The working principle and structure of the differential pressure transmitter, establish the mathematical model of the differential pressure transmitter fault, analyze the impact of diaphragm failure on the control loop, and combine the actual production experience to summarize the treatment methods under three different fault conditions. Through the combination of theory and practice, the fault can be better handled and the economic loss caused by the fault of the differential pressure transmitter can be reduced.
0 Introduction
Differential pressure control system is an important part of the automatic control system commonly used in chemical production. It will have wider application in intelligent production. If the differential pressure control system fails, it will lead to inaccurate measurement, resulting in production process and quality. Significant impact, fault analysis and processing of differential pressure control systems is particularly important. The fault of the differential pressure control system can be divided into the control valve fault, the controller fault, the transmitter fault, etc. according to the occurrence, and the most common one is the transmitter fault. The transmitter affects the performance of the entire control system and, if it fails, can greatly affect production safety. At present, capacitive differential pressure transmitters have been widely used due to their simple structure, high precision and easy maintenance. The capacitive differential pressure transmitter converts the detected liquid level, pressure and other data into a current signal. The key point of the measurement is the movable diaphragm measuring component, and the failure of the diaphragm is relatively common.
1 differential pressure transmitter principle
The differential pressure transmitter can be divided into single-chamber and double-chamber differential pressure measurement. After the diaphragm of the single-pressure measuring transducer is under pressure, the deformation of the diaphragm causes the resistance of the diaphragm to change, and the resistance of the resistor is tilted and amplified. The unit and the conversion unit perform the output 'output signal and the pressure value can be correspondingly corresponding. If the diaphragm fails and the transmitter fails, the entire feedback loop will have a transmission error, resulting in a differential pressure control system. The structure of the differential pressure transmitter is shown in Figure 1 .
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2 differential pressure transmitter fault model
When the diaphragm of the differential pressure transmitter fails, the capacitance is affected, and the relationship between the diaphragm and the output of the transmitter is established, and the relationship between the diaphragm and the capacitor is obtained. The capacitance between the diaphragm and the plate of the differential pressure transmitter is shown in Figure 2 . The current of the differential pressure transmitter is proportional to the differential pressure and inversely proportional to the elastic modulus of the diaphragm. The differential pressure transmitter usually has a fault condition when the transmitter operates for a long time and the differential pressure does not change. The current decreases as the modulus of elasticity increases, or the elastic diaphragm of the diaphragm increases, while the diaphragm output current is zero .
3 diaphragm failure on the control loop
When the transmitter works for a long time, the elastic modulus of the diaphragm will change, the elastic modulus increases, the smaller the degree of failure coefficient, the more serious the fault condition of the diaphragm, and the larger the degree of fault degree is established by using the Matlab platform. The simulation platform of the loop system, when the transmitter is in normal operation, the output current is linear with the tracking signal, and the output process is in a stable equilibrium state. If the fault degree coefficient changes, the transmitter output will increase with the increase of current. After a certain period of time, the output of the transmitter gradually decreases, and finally becomes 0 , and the value of the process object is increased to the maximum value.
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4 troubleshooting
4.1 Differential pressure transmitter shows high pressure value
During the production process, the process operator found that the pressure display value in the centralized control system was higher than the on-site display. The instrument maintenance personnel went to the site to check that the liquid pressure display value of the differential pressure transmitter was consistent with the pressure gauge display value. The DCS display was tested on site. The value is found to be a normal value. The detection instrument detects that the current value delivered by the transmitter is lower than the actual output current, and the fault occurrence point is determined to be a board fault. After the board is replaced, the display value of the centralized control system is consistent with the displayed value on site.
4.2 low pressure value failure
The operator of the centralized control system found that the pressure transmitter showed a low value. At the scene, the pressure transmitter display was consistent with the display value of the centralized control system. The fault point may be a problem with the field instrument. The reason for the above situation is pressure. The line contains liquid or the offset of the transmitter's scale is offset. The pressure line was drained at the site, and a small amount of liquid was found to be discharged. After the liquid was discharged, the field and the centralized control system showed basically the same.
4.3 No pressure value display failure
The operator of the centralized control system found that a certain pressure value was not displayed. The instrument personnel checked that the signal value transmitted by the pressure control circuit was low, the measurement circuit was faulty, and the terminal of the centralized control system was inspected on site, and an alarm light of the terminal block was displayed. After replacing the insurance, the pressure is normal. Conduct grounding measurement on the field wiring, and find that the negative terminal wiring has grounding phenomenon. Check the branch cable line and find that there is a crack in the wiring box interface line. Replace the line and the pressure value can be displayed stably.
5 Conclusion
The working principle and structure of the differential pressure transmitter are briefly described. The mathematical model of the differential pressure transmitter fault is established, and the influence of diaphragm failure on the control loop is analyzed. Combined with the actual production experience, three kinds of methods are summarized. failure processing method in different circumstances, through binding of the theory and practice in order to better handle the fault. In the production work, it is necessary to strengthen the operation of the differential pressure transmitter, record the fault according to the different situations of the fault, establish the maintenance mechanism of the differential pressure transmitter, standardize the workflow of the operator, and reasonable Arrange maintenance and repair programs, strengthen inspections at the production site, and reduce economic losses caused by faulty transmitters.
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