The working principle and structure of the differential pressure transmitter, establish the mathematical model of china differential pressure transmitter failure, analyze the effect of diaphragm failure on the control loop, and summarize the treatment methods under 3 different failure situations by combining practical experience. Through the combination of theory and practice, it can better deal with the failure and reduce the economic loss caused by the failure of differential pressure transmitter.

Introduction                     

Differential pressure control system is an important part of the automation control system commonly used in chemical industry, which will have wider application in intelligentization. If the failure of differential pressure control system occurs, it will lead to inaccurate measurements, and cause significant impacts on the process and quality, and the failure analysis and treatment of differential pressure control system is particularly important. Failure of differential pressure control system can be divided into regulating valve failure, controller failure, transmitter failure and so on according to the part that occurs, among which the common one is transmitter failure. Transmitter affects the performance of the whole control system, and if the failure occurs, it will affect the safety. At present, capacitive differential pressure transmitter has been widely used due to its simple structure, high accuracy and easy maintenance. Capacitive differential pressure transmitter by the detection of liquid level, pressure and other data converted to current signals, the key point of measurement is movable diaphragm measuring element, diaphragm failure is more common.

Differential pressure transmitter working principle

Differential pressure transmitter can be divided into single-chamber and double-chamber differential pressure measurement, single-pressure measurement transmitter diaphragm by the pressure, the diaphragm deformation leads to changes in the resistance of the diaphragm, the resistance of the change of inclination through the amplification unit and the conversion unit for the output of 'the output signal and the pressure value can be into the corresponding correspondence. If the diaphragm failure caused by the transmitter failure, the entire feedback loop will appear in the information transmission error, resulting in the paralysis of the differential pressure control system. The structure of the differential pressure transmitter is shown in Figure 1.

Differential pressure transmitter failure model

When the differential pressure transmitter diaphragm fails, the capacitance is affected, and the relationship between the diaphragm and the output of the transmitter is established to obtain the change relationship between the diaphragm and the capacitance. The capacitance between the diaphragm and the pole plate of the differential pressure transmitter china is shown schematically 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, and the differential pressure transmitter usually fails when the transmitter works for a long time, the differential pressure is unchanged and the current decreases with the increase of the elastic modulus, or the elastic diaphragm of the diaphragm increases, and the diaphragm output current is zero.

The effect of diaphragm failure on the control loop

When the transmitter working time is longer, the elastic modulus of the diaphragm will change, the increase of the elastic modulus, the smaller the failure degree coefficient, the more serious the failure of the diaphragm, the larger the failure degree coefficient using the Matlab platform to establish a simulation platform for the control loop system, the transmitter is in normal operation, the output current and the tracking signal is linear, the output process is in a stable equilibrium, if the degree of failure coefficient changes, the transmitter output will increase with the increase in current, and after a certain period of time, the output of the transmitter gradually decreases, and then becomes 0, while the value of the process object has been increased to a large value.

Troubleshooting

Differential pressure transmitter display pressure value is high failure

In the process process operators found that the centralized control system in the pressure display value than the field display is high, instrumentation maintenance personnel to the scene to check, found that the china differential pressure transmitter LCD display value and the pressure gauge display value is consistent with the field test DCS display value, found that the display value is also normal, through the detection of instrumentation testing and found that the current value of the transmitter delivery and the actual output of the current is lower, to determine the point of failure for the Circuit board failure. After replacing the circuit board, the display value of the centralized control system is the same as that of the field.

Low pressure value fault

Centralized control system operators found that the pressure transmitter display value is low, to the scene found that the pressure transmitter display and the centralized control system display value is consistent, the failure point may be the field instrumentation problems, the causes of the above situation are the pilot line contains liquid, or the zero point of the transmitter's scale shift. At the scene of the pilot line for liquid discharge, found that a small amount of liquid discharge, liquid discharge, the scene and the centralized control system display is basically the same.

No pressure value display failure

Centralized control system operators found that a pressure value is not displayed, the instrumentation staff to check the pressure control circuit to transmit the signal value is low, the measurement circuit failure, the field inspection of the centralized control system terminals, found that the terminal row of an insurance terminal light alarm display, replace the insurance, the pressure display is normal. Ground measurement of the field wiring, found that the negative end of the wiring there is a ground phenomenon, inspection of the branch cable line, found that the junction box interface line there are cracks, to replace the line processing, the pressure value can be stable display.

Conclusion

The working principle and structure of the differential pressure transmitter is briefly described, a mathematical model of the differential pressure transmitter failure is established, and the impact of diaphragm failure on the control loop is analyzed, combined with practical experience, summarizes and introduces three different kinds of faults under the treatment method, through the combination of theory and practice, in order to better handle the faults. And in the work of the differential pressure transmitter to strengthen the operation of the record, according to the different circumstances of the failure to investigate and deal with, the establishment of differential pressure transmitter maintenance mechanism, standardize the workflow of the operator, reasonable arrangement of maintenance programs, strengthen the site inspection, reduce the economic losses caused by the failure of the differential pressure transmitter.