Regardless of the type of instrument failure, we must first understand the production process and conditions of the installation location of the instrument, and understand the structural characteristics and performance of the instrument itself; before repairs, we must combine with the technicians to analyze and determine the real cause of the instrument failure ; At the same time, it is necessary to understand whether the instrument is accompanied by adjustment and interlocking functions, and the process should be kept as stable as possible during the maintenance process. Today, I will mainly talk about the failure analysis of temperature, pressure, flow, and liquid level field instruments.
temperature
â‘ : Sudden increase in temperature: This fault is mostly caused by thermal resistance (thermocouple) open circuit, loose connection terminals, (compensation) wire break, temperature failure, etc. At this time, it is necessary to understand the location of the temperature and the wiring layout. The resistance (millivolt) file of the multimeter can be quickly found out by measuring several sets of data at different positions.
â‘¡: Sudden decrease in temperature: This fault is mostly caused by thermocouple or thermal resistance short circuit, wire short circuit and temperature failure. Start with the weak points that are prone to failure, such as wiring ports and wire bends, and investigate them one by one. The temperature rises on site, but the general control indication remains unchanged, mostly due to the liquid (water) with a lower boiling point at the measuring element.
â‘¢: Large fluctuations or rapid fluctuations in temperature: At this time, the process operation should be mainly inspected (inspection and adjustment system involved in adjustment).
pressure
â‘ : The pressure suddenly decreases or increases or the indication curve does not change: At this time, check the transmitter pressure system, check whether the root valve is blocked, whether the pressure pipe is unblocked, whether there is abnormal medium in the pressure pipe, and the sewage plug is blocked. And whether the drain valve is leaking, etc. Medium freezing in winter is also a common phenomenon. The transmitter itself is very unlikely to fail.
â‘¡: Large pressure fluctuations: This situation must first be combined with the craftsman, which is generally caused by improper operation. The parameters involved in the adjustment should mainly check the adjustment system.
flow
â‘ : The minimum flow indication value: Generally caused by the following reasons: the detection element is damaged (the zero point is too low.; the display is faulty; the line is short-circuited or disconnected; the positive pressure chamber is blocked or leaked; the system pressure is low; the parameters involved in the adjustment need to be checked and adjusted , Regulating valve and solenoid valve.
â‘¡: The maximum flow indication: the main reason is that the negative pressure chamber pressure system is blocked or leaked. It is unlikely that the transmitter needs to be adjusted.
â‘¢: Large flow fluctuations: the flow parameters do not participate in the adjustment, usually due to process reasons; the PID parameters of the regulator can be checked if the flow parameters are involved in the adjustment; the parameters with the isolation tank, check whether there are bubbles in the pressure pipe, and the positive and negative pressure pipes Whether the liquid is the same height.
Liquid level
â‘ : The liquid level suddenly increases: Mainly check whether the pressure-inducing system of the negative pressure chamber of the transmitter is blocked, leaked, gas-collected, lack of liquid, etc. The specific method of filling liquid is: stop the watch first in the order of stopping the watch; close the positive and negative pressure root valve; open the positive and negative pressure drain valve to relieve pressure; open the double-chamber balance container filling plug; open the positive and negative pressure chamber drain plug; At this time, the liquid level indication is the maximum. Close the drain valve; close the drain plug in the positive and negative pressure chamber; slowly pour the same medium into the double-chamber balance container, then slightly open the drain plug to exhaust; until it is full, open the plug in the positive pressure chamber at this time, and change The transmitter indicates that it should return to zero. Then use the transmitter in the order of the meter.
â‘¡: The liquid level suddenly decreases: Mainly check whether the positive pressure chamber pressure system is blocked, leaking, gas gathering, lack of liquid, and whether the balance valve is closed. The specific method to check whether the pressure system is unblocked is to stop the transmitter, open the drain valve, and check the drain condition (except for the medium that cannot be leaked).
â‘¢: The instructions of the master control room do not match the on-site liquid level: First, determine whether the on-site level gauge is faulty. At this time, the liquid level can be artificially increased or lowered, and the cause of the problem can be specifically analyzed according to the on-site and master control instructions (root of the on-site level gauge) The valve is closed, blocked, and leaks easily cause inaccurate on-site instructions). The normal liquid level can be restored by checking the zero point, range, and filling. If it is still not normal, you can notify the process personnel to monitor on site to remove the transmitter to suppress the pressure adjustment.
â‘£: The liquid level fluctuates frequently: First, check the feeding and discharging conditions in conjunction with the process personnel, and after confirming that the process condition is normal, it can be stabilized by adjusting the PID parameters. The specific method is: adjust the control valve to manual state, first adjust the setting value to be consistent with the measured value to stabilize the fluctuation of the liquid level, and then slowly adjust the opening of the control valve to make the liquid level rise or fall slowly to meet the process requirements, and then adjust The set value is consistent with the measured value, and the regulating valve will automatically switch after the parameter is stable.
In short, once the instrument parameters are found to be abnormal, first of all, combine with the process personnel, start with the process operating system and the field instrument system, comprehensively consider, carefully analyze, especially consider the correlation between the measured parameter and the control valve, and divide the fault into account. Step by step judgment, it is easy to find out the problem, and solve the problem with the right medicine.
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