Analysis of Key Points for Electromagnetic Flowmeter Selection

I. Rangeability and Size
The rangeability of electromagnetic flowmeters is relatively large, typically not less than 20. For instruments with automatic range switching, the rangeability can exceed 50 to 100. Domestically available standard products have diameters ranging from 10 mm to 3000 mm. Although small and medium diameters are still the most common in practical applications, large-diameter instruments account for a significant proportion compared to most other flowmeter principles (such as positive displacement, turbine, vortex, or Coriolis mass flowmeters). Among nearly 10,000 flowmeters from a certain company, small diameters below 50 mm, medium diameters of 65–250 mm, large diameters of 300–900 mm, and extra‑large diameters above 1000 mm account for 37%, 45%, 15%, and 3% respectively.

II. Flow Velocity, Full‑Scale Flow Rate, Rangeability and Diameter Selection
The selected meter diameter does not necessarily have to match the pipe diameter; it should be determined based on the flow rate. For conveying low‑viscosity liquids such as water in the process industry, the pipeline flow velocity is generally the economic velocity of 1.5–3 m/s. When an electromagnetic flowmeter is used in such a pipeline, the sensor diameter can be the same as the pipe diameter. The liquid velocity at full‑scale flow can be selected within the range of 1–10 m/s, which is quite wide. The upper velocity limit is theoretically unrestricted, but it is usually recommended not to exceed 5 m/s, unless the lining material can withstand the scouring of the liquid velocity. In practice, velocities above 7 m/s are rare, and above 10 m/s are even rarer. The lower limit of the full‑scale flow velocity is generally 1 m/s, and for some models it is 0.5 m/s. For some new projects where the initial flow rate is low or for piping systems with inherently low flow velocities, the meter diameter should be reduced relative to the pipe diameter using a reducer, considering measurement accuracy.

III. Special Fluid Considerations
For fluids that tend to adhere, deposit, or scale, the selected flow velocity should be no less than 2 m/s, and preferably increased to 3–4 m/s or higher, to provide self‑cleaning and prevent adhesion and deposition. For highly abrasive fluids such as slurries, the common flow velocity should be kept below 2–3 m/s to reduce wear on the lining and electrodes. When measuring low‑conductivity liquids near the threshold, the flow velocity should be set as low as possible (below 0.5–1 m/s), because increasing the flow velocity increases flow noise and can cause output fluctuation.