Analysis steps for common faults in on-site instrument systems

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　　Currently, as the level of automation in industries such as petrochemicals, steel, papermaking, food, and pharmaceuticals continues to improve, higher technical skills are required from personnel responsible for maintaining field instruments. To reduce the time required to address instrument failures, ensure safe production, and improve economic efficiency, some experience in on-site instrument maintenance is presented below for reference by instrument maintenance personnel. 　　I. Basic analysis steps for faults in on-site instrument systems: The measurement parameters of on-site instruments generally fall into four categories: temperature, pressure, flow rate, and liquid level. 　　Now, based on the different measurement parameters, we analyze the causes of various faults in field instruments. 　　1. First, before analyzing faults in on-site instruments, it is necessary to have a thorough understanding of the production process, manufacturing methods, and conditions related to the instrument system in question. This includes knowing the design scheme and intent behind the instrument system, as well as its structure, characteristics, performance, and parameter requirements. 　　2. Before analyzing and checking for faults in the on-site instrument system, it is necessary to obtain information from the operators on site regarding the production load and changes in the parameters of the raw materials. The recording curves of the faulty instruments should also be examined, and a comprehensive analysis conducted to determine the cause of the instrument failure. 　　3. If the instrument’s recording curve is a straight line (a line with no changes at all is called a straight line), or if the curve, which was originally fluctuating, suddenly becomes straight; the fault is likely to be in the instrument system. Since most of the current recording instruments are DCS computer systems, they have very high sensitivity, allowing them to detect changes in parameters with great accuracy. At this point, the process parameters can be manually adjusted to observe the changes in the curve. If there is no change, it is almost certain that there is a problem with the instrumentation system; if there is normal variation, it is almost certain that there is no major issue with the instrumentation system. 　　4. When the process parameters are changed, it is observed that the recorded curve experiences a sudden change or jumps to its maximum or minimum value; faults in this case are often related to the instrumentation system. 　　5. Before the fault occurred, the instrument recording curve showed normal behavior; once fluctuations appeared, the curve became irregular or made it difficult to control the system, to the point where even manual operation was ineffective. In such cases, the fault may be caused by the process control system. 　　6. When it is found that the instruments displayed by the DCS are abnormal, one can go to the site to check the readings of the same physical instruments; if there is a large difference between them, it is likely that there is a fault in the instrument system. 　　In summary, when analyzing the causes of faults in field instruments, special attention should be paid to changes in the characteristics of the controlled process and control valves, as these can all be factors leading to failures in the field instrument system. Therefore, we need to conduct a comprehensive and careful analysis from both the perspective of the on-site instrument system and the process control system in order to identify the cause. 　　II. Steps for analyzing faults in the control systems of the four major measurement parameters: 1. Steps for analyzing faults in the temperature control instrument system When analyzing faults in the temperature control instrument system, two points should be noted first: the instruments in this system are mostly electric ones used for measurement, indication, and control; moreover, the measurement performed by these instruments often suffers from significant lag. 　　(1) If the reading of the temperature instrument system suddenly reaches the maximum or minimum value, it is generally due to a fault in the instrument system. Because the temperature instrument system has a large measurement lag, no sudden changes occur. At this time, the causes of failure are usually broken thermocouples, thermal resistors, compensation wires, or a malfunctioning transmitter amplifier. 　　(2) Rapid oscillations are observed in the indications of the temperature control instrument system, which are usually caused by improper adjustment of the PID control parameters. 　　(3) Significant and slow fluctuations in the indications of the temperature control instrument system are likely to be caused by changes in process operations. If there are no such changes in the process operations at that time, it is likely to be a fault within the instrument control system itself. 　　(4) Steps for troubleshooting the temperature control system itself: Check whether the input signal to the control valve changes; if the input signal does not change but the control valve still operates, it indicates that there is a leak in the diaphragm of the control valve. Check whether the input signal to the control valve’s positioner changes; if the input signal remains unchanged while the output signal does change, then the positioner is faulty. If the input signal to the positioner does change, then check whether there is a change in the output of the regulator; if the input to the regulator remains unchanged but its output changes, then it is a fault with the regulator itself. 　　2. Steps for fault analysis of pressure control instrument systems (1) When the readings of the instruments in the pressure control system exhibit rapid oscillations, first check whether there are any changes in the process operation; such changes are usually caused by issues with the process operation or by improper setting of the PID parameters of the regulator. 　　(2) If the instrument readings of the pressure control system show no response, and the pressure reading remains unchanged despite changes in process operations, the fault is usually located in the pressure measurement system. First, check whether there is any blockage in the pressure sensing conduit system; if not, then examine whether there are any changes in the output of the pressure transmitter. If there are such changes, the fault lies in the controller’s measurement and indication system. 　　3. Steps for analyzing faults in the flow control instrument system (1) When the indication value of the flow control instrument system reaches its minimum, first check the field measurement instruments; if they are functioning properly, then the fault lies with the display instrument. When the reading on the on-site measuring instrument is also at its minimum, check the opening degree of the control valve; if the opening degree is zero, it is usually due to a fault somewhere between the control valve and the regulator. When the readings from the on-site measuring instruments indicate a minimum value and the opening degree of the control valve is normal, the possible causes of the fault are insufficient system pressure, blockages in the system pipelines, the pump not being able to deliver sufficient flow, crystallization of the medium, or improper operation. If the issue lies with the instruments, the possible causes include: a blockage in the positive pressure gauge lead of the orifice plate differential pressure flow meter; a leak in the positive pressure chamber of the differential pressure transmitter; or in the case of mechanical flow meters, stuck gears or a clogged filter screen. 　　(2) When the indication value of the flow control instrument system reaches its maximum, the measuring instrument also often indicates a maximum value. At this point, the control valve can be manually adjusted to open or close more; if the flow rate can be reduced, it is generally due to process operation issues. If the flow rate cannot be reduced, it is due to problems with the instrumentation system. Check whether the control valves in the flow control instrumentation system are functioning properly; verify that the pressure measurement system of the instruments is working correctly; and check that the instrument signal transmission system is in good order. 　　(3) If the indication values of the flow control instrument system fluctuate frequently, the control can be switched to manual mode. If the fluctuations decrease, it is due to issues with the instruments or inappropriate PID control parameters; if the fluctuations remain frequent, it is caused by problems in the process operation. 　　4. Steps for fault analysis of the liquid level control instrument system (1) When the indication value of the liquid level control instrument system reaches its maximum or minimum value, it is possible to first check whether the sensing instrument is functioning properly. If the indication is normal, switch the liquid level control to manual remote control and observe how the liquid level changes. If the liquid level can be maintained within a certain range, the fault lies in the liquid level control system; if the liquid level cannot be stabilized, it is generally due to a fault in the process system, and the cause must be sought in the process aspects. 　　(2) When the indication of the differential pressure type level control instrument does not match that of the on-site direct-reading indicator, first check whether the on-site direct-reading indicator is functioning properly. If its indication is normal, then check for any leaks in the sealing fluid of the negative pressure pressure transfer tube of the differential pressure level instrument. If there are leaks, refill the sealant and adjust the zero point; if there are no leaks, it may be that the negative migration amount of the instrument is incorrect, so adjust this amount to make the instrument’s indication normal. 　　(3) When the indication values of the liquid level control instrument system fluctuate frequently, it is necessary first to analyze the capacity of the liquid level control object in order to determine the cause of the fault; a large capacity usually indicates a fault with the instrument itself. For those with low capacity, it is first necessary to analyze whether there have been any changes in the process operations; if so, frequent fluctuations are likely to be caused by the process itself. If there is no change, it may be caused by a malfunction in the instrument.