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Industry Information
Magnetic flowmeter for measuring the flow of sewage is a good choice
2012-3-5 10:18:46

Electromagnetic flowmeters have a history of more than 50 years and have been widely used worldwide.

Measurement principle

的工作原理是基于法拉第电磁感应定律。 The working principle of electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When a conductive metal rod moves at a certain speed perpendicular to the direction of the magnetic field lines, an induced voltage is generated. In the electromagnetic flowmeter, the conductive medium in the measuring tube is equivalent to the conductive metal rod in the Faraday test, and the two electromagnetic coils at the upper and lower ends generate a constant magnetic field. When a conductive medium flows, an induced voltage is generated. The two electrodes inside the pipe measure the induced voltage produced. The measuring pipe is electromagnetically isolated from the fluid and the measuring electrode by a non-conductive inner lining (rubber, Teflon, etc.). Assuming that the magnetic field strength B generated by the electromagnetic induction coil is constant, the induced voltage Ue is proportional to the fluid velocity v, and the cross-sectional area A of the pipe is known, then the volume flow Qv can be calculated by the following formula:

The electromagnetic flowmeter is composed of a flow sensor and a transmitter. The sensor measuring tube is provided with an excitation coil above and below, and a magnetic field is generated after the excitation current is passed through the measuring tube. A pair of electrodes are mounted on the inner wall of the measuring tube to contact the liquid, and the induced potential is drawn and sent to the transmitter. The excitation current is provided by the transmitter. According to the converter and sensor assembly methods, there are two types: separate and integrated. In the sewage treatment process, large-diameter flow meters are mostly split type, with one part installed underground and the other on the ground. The small caliber is more integrated. The functions of electromagnetic flowmeters on the market are also very different. The simple one is only to measure unidirectional flow, and only output analog signals to drive the rear instrument. The multi-function instrument has two-way flow measurement, range switching, upper and lower limit flow alarm, air pipe and power supply. Cut off alarm, small signal removal, flow display and total calculation, automatic check and fault self-diagnosis, communication with upper computer and motion configuration, etc. The serial digital communication function of some models of instruments can choose a variety of communication interfaces and application-specific chips (ASICs) to connect HART protocol systems, PROFTBUS, Modbus, FF fieldbus, etc.

The diameter range of the electromagnetic flowmeter 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 industry and food industry (P type). Food and pharmaceutical industry (H type), caliber range from 2mm to 2m. The transmitter can provide multiple options of 10, 50, 23, and 53. 10 is an economical type developed for the water industry. 50 is an ordinary type, 53 is an enhanced type, and 23 is a two-wire system. There are two choices of accuracy: 0.5% and 0.2%. Can measure forward and reverse bi-directional flow, also can measure pulsating flow. The premise of using an electromagnetic flowmeter is that the measured liquid must be conductive and cannot be lower than the threshold (ie, the lower limit). If the conductivity is lower than the threshold value, a measurement error will occur until it cannot be used. Even if it exceeds the threshold value, it can be measured. The displayed value error does not change much. The threshold value of the universal electromagnetic flowmeter is 10-4 ~ (5 × 10-6) S / cm Depending on the model. 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 solution is greater than 10-4S / cm, and the conductivity of acid, alkali and saline solution is between 10-4 ~ 10-1S / cm. There is no problem in use. Low-distilled water is 10-5S / cm is not a problem. Electromagnetic flow meters cannot measure liquids with very low conductivity, such as petroleum products and organic solvents. It is not possible to measure gases, vapors and liquids with large bubbles. From the information, we found that some pure liquids or aqueous solutions have low electrical conductivity and are considered unusable. However, in practice, we will encounter examples that can be used because they contain impurities. Such impurities are beneficial to increase the conductivity. For the aqueous solution, the conductivity in the data is measured in the laboratory with pure water ratio. The actual aqueous solution may be used with industrial water ratio. The conductivity will be higher than that found, which is also beneficial to flow measurement.

2.Characteristics of electromagnetic flowmeter

不受温度、压力、粘度等外界因素的影响,测量管内部无收缩或凸出部分的造成的压力损失,另外,流量元件检测出的最初信号,是一个与流体平均流速成精确线性变化的电压,它与流体的其他性质无关,具有很大的优越性。 The electromagnetic flowmeter is not affected by external factors such as temperature, pressure, viscosity, etc. There is no pressure loss caused by the shrinkage or protruding part inside the measuring tube. In addition, the initial signal detected by the flow element is an accurate linearity with the average flow velocity of the fluid. Changing voltage, which has nothing to do with other properties of the fluid, has great advantages. According to the characteristics of sewage with large flow changes, impurities, low corrosiveness, and certain conductivity, electromagnetic flowmeters are a good choice. It has a compact structure and small size. Easy to install, operate and maintain. The measuring system adopts intelligent design, and the overall seal is strengthened, which can work normally in the harsh environment. However, due to technical reasons, the vortex flowmeter is difficult to be larger, and the quality of pipeline ultrasonic flowmeters lacks competitive advantages over electromagnetics. Therefore, in the water and sewage industries, electromagnetic flowmeters, especially large-diameter electromagnetic flowmeters, have great advantages and have been widely used.

E + HPROMAG series electromagnetic flowmeter is powerful and easy to operate. The specific characteristics are as follows:

1) There are no obstructing flow parts in the measuring pipe, no pressure loss, and the bottom of straight pipe sections is required.
2) The measurement is not affected by changes in fluid density, viscosity, pressure, temperature, and conductivity.
3) Photosensitive key, can be operated without opening the cover; Chinese menu display, more convenient for domestic users.
4) Quick setting menu, guide the parameter setting, convenient and quick.
5) The lining has a variety of materials such as hard rubber, polyurethane, PTFE, PFA and so on.
6) The DC power supply / AC power supply of the transmitter, four-wire system / two-wire system, explosion-proof / non-explosion-proof, economical / standard, etc. can 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 and so on.

3. Installation requirements

提供防护等级IP67(防尘防浸水级)或IP68(防尘防潜水级)。 PROMAG series electromagnetic flowmeters provide protection class IP67 (dust-proof and submersible) or IP68 (dust-proof and submersible). Most of the large-caliber flowmeter sensors are installed underground in sewage plants, so it is recommended to choose IP68 (dust-proof and anti-submersible). Generally, the electromagnetic flow has the following requirements on the installation site:

1) When measuring mixed-phase fluid, choose a place that will not cause phase separation; when measuring two-component liquids, avoid installing downstream where the mixing is not uniform; when measuring chemical reaction pipelines, install it downstream of the fully completed reaction section Meet the straight pipe sections before and after not less than 5D and 2D;
2) Avoid negative pressure in the measuring tube as much as possible;
3) Avoid having large motors, transformers, etc. nearby, so as not to cause electromagnetic field interference;
4) A place where it is easy to achieve separate grounding of the sensor;
5) Avoid high concentrations of corrosive gases in the surrounding environment as much as possible;
6) Choose a place with small vibration, especially for integrated meters;
7) Avoid direct sunlight as much as possible