Instrument technicians know how to dissect instrument circuits and control system circuits, so troubleshooting is no problem

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Without an instrument circuit diagram, it is possible to analyze the instrument detection or control systems based on the actual conditions on site, and then draw an instrument circuit diagram. Such a diagram does not need to follow any strict format; it only needs to be convenient for the maintenance of the instruments and control systems. System circuits can be divided into electrical circuits, instrument air supply circuits, and pressure guiding tube circuits for process medium detection. 1. Circuit breakdown of the temperature control system. Figure 1 shows a typical temperature control system for a steam heater. The temperature of the hot water, measured by a TT thermoresistive integrated temperature transmitter, is sent to the TC regulator where it is compared with the set temperature. When the water temperature is lower than the set value, the output signal from the regulator causes the control valve to open, increasing the steam flow and thereby raising the water temperature. When the water temperature is higher than the set temperature, the output signal of the regulator reduces the flow through the control valve, thereby decreasing the steam flow and lowering the water temperature. http://yunrun.com.cn/upload/201806/21/201806211952355357.png Figure 1: Schematic diagram of the temperature control system for steam heaters. Information on the instrument circuits can be found at yunrun.com.cn/tech/2045.html. To visually illustrate the interactions and signal connections between the various components of this control system, a block diagram like the one in Figure 2 can be used; each box in this diagram represents one component of the system. http://yunrun.com.cn/upload/201806/21/201806211953324921.png Figure 2: Block diagram of the steam heating temperature control system. For better clarity, it is shown in terms of electrical signal circuits, as illustrated in Figure 3. In Figure 3: A is the output signal circuit of the thermal resistor of the temperature sensing element, and it is also the input signal circuit of the temperature transmitter. What is output is a resistance signal that is proportional to the change in hot water temperature; as the temperature rises, the resistance value increases, and as the temperature drops, the resistance value decreases. B is the output signal circuit of the temperature transmitter, as well as the input signal circuit of the regulator. What is output is a current signal that is proportional to the change in hot water temperature; as the temperature rises, the current value increases, and as the temperature drops, the current value decreases. C is the output signal circuit of the regulator and also the input signal circuit of the actuator. The output is a current signal generated based on the magnitude of the temperature deviation signal and a predetermined control rule; it is a current signal that acts in opposition to changes in the temperature of the hot water. As the temperature of the hot water rises, the value of this current signal decreases, which prompts the adjustment valve to be closed in order to reduce the steam flow and thus lower the temperature of the hot water ； As the temperature of the hot water drops, the current value increases; increasing the control valve helps to raise the steam flow rate, thereby raising the temperature of the hot water. http://yunrun.com.cn/upload/201806/21/201806211954183201.png Figure 3: Schematic diagram of the electrical circuit of the steam heating temperature control system. By understanding how each circuit functions, it is possible to determine what impact a fault in one circuit will have on the subsequent circuits. This approach makes it easier to analyze and identify system faults. This diagram omits the power supply circuit, the valve position feedback circuit, and the manual operator circuit. 2. DCS level control and interlock system circuit diagram. Figure 4 is a diagram of the reactor level control and interlock system. The system features level regulation and low-level interlock control functions. The working process is as follows: the level transmitter LT sends the level signal to the DCS. The LIC regulator in the DCS generates a current signal based on changes in the liquid level, and this signal is used, through an electrical converter, to control the opening degree of the pneumatic control valve, thereby maintaining a stable liquid level. When the supply volume of liquid in the process is insufficient and the liquid level drops to the specified value, the low-level switch LE activates, causing the output of the LS low-level interlock to lose pressure. This leads to the de-energization of the two-position three-way solenoid valve, which in turn closes the pneumatic control valve in order to ensure production safety. http://yunrun.com.cn/upload/201806/21/201806211957390187.png Figure 4: Diagram of the reactor level control and interlock system. To facilitate fault diagnosis, Figure 5 is provided, showing the electrical circuits, the circuits for instrumentation, and the circuits for monitoring process parameters. In Figure 5, the input and output terminals of the DCS card, as well as the control and interlock functions of the DCS, are all implemented through soft wiring; the signal relationships can be seen in the configuration diagram, and the circuit signals can still be understood and checked using the approach applicable to hard wiring. http://yunrun.com.cn/upload/201806/21/201806211958516321.png Figure 5 Schematic diagram of the reactor level control and interlock system circuit. ① Electrical circuit A is both the output signal circuit of the level transmitter and the input signal circuit for the DCS. What is output is a current signal that is proportional to the change in liquid level; as the liquid level rises, the current value increases, and as the liquid level drops, the current value decreases ； B is both the low liquid level detection output signal circuit and the input signal circuit for the DCS; it is a switch signal ； C is the control output signal circuit of the DCS, as well as the signal input circuit of the I/P converter. The output is a current signal generated based on the magnitude of the liquid level deviation signal and a predetermined control rule; it is a current signal that is proportional to the change in liquid level. The control current value increases when the liquid level rises, and it decreases when the liquid level falls ； D is the low liquid level interlock signal output circuit of the DCS, as well as the signal input circuit for the two-position three-way solenoid valve; both the output and input are switch signals. ②Instrument air circuit: Instrument air is referred to as the instrument air supply circuit, and pneumatic instruments have specific requirements regarding compressed air. The air circuit for instruments works on the principle that after compressed air enters the instrument, it still needs to be discharged back into the atmosphere; the air is compressed and then sent back to the atmosphere. If this compressed air circuit is not functioning properly, the instrument will still malfunction. E is the air supply circuit for the I/P converter ； F is the air supply circuit for the two-position three-way solenoid valve; an I/P converter converts the current output by the DCS regulator into the corresponding air pressure, which is then used to control the opening degree of the pneumatic control valve through G. When the two-way three-position solenoid valve is powered, the interlock does not activate; the output air pressure from the I/P converter controls the regulating valve, ranging from F to G. When the low liquid level interlock activates, the solenoid valve loses power, and the compressed air supplied to the regulating valve at point F is cut off. The diaphragm of the regulating valve then vents through path G to H, and under the force of the spring, the regulating valve remains in a fully closed state. ③In most process detection circuits, loop I serves as both the level detection sensor circuit and the input signal circuit for the level transmitter. Whether the output signal of the liquid level detection circuit is a differential pressure or buoyancy force, etc., depends entirely on the type of sensing instrument used. However, it outputs a current signal that is proportional to the liquid level change; when diagnosing faults in the liquid level detection circuit, attention must be paid to the issue of zero drift. J is both the low liquid level detection circuit and the input signal circuit for the level switch; it is a switch signal. 3. Decomposition of DCS configuration circuits: A DCS configuration circuit is actually a combination of various control algorithms. Since it is implemented using software, it can be a bit abstract to understand, but it can still be conceptualized using hard-wired circuits. Figure 6 is a configuration diagram of cascade control, where the output OUT of the main controller TIC100 is connected to the setpoint SET of the secondary regulator FIC100. %Z011101-%Z011103 are the hardware connection numbers for the control circuit; %Z011103 indicates that the signal comes from the first node, the first unit, the first slot, and the third channel. http://yunrun.com.cn/upload/201806/21/201806212000597872.png Figure 6: Configuration diagram of cascade control. By converting this configuration diagram into a conventional block diagram of a control system, as shown in Figure 7, it becomes much more intuitive and easier to understand. A cascade control system uses two PID regulators connected in series to stabilize a process parameter. Outside is the main circuit, which forms a closed loop consisting of the main transmitter, the main regulator, and the main process variable. Inside is the secondary circuit, which consists of a secondary transmitter, a secondary regulator, a control valve, and the secondary process variable, forming a closed loop. The output of the main regulator serves as the setpoint for the secondary regulator. The system controls the opening degree of the control valve through the output of the auxiliary regulator, thereby achieving control over the main parameter. The primary and secondary circuits have clear roles: the primary circuit handles the \"fine tuning\" while the secondary circuit performs the \"rough tuning,\" acting as a precursor. It can be understood as follows: the main circuit is a constant-value control system, while the secondary circuit is a follow-up system. From the above analysis, it is clear that the main regulator issues commands, and the secondary regulator carries out the adjustments; minor disturbances are eliminated by the secondary regulator, while larger disturbances are dealt with first by the secondary regulator, with the main and secondary regulators working together to handle the remaining disturbances. When it is known that a certain circuit is faulty and what impact it will have on another circuit, it is possible to conduct a rough analysis and judgment. Figure 7 Block diagram of the cascade control system. First, it is necessary to determine whether the fault occurs within the DCS or in the peripheral devices or connections. If it is within the DCS, the fault alarms and display curves from the upper-level computer can be used to analyze and diagnose faults; it is also possible to switch to manual operation for observation and to conduct inspections based on the symptoms of the faults. The fault occurs outside the DCS; it is necessary to check the three circuits %Z011101-%Z011103. First, check whether the current from the transmitter to the card is normal; second, check whether the transmitter itself is functioning properly; third, check whether the output current of the auxiliary regulator is normal. Based on signal status analysis, determine which circuit is faulty, and then inspect the relevant transmitters, wiring, and power supply. The above inspection methods for external circuits also apply to the cascade control CPID module. This module integrates two conventional PID control modules to form a function-rich and easy-to-use combined control module. It still has two input signals from the main transmitter and the secondary transmitter, as well as one output signal from the secondary regulator. 4. Process parameter detection and transmission circuit: Figure 8 shows a schematic diagram of the flow rate detection differential pressure transmission circuit, which is more intuitive than an electrical circuit diagram. This circuit is related to the measuring medium in the pressure guiding tube; it is the circuit for transmitting the differential pressure signal, as indicated by the dashed lines in the figure. Under normal conditions, there is a pressure difference before and after the flow control device that varies with the flow rate; the pressure in the positive pipe is always higher than that in the negative pipe. The greater the flow rate, the larger this pressure difference, and consequently the greater the output current of the transmitter. It is required that, within the specified measurement range, the pressure guiding tubes and valves remain unobstructed, and the measuring medium should fill the pressure guiding tubes without allowing it to flow. Flow of the measuring medium is only permitted during drain operations. The occurrence of flow in the measuring medium indicates a leakage issue in the flow detection system, such as internal leakage in the balance valve, or leaks in the sampling mesh, drain valve, pressure guide tube, or connections. Blockages in the valves or pressure guiding tubes, as well as the presence of gas or liquid in the pressure guiding tubes or the measurement chamber of the differential pressure transmitter, can all prevent smooth flow in the differential pressure transmission circuit. This leads to distortions in the transmission of the differential pressure signal, resulting in abnormal output currents from the transmitter. http://yunrun.com.cn/upload/201806/21/201806212004028210.png Figure 8: Schematic diagram of the differential pressure transmission circuit for flow measurement. During the transmission of differential pressure signals, these signals are affected by the resistance posed by pressure guiding components (including pressure pipes, valves, condensers, and isolators). This effect is particularly significant when measuring steam flow, as the resistance from these components has a greater impact on the measurement results. In many steam flow measurement systems, gas-liquid two-phase flows are present; factors such as the diameter of the pressure pipes, the type of valves, the installation location of the valves, the quality of welding at the connections of the pressure pipes, and the installation location of the transmitters all influence the level of resistance, thereby affecting the accuracy of the differential pressure signal transmission. When analyzing and diagnosing faults, first inspect the pipe fittings to ensure there are no issues, and only then check the transmitter and display instruments. The examples of breaking down the above 4 instrument circuits and control system circuits are merely intended to provide some ideas and methods. The goal of Changhui Instruments is to inspire everyone to apply these principles in similar situations and to utilize them in practice based on actual field conditions.