【2025 I&C】Examples of split-range control systems and methods to avoid sudden changes in flow rate at the split points

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In a split-control system, the output of one regulator controls two or more actuators or control valves. Usually, two actuators or control valves are the most common. The purpose is to expand the adjustable range and improve the adjustment quality.

Since control valves are divided into air-open and air-close types, there are the following four combinations when two control valves are used for split-range control. Figures (a) and (b) show that the two control valves operate in the same direction, while figures (c) and (d) show that they operate in opposite directions.

Step control involves dividing the output signal of a regulator into several segments; each segment controls an actuator, and multiple actuators working in sequence accomplish the same task. This type of control system is also known as continuous valve control.

Staged control is primarily used in systems that employ multiple control methods with logical relationships but share the same control objective; it is designed to coordinate the operational logic of these different control methods. It is also applicable to systems where an object’s properties are highly nonlinear, requiring a step-by-step approach for precise control. When more than one regulator is required to meet a certain target value, cascade control should be employed. For example, in temperature control, there are cold medium and hot medium; when the temperature is below a certain level, the valve for the hot medium is opened, and when the temperature is above that level, the valve for the cold medium is opened in order to maintain a constant system temperature.

The basic principle of a split-range control system is illustrated using one controller to operate two valves as an example.

To achieve the purpose of step control, it is necessary to use the valve positioners attached to each control valve, which are used for signal conversion. When 4~12mA: Valve A operates ； Valve B is fully open or fully closed. At 12~20mA: Valve B operates ； Valve A is fully open or fully closed.

The method for adjusting PID parameters is as follows: the default values are KP=1, TI=0, and TD=0. When running the process with default parameters, it was observed that the PV value of PIC711 adjusted very slowly and tended to stabilize. By increasing the P value to 10, it was found that the adjustment speed increased significantly. Based on past experience, for a pressure I value between 10 and 100, start by entering 10. The operating system detected rapid pressure fluctuations, so I was increased further to 30, while the proportional control value P was gradually increased to 20. Continuing to increase I to 50, the fluctuation trend slows down. Continuing to increase I to 80, it was found that the PV value of the PIC711 had stabilized and became equal to the SP value. By continuing to increase I to 100, it can be observed from the trend chart that the time required for flow balance is greatly extended; therefore, integral control at 80 is more reasonable.

In a split-control system where a large valve and a small valve are connected in parallel, if the leakage rate of the large valve is too high, the small valve will be unable to fulfill its control function properly, or even fail to provide any control at all. Therefore, control valves with low leakage or no leakage should be selected.