where the primary valve halts operation due to some problems the same operation is carried by the other valve called the secondary valve.

A complementary split-range control is observed in the case of an agitator tank where the mixing of two different fluids is carried out.

Let us consider an example of mixing base and pigment liquids together to form coloured paint.

The single controller output signal is sent to both the base valve and pigment valve. While the pigment valve is of Air-To-Open type.  And the base valve is of Air-To-Close type.

The below table represents the relationship between base and pigment valve opening for a particular task issued by the single controller output.

Exclusive Split-Range Control:

The behavior of valve sequencing in exclusive split-range control is to have an “EITHER OR” throttled path for process fluid. Here the process fluid will flow through only one valve at a particular instant of time but not through both valves simultaneously.

In this type of split-range control when the output signal of the controller is 50%, both the acid valve and caustic valve remain fully closed.

When the output signal of the controller is 100% only one valve will open and another valve will close fully. And vice versa.

Let us consider an example of a neutralization process in which the reagent is fed to bring the pH of the process liquid close to neutral by the addition of either acid or caustic.

The working of Exclusive split range control is as follows.

1. A pH analyzer monitors the pH value of the mixture, and a single pH controller commands two reagent valves to get open when needed.
2. If the pH value of the mixture rises above the desired set point, then the controller output signal will increase and the controller directs the acid valve to get open to maintain the desired pH value. The addition of acid to the mixture will have the effect of lowering the pH value of the mixture.
3. If the pH value of the mixture drops below the desired set point, then the controller output signal will decrease and the controller directs the caustic valve to get open to maintain the desired pH value. The addition of base (caustic) to the mixture will have the effect of raising the pH value of the mixture.
4. The pH value of the mixture in the tank is maintained by closing and opening either of the valves as directed by the controller.

The operating range of the acid valve and the caustic valve is as follows.

1. The acid valve called the Air-To-Open valve has an operating range of 9 to 15 PSI,
2. The caustic valve called the Air-To-Close valve has an operating range of 3 to 9 PSI.

The relationship between valve openings for each control valve and the controller’s output is shown in the below table.

Progressive Split-Range Control:

To expand the operating range of flow control of some kind of fluid above the range that a single control valve will deliver.

This type of control technique uses two valves one is the small acid valve and another one is a large acid valve.

For the controller output of 50%, the small acid valve opens gradually and becomes fully opened, during this time the large acid valve will remain closed until the controller output rises above 50%.

As the controller output rises above 50% the large acid valve will start opening and when the controller output reaches 100% both the small acid valve and large acid valve are opened fully.

Let us consider an example of controlling the pH of the incoming fluid.

When the pH of the incoming fluid is high this must be neutralized with the proper mixing of acid to control the pH to the desired set point. The pH analyzer AT is used to measure the pH of the inlet water.

When the controller output increases the small acid valve will start to open gradually and becomes fully open at 50% of the controller output. During this time the large acid valve will remain closed until the controller output rises above 50%.

When the controller output reaches 100% the large acid valve opens and small acid valve is fully closed to ensure that the pH of the incoming water is neutralized.

The below table represents the controller output and valve status for the proper sequencing of small and large acid control valves

What is pH?

 The pH of an aqueous solution is defined as the negative logarithm of its hydrogen ion concentration.

pH = – log {H⁺}

What are acid, neutral, and base?

• Acid: The solution where the pH value lies between pH 0.0 to pH 6.7
• Neutral: The solution where the pH value lies between pH 6.8 to pH 7.4
• Base: The solution where the pH value lies between pH 7.5 to pH 14.0

Split Range Control Signal Table Examples

In split range control systems, the controller output signal is divided between two or more control valves so that each valve operates within a specific signal range. These signal tables help operators and engineers understand how valve positions change with controller output during complementary, exclusive, and progressive split range control modes.

The controller output is usually represented as a percentage or pneumatic signal range such as 3 to 15 PSI or 4 to 20 mA. Based on the configured split points, one valve may open while another closes, or both valves may operate sequentially to maintain stable process conditions and accurate control performance.

Complementary Split Range Control Signal Table

Controller Output (%)	I/P Output (PSI)	Valve 1 Position	Valve 2 Position
0	3	Fully Closed	Fully Open
25	6	25% Open	75% Open
50	9	50% Open	50% Open
75	12	75% Open	25% Open
100	15	Fully Open	Fully Closed

Exclusive Split Range Control Signal Table

Controller Output (%)	I/P Output (PSI)	Acid Valve Position	Caustic Valve Position
0	3	Fully Closed	Fully Open
25	6	Fully Closed	Fully Open
50	9	Fully Closed	Fully Closed
75	12	Fully Open	Fully Closed
100	15	Fully Open	Fully Closed

Progressive Split Range Control Signal Table

Controller Output (%)	I/P Output (PSI)	Small Valve Position	Large Valve Position
0	3	Fully Closed	Fully Closed
25	6	50% Open	Fully Closed
50	9	Fully Open	Fully Closed
75	12	Fully Open	50% Open
100	15	Fully Open	Fully Open

These tables show how split range control sequences multiple valves using a single controller output signal. The valve operating ranges are selected according to the process requirement, allowing smooth transition and accurate control in industrial applications.

Common Problems in Split Range Control

Split range control systems can experience several operational problems if the valves, split points, or controller settings are not configured properly. Common issues include oscillation near the split point, valve hunting, poor process stability, and excessive wear on control valves and actuators.

These problems are often caused by incorrect PID tuning, poor valve sizing, dead band, stiction, actuator limitations, or improper split point selection. If not corrected, these issues may increase process variability, reduce control accuracy, and lead to higher maintenance costs in process industries.

Dead Band, Overlap and Tuning Tips

Dead band in split range control refers to a small gap between the operating ranges of two control valves where neither valve responds to the controller output. This condition can cause unstable process behavior and continuous switching between valves near the split point.

Overlap is the condition where both valves operate slightly together during the transition range to improve smoothness and reduce sudden process changes. Proper PID tuning, accurate split point selection, good valve calibration, and high-performance positioners help improve split range control stability and minimize oscillation problems.

Split Range Control Applications

Split range control is widely used in process industries where one controller must operate multiple control valves for efficient process regulation. Common applications include temperature control systems, pH neutralization systems, pressure control loops, reactor control, and flare gas handling systems.

In heating and cooling applications, one valve may control steam flow while another controls cooling water flow. In pH control systems, acid and caustic valves are operated through split range control to maintain the desired pH value accurately and improve overall process stability.

FAQ in Split range control in control valve applications

What is split range control in control valve applications?

Split range control is a control strategy where one controller output operates two or more control valves in different signal ranges to improve process control flexibility and operating range. It is widely used in temperature, pressure, flow, and pH control applications.

What are the common modes of split range control?

The three common modes are complementary, exclusive, and progressive split-range control. Each mode sequences the control valves differently based on the controller output and process requirement.

What is complementary split range control?

Complementary split range control operates two valves in opposite directions so that when one valve opens the other closes. This method is commonly used in mixing and blending applications.

What is exclusive split range control?

Exclusive split range control allows only one valve to operate at a time while the other remains closed. It is commonly used in heating and cooling or acid and caustic dosing systems.

What is progressive split range control?

Progressive split range control uses two valves in sequence to extend the flow control range. A smaller valve handles low flow rates first, and a larger valve opens when higher capacity is needed.

Why is split range control used in process industries?

Split range control improves process stability, increases control accuracy, and extends the operating range of the system. It also allows one controller to manage multiple final control elements efficiently.

What is dead band in split range control?

Dead band is a small gap between valve operating ranges used to prevent continuous switching and oscillation between valves. It helps improve loop stability near the split point.

What are the typical applications of split range control?

Split range control is commonly used in pH neutralization, reactor pressure control, temperature control, flare systems, and flow control applications. It is widely applied where multiple valves must respond to one controller output.

What is the difference between split range control and cascade control?

Split range control uses one controller output to operate multiple valves, while cascade control uses multiple controllers arranged in primary and secondary loops. Split range control focuses on valve sequencing, whereas cascade control improves disturbance rejection.

What problems can occur in split range control systems?

Common problems include oscillation near the split point, valve dead band, poor tuning, and incorrect split-point selection. Proper valve sizing and controller tuning help improve performance and stability.