Currently, with the continuous improvement of automation levels in petrochemical, steel, papermaking, food, and pharmaceutical enterprises, higher demands are placed on the technical skills of field instrument maintenance personnel. To shorten the time required to handle instrument failures, ensure safe production, and improve economic efficiency, this article presents some field instrument maintenance experience for reference by instrument maintenance personnel. I. Basic Analysis Steps for Field Instrument System Failures Field instrument measurement parameters are generally divided into four main parameters: temperature, pressure, flow rate, and liquid level. We will now analyze different field instrument failures based on the different measurement parameters. 1. First, before analyzing field instrument failures, it is necessary to thoroughly understand the production process, production technology, and conditions of the relevant instrument system, as well as the design scheme, design intent, structure, characteristics, performance, and parameter requirements of the instrument system. 2. Before analyzing and checking field instrument system failures, it is necessary to understand the production load and raw material parameter changes from the field operators, check the recorded curves of the faulty instruments, and conduct a comprehensive analysis to determine the cause of the instrument failure. 3. If the instrument recorded curve is a dead line (a line with no change at all is called a dead line), or if the recorded curve was originally fluctuating but has suddenly become a straight line, the failure is very likely in the instrument system. Because most recording instruments are currently DCS computer systems, they are highly sensitive and can react very quickly to changes in parameters. In this case, you can manually change the process parameters and observe the curve changes. If there is no change, it's basically certain that the instrument system has a problem; if there is a normal change, it's basically certain that the instrument system has no major problems. 4. When changing process parameters, if the recorded curve suddenly changes or jumps to its maximum or minimum, the fault is often in the instrument system. 5. If the instrument recording curve was normal before the fault occurred, but after fluctuations, the recording curve becomes irregular or makes the system difficult to control, even manually, the fault may be caused by the process operating system. 6. When the DCS display instrument is found to be abnormal, you can check the indication value of the same intuitive instrument in the field. If they differ significantly, it is very likely that the instrument system has a fault. In short, when analyzing the causes of field instrument faults, special attention should be paid to changes in the characteristics of the controlled object and control valve, as these may be the causes of field instrument system faults. Therefore, we need to comprehensively consider and carefully analyze both the field instrument system and the process operating system to check the cause. II. Fault Analysis Steps for the Four Major Measurement Parameter Instrument Control System 1. When analyzing the faults of a temperature control instrument system, two points should be noted first: the instruments in this system are mostly electric instruments for measurement, indication and control; the measurement of the instruments in this system often has a large lag. (1) If the indicated value of the temperature instrument system suddenly changes to the maximum or minimum, it is generally a fault of the instrument system. Because the measurement of the temperature instrument system has a large lag, it will not change suddenly. At this time, the fault is mostly caused by the thermocouple, resistance temperature detector, compensation wire breakage or transmitter amplifier failure. (2) If the temperature control instrument system shows rapid oscillation, it is mostly caused by improper adjustment of the control parameter PID. (3) If the temperature control instrument system shows large and slow fluctuations, it is likely due to changes in process operation. If there were no changes in process operation at that time, it is likely a fault of the instrument control system itself. (4) Fault analysis steps of the temperature control system itself: Check whether the input signal of the regulating valve changes. If the input signal does not change, the regulating valve will operate, and the diaphragm of the regulating valve will leak. Check whether the input signal of the regulating valve positioner changes. If the input signal does not change, but the output signal changes, the positioner is faulty. Check whether the input signal of the positioner changes, and then check whether the output of the regulator changes. If the regulator input does not change, but the output changes, then the regulator itself is faulty. 2. Fault analysis steps of the pressure control instrument system (1) When the pressure control system instrument indication shows rapid oscillation, first check whether the process operation has changed. This change is mostly caused by poor process operation and regulator PID parameter tuning. (2) The pressure control system instrument indication shows a dead line. The pressure indication does not change even after the process operation changes. Generally, the fault is in the pressure measurement system. First check whether the pressure measuring guide pipe system is blocked. If it is not blocked, check whether the pressure transmitter output system has changed. If it has changed, the fault is in the controller measurement indication system. 3. Fault Analysis Steps for Flow Control Instrument System (1) When the indicated value of the flow control instrument system reaches the minimum, first check the field detection instrument. If it is normal, the fault lies in the display instrument. When the field detection instrument also indicates the minimum, check the opening of the regulating valve. If the opening of the regulating valve is zero, the fault is usually between the regulating valve and the regulator. When the field detection instrument indicates the minimum and the opening of the regulating valve is normal, the fault is likely caused by insufficient system pressure, blockage of system pipeline, pump not delivering, medium crystallization, improper operation, etc. If it is an instrument-related fault, the causes are: the orifice plate differential pressure flow meter may have a blocked positive pressure tapping conduit; the differential pressure transmitter may have a leaking positive pressure chamber; the mechanical flow meter may have a stuck gear or a blocked filter screen, etc. (2) When the indicated value of the flow control instrument system reaches the maximum, the detection instrument will often also indicate the maximum. At this time, the regulating valve can be manually opened or closed remotely. If the flow can be reduced, it is generally caused by process operation. If the flow value cannot be reduced, it is caused by the instrument system. Check whether the regulating valve of the flow control instrument system is working; check whether the instrument measurement tapping system is normal; check whether the instrument signal transmission system is normal. (3) If the flow control instrument system indicates frequent fluctuations, the control can be switched to manual. If the fluctuations decrease, it is due to the instrument or the PID control parameter is not suitable. If the fluctuations are still frequent, it is due to the process operation. 4. Fault analysis steps of liquid level control instrument system (1) When the liquid level control instrument system indicates the maximum or minimum value, check the detection instrument first to see if it is normal. If the indication is normal, switch the liquid level control to manual remote control and observe the liquid level change. If the liquid level can be stabilized within a certain range, the fault is in the liquid level control system. If the liquid level cannot be stabilized, it is generally a fault caused by the process system. The cause should be found from the process. (2) When the indication of the differential pressure liquid level control instrument and the indication of the field direct reading instrument do not match, first check if the field direct reading instrument is normal. If the indication is normal, check if the sealing liquid of the negative pressure guide pipe of the differential pressure liquid level instrument is leaking. If there is leakage, refill the sealing liquid and adjust the zero point. If there is no leakage, the negative migration amount of the instrument may be incorrect. Readjust the migration amount to make the instrument indication normal. (3) When the indicated value of the liquid level control instrument system fluctuates frequently, the first step is to analyze the capacity of the liquid level control object to determine the cause of the fault. A large capacity is generally caused by an instrument malfunction. For a small capacity, the first step is to analyze whether there have been changes in the process operation. If there have been changes, the frequent fluctuations are likely caused by the process. If there have been no changes, the fault may be caused by an instrument malfunction. The above is only the field fault analysis of the four main parameters individually controlled by the instrument. In actual field operations, there are also some complex control loops, such as cascade control, split-range control, program control, interlocking control, etc. The analysis of these faults is more complex and requires specific analysis.