Analysis of common faults in on-site instrument systems

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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. It is also important to know the design scheme and intentions behind the instrument system, as well as its structure, characteristics, performance, and parameter requirements. 3. If the instrument’s recorded 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 a straight line ； The fault is likely in the instrumentation 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 are normal variations, it can be generally concluded that there is no major issue with the instrumentation system. 5. Before the fault occurred, the instrument recording curve showed normal behavior; after 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. Steps for fault analysis of temperature control instrument systems: (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. (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. Steps for fault analysis of pressure control instrument systems (2): If the readings on the instruments in the pressure control system remain 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. (1) When the indication value of the flow control instrument system reaches its minimum, first check the field measurement instrument; if it is functioning properly, 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 on 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 it is a problem with the instruments, the possible causes include: the orifice plate differential pressure flow meter may have a clogged positive pressure gauge lead ； Leak in the positive pressure chamber of the differential pressure transmitter ； Mechanical flow meters suffer from issues such as seized gears or clogged filters. (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. (1) When the indication value of the liquid level control instrument system reaches its maximum or minimum value, it is possible to first check the sensing instrument to see if it 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, then 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 should be sought from a process perspective. (3) When the indication values of the liquid level control instrument system fluctuate frequently, it is necessary to first 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. The above is only an analysis of on-site failures related to the individual control instruments for the four main parameters at the site; in actual situations, there are also more complex control circuits, such as cascade control, proportional control, program control, interlock control, and so on. The analysis of these faults is even more complex and requires a detailed examination.