Magnetic flowmeters apply Faraday’s Law of Electromagnetic Induction to measure fluid movement through a pipe. A magnetic flowmeter produces and directs a magnetic field into the fluid in the pipe. Using Faraday’s Law, a voltage signal can be detected using electrodes on the walls of a flow tube while a conductive liquid moves through a magnetic field.
More voltage is produced when the fluid velocity increases. According to Faraday’s Law, the voltage produced is directly related to the liquid’s speed. The electronic transmitter analyses the voltage signal to calculate the velocity of the flowing liquid. Learn the benefits of measuring flow rates and how to determine the flow rate using an electromagnetic flow meter.
Tips for Using Magnetic Flowmeters
Pipes carrying conductive liquids like water, acids, caustics, and slurries can measure their flow rates with magnetic flowmeters. When the liquid’s electrical conductivity is more than about 5 S/cm, a magnetic flowmeter will provide accurate readings. Deionized water, boiler feed water, and hydrocarbons all have low conductivities, thus it’s important to be cautious using a magnetic flowmeter to assess their flow rates.
Because it does not restrict fluid movement, this flowmeter can be used with any liquid, including clean, unclean, dirty, corrosive, or abrasive. Since hydrocarbons and gases are both non-conductive and in a gaseous form, magnetic flowmeters can only monitor the flow of non-gaseous liquids.
Because Electromagnetic flow meter don’t need a long straight line upstream or downstream, they can be put in locations with limited piping length. Straight runs upstream of the magnetic flowmeter electrodes should be between three and five diameters, while downstream should be between zero and three.
Magnetic Flowmeter Applications
Never push an electromagnetic flow meter to the edge of its electrical conductivity range, as doing so could cause the device to shut down. Allow for the possibility that the liquid’s electrical conductivity will vary as a function of its composition and operating conditions.
Typically, the greatest flow rate measured by an electromagnetic flow meter is two to three meters per second. Limitations on differential pressure and/or other process conditions may prevent this basic rule from being implemented. For instance, a larger magnetic flowmeter may be necessary for gravity-fed pipes to prevent the liquid from backing up the piping system as it passes through the magnetic flowmeter. In this scenario, the larger flowmeter will have a lower liquid velocity when running at the same flow rate as the smaller flowmeter.
Selecting a Magnetic Flow Metre
As long as the liquid or slurry’s conductivity is greater than the minimum necessary for the design, a magnetic flowmeter can detect its flow of it, regardless of whether it is clean, multi-phase, unclean, corrosive, erosive, or viscous. The best designs typically have an estimated accuracy and rangeability of 0.2-1% of the rate throughout a range of 10:1 to 30:1 if the flow velocity is greater than 1 ft/sec. The measurement error grows at decreasing flow velocities (below 0.1 ft/s), yet the findings are still repeatable.
The process fluid needs to have consistent conductivity. Suppose one of the blended fluids has a considerably different conductivity from the other process fluid. In that case, the two fluids must be thoroughly mixed before being introduced to the mag meter. The output signal quality will suffer if the blend is inconsistent. To avoid this, a static mixer can be placed upstream of the mag meter to minimize areas of variable conductivity.
Electromagnetic Flow Metre Setup
There must always be fluid in the magnetic flowmeter. Therefore, magnetometers should ideally be placed along vertical upward flow lines. If the pipe section is at a low place and the electrodes are not at the top of the pipe, then horizontal installations are fine. By doing so, the air is kept from touching the electrodes. To prevent solids from settling and coating the internals of the mag meter during long periods of shutdown, it is best to remove the mag meter from its low-mounted location when the process fluid is a slurry.
To keep an electromagnetic flow meter accurate and reduce liner wear, five diameters of straight pipe are needed upstream, and two diameters are needed downstream. The cutting edge of the liners can be shielded against wear caused by process fluids by using a liner protector. When installing a mag meter in a horizontal pipe longer than 30 feet, it is recommended that the pipe be braced on both sides of the meter.