Introduction Differential pressure transmitter is a remote pressure measuring instrument that converts pressure difference into DC standard signal. It is widely used in industrial production and scientific research. It is the hardware for acquiring, processing, and transmitting various information. With the continuous improvement of the level of enterprise automation, its application is more and more extensive, and more and more problems are encountered in production. According to the author's many years of practical experience, in order to properly install, use, and maintain the differential pressure sensor, it is necessary to understand its working principle, specify how it performs data collection and common troubleshooting methods.
1. Working principle and monitoring system 1.1 Working principle The differential pressure from the double sided pressure tube directly acts on the diaphragms of the transmitter's sensors on both sides, and is conducted to the measuring element through the sealing liquid in the diaphragm. The measuring element The measured differential pressure signal is converted into an electrical signal corresponding to it and transmitted to the converter, which undergoes processing such as amplification and becomes a standard electrical signal output. Several differential pressure transmitter application measurement methods: (1) combined with the throttling element, using the differential pressure value generated before and after the throttling element to measure the liquid flow; (2) using the pressure difference generated by the liquid's own gravity, measuring the liquid The height; (3) direct measurement of the pressure difference between the different pipes, tank liquids.
1.2 monitoring system through the pressure device, the pressure signal into the differential pressure transmitter, differential pressure transmitter will convert the signal to 0 ~ 5V or 4mA ~ 20mA electrical signal output, collected by the A / D conversion module is converted to Digital signal output. The A/D conversion module adopts Beijing Dingsheng Lichuang R8017. The digital signal output by R8017 is converted into a computer-recognizable signal by the data communication module R8520 and input into the computer. The specific hardware structure is shown in Figure 1.
The R8017 is an 8-channel analog acquisition module. The input is a 0-5V DC voltage or a 4mA-20mA DC current signal. The R8520 module uses 9-pin data cable and serial port communication for RS232/RS485 conversion module and computer connection. Set the address of each module, read the data of each channel of each module, and process it by computer software to obtain the monitoring data.
2. Fault handling Differential pressure transmitters often have some faults in the measurement process. The timely determination, analysis and processing of faults are crucial to the equipment being used. To a certain extent, it affects the normal production and even jeopardizes production safety. According to the author's years of practical experience, summed up some common fault determination analysis and solution.
2.1 Line fault When the computer shows the value is not normal, first open the differential pressure transmitter terminal box, check whether the line is virtual, short or disconnected, you can measure the power, the amount of resistance, shake insulation, etc. Fault diagnosis and processing.
2.2 Acquisition module or differential pressure sensor failure When the line fault is removed, it depends on whether the acquisition module or differential pressure sensor is faulty. Use a multimeter to check whether the differential pressure transmitter working power is normal, and at the same time measure whether the output voltage of the differential pressure transmitter is within 4mA to 20mA (if the output voltage value is measured, whether the measurement is in the range of 0-5V), and confirm whether the output value is normal. If there is no output value, the differential pressure transmitter is damaged and the differential pressure transmitter needs to be replaced. If the on-site measured value conversion matches the actual experience value, there will be no problem with the field instrument and the measuring point, the module is damaged, and the module needs to be replaced. When the field measured value conversion is the same as the computer display value, there is a problem with the pressure tube or differential pressure transmitter.
2.3 Impulse pressure tube failure (1) Impulse pressure tube plugging In the actual production use and maintenance, due to the timeliness of the discharge, or the medium is dirty, sticky, or with particles, powder, etc., it takes a long time, and some will solidify, causing The pressure tube is clogged so that the measurement cannot be performed normally. Therefore, in order to ensure the normal operation of the transmitter, it is necessary to periodically try to clear the pressure tube.
(2) Leakage of the pressure pipe Because the differential pressure transmitter contact, stop valve and other accessories are more, the leakage point increases and the maintenance workload increases. Therefore, it is necessary to reasonably do a good job of anti-corrosion of the pressure pipe and reduce the appearance of trachoma in the pressure pipe. The screws at the joint of the pressure pipe should be tightened to prevent loosening.
(3) Pressure eliminating fluids Because the pressure of gas flow is not taken or the installation of the pressure tube is not in compliance with the requirements, the phenomenon of liquid accumulation inside the pressure tube is often caused. The appearance of this phenomenon will often result in inaccurate measurements. If the transmitter's range is very small, it may even cause some fluctuation in the output of the transmitter. It has been pointed out in the literature that the correct installation of the pressure tube should be continuous with the gradient with a level not less than 1:12, there is a height difference between the measurement point and the installation point, and the pressure take point should be perpendicular to the wall of the device under test. In order to make the measurement more accurate, a number of pressure measuring heads can be set. All the pressure measuring heads are connected together to form a pressure equalizing pipe, which is then led out through the pressure guiding pipe. A discharge valve is arranged at the lowest point of the pressure equalizing pipe.
In addition, in the actual production and use of the site, there will also be problems such as the heat-supply of the pressure tube and the reverse of the pressure tube.
2.4 Variable Frequency Interference During the field installation and wiring, various signal cables are bundled together or take the same multi-core cable. The signal will be interfered with, especially if the signal line and the power line go together in a long pipeline. In this case, the differential pressure transmitter will appear on the scene without communication, or even misinformation. Therefore, during the installation process, similar misoperations should be avoided, and the distance between the instrument cable and the power cable tray should be increased.
2.5 Electrical signal transmission failure Due to certain differences in the level of installation, use, and maintenance personnel, in order to save manpower, material resources, financial resources, or consider other factors, the differential pressure transmitter is often placed near the device under test, causing electricity. The signal transmission distance is too long, making the electrical signal have the phenomenon of interference and attenuation. Therefore, it is necessary to appropriately increase the cross-sectional area of ​​the cable to solve this problem.
3. Conclusions The working principle and parameter monitoring system of the differential pressure transmitter were discussed above, and the common faults in the production process were analyzed. The on-site installation, maintenance, and use of personnel can only eliminate the occurrence of faults and improve the quality of maintenance from the roots if they have mastered the above knowledge.
1. Working principle and monitoring system 1.1 Working principle The differential pressure from the double sided pressure tube directly acts on the diaphragms of the transmitter's sensors on both sides, and is conducted to the measuring element through the sealing liquid in the diaphragm. The measuring element The measured differential pressure signal is converted into an electrical signal corresponding to it and transmitted to the converter, which undergoes processing such as amplification and becomes a standard electrical signal output. Several differential pressure transmitter application measurement methods: (1) combined with the throttling element, using the differential pressure value generated before and after the throttling element to measure the liquid flow; (2) using the pressure difference generated by the liquid's own gravity, measuring the liquid The height; (3) direct measurement of the pressure difference between the different pipes, tank liquids.
1.2 monitoring system through the pressure device, the pressure signal into the differential pressure transmitter, differential pressure transmitter will convert the signal to 0 ~ 5V or 4mA ~ 20mA electrical signal output, collected by the A / D conversion module is converted to Digital signal output. The A/D conversion module adopts Beijing Dingsheng Lichuang R8017. The digital signal output by R8017 is converted into a computer-recognizable signal by the data communication module R8520 and input into the computer. The specific hardware structure is shown in Figure 1.
The R8017 is an 8-channel analog acquisition module. The input is a 0-5V DC voltage or a 4mA-20mA DC current signal. The R8520 module uses 9-pin data cable and serial port communication for RS232/RS485 conversion module and computer connection. Set the address of each module, read the data of each channel of each module, and process it by computer software to obtain the monitoring data.
2. Fault handling Differential pressure transmitters often have some faults in the measurement process. The timely determination, analysis and processing of faults are crucial to the equipment being used. To a certain extent, it affects the normal production and even jeopardizes production safety. According to the author's years of practical experience, summed up some common fault determination analysis and solution.
2.1 Line fault When the computer shows the value is not normal, first open the differential pressure transmitter terminal box, check whether the line is virtual, short or disconnected, you can measure the power, the amount of resistance, shake insulation, etc. Fault diagnosis and processing.
2.2 Acquisition module or differential pressure sensor failure When the line fault is removed, it depends on whether the acquisition module or differential pressure sensor is faulty. Use a multimeter to check whether the differential pressure transmitter working power is normal, and at the same time measure whether the output voltage of the differential pressure transmitter is within 4mA to 20mA (if the output voltage value is measured, whether the measurement is in the range of 0-5V), and confirm whether the output value is normal. If there is no output value, the differential pressure transmitter is damaged and the differential pressure transmitter needs to be replaced. If the on-site measured value conversion matches the actual experience value, there will be no problem with the field instrument and the measuring point, the module is damaged, and the module needs to be replaced. When the field measured value conversion is the same as the computer display value, there is a problem with the pressure tube or differential pressure transmitter.
2.3 Impulse pressure tube failure (1) Impulse pressure tube plugging In the actual production use and maintenance, due to the timeliness of the discharge, or the medium is dirty, sticky, or with particles, powder, etc., it takes a long time, and some will solidify, causing The pressure tube is clogged so that the measurement cannot be performed normally. Therefore, in order to ensure the normal operation of the transmitter, it is necessary to periodically try to clear the pressure tube.
(2) Leakage of the pressure pipe Because the differential pressure transmitter contact, stop valve and other accessories are more, the leakage point increases and the maintenance workload increases. Therefore, it is necessary to reasonably do a good job of anti-corrosion of the pressure pipe and reduce the appearance of trachoma in the pressure pipe. The screws at the joint of the pressure pipe should be tightened to prevent loosening.
(3) Pressure eliminating fluids Because the pressure of gas flow is not taken or the installation of the pressure tube is not in compliance with the requirements, the phenomenon of liquid accumulation inside the pressure tube is often caused. The appearance of this phenomenon will often result in inaccurate measurements. If the transmitter's range is very small, it may even cause some fluctuation in the output of the transmitter. It has been pointed out in the literature that the correct installation of the pressure tube should be continuous with the gradient with a level not less than 1:12, there is a height difference between the measurement point and the installation point, and the pressure take point should be perpendicular to the wall of the device under test. In order to make the measurement more accurate, a number of pressure measuring heads can be set. All the pressure measuring heads are connected together to form a pressure equalizing pipe, which is then led out through the pressure guiding pipe. A discharge valve is arranged at the lowest point of the pressure equalizing pipe.
In addition, in the actual production and use of the site, there will also be problems such as the heat-supply of the pressure tube and the reverse of the pressure tube.
2.4 Variable Frequency Interference During the field installation and wiring, various signal cables are bundled together or take the same multi-core cable. The signal will be interfered with, especially if the signal line and the power line go together in a long pipeline. In this case, the differential pressure transmitter will appear on the scene without communication, or even misinformation. Therefore, during the installation process, similar misoperations should be avoided, and the distance between the instrument cable and the power cable tray should be increased.
2.5 Electrical signal transmission failure Due to certain differences in the level of installation, use, and maintenance personnel, in order to save manpower, material resources, financial resources, or consider other factors, the differential pressure transmitter is often placed near the device under test, causing electricity. The signal transmission distance is too long, making the electrical signal have the phenomenon of interference and attenuation. Therefore, it is necessary to appropriately increase the cross-sectional area of ​​the cable to solve this problem.
3. Conclusions The working principle and parameter monitoring system of the differential pressure transmitter were discussed above, and the common faults in the production process were analyzed. The on-site installation, maintenance, and use of personnel can only eliminate the occurrence of faults and improve the quality of maintenance from the roots if they have mastered the above knowledge.
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