Flow tutorial: Magnetic flowmeters

07/19/2007


In our last issue, we discussed turbine flowmeters because they had tied with magnetic flowmeters as the most commonly used in a recent Control Engineering product research survey . This month we look at magnetic flowmeters and examine what makes them so popular.

Magnetic flowmeters work on the principal that a conductor moving through a magnetic field generates a current in proportion to the speed at which it moves. In this case, the wire is replaced by the fluid which moves through magnetic coils on opposite sides of the pipe. Knowing the principal is not as critical as knowing how it can be put to work for you. The approach is popular for many practical reasons:

  • No moving parts;

  • Most designs do not obstruct the flow;

  • Can be bi-directional;

  • Unaffected by pressure, viscosity, or fluid density;

  • Can be very precise (although this varies by design);

  • Can operate with aggressive liquids;

  • Is relatively compact; and

  • Is available from a wide range of manufacturers.

There are some restrictions to their use:

  • The process liquid must be conductive (verify value with supplier);

  • Cannot measure gasses or two-phase flow;

  • Partially filled pipes are a problem;

  • Should have straight pipe sections up and downstream;

  • Measurement is based on velocity, not mass; and

  • Can be fouled by solids deposits.

Bear in mind that these comments are generalities. Review specifics carefully with your prospective suppliers.

Most designs do not obstruct the liquid flow, but there are insertion type designs that are useful for specific applications. Their insert is generally small and does not cause a major obstruction, however.

Magnetic sensors are typically constructed using stainless steel and plastic components for electrical insulation purposes, so make sure these are compatible with your process fluids and temperatures. Failure of the plastic component can allow liquid to get where it shouldn’t.

Accuracy varies, often with price, and new series are available that have accuracies competitive with more complex technologies. Magnetic sensors can now compete with more expensive Coriolis technologies under the right conditions.

A unit’s turndown ratio is determined by its ability to read low velocities. This varies between manufacturers. At the high end, units can frequently handle velocities of 30 ft/s. While keeping the liquid moving helps prevent solids deposits, velocities higher than 15 ft/s will cause premature wear in the sensor as well as supporting piping.

AC and DC powered units are available. Each type has its particular capabilities in more complex applications.
The ability to tolerate small amounts of bubbles or entrained air varies by design, so if this is a factor of your application, discuss it with your provider.

When installed, units need grounding, which can involve electrical jumpers to surrounding piping, etc.

Here is a selection of suppliers who offer magnetic flowmeters:

ABB
Dynasonics
Endress+Hauser
Foxboro
Honeywell

McCrometer
Rosemount (Emerson)
Siemens
Yamatake

Yokogawa

You can also search online at the Control Engineering Supplier Search.

—Peter Welander, process industries editor, PWelander@cfemedia.com , Control Engineering Weekly News





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