Electromagnetic Flow Meter
Electromagnetic flow meters are among the most extensively utilized flow meters types today. If conditions are suitable for their use, mag meters deliver excellent results! As they rely on a measurement principle that is non-intrusive and allows no pressure decline. Manas Microsystems, Electromagnetic flow meters may be used on all commercially available pipe dimensions.
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 channelled into the liquid flowing through the pipe. Following Faraday’s Law, the 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. They represent about 23% of all flowmeters sold.
Plusses and Minuses:
- The magnetic flow meter is average in accuracy therefore not commonly used for commodity transfer except for some special cases where the fluid is not expensive like water.
- Can be adapted for sanitary uses.
- They have large line sizes available.
- No pressure drops induced.
- Can work in Dirty liquids and even slurries.
- Magnetic meters are very reliable. On the other hand, don’t work on nonconductive fluids such as oils.
- Steam or gas flows don’t register by them.
- Electrodes can become coated.
How to Use Magnetic Flowmeters or How not to break them!
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 5μS/cm. Be careful because using magnetic flowmeters on fluids with low conductivity, such as demonized water, boiler feed water, or hydrocarbons, can cause the flowmeter to turn off and measure zero flow.
These flowmeters do 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 chemical 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 wastewater 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 filled with liquid. Failure to ensure that the flowmeter is filled with liquid can significantly affect the flow measurement.
You should 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.
Electromagnetic flow meters in Industrial Use:
In order of usage, water/wastewater industry, chemical, food and beverage, oil and gas (although not for oil and gas fluids but in support of the processes), power, pulp and paper, metals and mining, and pharmaceutical.
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 the 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 the 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 the 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 Magnetic flow meters 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 somewhat less accurate.
Magnetic Flow Meter Options:
Manas Microsystems offers a comprehensive line of magnetic flow meters, including:
- General purpose mag meter
- Battery-powered mag meter
- High pressure mag meter
- Mag meter with integrated BTU measurement
- Sanitary grade flow meters
- Insertion type mag meter
Specific advantages that Manas Microsystems make magnetic flow meter technology provides include:
- High accuracy.
- Low cost.
- Little to no pressure loss.
- Readings that are unaffected by changes in density or viscosity.
- No susceptibility to bearing wear or other mechanical wear-and-tear issues.
For more info on our magnetic flow meters series you can download our manuals by visiting following page: