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How to maintain the selection of electromagnetic flowmeters and the prerequisites for selecting electromagnetic flowmeters
2012-3-9 12:47:56

的测量原理基于法拉第电磁感应定律。 The measurement principle of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction. When measuring the flow, the fluid flows through a magnetic field perpendicular to the direction of flow. The flow of the conductive liquid induces a voltage proportional to the average flow rate. The induced voltage signal passes through two electrodes on the wall of the fixed measuring tube that are in direct contact with the liquid. After detection, the signal is transmitted to the converter, and the converter performs signal amplification, sampling, anti-interference and other processing, and then converts it into a standard DC current signal that has a linear relationship with the flow. Reasonable selection of electromagnetic flowmeters is important to ensure measurement accuracy and extend service life. The following is a brief introduction to the selection principles of electromagnetic flowmeters:

Prerequisites for selecting an electromagnetic flowmeter

① The measured medium should not contain more ferromagnetic media or a lot of air bubbles.
② The measured medium must be a conductive liquid (that is, the measured fluid must have a minimum conductivity).

Choice of caliber and range

①The diameter of the transmitter is usually the same as that of the piping system. If the pipeline system is to be designed, the caliber can be selected according to the flow range and velocity. For electromagnetic flowmeters, a flow velocity of 2-4m / s is more appropriate. In special cases, such as solid particles in the liquid, considering the wear conditions, the commonly used flow rate ≤ 3m / s can be selected. For fluids that are easy to manage. Optional flow velocity ≥2m / s. After the flow rate is determined, the transmitter diameter can be determined according to qv = D2.

② The range of the transmitter can be selected according to two principles: one is that the full scale of the meter is greater than the expected maximum flow value; the other is that the normal flow is greater than 50% of the full scale of the meter to ensure a certain measurement accuracy.

Choice of temperature and pressure

① There are certain restrictions on the fluid pressure and temperature that the electromagnetic flowmeter can measure. When selecting, the operating pressure must be lower than the working pressure specified by the flowmeter. At present, the working pressure specifications of domestically produced electromagnetic flowmeters are:

Caliber less than 50mm, working pressure is 1.6MPa;
900 mm caliber, working pressure is 1MPa;
Larger than 1000mm, working pressure is 0.6MPa.

If there are special requirements for the pressure resistance of the transmitter, you can consult the manufacturer specifically.

② The operating temperature of the electromagnetic flowmeter depends on the lining material used, generally 5-70 ° C. Such as special treatment, it can exceed the above range, such as the wear-resistant and corrosion-resistant electromagnetic flowmeter produced by Tianjin Automation Instrument Factory. The transmitter allows the measured medium temperature to be -40 to 130 ° C.

How to maintain and maintain the electromagnetic flowmeter in a stopped state

在停机即不使用的时候,我们要对电磁流量计采取科学合理的维护和保养方法。 When the electromagnetic flowmeter is not used when it is stopped, we must adopt scientific and reasonable maintenance and maintenance methods for the electromagnetic flowmeter. In providing electromagnetic flowmeter technical services to users, users often encounter inaccurate measurement data after purchasing the electromagnetic flowmeter for a period of use. In fact, this is not a quality problem of the electromagnetic flow meter itself. Because the user does not follow the post-use care and maintenance methods in the manual.

I. Zero check and adjustment of electromagnetic flowmeter maintenance:

Before the electromagnetic flowmeter is put into operation, the zero point must be adjusted after the electromagnetic flow sensor is full of liquid after the power is turned on. After being put into operation, it is necessary to periodically check the zero point according to the conditions of use. Especially for precipitates and easily polluted electrodes, non-cleaning liquids containing solid phases should be checked at the beginning of operation to obtain experience to determine the normal inspection cycle. The AC magnetic field flowmeter and rectangular wave excitation ratio are more likely to produce zero drift, so more attention should be paid to inspection and adjustment. Take two examples of application errors where the deposits have failed. One is that in the oil drilling and cementing project, the total flow of cement slurry is an important process parameter, and high pressure electromagnetic flow meters are often used. Use the meter intermittently. Rinse the sensor measuring tube with clean water after use. The remaining time is empty. Due to incomplete cleaning, the residual cement slurry on the inner wall of the measuring tube has solidified into a thin layer, which has accumulated in the past two months to form an insulating layer, covering the entire electrode surface, resulting in abnormal operation and eventually failure to work. The other is an electrolytic cutting process inspection device, which uses an electromagnetic flowmeter to control the flow of saturated saline solution. After a period of use, the flow signal is gradually weakened, and the signal is zero after 2 months. The reason is that iron oxide deposits on the tube wall during electrolytic cutting, resulting in a short circuit. Clearing the stratum immediately returned to normal.

维护之定期检查传感器电性能: Second, the periodic inspection of the electrical performance of the electromagnetic flowmeter maintenance sensor:

First, roughly measure the resistance between electrodes. Disconnect the signal connection between the sensor and the converter, the sensor is filled with liquid, and use a multimeter to measure the resistance value of the two electrodes and the ground terminal, whether it is within the range specified by the manufacturer, and the measured two values are substantially the same. Record the resistance value measured for the first time. This value is useful for later determining the cause of the sensor failure (such as whether the deposited layer is conductive or insulating). Secondly, drain the sensor from the liquid and wipe the inner wall. After it is completely dry, use a megohmmeter to measure the resistance between the two electrodes and the ground terminal. Finally, check the insulation resistance of the excitation coil, remove the sensor excitation coil, wire the terminal indirectly to the converter, and measure the insulation resistance of the coil with a megohmmeter