Stepwise adjustment

One regulator controls two or more control valves simultaneously

The 4–20mA signal output by a controller is divided into several signal segments, which are sent to various control valves. In other words, multiple control valves share one 4–20mA signal; each control valve receives one of these signal segments, and this is what is referred to as split-range control.

It’s just a few control valves that regulate one parameter.

This post was last edited by Yunmu on 2019-4-11 at 11:05. Definition: Segmented 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 a cooling agent and a heating agent: the heating agent valve is opened when the temperature is below a certain level, and the cooling agent valve is opened when the temperature is above that level, in order to maintain a constant system temperature. Step control enables safe operation during the initial startup phase or in case of an accident, providing automatic protection. It can ensure normal control and operation under conditions with large load fluctuations, as it can utilize control valves of different sizes to achieve a tunable range of nearly 2500:1. Type: In proportional control, the opening/closing mechanism of the control valves can generally be divided into two types: simultaneous action of the valves. With simultaneous action control, as the output signal from the regulator increases (or decreases), both valves open wider (or close more). Abnormal operation of the valve: The abnormal operation of the valve involves step control, meaning that as the output signal of the regulator increases (or decreases), the valve always operates in such a way that one valve closes while the other opens. Objective: 1. Expand the adjustable range of the control valve and improve the control quality; expand the adjustable range of the control valve to enhance control quality. When split-range control is used to expand the adjustable range, two split-range control valves that operate in the same direction are always installed in parallel on the same fluid pipeline. 2. Meet some specific requirements in the production process. For example, when controlling the pressure at the top of the stabilizer, the process requires that if the pressure becomes too high, the heat bypass valve should be closed first, followed by opening the non-condensable gas vent valve. In such cases, a split-range control system can be used, with different signal ranges from the pressure controller used to control these two valves. Precautions: Choose the flow characteristic of the control valves correctly. At the split points between the two control valves (i.e., the points where control shifts from one valve to another), the system requires a smooth change in flow rate; otherwise, it will be detrimental to system control. However, due to the different amplification coefficients of the control valves, there is a sudden change in the flow characteristics at the split points, which is particularly evident when the large and small valves operate in parallel. If both control valves are linear valves, the sudden changes become very severe ； The situation is better when logarithmic valves are used in all cases. In proportional control, the selection of the flow characteristic of the control valve is extremely important. To achieve a relatively smooth overall flow characteristic, logarithmic valves are generally preferred. If the flow capacities of the two control valves are similar and the range of controllability of the valves is limited, linear valves can be used. The issue of control valve leakage: In proportional control, the amount of leakage from the valve is a very important factor. 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. The selection of control laws and parameter tuning: A multi-zone control system is essentially still a simple control system; the selection of control laws for the controller and its parameter tuning can be handled using the methods applicable to simple control systems. However, when the characteristics of the two control channels differ, the properties of the object under normal conditions should be taken into account, and the controller parameters should be set according to normal operating conditions; the other valve only needs to operate within the range permitted by the process.

Step control means that one regulator controls two or more valves: 0-50 controls valve A, and 50-100 controls valve B

For the A valve and B valve during split operation, there can be a dead zone between the two valves, as well as a coverage area between them; the two valves may divide the flow at a certain point, resulting in these three possible patterns of split operation.

The explanation is very detailed; it’s informative