How Magnetic Flowmeters Work
Magnetic flowmeters use Faraday’s Law of Electromagnetic Induction to determine the flow of liquid in a pipe. In a magnetic flowmeter, a magnetic field is generated and channeled into the liquid flowing through the pipe. Following Faraday’s Law, flow of a conductive liquid through the magnetic field will cause a voltage signal to be sensed by electrodes located on the flow tube walls. When the fluid moves faster, more voltage is generated. Faraday’s Law states that the voltage generated is proportional to the movement of the flowing liquid. The electronic transmitter processes the voltage signal to determine liquid flow.
In contrast with many other flowmeter technologies, magnetic flowmeter technology produces signals that are linear with flow. As such, the turndown associated with magnetic flowmeters can approach 20:1 or better without sacrificing accuracy.
How to Use Magnetic Flowmeters
Magnetic flowmeters measure the velocity of conductive liquids in pipes, such as water, acids, caustic, and slurries. Magnetic flowmeters can measure properly when the electrical conductivity of the liquid is greater than approximately 5?S/cm. Be careful because using magnetic flowmeters on fluids with low conductivity, such as de-ionized water, boiler feed water, or hydrocarbons, can cause the flowmeter to turn off and measure zero flow.
This flowmeter does not obstruct flow, so it can be applied to clean, sanitary, dirty, corrosive and abrasive liquids. Magnetic flowmeters can be applied to the flow of liquids that are conductive, so hydrocarbons and gases cannot be measured with this technology due to their non-conductive nature and gaseous state respectively.
Magnetic flowmeters do not require much upstream and downstream straight run so they can be installed in relatively short meter runs. Magnetic flowmeters typically require 3-5 diameters of upstream straight run and 0-3 diameters of downstream straight run measured from the plane of the magnetic flowmeter electrodes.
Applications for dirty liquids are found in the water, wastewater, mining, mineral processing, power, pulp and paper, and chemicals industries. Water and wastewater applications include custody transfer of liquids in force mains between water/wastewater districts. Magnetic flowmeters are used in water treatment plants to measure treated and untreated sewage, process water, water and chemicals. Mining and mineral process industry applications include process water and process slurry flows and heavy media flows.
With proper attention to materials of construction, the flow of highly corrosive liquids (such as acid and caustic) and abrasive slurries can be measured. Corrosive liquid applications are commonly found in the chemical industry processes, and in chemical feed systems used in most industries. Slurry applications are commonly found in the mining, mineral processing, pulp and paper, and waste water industries.
Magnetic flowmeters are often used where the liquid is fed using gravity. Be sure that the orientation of the flowmeter is such that the flowmeter is completely filled with liquid. Failure to ensure that the flowmeter is completely filled with liquid can significantly affect the flow measurement.
Be especially careful when operating magnetic flowmeters in vacuum service because some magnetic flowmeter liners can collapse and be sucked into the pipeline in vacuum service --- catastrophically damaging the flowmeter. Note that vacuum conditions can occur in pipes that seemingly are not exposed to vacuum service such as pipes in which a gas can condense (often under abnormal conditions). Similarly, excessive temperature in magnetic flowmeters (even briefly under abnormal conditions) can result in permanent flowmeter damage.
Application Cautions for Magnetic Flowmeters
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.