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There should be a clear number of measurement requirements, mainly: measuring medium, flow m3 / h (zui small, working point, zui large), medium temperature ° C, medium pressure MPa, installation form (pipeline or plug-in).
Second, the preconditions for the selection of electromagnetic flowmeter
1. The medium to be tested must be a conductive liquid (ie, the fluid to be tested is required to have a low conductivity).
2. The measured medium should not contain more ferromagnetic media or a large number of bubbles.
Third, we should understand the measurement principle of electromagnetic flowmeter
The measurement principle is based on Faraday's law of electromagnetic induction, that is, when the flow rate is measured, the fluid flows through a magnetic field perpendicular to the flow direction, and the flow of the conductive fluid induces a voltage proportional to the average flow velocity (also called volume flow), and the voltage signal passes. Two electrodes in direct contact with the fluid are detected and transmitted through the cable to the amplifier, which is then converted to an output signal.
Fourth, should understand the main characteristics of electromagnetic flowmeter
Its main features are: (1) There is basically no pressure loss in the measuring tube, it is not easy to block, and it has unique adaptability to the measurement of slurry; (2) Low requirements for straight pipe section; (3) Low-frequency rectangular wave excitation, which is not affected by power frequency and on-site distributed interference, work Stable and reliable; (4) The transmitter body can be made of all stainless steel, with anti-acid, anti-alkali and anti-corrosion ability after adding lining material; (5) The field display type converter can use special intelligent chip, parameter setting is convenient; (6) The self-calibration system can be set inside the device, and the transmitter constant and the factory calibration value can be self-calibrated at any time for easy debugging and maintenance; (7) The measurement range is wide, and the full-scale flow rate can be set in the range of 0.3m/s to 12m/s. (8) The plug-in type can be installed or disassembled under continuous flow; (9) Wide range of use: It can be applied to chemical, metallurgy, papermaking, food, petroleum, urban water supply and other fields.
Five, the choice of model points
1. The first and foremost is to choose a pipeline geomagnetic flowmeter or a plug-in electromagnetic flowmeter.
2. Under normal circumstances, the on-site non-display type electromagnetic flowmeter is selected, and its output 4-20 mA (or 0-10 mA) current signal is sent to the secondary meter of the control room and the flow rate and total amount can be displayed.
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 environmental requirements or measurement accuracy requirements are high, the safety voltage intelligent electromagnetic flowmeter can be selected.
5, in the 200mm or more large pipe diameter measurement flow or continuous flow state assembly and disassembly, the preferred plug-in or enhanced plug-in electromagnetic flowmeter.
Sixth, the choice of sensor caliber
1. Select the diameter of the sensor to be the same as the diameter of the connected process pipe.
The advantage is that the installation is convenient (no need for a reducer); the premise is that the flow rate of the fluid in the pipe must be in the range of 0.3 m / s - 10 m / s; the applicable state is the use of the pre-engineering period and the flow velocity of the pipe is at a lower state.
2. The diameter of the selected sensor is different from the diameter of the connected process pipe.
Its applicable state: (1) low flow rate, stable flow; (2) low cost performance.
Seven, the selection of lining materials
According to the corrosiveness, wear and temperature of the tested medium of the enterprise, the order is selected by the subscriber, please refer to the “lining material performance and applicable range table†provided by each manufacturer.
Eight, the choice of electrode materials
According to the corrosiveness of the tested medium of the enterprise, it is selected by the orderer. Please refer to the “Electrode Corrosion Resistance Table†provided by each manufacturer.
Nine, the choice of protection level
According to the national standard GB4208-84, the protection level of the enclosure is: if the instrument is installed below the ground and is often flooded, the submersible type should be selected; if the instrument is installed on the surface, the anti-spray type should be selected.
Ten, the choice of the upper limit flow
1. Generally, the diameter of the selected transmitter is the same as the diameter of the connected piping. Since there is a strict function relationship between the path, the flow rate and the flow rate, please refer to the upper limit flow meter of the electromagnetic flowmeter provided by the manufacturer when selecting, and select the upper limit flow value under the corresponding path.
2. When the flow rate in the process piping is low and the small upper limit flow value of the corresponding diameter in "1" is not reached, the diameter of the transmitter can be selected to be smaller than the diameter of the process piping, that is, before and after the transmitter. Connect the reducer.
XI, the choice of installation form
1. Integral type: The sensing part and the conversion part of the flow meter are integrated into one body, and the advantage is that it is easy to install.
2. Separate type: Install the sensing part of the flowmeter on the pipe to be tested, and install the conversion part in a room that is easy to operate. Applicable to occasions with poor on-site environment (but the separation length should be less than 30m).
Twelve, the choice of grounding ring
If the pipe connecting the meter is insulated relative to the medium being tested, you need to use a grounding ring.
Thirteen, the choice of the ball valve
For the process where the fluid flow in the pipeline must not be stopped, and the medium is required to not overflow, a ball valve is required. However, if the flow is allowed to be interrupted when the flowmeter is installed or disassembled (or the assembly and disassembly operation does not affect the work of the pipeline), the flowmeter is directly installed on the short flange of the flange without adding a ball valve.
Fourteen, installation attention points
1. The electrode axis must be kept at an approximate level;
2. Ensure that the measuring tube must be filled at all times;
3. Ensure that there are enough installation space for bolts and nuts near the pipe flange;
4. Pipeline support should be provided in the pipe section where the flowmeter is installed to reduce the running vibration of the pipeline;
5. Strong electromagnetic fields should be avoided near the flowmeter;
6. For long pipelines, control valves and shut-off valves should be installed downstream of the flowmeter;
7. In the case of “open feed or dischargeâ€, the instrument should be installed in the lower section of the pipeline;
8. Based on the electrode axis, the inlet straight pipe section should be greater than or equal to 5 times the measuring pipe diameter, and the outlet pipe should be greater than or equal to 2 times the measuring pipe diameter.
15. Grounding requirements for the flowmeter
The sensor should have a good separate grounding wire (copper core section ≥16mm2), and its grounding resistance is <10 ohms; if the pipe connecting the sensor is coated with an insulating layer or a non-metallic pipe, a grounding ring (grounding wire) should be provided on both sides of the sensor. Copper core section ≥16mm2).
Sixteen, cable and electrical connection attention points
Separate connecting cables (signal transmission and excitation) are as short as possible from the perspective of reducing interference and saving money. Generally, the limitations of the dielectric conductivity, the cross section of the excitation cable, and the type (core, layer, and shield) of the signal cable must be considered.
First, the primary determination of the unit's measurement needs