Understand the Attraction of Magmeters

By David W. Spitzer

Magnetic flowmeters are among the most versatile of flowmeter technologies. These meters measure liquid velocity, from which the volumetric flow rate is inferred. The measurement is linear with liquid velocity and exhibits a relatively large turndown. In addition, the range of accurate flow measurement is relatively large and easy to change after installation.

Straight-run requirements are relatively short, so magnetic flowmeter technology can be applied where limited straight run is available. In addition, the technology has no Reynolds number constraints, so it can be used for liquids with high or varying viscosity. However, liquid electrical conductivity constraints must be satisfied for these flowmeters to function.

The only wetted parts of the flowmeters are the liner and electrodes, both of which can be made from materials that can withstand corrosion. This makes the flowmeters suitable for use in chemical plants where corrosion may be a concern. Two-wire magnetic flowmeters are available that do not require power wiring. These can replace an existing flowmeter using the existing conduit or wiring with little or no electrical rework.

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Principle of operation
Magnetic flowmeters use Faraday's Law of electromagnetic induction to determine the velocity of a liquid flowing through a pipe. Following Faraday's Law, flow of a conductive liquid through a magnetic field will generate a voltage signal. This signal is sensed by electrodes located on the flow tube walls. When the coils are located externally, a non-conductive liner is installed inside the pipe to electrically isolate the electrodes and prevent the signal from being shorted. For similar reasons, non-conductive materials are used to isolate the electrodes for internal coil designs.

The fluid itself is the conductor that will move through the magnetic field and generate a voltage signal at the electrodes. When the fluid moves faster, it generates more voltage. Faraday's Law states that the voltage generated is proportional to the movement of the flowing liquid. The transmitter processes the voltage signal to determine liquid flow.

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Selection factors
Many factors must be considered when selecting a flowmeter, including the ambient conditions to which the flowmeter primary and transmitter will be exposed. For the most part, the ambient temperature rating of the flowmeter primary is higher than that of the transmitter and does not limit applicability. Many primary and transmitter enclosures that are rated for NEMA 4X or IP67 provide adequate protection against ambient humidity and precipitation encountered in outdoor installations.

Operating conditions inside the pipe include pressure, temperature and liquid conductivity. In addition, the liquid can be corrosive or abrasive. These conditions are typically addressed using appropriate mechanical design and material selection. Pressure requirements are addressed by appropriate design of the flow tube for the application. One supplier makes a specially designed magnetic flowmeter that can withstand 1,500 to
2,000 bar (more than 20,000 psi).

Many primaries are available with polytetrafluoroethylene (PTFE) or perfluoroalkoxy (PFA) liners that are rated to about 266 Degrees F and 356 Degrees F
(130 Degrees C and 180 Degrees C), respectively. Less expensive liners rated to lower temperatures are often available to handle less demanding applications. Appropriate electrode and liner material selection can reduce the effects of corrosion and abrasion. Take care when using ceramic liners because they can shatter when temperature gradient constraints are exceeded.

Whereas the conductivity of the liquid in a typical magnetic flowmeter must be maintained above about 5 mixro-Siemens/cm (micro-S/cm), special low-conductivity designs are available that operate as low as about 0.01
micro-S/cm. Some flowmeters require more than 50 micro-S/cm, however, they are low-cost units that are often applied to water or wastewater service where this conductivity is usually not a constraint.

The amount of straight-run pipe required to achieve the stated accuracy of the flowmeter is a reflection of the quality of the design and the tightness of the accuracy specification. In many applications, these flowmeters will function accurately with about three nominal pipe diameters upstream and two nominal pipe diameters downstream of the electrode.

Magnetic flowmeter operation requires good electrical connections between the electrodes and the liquid. The quality of this connection can degrade if an electrode becomes coated or corroded; this can compromise AC flowmeter accuracy by shifting the zero, and may cause the flowmeter to fail to operate. The advent of DC-pulse excitation transmitters reduced much of the need to address this issue. In addition, some manufacturers have designed their transmitters to exhibit a relatively high input impedance to help decrease the effects of connection quality.

Magnetic flowmeter coils can use and store significant amounts of energy relative to the amount of energy needed to cause ignition. Most magnetic flowmeter transmitters are designed to be non-incendive, so normal transmitter operation will not cause ignition. However, when installed in some hazardous locations, formal approval is required, and the transmitter must be designed and installed to address the hazard.

A hazard may be present not only in the general location of the primary and transmitter, but also inside the pipe where the electrodes can provide a source of ignition. To mitigate this hazard, the circuits of some designs limit the energy available at the electrodes to an amount less than that required for ignition.