Key factors to consider when selecting magnetic inductive flow meters in the chemical industry include the measured medium, flow range, accuracy requirements, operating pressure and temperature, and safety requirements.
Qinghai Lithium Industry Company is a salt lake chemical enterprise specializing in the production of lithium carbonate, boric acid, potash fertilizers, and other salt lake chemical products. The raw materials used in salt lake chemical production are characterized by high conductivity (averaging 150 ms/cm), strong corrosiveness, and a tendency to crystallize. The auxiliary materials required in production primarily include strong acids and bases. The pipelines in the production workshop are made of corrosion-resistant plastic materials, with environmental temperatures ranging from a high of 90°C to a low of -30°C. Given these working conditions, magnetic inductive flow meters offer advantages over other flow meters, including high accuracy, long service life, easy maintenance, and cost-effectiveness.
Selection Criteria for Magnetic Inductive Flow Meters
1. Diameter Selection
The diameter of a magnetic inductive flow meter does not necessarily have to match the pipeline diameter; instead, it should be determined based on flow rate and velocity. However, since salt lake chemical raw materials and intermediates have high viscosity and relatively low flow velocities (typically 2.0–4.0 m/s), the sensor diameter generally matches the pipeline diameter. If the flow velocity is below 1.0 m/s, a meter with a smaller diameter may be chosen to maintain measurement accuracy while reducing investment costs.
2. Flow Velocity and Range Selection
Salt lake chemical processes mainly use corrosion-resistant plastic pipelines. Considering the lining material of the flow meter, the flow velocity is generally controlled at around 2 m/s. For materials prone to crystallization (such as sodium carbonate solution and feedstock solutions where crystallization mainly involves boron-magnesium double salts), measures are taken to increase the flow velocity to 3.0–4.0 m/s. However, higher flow velocity increases flow noise and pipeline vibrations, which can impact measurement accuracy. To mitigate this, vibration dampers should be installed before and after the flow meter. The meter’s full-scale range should exceed the expected maximum flow rate, typically by 1.2 times. The normal operating flow rate should be at least 50% of the meter’s full-scale range to ensure accurate measurement.
3. Electrode and Lining Selection
For media that do not cause crystallization, caking, or scaling on the electrodes, standard electrodes should be used. Otherwise, scraper electrodes are recommended. In salt lake chemical processes, except for solutions like soda ash, feedstock solutions (boron-magnesium double salt crystallization), and washing liquids that require scraper electrodes, standard electrodes are used for other applications. Since most workshop pipelines are made of plastic insulation materials, two-electrode flow meters must be equipped with grounding rings to ensure measurement accuracy. However, conventional grounding rings are prone to corrosion, while corrosion-resistant titanium or tantalum grounding rings are costly. To balance accuracy and cost-effectiveness while ensuring easy installation and maintenance, three-electrode magnetic inductive flow meters are preferred.
4. Flow Meter Type Selection
Magnetic inductive flow meters come in two types: integrated and remote (split-type). In Qinghai, winter temperatures can drop to -30°C, and many flow meters are installed outdoors. In lithium and boron production systems, the magnesium-lithium separation, filtration, and washing processes occur in high-humidity environments, while refining, evaporation, and conversion processes involve temperatures as high as 90°C. Under conditions of high humidity, extreme temperatures, or vibration sources, remote flow meters should be used to protect measuring components and ensure measurement accuracy and meter longevity. However, the signal cable between the sensor and transmitter should not be excessively long and must use the manufacturer’s specialized cable to prevent measurement errors. For other conditions, integrated flow meters are the preferred choice.
5. Pressure and Temperature Considerations
The actual working pressure of the fluid in the pipeline must be lower than the rated pressure of the flow meter. Additionally, potential negative pressure conditions (such as in evaporation processes with vacuum levels up to -60 kPa) should be communicated with the manufacturer during ordering to ensure appropriate lining material selection. The actual operating temperature should also comply with the flow meter’s specifications.
6. Power Supply Selection
The power supply for magnetic inductive flow meters should be chosen based on the meter’s requirements and the site environment. If site conditions allow, the power supply should be selected for convenience, safety, and ease of maintenance. As salt lake chemical enterprises operate with over 90% of their raw materials and process stages in liquid phase, and workshop humidity is relatively high with no flammable gases present, a 24V DC power supply is the safer option. For dry outdoor environments, an AC 220V power supply is preferable due to easier wiring and lower maintenance costs. In explosion-proof areas, such as gas boiler rooms, battery-powered electromagnetic flow meters are the ideal choice.
7. Protection Level Selection
The protection level of the magnetic inductive flow meter should be selected based on actual conditions. If the sensor is installed underground and frequently exposed to water, an IP68-rated meter should be used. For sensors installed above ground, IP65 or IP67 protection is sufficient. The display unit of the flow meter should never be submerged or exposed to water, as this could damage the instrument. In salt lake chemical production, where workshop humidity is high, an IP65-rated flow meter (which protects against water spray) is sufficient to meet measurement requirements.
8. Connection Method Selection
Magnetic inductive flow meters are available with threaded, flanged, and clamp-type connections. In salt lake chemical applications, flanged connections are generally preferred. When selecting a meter, it is essential to ensure that the flanges match the process piping’s engineering pressure and standard requirements.
As an essential industrial flow measurement instrument, proper selection of a magnetic inductive flow meter is critical to achieving its intended role in a DCS control system. A well-chosen meter provides accurate measurement data, reduces labor costs, and enhances production efficiency for enterprises.
