Application Cautions for Magnetic Flowmeters
2019-05-23
Do not operate a magnetic flowmeter near its electrical conductivity limit because the flowmeter can turn off. Provide an allowance for changing composition and operating conditions that can change the electrical conductivity of the liquid.
In typical applications, magnetic flowmeters are sized so that the velocity at maximum flow is approximately 2-3 meters per second. Differential pressure constraints and/or process conditions may preclude application of this general guideline. For example, gravity fed pipes may require a larger magnetic flowmeter to reduce the pressure drop so as to allow the required amount of liquid to pass through the magnetic flowmeter without backing up the piping system. In this application, operating at the same flow rate in the larger flowmeter will result in a lower liquid velocity as compared to the smaller flowmeter.
For slurry service, be sure to size magnetic flowmeters to operate above the velocity at which solids settle (typically 1 ft/sec), in order to avoid filling the pipe with solids that can affect the measurement and potentially stop flow. Magnetic flowmeters for abrasive service are usually sized to operate at low velocity (typically below 3 ft/sec) to reduce wear. In abrasive slurry service, the flowmeter should be operated above the velocity at which solids will settle, despite increased wear. These issues may change the range of the flowmeter, so its size may be different than the size for an equivalent flow of clean water.
The closest technology to Mag that could possibly handle similar applications more cost effectively would be vortex shedding. They can handle light particulate, have a higher pressure drop, lower rangeability and are slightly less accurate.