Slurry electromagnetic flowmeters

Electromagnetic flowmeters can be applied only to solid/liquid slurries that contain electrical conducting liquids. A common problem of EM flowmeters is their temperature dependence for magnet stability. Thermal flowmeters, which measure the heating power that is required to maintain a constant temperature difference between two points along the stream, have not been successful in multiphase streams because varying composition changes the thermal conductivity of the flow rate. 

Electromagnetic flowmeters are mainly applied to single-phase conducting fluids, for example liquid metals, water-based industrial liquids, and blood. However, they are also being used to measure solid/liquid flows such as water-based slurries and sludge as long as conductivity of the liquid medium exceeds 50 microsiemens per centimeter (pS/cm). Output signals from an EM flowmeter are basically proportional to the volumetric flow rate, which is applicable to solid/liquid flows only if the solids particles are uniformly distributed in the liquid. Moreover, to obtain the mass flow rate of a solid/liquid flow, the EM flowmeter requires other means to measure solids concentration. 

Bulk Velocity. Electromagnetic Flowmeters. An electromagnetic flowmeter is based on the principle of electromagnetic induction as expressed by Faraday s law. An electric current is induced in a conductor when it moves across a magnetic field. The flowmeter consists of a pair of electrodes placed diametrically opposite one another on the same section of pipe (Figure 23). A conductive slurry flowing through the... 

For slurry systems, electromagnetic flowmeters are often installed in a vertical pipe to reduce the risk of solids deposition on one of the electrodes and to avoid inaccuracies caused by asymmetric velocity profiles encountered in horizontal pipes. However, if horizontal mounting is unavoidable, then the meter should never be installed with the electrodes at the ends of a vertical diameter, because the electrode at the top of the pipe would be affected by the occasional air bubble passing (24). 

Electromagnetic flowmeters (Fig. 18.14) are useful for slow moving flows of liquids, sludges, or slurries. The flow material must support electrical conduction between the electrodes, and so in some cases it is necessary to ionize the flow upstream from the measurement point in order to use an electromagnetic flowmeter. 

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