Flow Measurement

Magnetic flowmeter

Magnetic flowmeters are velocity type of flowmeters. An electromagnetic flowmeter can work with almost all liquids and slurries, as long as the liquid being measured is electrically conductive. Electromagnetic flowmeters can be employed to measure more or less all electrically conducting fluids, pastes, slurries, acids, lyes, juices and emulsions including fluids with a conductivity as low as 0.5µS/cm i.e. 0.5µmhos.

Principle:

As in case of many underlying principles of electromagnetic flowmeter is Faraday’s law of electromagnetic induction.

This law state that if a conductor of length l(m) is moving with a velocity v, perpendicular to a magnetic field of flux density B (Tesla), then the induced voltage e, across the end of the conductor can be expressed by

Working:

Electromagnetic flowmeter uses Faraday’s law of electromagnetic induction to determine the flow of a liquid through a pipeline. The magnetic flow meters are also known as magmeters.

Electrically conductive process fluid is passed through a magnetic field induced by coils that are positioned around a section of pipe.

The process fluid is electrically insulated from the pipe with a suitable lining, in the case of a metal pipe, so that the generated voltage is not dissipated through the pipeline. The electrodes are located in the pipe and a voltage is generated across these electrodes that are directly proportional to the average velocity of the liquid passing through the magnetic field.

The coils are energised with ac power or pulsed dc voltage, so consequently the magnetic field and resultant induced voltage respond accordingly. The generated voltage is protected from interference, amplified and converted into a dc current signal by the transmitter. Line voltage variations are accounted for by the sensing
circuits.

The advantages of magnetic flowmeters are that they have no obstructions or restrictions to flow, and therefore no pressure drop and no moving parts to wear out. They can accommodate solids in suspension and have no pressure sensing points to block up. The magnetic flowmeter measures volume rate at the flowing temperature independent of the effects of viscosity, density, pressure or turbulence.

• No restrictions to flow.
• No pressure loss.
• No moving parts.
• Good resistance to erosion.
• Independent of viscosity, density, pressure and turbulence.
• Good accuracy.
• Bi-directional.
• Large range of flow rates and diameters

• Expensive.
• Most require a full pipeline.
• Limited to conductive liquids.

Sivaranjith

Instrumentation Engineer