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 that acquires, processes and transmits various information. With the continuous improvement of the level of enterprise automation, its application range is more and more extensive, and more and more problems are encountered in production. According to the actual experience of the Huaheng Instrument's field for many years, in order to properly install, use and maintain the differential pressure sensor ( dp sensor ), it is necessary to understand its working principle, how to carry out data acquisition and common fault handling methods.
1. Working principle and monitoring system of differential pressure transmitter
1.1 Differential pressure transmitter working principle
The differential pressure from the double-sided pressure guiding tube directly acts on the double-sided isolating diaphragm of the transmitter sensor, and is transmitted to the measuring component through the sealing liquid in the diaphragm, and the measuring component converts the measured differential pressure signal into corresponding The electrical signal is transmitted to the converter, and is converted into a standard electrical signal output by amplification or the like. Several applications of differential pressure transmitters are measured: (1) combined with throttling elements, using the differential pressure generated before and after the throttling element to measure the flow of liquid; (2) using the pressure difference generated by the gravity of the liquid, measuring the liquid (3) directly measure the pressure difference between different pipes and tank liquids.
1.2 Monitoring System
Through the pressure device, the pressure signal is introduced into the differential pressure transmitter, and the differential pressure transmitter converts the signal into an electrical signal output of 0 to 5V or 4 mA to 20 mA, which is converted and converted into a digital signal output by the A/D conversion module. The A/D conversion module adopts Beijing Dingshengli R8017, and the digital signal output by R8017 is converted into a computer identifiable signal by the data communication module R8520 and input into the computer.
The R8017 is an 8-channel analog acquisition module with an input of 0 to 5 VDC or a 4 mA to 20 mA DC current signal. The R8520 module uses a 9-pin data cable and serial communication for the connection between the RS232/RS485 conversion module and the computer. Set the address of each module, read the data of each channel of each module, and process it by computer software to get the monitoring data.
2, Differential pressure transmitter troubleshooting
Differential pressure transmitters often have some faults during the measurement process. The timely determination and processing of faults is crucial for the equipment being used. To a certain extent, it affects the normal production and even jeopardizes production safety. According to the author's many years of practical experience, I summarized some common fault judgment analysis and solutions.
2.1 Differential pressure transmitter line fault
When the computer display value is not normal, first open the junction box of the differential pressure transmitter to check whether the line is connected, shorted or disconnected. The fault can be judged by measuring the power supply, measuring the resistance, shaking the insulation, etc. deal with.
2.2 Differential pressure transmitter acquisition module or differential pressure sensor failure
When the line is faulty, it depends on whether the acquisition module or the differential pressure sensor is faulty. Use a multimeter to check whether the differential pressure transmitter is working properly, and measure whether the output current of the differential pressure transmitter is between 4 mA and 20 mA (if the output voltage value is 0 to 5 V), 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, the field instrument and measuring point are no problem, the module is damaged, and the module needs to be replaced. When the field measurement value conversion is the same as the computer display value, there is a problem with the pressure tube or differential pressure transmitter.
2.3 Differential pressure transmitter lead pipe failure
(1) Differential pressure transmitter is blocked by the pressure tube
In actual production and maintenance, due to untimely discharge or dirty or sticky medium or particles, powder, etc., it will take a long time, and some will solidify, causing the pressure tube to clog, making the measurement impossible. Therefore, in order to ensure the normal operation of the transmitter, it is necessary to regularly clear the impulse tube.
(2) Differential pressure transmitter leakage pipe leakage
Due to the large number of accessories such as differential pressure transmitter contacts and shut-off valves, the number of leaks increases and the maintenance workload increases. Therefore, it is necessary to properly prevent the corrosion of the pressure pipe and reduce the occurrence of trachoma in the pressure pipe; the screw at the interface of the pressure pipe should be tightened to prevent loosening.
(3) Differential pressure transmitter impulse tube effusion
Because the gas flow pressure is not correct or the pressure pipe installation does not meet the requirements, it often causes the liquid to accumulate inside the pressure pipe. The appearance of this phenomenon often leads to inaccurate measurement. If the transmitter has a small range, it may even cause some fluctuations in the output of the transmitter. It is pointed out in the literature that the correct installation of the impulse tube should be continuous with the slope of the horizontal not less than 1:12. There is a height difference between the measurement point and the installation point. The pressure point should be perpendicular to the wall of the device under test. In order to make the measurement more accurate, a plurality of pressure measuring heads can be arranged, and all the pressure measuring heads are connected together to form a pressure equalizing tube, which is then led out through the pressure guiding tube, and a drain valve is arranged at a low position of the pressure equalizing tube.
In addition, during the actual production and use of the site, there will be problems such as the pressure of the pressure tube and the reverse of the pressure tube.
2.4 Inverter interference
During the installation and wiring process on site, various signal lines are tied together or the same multi-core cable is used, and the signal will be disturbed, especially when the signal line and the power line go together in a long pipeline. In this case, the differential pressure transmitter will not appear on-site communication or even false indication. Therefore, similar misoperations should be avoided during the installation process, and the distance between the instrument cable and the power cable tray should be increased.
2.5 Differential pressure transmitter electrical signal transmission failure
Due to the difference in the level of installation, use and maintenance personnel, in order to save manpower, material resources, financial resources or other factors, the differential pressure transmitter is often placed in the vicinity of the device under test, causing the electrical signal transmission distance to be too far. There is interference and attenuation of electrical signals. Therefore, it is necessary to appropriately increase the cross-sectional area of the cable to solve this problem.
3, The conclusion
The above discusses the working principle and parameter monitoring system of the differential pressure transmitter, and analyzes the common faults in the production process. On-site installation, maintenance, and use personnel can only prevent the occurrence of faults from the root cause and improve the maintenance quality.