【2025 I&C】Requirements of proportional control systems for control valves and examples
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The attachment 1.jpg is: the requirements of a distributed control system for control valves along with examples.
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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 incorporate 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 parameter, 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 valve positioners attached to each control valve, which are used for signal conversion. 4~12mA: Valve A operates ； Valve B is fully open or fully closed. When 12~20mA: Valve B operates ； Valve A is fully open or fully closed.