According to the working principle and structure of the differential pressure transmitter, establish the mathematical model of differential pressure transmitter failure, analyse the impact of diaphragm failure on the control loop, combined with the actual production experience, summed up the treatment method in three different failure situations

Through the combination of theory and practice, it is possible to better deal with faults and reduce the risk of economic loss that may be caused by abnormalities in the differential pressure transmitter.

Differential pressure control system is an important part of the automation control system commonly used in chemical production, which will have wider application in intelligent production. If abnormalities occur in the differential pressure control system, such as inaccurate measurements, it will affect the industrial production, so the fault analysis and treatment of the differential pressure control system is particularly important.

Abnormalities in the differential pressure control system can be divided into control valve abnormalities, controller abnormalities, transmitter abnormalities, etc., according to the part of the occurrence of abnormalities, of which the more common is the transmitter abnormalities.

The transmitter affects the performance of the whole control system, and if it fails, it will greatly affect the production. Currently capacitive differential pressure transmitter has been widely used due to its simple structure, high accuracy and easy maintenance. Capacitive differential pressure transmitter by its level, pressure and other data converted to current signals, the key point of measurement is movable diaphragm measuring element, diaphragm abnormalities are more common.

I. Working Principle of Differential Pressure Transmitter

Differential pressure transmitter can be divided into single-chamber and double-chamber differential pressure measurement. After the diaphragm of single-pressure measurement transmitter is subjected to pressure, the deformation of the diaphragm leads to changes in the resistance of the diaphragm, and the output signals and the pressure values can become corresponding correspondences. If the diaphragm abnormality causes the transmitter to malfunction, the entire feedback loop will have an information transmission error, resulting in paralysis of the differential pressure control system.

Second, the differential pressure transmitter failure 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 to obtain the relationship between the diaphragm and the change in capacitance. 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 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 elasticity diaphragm of the diaphragm increases, and the output current of the diaphragm is zero.

III. Impact of diaphragm failure on the control loop

When the transmitter works for a long time, the elastic modulus of the diaphragm will change, the increase of the elastic modulus, the smaller the coefficient of the degree of failure, the more serious the failure of the diaphragm, the larger the coefficient of the degree of failure using the Matlab platform to establish the simulation platform of the control loop system, the transmitter is in the normal operation of the output current and the tracking signal is in a linear relationship, the output process is a stable equilibrium, if the coefficient of the degree of failure change, the output of the transmitter will be with the increase of the current increases, and after a certain period of time, the transmitter's output is gradually decreasing, and then becomes 0, and the process of the object of the value of the value of the process has been increased to the maximum value.

IV. Troubleshooting

1、Differential pressure transmitter display pressure value is high failure

In the production process process implementers found that the centralised control system in the pressure display value than the field display is high, instrumentation maintenance personnel to the scene to check, found that the 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 a normal value, through the instrumentation found that the transmitter to deliver the value of the current and the actual output of the current is lower, to determine the point of failure occurs for circuit boards Failure. After replacing the circuit board, the display value of the centralised control system was the same as the display value on site.

2、Pressure value is low fault

Centralised control system staff found that the pressure transmitter display value is low, to the scene found that the pressure transmitter display and the centralised control system display value is the same, the point of failure may be the field instrumentation problems, the reasons for the above situation are the pilot line contains liquid, or the zero point of the scale of the transmitter has shifted. At the scene of the pilot line for liquid discharge, found a small amount of liquid discharge, liquid discharge, the scene and the centralised control system display is basically the same.

3、No pressure value display fault

The centralised control system staff found that a pressure value was not displayed, the instrumentation staff checked that the signal value transmitted by the pressure control circuit was low and the measurement circuit was faulty. On-site inspection of the centralised control system terminals revealed that one of the fuse terminals in the terminal row had an alarm display, and after replacing the fuse, the pressure display was 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.

concluding remarks

The differential pressure transmitter working principle and structure is briefly described, a mathematical model of differential pressure transmitter failure is established, and the impact of diaphragm failure on the control loop is analysed, combined with practical production experience, summarizes and introduces the three different faults under the treatment method, through the combination of theory and practice, in order to better handle the fault. And in the production work to strengthen the operation of the differential pressure transmitter to 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 staff, a reasonable arrangement of maintenance programmes, strengthen the inspection of the production site, to reduce the economic losses caused by the failure of the differential pressure transmitter.