1. Introduction Instrument malfunctions frequently occur in chemical production processes. Due to the complexity of these malfunctions during detection and control, accurate diagnosis and timely handling of instrument malfunctions are crucial. They directly impact the safety and stability of chemical production, as well as the quality and consumption of chemical products. Furthermore, they best reflect the actual work ability and professional level of instrument maintenance personnel, and are key to gaining the trust of process operators and fostering close cooperation. 2. Instrument Malfunction Diagnosis Approach Due to the pipelined, process-oriented, and fully enclosed nature of chemical production operations, especially the high level of automation in modern chemical enterprises, process operations are closely related to monitoring instruments. Process personnel use various process parameters displayed on instruments, such as reaction temperature, material flow rate, container pressure and level, and raw material composition, to determine whether the process is normal and whether the product quality is up to standard. Based on instrument readings, they adjust production levels, increase or decrease output, or even shut down the plant. Abnormal instrument readings (indicating excessively high or low readings, no change, instability, etc.) inherently involve two factors: first, process factors, where the instrument accurately reflects the abnormal process situation; and second, instrument factors, where a malfunction in a part of the instrument (measuring system) causes the indicated process parameters to deviate from reality. These two factors are often intertwined, making it difficult to immediately pinpoint the exact location of the fault. To improve their fault diagnosis capabilities, instrument maintenance personnel need to be familiar not only with the instrument's working principles, structure, and performance characteristics, but also with every link in the measurement system. Furthermore, they should have some understanding of the process flow, the characteristics of the process medium, and the characteristics of chemical equipment. This will help them broaden their thinking and facilitate the analysis and diagnosis of fault phenomena. 2.1 Temperature Detection Fault Diagnosis Fault Phenomenon: Abnormal temperature indication, either too high or too low, or slow or no change. This is explained using a thermocouple as the measuring element. First, the process conditions should be understood. Inquire with process engineers about the measured medium and the instrument's installation location—whether it's in the gas phase, liquid phase, or other process conditions. Since this is a fault occurring during normal production and not a newly installed thermocouple, factors such as reversed polarity of the thermocouple compensating wires or incompatibility between the thermocouple and the compensating wires can be ruled out. After eliminating these factors, the diagnosis and inspection can be carried out according to the approach shown in Figure 1. [align=center]Figure 1 Temperature Detection Fault Diagnosis[/align] 2.2 Flow Detection Fault Diagnosis Fault Phenomenon: Abnormal flow indication, either too high or too low, or zero indication or fluctuating indication. Take a differential pressure flow transmitter (1151D) as an example. When handling faults, instrument maintenance personnel should consult with process operators to understand the fault situation and process conditions, such as the measured medium, pump conditions, and process flow. Through a detailed understanding of the process startup situation, fault handling can be performed according to the approach shown in Figure 2. [align=center]Figure 2 Flow Detection Fault Diagnosis[/align] 2.3 Pressure Detection Fault Diagnosis Fault Phenomenon: Abnormal pressure indication of a chemical container, either too high or too low, or zero indication or no change. Take an electric pressure transmitter (3051C) as an example. First, it should be determined whether the measured medium is gas, liquid, or steam, and the process startup situation and simple process flow should be understood. Based on the understanding of the process conditions and the instrument fault phenomenon, the instrument fault can be diagnosed and handled. Fault diagnosis and handling can be performed according to the approach shown in Figure 3. [align=center]Figure 3 Pressure Detection Fault Diagnosis[/align] 2.4 Liquid Level Detection Fault Diagnosis Fault Phenomenon: The liquid level indication does not change, is too high or too low, or has no indication. A differential pressure level transmitter is used as the detection instrument. First, it is necessary to understand the process conditions, process medium, and whether the object being measured is a distillation column, reactor, storage tank, or reactor. When using a differential pressure level gauge to measure the liquid level, a glass level gauge is often configured simultaneously. Process operators use the on-site glass level gauge as a reference to judge whether the value measured by the differential pressure level transmitter is too high or too low, because the glass level gauge is more intuitive. Instrument maintenance personnel should make judgments and checks based on the process conditions and instrument fault phenomena. The troubleshooting approach for liquid level (material level) detection faults can be referred to Figure 4. [align=center]Figure 4 Liquid Level Detection Fault Diagnosis[/align] 2.5 Simple Control System Fault Diagnosis Fault Phenomenon: The control system is unstable, and the input signal fluctuates greatly. Taking a simple flow control system as an example, the control system consists of an electric differential pressure transmitter, a single-loop regulator, and a pneumatic diaphragm control valve with an electric valve positioner. When dealing with this type of fault, the instrument maintenance personnel should be very clear about the composition of the flow control system and understand the process conditions, such as the process medium, the simple process flow, whether it is the feed flow, the discharge flow, or the return flow of the tower; whether it is liquid, gas, or steam. For the troubleshooting steps, see Figure 5. [align=center] Figure 5 Fault Judgment of Control System[/align] 2.6 Common Faults and Causes of Control Valves In the chemical production process, the structure of the control valve is relatively simple compared to other parts of the automatic control system, but it is in direct contact with the process medium, so the failure rate is high. The control valve has the following common faults during use: (1) The control valve does not operate: the reason may be that there is no signal pressure or although there is signal pressure, the diaphragm is cracked and leaks air. It may also be that the valve core is stuck with the valve seat or bushing, or the valve stem is bent. (2) The control valve operates normally but does not regulate: The reason is that the valve core is detached. At this time, although the valve stem moves normally, the valve core does not move, so there is no regulating effect. Generally, for the positive-acting air-to-close valve and the reverse-acting air-to-open valve, the control valve is always in the closed state and cannot be opened; for the positive-acting air-to-open valve, the control valve is always in the fully open state and cannot be closed. In addition, pipeline blockage will also cause the control valve to not regulate. (3) The control valve is unstable or oscillates: The reason may be that the diameter of the control valve is too large, it often works at a small opening, or the medium in a single-seat valve flows in the same direction as the closing direction. In the case of severe wear of the valve core and the bushing, the control valve may also oscillate at any opening. The presence of an oscillation source nearby is an external factor for the oscillation of the control valve. (4) The control valve operates slowly or jumps: Due to the aging or drying of the sealing packing, the dry friction between the valve stem and the packing increases, which will cause slow operation or jumps. Sometimes it may also be caused by the presence of highly viscous dirt in the valve body, as well as blockage, coking, etc. Leakage at the diaphragm and "O" ring can also cause sluggish action, but this often manifests as sluggish action in one direction. (5) Large leakage of the control valve: The main reason is corrosion and wear of the valve core and valve seat. Sometimes, it may also be due to foreign objects in the valve body, which can cause the valve core to be blocked and not close tightly, resulting in large leakage. (6) Other reasons: Faults in the valve positioner or regulator of the control valve may also cause other faults such as the control valve not operating. 3. Examples of common instrument fault handling In the chemical production process, there are many process parameters involved in instrument detection. In order to better explain how to judge and handle instrument faults, we will take some examples of temperature and liquid level faults handled in the production process as examples. 3.1 Temperature detection fault handling 3.1.1 Temperature indication is zero (1) Process: The temperature indication system uses thermocouples as temperature measuring elements and uses temperature transmitters to convert the signal into a standard 4-20mA signal and send it to the DCS display. (2) Fault phenomenon: The temperature display on the DCS system is zero. (3) Analysis and judgment: First, check the module input signal of the DCS system. The input signal is 4mA, which means that the output signal of the temperature transmitter is 4mA. In order to further determine whether the fault is in the temperature transmitter or the temperature sensing element, the mv signal of the thermocouple is measured. From the measured mv signal, it can be seen that the temperature sensing element is not the problem, which means that the temperature transmitter is faulty. Because the temperature transmitter is faulty, the output of the temperature transmitter is 4mA, which causes the temperature to be displayed as zero on the DCS system. (4) Handling method: The problem is found. The handling method is to send the temperature transmitter for inspection and repair. If it cannot be repaired after inspection, the only way is to replace the temperature transmitter. 3.1.2 The temperature indication in the control room is lower than the temperature indication on the field (1) Process: The temperature indication and adjustment system uses thermocouples as temperature sensing elements. In addition to thermocouples, bimetallic thermometers are used on the device for local display. (2) Fault phenomenon: The temperature indication in the control room does not match the temperature indication on site. The temperature indication in the control room is 50°C lower than the temperature indication on site. (3) Analysis and judgment: Bimetallic thermometers are relatively simple and intuitive. Start with the temperature indication in the control room. Measure the thermoelectric potential at the thermocouple terminal on site and compare it with the corresponding temperature. It is determined that the temperature is too low, indicating that the problem is not with the regulator indication system, but with the thermocouple temperature sensing element. Remove the thermocouple for inspection and find that there is water accumulation inside the thermocouple protective sleeve. The water accumulation causes a short circuit at the lower end. Firstly, the thermoelectric potential decreases. Secondly, the thermocouple measures the point temperature, that is, the temperature at the thermocouple measuring point. Due to the water accumulation, the water part short-circuits, causing the thermocouple measuring point to change, resulting in a change in the measured temperature. (4) Handling method: The water inside the protective sleeve should be thoroughly wiped dry or blown dry with instrument air. The thermocouple should be installed after drying. After reinstallation, pay attention to the sealing of the thermocouple junction box and the wiring requirements of the compensating wire to prevent rainwater from entering the protective sleeve again. 3.1.3 Temperature indication does not change (1) Process: The sulfuric acid incinerator temperature indication has three points to measure the temperature at the furnace head, furnace middle and furnace tail respectively. Thermocouples are used as temperature measuring elements, and the signals are directly sent to the DCS system for display. (2) Fault phenomenon: The temperature indication at one of the three points does not change, while the temperature indications at the other two points are normal. (3) Analysis and judgment: The three temperatures are measured at the same time. Two of them are normal, while the value of the other point does not change, indicating that there is indeed a problem with the temperature indication at that point. First, the mv signal of the temperature at that point is measured behind the panel. From the measured value, there is no problem with the thermocouple. The thermocouples on site are also checked and no problems are found. In order to further confirm, the temperature at that point is connected to the channel of the other two temperature points that are displayed normally, and the temperature indication is normal. This indicates that the temperature measuring element at that point is not the problem. The problem lies in the module input channel or system configuration. When checking the system configuration, it was found that the output parameter of the configuration module for that point temperature was in manual mode. Because the configuration module output parameters are in manual mode, the module output value remains unchanged, resulting in no change in the temperature indication value. (4) Solution: Once the problem is found, the solution is relatively simple. Set the configuration module output parameters to automatic mode, the problem is solved, and the temperature indication returns to normal. 3.2 Level detection fault handling 3.2.1 Inaccurate boiler drum liquid level indication (1) Process: Boiler drum liquid level indication uses a differential pressure transmitter to detect the liquid level, and a glass plate liquid level gauge is installed on the other side of the drum. (2) Fault phenomenon: When starting up, the output of the differential pressure transmitter is much higher than the indication of the glass plate liquid level gauge. (3) Analysis and judgment: When using a differential pressure transmitter to detect the liquid level of a closed container, the pressure guide tube is filled with condensate. Use 100% negative migration to migrate the liquid column in the negative pressure chamber that is greater than that in the positive pressure tube, so that the positive and negative pressure difference of the differential pressure transmitter ΔP=r＊h, where h is the liquid level height and r is the density of water. The range of the differential pressure transmitter is Hr, where H is the distance between the upper and lower pressure tapping valves of the steam drum. During calibration, the density of water is taken as the value of boiling state during normal boiler production, r = 0.76. When the boiler is first started, the temperature and pressure inside the boiler have not reached the design value. At this time, the density of water is r = 0.98. Although h remains unchanged, the value of h * r increases, ΔP = r * h, the differential pressure of the differential pressure transmitter increases, and the transmitter output increases. The glass plate level gauge is only related to h, so it indicates normally, resulting in the differential pressure transmitter indicating a liquid level height greater than the glass plate level gauge height. (4) Handling method: This situation is a temporary phenomenon. After a period of time, when the boiler reaches normal operation, the two gauges will reach the same level, so there is no need to handle it. However, it is necessary to explain it clearly to the process operators. Here, it should be noted that since the instrument personnel cannot explain the cause of this phenomenon, and the process operators insist on the two gauges indicating the same level, in order to achieve the same level, the instrument personnel lower the zero position of the differential pressure transmitter until the two gauges indicate the same level. After the boiler has been running for a period of time, remember to adjust the zero position of the transmitter back. Otherwise, the measured value of the differential pressure transmitter will be too low. 3.2.2 A few indicator lights on the electrode point level gauge display instrument are always on (1) Process: The electrode point level gauge measures the liquid level of the boiler drum. (2) Fault phenomenon: A few indicator lights on the display (secondary) instrument are always on. (3) Analysis and judgment: The electrode point level gauge takes advantage of the large difference in conductivity between the liquid phase (water) and the gas phase (steam) of the measured medium. This causes the impedance of the electrode on the measuring cylinder to change by an order of magnitude when immersed in the gas phase (steam). This converts the liquid level of the measured container into an electrical signal. After amplification, the liquid level height range is indicated by the "on" or "off" indicator lights on the indicator instrument. (4) Handling method: First, determine whether the fault is caused by the indicator instrument or the electrode. Disconnect the wiring corresponding to the constantly lit indicator light on the indicator instrument. If the indicator light continues to be lit, the fault should be in the indicator instrument. Otherwise, the electrode circuit should be checked. If the wiring of the electrode corresponding to the constantly lit indicator light on the indicator instrument is disconnected and the indicator light goes out, it indicates that there is a problem with the electrode circuit. First, the electrode measuring cylinder can be flushed to remove the fault caused by the contamination of the electrode insulation terminal. If the fault is not eliminated, the electrode can be removed while the measuring cylinder is stopped to check the insulation resistance between the inner and outer electrodes. Generally, the fault is caused by the insulation resistance being too low, and the electrode needs to be replaced. When replacing the electrode, the rated working pressure, working temperature and length of the electrode should be consistent with the design parameters of the boiler drum water level measuring cylinder. If the electrode is too long or too short, the distance between the inner electrode and the measuring cylinder wall will be too close, which may cause the indicator light corresponding to that point to be lit. 3.2.3 The copper tower liquid level of the synthetic ammonia fluctuates greatly (sometimes high and sometimes low) and the indication is unstable. (1) Process: The copper tower liquid level regulation system is composed of a nuclear liquid level gauge and the control system in the control room. (2) Fault Phenomenon: During the production process, the copper tower liquid level indicator is unstable, fluctuating between high and low, causing the regulating system to malfunction and affecting the normal operation of the process. (3) Analysis and Judgment: The copper tower liquid level control system is to ensure that the copper tower liquid level is controlled within the effective range. If the liquid level is higher than the upper limit of the control range, it will cause liquid to be carried by the compressor. If the liquid level is lower than the lower limit of the control range, high-pressure gas will enter the low-pressure system, and the consequences will be unimaginable. The process requires that the liquid level regulating system must be sensitive, accurate, and stable. If the copper tower liquid level is unstable, the purpose of normal system control cannot be achieved. According to the fault judgment approach, the regulating system was first switched to manual position for manual adjustment to see if the liquid level could be stabilized, so as to determine whether the liquid level gauge, regulator, or regulating valve was faulty. Through manual adjustment, the liquid level gradually stabilized and no more fluctuations occurred. This indicates that the liquid level gauge and regulating valve are not the problem, and the liquid level fluctuation is caused by improper PID parameter settings of the regulating system. (4) Handling Method: Set the regulating system to manual mode for adjustment. After the process conditions and liquid level indication stabilize, readjust the PID parameters of the regulating system. Then, restore the regulating system to automatic control and observe the recorded curves to see if the PID parameter settings are reasonable. The problem was solved by adjusting the PID parameters of the regulating system. 4. Conclusion This paper discusses the approach to diagnosing instrument faults in chemical production processes and provides examples of corresponding instrument fault handling. It illustrates how to check and handle instrument faults during production and provides a working approach and method for handling and diagnosing common instrument faults. Since the fault phenomena that occur during instrument detection and control are relatively complex, correctly diagnosing and promptly handling instrument faults during production is an essential skill for instrument maintenance personnel. Only through continuous learning and summarizing experience in practical work can one improve their work ability and professional level.