Illustrated explanation of the principle of split-range control and its applications

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This post was last edited by yunrun on 2020-6-18 at 15:58. Split-range control is quite common in petrochemical production; Changhui Instruments shares in this article knowledge regarding both the principles of split-range control and its applications, to help instrument technicians design and make effective use of split-range control systems. Staged control yunrun.com.cn/tech/2611.html A simple negative feedback control system, in which the control output of the regulator typically acts to control only one actuator or control valve, as shown in Figure 1. http://yunrun.com.cn/upload/201907/11/201907111756353175.png Figure 1: Block diagram of the control system for a single-loop configuration. http://yunrun.com.cn/upload/201907/11/201907111613141692.png Figure 2: Schematic illustration of the characteristics of a control valve. If the output of a regulator is sent to two control valves simultaneously, a split-control system can be established, as shown in Figures 3 and 4. These two valves are used in parallel, and both are air-actuated valves. The two valves move from fully open to fully closed across the entire stroke in different ranges of control signals. http://yunrun.com.cn/upload/201907/11/201907111627181650.png Figure 3: Block diagram of the split-control system. http://yunrun.com.cn/upload/201907/11/201907111629546169.png Figure 4: Operating characteristics of the split-control system. Since pneumatic control valves can operate in either air-open or air-close mode, there are four possible combinations for these two valves. As shown in Figures 5 to 8. Figures 5 and 6 show the valves moving in the same direction, while Figures 7 and 8 show the two valves moving in opposite directions. Although split-range control can involve more than two valves for control, two valves are generally used for this purpose. http://yunrun.com.cn/upload/201907/11/201907111643377379.png http://yunrun.com.cn/upload/201907/11/201907111645371246.png Figure 5 Figure 6 http://yunrun.com.cn/upload/201907/11/201907111648000331.png http://yunrun.com.cn/upload/201907/11/201907111649004943.png Figure 7 Figure 8 Applications of stepped control 1. Improving the adjustability ratio of valves: If a control valve can regulate a minimum flow rate of Qmin and a maximum flow rate of Qmax, then the adjustability ratio or range R is defined as R = Qmax/Qmin. The adjustability ratio of most domestically produced valves is 30; in some cases this value is not sufficient. To meet the requirements, to handle large variations in load, and to improve control quality, stepped control can be employed. http://yunrun.com.cn/upload/201907/11/201907111643377379.png Taking the air-operated valve shown in the above image as an example for analysis, assuming the maximum controllable flow rate is 200 and R=30, the minimum controllable flow rate is 200/30=6.67. With split-range control, the two valves operate within different control signal ranges. For the entire system formed by connecting these two valves in parallel to achieve step control: the minimum flow rate that can be controlled is Q′min = Qmin = 6.67 ； The controllable maximum flow rate is Q′max=2×Qmin=400. Then the adjustable ratio R′=Q′max/Q′min=400/6.67=60, which shows that the adjustable ratio has doubled. 2. Alternating between different operating modes: In industrial production processes, it is sometimes necessary to use various control methods, as shown in the examples in Figures 9 to 10. http://yunrun.com.cn/upload/201907/11/201907111712531944.png http://yunrun.com.cn/upload/201907/11/201907111722140027.png Figure 9: Step-controlled scheme for nitrogen sealing of storage tanks (Valve A is air-shut, Valve B is air-open). Figure 10: Characteristic diagram of step-controlled nitrogen sealing. Nitrogen sealing refers to the practice of filling the tops of certain oil storage tanks with nitrogen in order to prevent the oil from coming into contact with oxygen in the air. The top of the storage tank is filled with nitrogen to maintain a slight positive pressure. As the liquid level changes, the pressure at the top will change as well. ◆As the liquid level rises, the pressure increases; as the liquid level drops, the pressure decreases. It is not acceptable for the pressure in nitrogen-sealed oil storage tanks to increase or decrease significantly. ◆Nitrogen sealing step-controlled process: When the liquid level rises, valve B closes and valve A opens to release nitrogen and maintain constant pressure. ◆ When the liquid level drops, valve B opens and valve A closes to supply nitrogen and maintain constant pressure. ◆ When the liquid level fluctuates within a small range, the pressure also fluctuates within a small range; the control system does not take action, meaning no nitrogen is supplied or released, and this is referred to as the safe zone. It can prevent frequent valve operation and maintain system stability. 3. Meeting the requirements of different stages in the production process: For exothermic chemical reaction processes, it is necessary to heat the materials at the initial stage of the reaction in order to initiate it ； Since it is an exothermic reaction, heat continues to accumulate in the reactor; therefore, the amount of heat that needs to be supplied gradually decreases. Once the heat released exceeds the amount required for the reaction process, it is no longer possible to supply additional heat, and instead the reactor must be cooled in order to remove the excess heat generated during the reaction. For such processes, split-control as shown in Figures 11 to 12 can be employed. http://yunrun.com.cn/upload/201907/11/201907111740500933.png http://yunrun.com.cn/upload/201907/11/201907111741591468.png Figure 11: Temperature zoning control system for batch reactors Figure 12: Characteristic diagram of the temperature zoning control system for batch reactors http://yunrun.com.cn/upload/201907/11/201907111749274616.png Figure 13: Block diagram of the temperature zoning control system for batch reactors Note: In the temperature zoning control system for batch reactors, the regulator operates in a feedback manner ； Valve A: Air-shut ； Valve B: Air-operated open ； Object 1: Reaction ； Object 2: Positive effect. The temperature zoning control system of the batch reactor is negative feedback whenever any valve is in operation. By combining these three examples of cascade control along with the relevant system diagrams, it is believed that everyone now has a basic understanding of the principles of cascade control as well as cascade control systems. It is recommended that you read the article titled \"If You Can’t Calculate and Determine the Cascade Points, Don’t Claim to Understand Cascade Control\" for further information on this topic