Overview of electromagnetic flowmeter
The electromagnetic flowmeter is an instrument for measuring conductive liquid according to Faraday's electromagnetic induction law. When the fluid flows through a magnetic field, it will interact with the magnetic field and generate charges. The flow rate is determined according to the measurement of the charges. With the continuous development of electronic technology, the electromagnetic flowmeter, as a new type of flow measurement instrument, is characterized by a smooth straight pipe as the measurement channel. It is not easy to block when conducting conductor measurement; It can be used for liquid-solid fluid containing solid particles or fibers; In the process of flow monitoring, there will be no pressure loss, and the measurement results will not be affected by external factors; The measuring range is between 20:1 and 50:1; However, it is not allowed to measure the liquid with too high or too low temperature.
The basic principle of electromagnetic measurement is mainly based on Faraday's virtual law of electromagnetic induction. In the magnetic field, the conductive liquid moves by cutting the magnetic line of force, and the conductor generates an induced electromotive force. The induced electromotive force is related to the constant coefficient of the instrument, magnetic induction intensity, the inner diameter of the measuring tube, and the average flow velocity at the interface of the measuring tube. The components of an electromagnetic flowmeter mainly include a flow sensor and converter. The flow sensor mainly includes the shell, magnetic circuit system, measuring pipe, lining, electrode, and other related components.
The upper and lower parts of the measuring tube are equipped with excitation coils. After the excitation current, the magnetic field appears to pass through the measuring tube, and then the electrode contacts with the liquid to generate induced potential; The converter mainly has the functions of amplifying the induced potential and preventing interference signals, that is, amplifying the millivolt signal output by the electromagnetic flowmeter transmitter and converting it into a standard current signal or frequency signal.
Reasonable selection and installation of electromagnetic flowmeter
Selection of electromagnetic flowmeter
Reasonable selection of electromagnetic flowmeter is of great significance. It can not only promote the accurate measurement of data but also promote the in-depth development of electromagnetic flowmeters in the steel industry. When selecting the electromagnetic flowmeter, the installation diameter is mostly considered.
Selection principle of caliber and range
When determining the transmitter diameter, generally ensure that its diameter is consistent with the pipeline system. If the pipeline system is still unknown, the diameter shall be determined in combination with the variation range of flow rate and flow rate, and the flow rate of 2~4m/s is appropriate. In case of special circumstances such as solid particles in the liquid, the common flow rate ≤ 3m/s can be selected in consideration of wear; For easily attached pipe fluid, the flow rate ≥ 2m/s can be selected.
The selection principle of transmitter range is: on the one hand, ensure that the full range of the instrument is greater than the expected maximum flow value; On the other hand, the normal flow shall be greater than 50% of the full range of the instrument to ensure a certain measurement accuracy.
Selection range of pressure and temperature
The fluid pressure and temperature that can be measured by the electromagnetic flowmeter are limited. When selecting, ensure that the working pressure is lower than the specified working pressure. At present, the pressure specifications of the electromagnetic flowmeter commonly used in China are: less than 500mm diameter and the working pressure is 1.6MPa; (500~900) mm diameter, the working pressure is 1MPa, and the working pressure is 0.6MPa when the diameter is greater than 1000mm. If the pressure resistance of the transmitter has special requirements, the manufacturer shall negotiate with the manufacturer to manufacture the transmitter that meets the requirements.
At present, the strongest pressure resistance is the electromagnetic flow transmitter with a pressure resistance of 32MPa. The lining material of the electromagnetic flowmeter is an important factor to determine its working temperature, which is generally 5-70 ℃. Through special treatment of the lining material, its tolerance range can be appropriately improved, mainly because the transmitter allows the measured medium temperature to be - 40~+130 ℃.
Selection of lining materials and electrode tree materials
During the selection of lining materials and electrode materials for the transmitter, the physical and chemical properties of the medium shall always be followed, so as to fundamentally avoid instrument damage caused by corrosion of the lining and electrode. It should be noted that inappropriate material selection may also cause leakage of highly corrosive media, which may lead to safety accidents, which should be paid attention to. Therefore, the electrode and lining materials must be carefully selected according to the specific measuring medium in the production process.
Installation of electromagnetic flowmeter sensor
The transmitter shall be installed indoors in a dry and ventilated place
It shall not be installed in a place where the ambient temperature is too high. It shall not be subjected to strong vibration. It shall avoid objects with strong magnetic fields such as large motors and transformers as far as possible. Avoid installation in places with corrosive gas, and ensure that the installation site is convenient for maintenance, so as to provide good environmental conditions for the normal operation of the transmitter.
In order to ensure that the measuring pipe of the transmitter is full of the measured medium and plays a proper role in the use process, the transmitter should be vertically installed, especially for the liquid-solid two-phase flow, to ensure that its flow direction is bottom-up. If limited by objective conditions, only horizontal installation is allowed, it must be ensured that two electrodes are installed with valves and bypass at both ends of the transmitter on the same horizontal plane.
In order to conduct the measurement more stably, both ends of the transmitter housing and metal pipe shall be well grounded, and the converter housing shall also be ensured to be grounded. The grounding resistance shall not be greater than 10, and the grounding wire shall not be shared with other electrical equipment. If it is impossible to ensure good contact between the transmitter housing and the metal pipe during installation, it can be reliably grounded after connecting it with metal wires. In order to avoid interfering signals, the signals between the transmitter and the converter must be transmitted with shielded wires, and the phenomenon that the signal cable and the power line are placed in the same cable steel pipe in parallel is not allowed.
The ambient temperature of the converter installation site is - 20~50 ℃, without corrosive gas, and the relative humidity is not more than 80%. At the same time, it should be noted that the length of the signal cable should not exceed 30m generally. In addition, in order to avoid the influence of the flow rate on the measurement, the flow regulating valve should be set downstream of the transmitter.
Application and maintenance of electromagnetic flowmeter in the iron and steel industry
In order to effectively maintain the good application of electromagnetic flowmeters in the iron and steel industry, it is necessary to clearly analyze the causes of its failure when it occurs, and the failure parts are mostly failures during commissioning and operation.
Faults of an electromagnetic flowmeter during commissioning
In general, when the electromagnetic flowmeter fails, the installation position of the sensor is incorrect. Common sensor installation errors are mainly shown in placing it at the highest point of the gas gathering pipeline, on the vertical pipe from top to bottom, or without back pressure behind the sensor. When the debugging environment of the electromagnetic flowmeter is interfered with by the stray current of the pipeline, strong electromagnetic waves in space, and the magnetic field of a large motor, the measurement results will be seriously affected. In case of this situation, corresponding measures shall be taken to solve or reduce the interference. In case of pipe stray current interference, separate grounding protection measures can be taken. In case of a strong stray current, timely ensure that the flow sensor is insulated from the pipe. When there are tiny and uniform bubbles in the measured liquid, the normal use and measurement of the electromagnetic flowmeter will not be affected. However, if the bubble continues to increase, the output signal of the electromagnetic flowmeter will fluctuate; When the whole electrode surface is covered by bubbles, when the bubbles contact the electrode, the electrode circuit will open circuit, which will affect the normal operation of the output signal.
Fault of an electromagnetic flowmeter during operation
During the operation of the electromagnetic flowmeter, it is mainly affected by changes in environmental conditions, or the attachment layer on the inner wall of the sensor of the electromagnetic flowmeter fails or is struck by lightning. Since an electromagnetic flowmeter is mostly used for flow measurement of industrial water, after a period of operation, the attachment layer on the inner wall of the sensor will produce scaling, which will affect the normal operation of the electromagnetic flowmeter, resulting in the failure of the measuring instrument and affecting the working efficiency. Therefore, it is necessary to take timely removal measures to ensure that the measuring pipe is clean and smooth. In the instrument circuit, a lightning strike is easy to induce high voltage and strong current, thus causing the electromagnetic flowmeter to be damaged by a lightning strike during operation. The main ways for lightning to enter the electromagnetic flowmeter are the power line, excitation coil, and flow signal line.
Through the conductor in the line, the current increases instantaneously, exceeding the current bearing capacity of the electromagnetic flowmeter, causing damage to the electromagnetic flowmeter. During operation, electromagnetic flowmeter measurement failure is also affected by changes in environmental conditions. During the commissioning period, the environmental conditions are good and have no impact on the measurement work; However, in the process of operation, new interference sources or changing conditions that hinder the measurement will cause instability of the output signal of the electromagnetic flowmeter, thus affecting the normal operation of the entire measurement.