With the acceleration of China's industrialization, the problem of environmental pollution is becoming more and more serious, and the society's call for sustainable development is getting higher and higher. Because in the sewage treatment process, it is necessary to measure and control the discharged sewage, additives, sludge, etc., so flow measurement is very important. Among the many flowmeter products used for sewage measurement, the electromagnetic flowmeter has an absolute advantage, both from the perspective of principle and the accuracy of measurement.
Electromagnetic flowmeter consists of two parts: flow sensor and transmitter. The sensor measuring tube is equipped with excitation coils above and below, and a magnetic field is generated after the excitation current is turned on through the measuring tube, and a pair of electrodes are installed on the inner wall of the measuring tube to contact the liquid, which induces the induced potential and sends it to the transmitter. The excitation current is provided by the transmitter. According to the assembly method of converter and sensor, there are two types: separate type and integrated type. In the sewage treatment process, large-diameter flowmeters are mostly split type, one part is installed underground and the other part is above ground. Small calibers are mostly integrated. The working principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When the conductive metal rod moves perpendicular to the magnetic field line at a certain speed, an induced voltage will be generated. If the magnetic field strength is B, the length of the metal rod is L, and the velocity is v, then the induced voltage is generated.
At present, the functions of electromagnetic flowmeters on the market are also very different, simply measuring one-way flow, only outputting analog signals to drive the rear instrument; The intelligent electromagnetic flowmeter can measure bidirectional flow, range switching, upper and lower limit flow alarms, air traffic control and power cut-off alarms, small signal cutting, flow display and total calculation, automatic verification and fault self-diagnosis, communication with the host computer and motion configuration, etc. Some models of electromagnetic flowmeter serial digital communication functions can be selected with a variety of communication interfaces and specialized chips (ASICs) to connect to HART protocol systems, PROFTBUS, Modbus, FF fieldbus, etc.
The premise of using an electromagnetic flowmeter is that the measured liquid must be conductive, and the conductivity below the threshold will produce measurement error until it cannot be used. However, the conductivity of industrial water and its aqueous solution is greater than 10-4S/cm, and the conductivity of acid, alkali and salt solution is between 10-4~10-1S/cm, and there is no problem in the use, and there is no problem with low-distilled water of 10-5S/cm. Usually sewage has high conductivity because it contains many ions, which is one of the advantages of electromagnetic flowmeters for measuring sewage, which is determined by its measurement principle.
In addition, some characteristics of the electromagnetic flowmeter also determine that it is more suitable for sewage measurement than other flowmeter products. The electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, etc., and there is no pressure loss caused by shrinkage or protruding part inside the measuring tube. According to the characteristics of sewage with large flow changes, impurities, small corrosiveness, and certain conductivity, the electromagnetic flowmeter can be said to be the best choice for measuring the flow rate of sewage.
In addition, the electromagnetic flowmeter has a compact structure, small size, easy installation, operation and maintenance, and the measurement system adopts intelligent design, the overall seal is strengthened, and it can work normally in a harsh environment. Compared with other flowmeter products, vortex flowmeter is difficult to enlarge the caliber due to technical reasons, quality, and pipeline ultrasonic flowmeter lacks competitive advantage in price compared to electromagnetism. Therefore, in the water supply and drainage and sewage treatment industry, electromagnetic flowmeters, especially large-diameter electromagnetic flowmeters, have great advantages and have been widely used.
The sewage treatment process is mainly through biological oxidation, flocculation, sedimentation and other processes to achieve the purpose of sewage purification, and a certain flow meter is required for measurement at each stage. In the sewage treatment process, it is necessary to configure the corresponding detection instruments according to the process requirements to form a reliable control system.
Generally, wastewater treatment plants need to be equipped with flow meters in the following process testing processes:
(1) Water inlet pipe;
(2) Detection of mud volume from the primary sedimentation tank to the mud storage tank;
(3) Sludge pump room residual and sludge flow detection;
(4) Sludge flow detection in the return flow of sludge pumping room;
(5) Outlet flow detection of secondary sedimentation tank distribution well;
(6) Digester mud intake detection;
(7) Detection of mud output from digester;
(8) Flocculation tank agent intake detection, etc.
Therefore, sewage treatment is inseparable from electromagnetic flowmeters. With the increasing attention of the country to environmental protection, the electromagnetic flowmeter used in sewage treatment will also usher in an unprecedented prosperous market.
