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Electromagnetic flowmeter selection of the elements to be considered

Electromagnetic flowmeters continue to hold high market share because of their incomparable advantages. The structure is simple, the fluid flow through the flowmeter does not cause pressure loss; the dirty medium, the turbid medium and the liquid-solid two-phase flow of the corrosive medium can be measured; the measurement process is not easily affected by the density, temperature and viscosity of the liquid; the speed measurement scale is relatively wide. Of course, electromagnetic flowmeters are perfected from time to time, but there are still some shortcomings: steam, gas and gas-liquid two-phase media cannot be measured; they are simply affected by external magnetic field disturbance; media with lower conductivity cannot be measured, such as organic solvents or dairy products. .

1.1 Liquid conductivity
The conditions used by the electromagnetic flowmeter must be conductive and cannot be lower than the threshold. The conventional flowmeter threshold is generally 10-4 to 10 s/m, which should be determined according to the model. Organic solvents and petroleum products have low electrical conductivity and therefore cannot be measured with an electromagnetic flow meter. Depending on the experience, the conductivity of the liquid to be tested should be at least an order of magnitude greater than the magnitude of the devaluation by the manufacturer. Otherwise it will cause a sway.

1.2 Accuracy level
The accuracy of the electromagnetic flowmeter is about ±0.5% to ±1%, and the flowmeter with low accuracy is about ±1.5% to ±2.5%, and the price is about 2 times different. Therefore, the flowmeter should be reasonably selected. In the center where the precision is not high, the lower price is used to cut the cost. Some flowmeters have higher precision, but the equipment conditions are very high, such as temperature 20"C ~ 22 °C, so that DC calibration should be done to reduce the impact.

1.3 Selection of flow rate and caliber
The diameter of the flowmeter may not be different from the diameter of the pipe, and should be determined according to the flow rate.
The liquid economic flow rate of the pipeline is generally 1.5 to 3 m/s. At this flow rate, the diameter and diameter of the flowmeter are not matched. The on-line flow rate is theoretically unlimited, but it is recommended that the fabric cannot withstand fluid flushing and does not exceed 5 m/s. For simple accumulation of fouling liquid, the flow rate should be no less than 2m/s.

2. Elements that affect the accuracy of electromagnetic flow measurement
Assuming that the electromagnetic flowmeter equipment and improper use, it will cause the error to become large, resulting in damage.

2.1 Selection of grounding ring and electrode raw materials
Because the measured liquid and the electrode or the grounding ring do not match, the mismatch, in addition to the corrosion problem, as well as the appearance of the electrode, mainly includes: chemical reaction, polarization, catalyst effect. Grounding rings have the same problems, but the level of impact is smaller.
2.2 The tube is not filled with liquid
Poor orientation of the flowmeter device or lack of back pressure, causing the liquid not to fill the pipe, resulting in a smaller measured value than the actual value. Assuming that the tube is not severe enough to allow the electrode to leak out of the liquid level, there is no way to measure the liquid. It is assumed that the activity is a plug flow or a bubble flow, which not only makes the measured value inaccurate, but also constitutes output sloshing because the bubble instantaneously hits the surface of the electrode.

2.3 Solid phase in liquid
The shortcomings of the liquid that are supposed to contain particles may be: The cover layer of the electrode and the fabric is covered, the fabric is worn, and the cross-sectional area of the pipe is reduced. Some relatively simple crystallized liquids cause a layer of solids on the inner wall when the fluid flows through the flowmeter. Because other types of flowmeters have the same problem, the flowmeters that are very short and can be easily disassembled can be selected. It can be easily disassembled after crystallization to stop the protection.

2.4 Equipment
During the process of the equipment, the flow direction of the flowmeter should be inconsistent with the flow direction of the fluid. Prevent the device from being in the center of a strong magnetic field. The flow meter should not be placed at the highest point of the pipe that is not simple to accumulate gas, or the straight pipe from top to bottom, so that the emptying is simple. The flow meter should not directly discharge the liquid into the atmosphere, but should have back pressure to prevent the pipe from being filled.

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