First, we must clearly define the metrological needs of our unit.
Several measurement requirements should be clearly defined, including: the measuring medium, flow rate (m3/h) (minimum, operating point, maximum), medium temperature (°C), medium pressure (MPa), and installation method (pipeline or insertion type).
II. Prerequisites for Selecting an Electromagnetic Flowmeter
1. The medium being measured must be a conductive liquid (i.e., the fluid being measured must have a minimum conductivity).
2. The medium being tested should not contain a large amount of ferromagnetic media or a large number of air bubbles.
Third, it is necessary to understand the measurement principle of electromagnetic flowmeters.
Its measurement principle is based on Faraday's law of electromagnetic induction, which states that when measuring flow rate, the fluid flows through a magnetic field perpendicular to the flow direction. The flow of conductive fluid induces a voltage that is proportional to the average flow velocity (also known as volumetric flow rate). This voltage signal is detected by two electrodes that are in direct contact with the fluid and transmitted to an amplifier via a cable, and then converted into an output signal.
IV. The main characteristics of electromagnetic flowmeters should be understood.
Its main features include: (1) Minimal pressure loss within the measuring tube, making it less prone to clogging and uniquely adaptable to slurry measurements; (2) Low straight pipe section requirements; (3) Low-frequency rectangular wave excitation, unaffected by power frequency and field-based distributed interference, ensuring stable and reliable operation; (4) The transmitter body can be made entirely of stainless steel, and with added lining material, it has acid, alkali, and corrosion resistance; (5) The field display converter can use a dedicated intelligent chip, facilitating parameter setting; (6) The transmitter can be equipped with an internal self-calibration system, allowing for self-calibration of the transmitter constant and factory calibration values at any time, facilitating debugging and maintenance; (7) Wide measurement range, with full-scale flow rate settings ranging from 0.3 m/s to 12 m/s; (8) Its insertion type allows for installation or removal without interrupting flow; (9) Wide range of applications: applicable to chemical, metallurgical, papermaking, food, petroleum, and urban water supply industries.
V. Key Points for Model Selection
1. The first thing to clarify is whether to choose a pipeline-type geomagnetic flow meter or an insertion-type electromagnetic flow meter.
2. Generally, a field-display-free electromagnetic flowmeter is selected. Its output 4-20mA (or 0-10mA) current signal is sent to the secondary instrument in the control room, where the flow rate and total flow can be displayed.
3. If easy observation of the flow rate in the pipeline during on-site operation is emphasized, a field display electromagnetic flow meter can be selected.
4. When environmental requirements or measurement accuracy requirements are high, a safe voltage intelligent electromagnetic flowmeter can be selected.
5. For measuring flow in pipes with a diameter of 200mm or more, or for installation and disassembly under continuous flow conditions, insertion type or enhanced insertion type electromagnetic flow meter is preferred.
VI. Key Points for Selecting Sensor Aperture
1. Select a sensor with the same diameter as the connected process piping.
Its advantages are easy installation (no reducer required); the prerequisite is that the flow velocity of the fluid in the pipe must be in the range of 0.3m/s to 10m/s; it is suitable for use in the early stage of the project when the flow velocity of the fluid in the pipe is low.
2. The diameter of the selected sensor is different from the diameter of the connected process pipeline.
Its applicable conditions are: (1) low flow rate and stable flow rate; (2) reduced cost-effectiveness.
VII. Key Points for Selecting Lining Materials
The lining material is selected by the customer based on the corrosiveness, abrasiveness, and temperature of the medium being tested. Refer to the "Properties and Applications of Lining Materials" provided by each manufacturer.
VIII. Selection of Electrode Materials
The electrode material is selected by the customer based on the corrosiveness of the medium being tested. Refer to the "Corrosion Resistance Table of Electrode Materials" provided by each manufacturer.
IX. Selection of Protection Level
According to the national standard GB4208-84, the enclosure protection levels are as follows: if the instrument is installed below ground and is frequently flooded, a submersible type should be selected; if the instrument is installed on the ground, a water-spray resistant type should be selected.
10. Choosing the Maximum Traffic Limit
1. Generally, the transmitter's pipe diameter should be the same as the connected piping diameter. Since there is a strict functional relationship between pipe diameter, flow velocity, and flow rate, the upper limit flow rate table for electromagnetic flowmeters provided by the manufacturer can be consulted to select the upper limit flow rate value for the corresponding pipe diameter.
2. When the flow velocity in the process piping is too low to reach the minimum upper limit flow value under the corresponding pipe diameter in "1", the transmitter pipe diameter can be selected to be smaller than the process piping pipe diameter, that is, add reducers before and after the transmitter.
XI. Selection of Installation Method
1. Integrated type: The sensing part and the conversion part of the flow meter are integrated into one unit. The advantage is that it is easy to install.
2. Separate type: The sensing part of the flow meter is installed on the pipe being measured, while the conversion part is installed indoors for easy operation. Suitable for situations with harsh field environments (but its separation length should be less than 30m).
12. Selection of Grounding Ring
If the pipe connecting the instrument is insulated relative to the measured medium, a grounding ring must be selected.
Thirteen, Prerequisites for Selecting Ball Valves
For applications where the process requires uninterrupted fluid flow in the pipeline and that the medium must not overflow, a ball valve is necessary. However, for applications where flow interruption is permissible during installation or removal (or where installation or removal does not affect pipeline operation), a ball valve is unnecessary, and the flow meter can be directly installed on the flange short pipe.
XIV. Installation Precautions
1. The electrode axis must be kept approximately horizontal;
2. Ensure the measuring tube is filled at all times;
3. Ensure sufficient space is left near the pipe flange for bolt and nut installation;
4. Pipeline sections with flow meters installed should be supported to reduce pipeline vibration during operation;
5. Strong electromagnetic fields should be avoided near the flow meter;
6. For long pipelines, control valves and shut-off valves should be installed downstream of the flow meter;
7. In the case of "open feed or discharge", instruments should be installed in the lower section of the pipeline;
8. With the electrode axis as the reference, the inlet straight pipe section should be greater than or equal to 5 times the diameter of the measuring pipe, and the outlet pipe should be greater than or equal to 2 times the diameter of the measuring pipe.
15. Grounding requirements for flow meters
The sensor should have a good independent grounding wire (copper core cross-section ≥16mm2) with a grounding resistance <10 ohms; if the pipe connected to the sensor is coated with an insulating layer or is a non-metallic pipe, grounding rings (grounding wire copper core cross-section ≥16mm2) should be installed on both sides of the sensor.
XVI. Important Notes on Cables and Electrical Connections
For separate connecting cables (signal transmission and excitation), the shorter the better, from the perspective of reducing interference and saving money. Generally, it is necessary to take into account the limitations of the conductivity of the medium, the cross-section of the excitation cable, and the type of the signal cable (core, layers, shielding).
