Decomposition and practical application of instrument loops and control system loops

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Decomposition and practical application of instrument loops and control system loops [Copy link]

If there is no instrument loop diagram, you can decompose the instrument detection or control system according to the actual situation on site, and then draw the instrument loop diagram. Such a loop diagram does not need to be restricted by form, as long as it is convenient for instrument and control system maintenance. The system loop can be divided into three types: electrical loop, instrument gas source loop, and pressure pipe loop for process medium detection. 1. Temperature control system loop decompositionyunrun.com.cn/tech/2045.html Figure 1 is a common steam heater temperature control system. The hot water temperature measured by the TT thermal resistor integrated temperature transmitter is sent to the TC regulator and compared with the given temperature. When the water temperature is lower than the given temperature, the output signal of the regulator will open the regulating valve, increase the steam flow rate and increase the water temperature. When the water temperature is higher than the given temperature, the output signal of the regulator will close the regulating valve, reduce the steam flow rate and reduce the water temperature. Figure 1 Schematic diagram of steam heater temperature control system In order to intuitively see the mutual influence and signal connection between the various components of this control system, the block diagram of Figure 2 can be used to represent the composition of the control system, and each block in the figure represents a link of the system. Figure 2 Block diagram of steam heating temperature control system In order to be more intuitive, we draw it according to the electrical signal circuit, as shown in Figure 3. In Figure 3: A is the output signal circuit of the temperature sensing element thermal resistor, and it is also the input signal circuit of the temperature transmitter. The output is a resistance signal proportional to the change in hot water temperature. The resistance value increases when the temperature rises, and decreases when the temperature drops. B is the output signal circuit of the temperature transmitter, and it is also the input signal circuit of the regulator. The output is a current signal proportional to the change in hot water temperature. The current value increases when the temperature rises, and decreases when the temperature drops. C is the output signal circuit of the regulator, and it is also the input signal circuit of the actuator. The output is a current signal issued according to the size of the temperature deviation signal and the predetermined control law. It is a current signal that reacts inversely to the change in hot water temperature. When the hot water temperature rises, the current value decreases, and the regulating valve is closed to reduce the steam flow, so that the hot water temperature drops; when the hot water temperature drops, the current value increases, and the regulating valve is opened to increase the steam flow, so that the hot water temperature rises. Figure 3 Schematic diagram of the electrical circuit of the steam heating temperature control system. Knowing how each circuit works, you can know what kind of impact will be caused to the next circuit when the circuit fails. In this way, the idea of analyzing and judging the system failure will be clear. This figure omits the power supply circuit, valve position feedback circuit, and manual operator circuit. 2. DCS liquid level control and interlocking system circuit decomposition diagram 4 is a reactor liquid level control and interlocking system diagram. The system has liquid level regulation and low liquid level interlocking control functions. The working process is: the liquid level transmitter LT transmits the liquid level signal to the DCS, and the DCS LIC regulator outputs a current signal according to the liquid level change, which controls the opening of the pneumatic control valve through the electrical converter to maintain the stability of the liquid level. When the process is short of liquid supply, when the liquid level drops to the specified value, the low liquid level switch LE is activated, causing the output of the LS low liquid level interlock to lose pressure, causing the two-position three-way solenoid valve to be de-energized and the pneumatic control valve to be closed to ensure production safety. Figure 4 Reactor liquid level control and interlocking system diagram In order to facilitate fault inspection, Figure 5 is drawn according to the electrical circuit, instrument gas circuit, and process parameter detection circuit. The input and output terminals of the DCS card in Figure 5, the control and interlock of the DCS are all soft wiring, and the signal relationship can be seen in the configuration diagram. The circuit signal can still be understood and checked according to the idea of hard wiring. Figure 5 Schematic diagram of reactor liquid level control and interlocking system circuit ①Electrical circuit A is the output signal circuit of the liquid level transmitter, and it is also the input signal circuit of the DCS. The output is a current signal proportional to the liquid level change. The current value increases when the liquid level rises, and the current value decreases when the liquid level drops; B is the low liquid level detection output signal circuit, and it is also the input signal circuit of the DCS, which is a switch signal; C is the control output signal circuit of the DCS, and it is also the signal input circuit of the I/P converter. The output is a current signal issued according to the size of the liquid level deviation signal and the predetermined control law. It is a current signal proportional to the liquid level change. When the level rises, the control current value is turned on, and when the level drops, the control current value decreases; D is the low liquid level interlocking signal output circuit of the DCS, and it is also the signal input circuit of the two-position three-way solenoid valve. The output and input are switch signals. ②Instrument air circuit Instrument air is called instrument air source circuit. Pneumatic instruments have requirements for compressed air. The instrument air circuit is based on the fact that after compressed air enters the instrument, it still needs to be emptied to the atmosphere. After the air is compressed into the instrument, it returns to the atmosphere. If the compressed air circuit is not smooth, the instrument will still fail. E is the air supply circuit of the I/P converter; F is the air supply circuit of the two-position three-way solenoid valve. The I/P converter converts the current output by the DCS regulator into the corresponding air pressure, and controls the opening of the pneumatic control valve through G. When the two-position three-way solenoid valve is powered on, the interlock does not work. The output air pressure of the I/P converter controls the control valve from F to G. When the low liquid level interlock is activated, the solenoid valve is powered off, and the compressed air F to the control valve is shut off. The control valve diaphragm is emptied through G to H. Under the action of the spring force, the control valve will be in a fully closed state. ③The majority of process detection circuits I is a liquid level detection sensor circuit, and it is also the input signal circuit of the liquid level transmitter. Whether the output signal of the liquid level detection sensor circuit is differential pressure or buoyancy, etc., depends entirely on the type of detection instrument used. But its output is a current signal proportional to the liquid level change. When judging the fault of the liquid level detection circuit, attention should be paid to the zero point migration problem. J is the low liquid level detection circuit, and it is also the input signal circuit of the liquid level switch. It is a switch signal. 3. DCS configuration circuit decomposition DCS configuration circuit is actually a combination of control algorithms. Since it is implemented by software, it is a bit abstract to understand, but it can still be understood by hard-wired circuits. Figure 6 is a configuration diagram of cascade control. The output OUT of the main controller TIC100 is connected to the given end SET of the auxiliary regulator FIC100. %Z011101-%Z011103 is the hardware connection number of the control circuit. %Z011103 means: the signal comes from the first node, the first unit, the first slot, and the third channel. Figure 6 Configuration diagram of cascade control We convert the configuration diagram into a conventional control system block diagram, as shown in Figure 7, which is very intuitive and easy to understand. The cascade control system uses two PID regulators connected in series to stabilize a process parameter. The main loop is outside, which is composed of the main transmitter, the main regulator, and the main object to form a closed loop. The sub-loop is inside, which is composed of the sub-transmitter, the sub-regulator, the control valve, and the sub-object to form a closed loop. The output of the main regulator is used as the given value of the sub-regulator. The system controls the opening of the control valve through the output of the sub-regulator to achieve control of the main parameters. The division of labor between the main and sub-loops is clear. The main loop completes the "fine adjustment" task, the sub-loop completes the "coarse adjustment" task, and the sub-loop plays a leading role. It can be understood that the main loop is a fixed value control system, and the sub-loop is a follow-up system. Through the above analysis, it can be known that the main regulator issues a command, the sub-regulator makes adjustments, small interference is eliminated by the sub-regulator, and the sub-regulator takes the lead in adjusting large interference, and the remaining main and sub-loops are adjusted together. When you know that a certain circuit has a fault, you can roughly analyze and judge what impact it will have on another circuit. Figure 7 Cascade control system block diagram First, you need to determine whether the fault occurs inside the DCS, or in peripheral equipment or wiring. If it is inside the DCS, you can use the fault alarm and display curve of the host computer to analyze and determine the fault, and you can also switch to manual operation for observation and check according to the fault phenomenon. If the fault occurs outside the DCS, you need to check the three circuits of %Z011101-%Z011103. First, check whether the current from the transmitter to the card is normal, second, check whether the transmitter is normal, and third, check whether the output current of the auxiliary regulator is normal. According to the signal status analysis, determine which circuit the fault occurs in, and then check the relevant transmitters, wiring, and power supply. The above external circuit inspection method is also applicable to the cascade control CPID module. This module integrates two conventional PID control modules to form a combination control module with rich functions and easy to use. It still has two input signals of the main transmitter and the auxiliary transmitter and one output signal of the auxiliary regulator. 4. Process parameter detection transmission circuit Figure 8 is a schematic diagram of the flow detection differential pressure transmission circuit, which is more intuitive than the electrical circuit diagram. The circuit here is for the measuring medium in the pressure pipe, that is, the transmission circuit of the differential pressure signal, as shown by the dotted line in the figure. Under normal circumstances, there is a pressure difference before and after the throttling device that changes with the flow rate. The pressure of the positive pipe is always greater than that of the negative pipe. The greater the flow rate, the greater the differential pressure, and the greater the output current of the transmitter. It is usually required that the instrument is within the specified measurement range, the pressure pipe and the valve are unobstructed, and the measuring medium must fill the pressure pipe but cannot be allowed to flow. The measuring medium is allowed to flow only during the sewage operation. The flow of the measuring medium indicates that there is a leakage fault in the flow detection system, such as internal leakage of the balancing valve, leakage of the sampling network, sewage valve, pressure pipe or joint. If the valve or pressure pipe is blocked, or there is gas or liquid in the pressure pipe or differential pressure transmitter measurement chamber, the differential pressure transmission circuit will be blocked, which will cause the differential pressure signal transmission to be distorted and the output current of the transmitter to be abnormal. Figure 8 Schematic diagram of flow detection differential pressure transmission circuit During the transmission process, the differential pressure signal will be affected by the resistance of the pressure pipe fittings (including pressure pipes, valves, condensers, and isolators), especially in the measurement of steam flow. The resistance of the pressure pipe fittings has a greater impact on the measurement. Many steam flow measurement systems have gas and liquid two-phase flows. The thickness of the pressure pipe, the type of valve, the change of the valve installation position, the welding quality of the pressure pipe joint, the installation position of the transmitter, etc., will affect the size of the key resistance, thereby affecting the correctness of the differential pressure signal transmission. When analyzing and judging the fault, first check the pipe fittings to make sure there is no problem before checking the transmitter and display instrument. The above four examples of instrument loops and control system loop decomposition only provide some ideas and methods. The purpose of Changhui Instruments is to inspire everyone to draw inferences from one example and to apply it in combination with the actual situation on site.

长沙威伟