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First, the first clear that the measurement needs of the unit should have a clear number of measurement needs, mainly include: measuring medium, flow m3 / h (minimum, operating point, maximum), medium temperature °C, medium pressure MPa, installation form (pipeline or Plug-in) etc.
Second, the selection of electromagnetic flowmeter preconditions
1. The medium to be measured must be a conductive liquid (ie, the fluid to be tested has a minimum conductivity).
2. The measured medium should not contain more ferromagnetic medium or a large number of bubbles.
Third, should understand the measurement principle of electromagnetic flowmeter The measurement principle is based on Faraday's law of electromagnetic induction, that is, when measuring flow, the fluid flows through the magnetic field perpendicular to the direction of flow, and the flow of conductive fluid induces an average flow velocity (also called volume flow). Proportional to the voltage, the voltage signal is detected by two electrodes in direct contact with the fluid, and transmitted to the amplifier through the cable, and then converted into an output signal.
Fourth, should understand the main characteristics of the electromagnetic flowmeter its main features are: (1) the measuring tube is basically no pressure loss, easy to plug, has a unique adaptability to the grout type measurement; (2) low straight pipe requirements; (3) low-frequency rectangular wave excitation, not It is stable and reliable due to the influence of power frequency and on-site distribution interference; (4) The body of the transmitter can be made of stainless steel, and it has anti-acid, anti-alkali and anti-corrosion capabilities after the lining material is installed; (5) The field display converter can use dedicated intelligence. Chips, convenient parameter setting; (6) The transmitter can be set up within the self-school system, the transmitter constants and factory calibration values ​​can be self-calibrated at any time, easy to debug and repair; (7) wide measurement range, full-scale flow rate settings can be In the range of 0.3m/s to 12m/s; (8) The plug-in type can be installed or disassembled under constant flow; (9) Wide range of applications: Can be used in chemical, metallurgy, papermaking, food, petroleum, urban water supply and other fields.
Fifth, the choice of model points
1, the first clear is to choose the pipeline electromagnetic flowmeter , or plug-in electromagnetic flowmeter .
2. Normally, the on-site non-display electromagnetic flowmeter is selected , and the 4-20 mA (or 0-10 mA) current signal output to the secondary meter of the control room can display the flow and the total amount.
3. If it is emphasized that it is convenient to observe the flow in the pipeline during on-site operation, the on-site display type electromagnetic flowmeter can be selected.
4. When the requirements of the environment or measurement accuracy are high, the safe voltage intelligent electromagnetic flowmeter can be selected.
5, in 200mm or more large diameter measuring flow or continuous flow state assembly and disassembly, can be preferred to insert or enhanced plug-in electromagnetic flowmeter .
Sixth, the choice of sensor diameter
1. The choice of the sensor's diameter is the same as the diameter of the connecting process pipe. The advantage is that it is easy to install (no need for different diameter pipe); the premise is that the flow rate of the fluid in the pipe must be in the range of 0.3m/s to 10m/s; the applicable state is It is used early in the project and the fluid flow rate in the pipe is low.
2. The caliber of the selected sensor is not the same as the diameter of the connected process pipe. Applicable status: (1) Low flow rate and stable flow rate: (2) Reduced cost performance:
7. Selection of lining materials According to the corrosivity, wearability, and temperature of the medium to be measured by the company, the choice of the ordering party may refer to the “performance and scope of application of lining materials†provided by various manufacturers.
VIII. Selection of Electrode Material According to the corrosivity of the medium to be measured by the company, the choice of the orderer may refer to the “Corrosion Resistance of Electrode Materials†provided by various manufacturers.
Nine, the choice of protection level According to the national standard GB4208-84, the enclosure protection class is: if the meter is installed below the ground and often flooded should choose submersible type; if the meter is installed on the surface, it should be selected to prevent water spray type .
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Tiankang introduces the selection of electromagnetic flowmeters and places that should be taken into account when installing and using electromagnetic flowmeters.
In the measurement practice of the enterprise's application of electromagnetic flowmeters , some units of measurement personnel do not pay attention to their selection and installation points, and often fail to achieve the expected purpose of measurement.