Basic knowledge of Vacorda efficient customized electromagnetic flowmeter

Electromagnetic flow meters have unparalleled advantages in measuring the liquid flow rate of conductive media. In recent years, electromagnetic flow meters have been widely used in various industries, and maintenance instrument workers are increasingly required to master more knowledge about electromagnetic flow meters. Below, Jinhu Zhongding Instrument Co., Ltd. will give you a detailed analysis:
Measurement principle
The working principle of electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When a conductive metal rod moves perpendicular to the direction of magnetic field lines at a certain speed, an induced voltage is generated.
Assuming that the magnetic field strength B generated by the electromagnetic induction coil is constant, the induced voltage Ue generated is proportional to the fluid velocity v, and the cross-sectional area A of the pipeline is known, the volumetric flow rate Qv can be calculated by the following formula:
An electromagnetic flowmeter consists of two main parts: a flow sensor and a transmitter. The sensor measuring tube is equipped with excitation coils on top and bottom, which generate a magnetic field through the measuring tube after passing through the excitation current. A pair of electrodes are installed on the inner wall of the measuring tube and in contact with the liquid, leading out the induced potential and sending it to the transmitter. The excitation current is provided by the transmitter. According to the assembly method of converters and sensors, there are two types: separated type and integrated type. In the sewage treatment process, large-diameter flow meters are mostly split type, with some installed underground and others above ground. Small caliber is mostly integrated.
The functions of electromagnetic flow meters on the market also vary greatly. Simple ones only measure one-way flow and output analog signals to drive the rear instrument; Multi functional instruments include bidirectional flow measurement, range switching, upper and lower limit flow alarm, air traffic control and power cut-off alarm, small signal cut-off, flow display and total calculation, automatic verification and fault self diagnosis, communication with the upper computer, and motion configuration. The serial digital communication function of some models of instruments can be equipped with multiple communication interfaces and dedicated chips (ASICs) to connect to HART protocol systems, PROFTBUS, Modbus, FF fieldbus, etc.
The aperture range of electromagnetic flow meters is wider than other types of flow meters. E+H company provides three types of sensors: W, P, and H, which are respectively used in the water and sewage industry (W type), chemical and food industry (P type), and food and pharmaceutical industry (H type), with aperture ranges from 2 millimeters to 2 meters. The transmitter can provide multiple options including 10, 50, 23, and 53. 10 is an economical type developed for the water industry, 50 is a regular type, 53 is an enhanced type, and 23 is a two-wire system. There are two options for accuracy: 0.5% and 0.2%. It can measure both positive and negative flow rates, as well as pulsating flow rates.
The premise of using an electromagnetic flowmeter is that the measured liquid must be conductive and cannot be lower than the threshold (i.e. lower limit). Conductivity below the threshold will result in measurement errors until it cannot be used. Even if it changes beyond the threshold, it can still be measured with little change in indication error. The threshold of a universal electromagnetic flowmeter is between 10-4 and (5 × 10-6) S/cm, depending on the model. When used, it also depends on the length of the flow signal line and its distributed capacitance between the sensor and the converter. The manufacturer's instruction manual usually specifies the length of the signal line corresponding to the conductivity.
The conductivity of industrial water and its aqueous solutions is greater than 10-4S/cm, while the conductivity of acid, alkali, and salt solutions is between 10-4 and 10-1S/cm. There is no problem with their use, and the conductivity of low distilled water is also between 10-5S/cm. Electromagnetic flow meters cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. Cannot measure gases, vapors, and liquids containing a large number of bubbles. It has been found from the data that some pure liquids or aqueous solutions have low conductivity and are considered unusable. However, in practical work, there may be instances where they can be used due to the presence of impurities, which are beneficial for increasing conductivity. For aqueous solutions, the conductivity in the data was measured in the laboratory using pure water ratio. The actual aqueous solution used may use industrial water ratio, and the conductivity will be higher than the measured value, which is also beneficial for flow measurement.
2. Characteristics of electromagnetic flowmeter
Electromagnetic flow meters are not affected by external factors such as temperature, pressure, viscosity, etc., and there is no pressure loss caused by shrinkage or protrusion inside the measuring tube. In addition, the initial signal detected by the flow element is a voltage that varies linearly with the average flow velocity of the fluid, which is independent of other properties of the fluid and has great advantages. According to the characteristics of large flow rate changes, impurities, low corrosiveness, and certain conductivity of sewage, electromagnetic flowmeter is a good choice for measuring the flow rate of sewage. It has a compact structure and small volume. Easy to install, operate, and maintain, the measurement system adopts intelligent design, and the overall sealing is strengthened, which can work normally in harsh environments. Due to technical reasons, it is difficult to enlarge the diameter of vortex flowmeters, and pipeline ultrasonic flowmeters lack competitive advantages in price compared to electromagnetic flowmeters in terms of quality. So in the water and sewage industry, electromagnetic flow meters, especially large-diameter electromagnetic flow meters, have great advantages and have been widely used.
Electromagnetic flowmeter has powerful functions and simple operation. Its specific features are as follows:
1) Measure that there are no obstructing flow components or pressure loss inside the pipeline, and the straight pipe section requires a bottom.
2) The measurement is not affected by changes in fluid density, viscosity, pressure, temperature, or conductivity.
3) Light sensitive key, can be operated without opening the cover; Chinese menu display makes it more convenient for domestic users to use.
4) Quick setting menu, guiding the completion of parameter settings, convenient and fast.
5) There are various materials available for lining, such as hard rubber, polyurethane, PTFE, PFA, etc.
6) The transmitter's DC/AC power supply, four wire/two-wire system, explosion-proof/non explosion-proof, economical/standard type and other subdivisions meet your different needs.
7) High measurement reliability, good repeatability, and long-term maintenance free. Range ratio up to 1000:1
8) Provide various output modes such as current, frequency, pulse, Hart, Profibus PA, Profibus DP, FF, etc. for selection.
3. Installation requirements
The electromagnetic flowmeter provides a protection level of IP67 (dustproof and waterproof) or IP68 (dustproof and waterproof). In sewage treatment plants, large-diameter flow meter sensors are mostly installed underground, so it is recommended to choose IP68 (dustproof and anti diving level). Usually, electromagnetic flow meters have the following requirements for installation sites:
1) When measuring mixed phase fluids, choose a location that will not cause phase separation; When measuring two-component liquids, avoid placing them downstream where mixing is not yet uniform; When measuring chemical reaction pipelines, they should be installed downstream of the fully completed reaction section; Try to ensure that the front and rear straight pipe sections are not less than 5D and 2D respectively;
2) Try to avoid negative pressure inside the measuring tube as much as possible;
3) Avoid large motors, transformers, etc. nearby to prevent electromagnetic interference;
4) Places that are easy to implement separate grounding for sensors;
5) Try to avoid high concentrations of corrosive gases in the surrounding environment as much as possible;
6) Choose a location with low vibration, especially for integrated instruments;
7) Try to avoid direct sunlight as much as possible