Operating principle and structure of the differential pressure transmitter, creation of a mathematical model for the failure of the differential pressure transmitter, analysis of the influence of the diaphragm failure on the control circuit, combination of actual production experience, summary of the method of dealing with three different failure situations.
By combining theory and practice, errors can be better treated and economic losses due to errors in the differential pressure transmitter can be reduced.
The differential pressure control system is an important part of the automatic control system, commonly used in chemical production, and more widely used in intelligent production;
A failure of the differential pressure control system leads to inaccurate measurements and has a significant impact on the production process and quality. Error analysis and handling of the differential pressure is particularly importantcontrol system.
The faults of the differential pressure control system can be divided into control valve faults, regulator faults, sender faults, etc. according to the parts that occur, the most common faults being sender faults.
The transmitter affects the performance of the entire control system, and if it fails, there will be a significant impact on production safety. At present, the capacitive differential pressure transmitter is widely used because of its simple structure, high precision and easy maintenance.
The capacitive differential pressure transmitter converts the detected liquid level, pressure and other data into current signals. The decisive point of the measurement is the movable membrane measuring element. Failures are more common.
1 Principle of the differential pressure transmitter
Differential pressure transmitters can be divided into single-chamber and double-chamber differential pressure measurement, single pressure According to the Will diWhen pressure is applied to the diaphragm of the transmitter, the resistance of the diaphragm changes due to the deformation of the diaphragm and the change in resistance is output via the amplification unit and the conversion unit. The output signal and the pressure value can form a corresponding value relationship.
If the diaphragm fails and the transmitter fails, there will be errors in information transmission throughout the feedback loop, resulting in paralysis of the differential pressure control system.
2 Differential pressure transmitter failure model
If the diaphragm of the differential pressure transmitter fails, it will affect the capacitance and the relationship between the diaphragm and The variable The relationship between the Output of the transmitter is obtained to obtain the relationship between the membrane and the capacitance.
The schematic diagram of the capacitance between the diaphragm and the plate of the differential pressure transmitter is shown in Ashown in Figure 2A.
The current of the differential pressure transmitter is proportional to the differential pressure and inversely proportional to the modulus of elasticity of the diaphragm. The failure of the differential pressure transmitter is usually in the operation of the transmitter time is long, the differential pressure remains unchanged, and the current decreases as the elastic modulus increases, or the elastic membrane of the membrane increases, but the output current of the membrane is 0.
3 The influence of a diaphragm failure on the control loop
When the transmitter works for a long time, the modulus of elasticity of the diaphragm changes. As the Young's modulus increases, the smaller the failure coefficient, the more serious the failure state of the membrane, and the larger the failure coefficient when using MaThe tlab platform has established a closed-loop system simulation platform.
If the transmitter works normally, the output strom and the tracking signal in a linear relationship, and the output process is in a stable and balanced state;
When the error degree coefficient changes, the output of the transmitter increases with increasing current and after a After a certain period of time, the output of the sender's value gradually decreases and becomes 0, while the process object's value is increased until the value is reached.
Change in capacitance between diaphragm and plate of the differential pressure transmitter
4.1 The differential pressure transmitter indicated a high-pressure error
During the production process, the process operator found that the pressure reading in the central control system was higher than the reading on site, and the instrument maintenance personnel went to the website to check ;
It was found that the liquid crystal display value of the differential pressure transmitter matched the display valueof the pressure gauge matched and the reading of the DCS was also correct. Tested on site and found that the reading was also a normal value. The current value supplied by the transmitter and the actual output current are relatively low, and the fault is judged to be a circuit board fault.
After replacing the circuit board, the display value of the central control matches the display value on site.
4.2 Low pressure error
The operator of the central control system found that the reading of the pressure transmitter was low and found the pressure transmitter on site The reading agrees match the reading of the central control system, and the fault point may be a problem with the field instrument;
The reason for the above situation is that there is liquid in the pressure line or in the transmitter Der The zero point of the scale has shifted.
Drained the penstock on site and found a small amount of liquid leaking. After draining the liquid, the on-site display and the central control system essentially matched.
4.3 Pressure value display error
The operator of the central control system found that a certain pressure value was not displayed and checked it through the Instrumentation personnel The signal value transmitted from the pressure control circuit is low, the measuring circuit is faulty. On site inspection of the terminals of the central control system, it was found that an insurance terminal light in the row of terminals was alarming. After replacing the insurance, the pressure indicator lit was normal.
Carry out a grounding measurement on the field wiring and note that there is a grounding phenomenon in the negative pole wiring. Check the drop cable line and adjust fest that there is a crack in the interface line of the junction box, replace the line, and the pressure value can be stabilized.
The working principle and structure of the differential pressure transmitter are briefly described, and the mathematical model of the error of the differential pressure transmitter is created, and analyzed the influence of the Diaphragm failure on the control circuit, combined with actual production experience, summarized and introduced the treatment methods for three different failure situations, and through the combination of theory and practice, the failure can be better treated.
And in the production work, it is necessary to improve the record of the operating status of the differential pressure transmitter, conduct investigation and treatment according to the various failure situations, and set up a differential pressure transmitter maintenance mechanism;