Electromagnetic flowmeter is a kind of precision inductive instrument. If the type selection or installation is unreasonable, it will have a great influence on the accuracy of the measurement and the service life of the instrument. The following is a brief introduction of the electromagnetic flowmeter selection, installation conditions and precautions.
(I) Selection principle of electromagnetic flowmeter Selection of electromagnetic flowmeter is mainly due to the correct selection of sensors, and the converter only needs to be matched with it.
1. Choice of Diameter and Span The diameter of the sensor is usually the same as the diameter of the pipe system. If the pipe system is to be designed, the diameter can be selected according to the flow range and flow rate. For electromagnetic flowmeters, flow rates of 2 to 4 m/s are more appropriate. In special circumstances, such as liquid with solid particles, taking into account the situation of wear, the optional common flow rate ≤ 3m / s, for easy to attach the management of the fluid, the choice of flow rate ≥ 2m / s. After the flow rate is determined, the sensor caliber can be determined.
The range of the sensor can be selected according to two principles: First, the full-scale range of the instrument is greater than the expected maximum flow value; second, the normal flow is greater than 50% of the full-scale range of the instrument to ensure a certain measurement accuracy.
2. The choice of temperature and pressure The electromagnetic flowmeter can measure the fluid pressure and temperature is a certain limit, when used, the use of pressure must be lower than the working pressure specified by the flowmeter. At present, the working pressure specifications of electromagnetic flowmeters produced in China are:
Less than 50mm diameter, working pressure is 1.6MPa;
900mm diameter, working pressure is 1MPa;
More than 1000mm diameter, working pressure is 0.6MPa.
The working temperature of the electromagnetic flow meter depends on the lining material used, and is generally 5-70°C. If you do special treatment, you can exceed the above range, the sensor allows the measured medium temperature is -40-+130 °C.
3. Selection of lining material and electromagnetic material The lining material and electrode material of the sensor must be correctly selected according to the physical and chemical properties of the medium, otherwise the instrument will be damaged quickly due to the corrosion of the lining and the electrode, and the corrosive medium is very strong. Once leaked, it can easily cause an accident. Therefore, according to the specific measurement medium in the production process, carefully select the electrode and lining material (2) The installation of the electromagnetic flowmeter should ensure the measurement accuracy of the electromagnetic flowmeter, and the correct installation is very important.
1 The sensor should be installed in a dry and ventilated room. Avoid installation in places where the ambient temperature is too high. Do not subject to strong vibration. Try to avoid devices with strong magnetic fields, such as large motors, transformers, etc. Avoid installation in corrosive gases. The installation site is easy to overhaul. This is the environmental condition that guarantees the normal operation of the sensor.
2 In order to ensure that the sensor measuring tube is filled with the measured medium, the transmitter is preferably installed vertically and flows from bottom to top. Especially for liquid-solid two-phase six, it must be installed vertically. If the site only allows horizontal installation, it must ensure that both electrodes should be installed with valves and bypass at both ends of the same level 3 sensor.
4 The millivolt AC potential measured by the electrodes of the electromagnetic flow sensor is based on the internal potential of the sensor. In order to make the liquid potential stable and the position sensor and the fluid maintain equipotentiality, in order to ensure stable measurement, both ends of the sensor housing and the metal tube should be well grounded, and the converter housing should also be grounded. The grounding resistance must not be greater than 10 ∩ and it cannot be shared with the grounding wire of other electrical equipment. If the sensor housing cannot be guaranteed to make good contact with the metal pipe, use metal wires to connect them. And then reliably grounded.
5 In order to avoid interference signals, the signal between the sensor and the converter must be transmitted with shielded wires. It is not allowed to place signal cables and power cables in parallel in the same cable duct. Signal cable length must not exceed 30m
6 The converter shall be installed in a place where strong magnetic fields and vibrations of AC and DC are to be avoided. The ambient temperature is -20-50°C, does not contain corrosive gas, and the relative humidity is not heavy 80%.
7 In order to avoid the influence of the flow rate relative measurement, the flow control valve should be set downstream of the sensor. For a small-diameter sensor, since the distance from the center of the electrode to the inlet end of the flowmeter is equivalent to several times the length of the diameter D, the upstream straight tube may not be specified. However, for larger diameter flowmeters, there should be more than 5D straight pipe sections in the upstream, and no straight pipe requirements in the downstream.
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