By Ralf Haut, KROHNE, Inc.
Electromagnetic flowmeters (EMFs) are the leading choice for recording the volume flow of electrically conductive liquids in a wide range of industries, including chemical, pharmaceutical, water/wastewater and food. Like all electrical equipment, EMFs must be grounded for safety reasons. Grounding is usually done by using the conductive non-lined pipe flange, grounding rings, or occasionally with grounding electrodes.
In some applications, however, these standard grounding methods can pose problems. For example, in lines with cathodic corrosion protection or in galvanization plants, voltage is generated between the electrodes and the earth. Or, when using aggressive media in the application, the grounding rings for conventional procedures must usually be manufactured from special materials that are very expensive, which adds significantly to costs when dealing with large nominal widths.
With a new method called virtual reference, also known as virtual grounding, EMFs can be installed in any type of pipeline, without grounding rings or electrodes. The method facilitates the use of less expensive plastic nonconductive piping, which would otherwise require grounding rings or disks that can be quite expensive, reducing the cost savings inherent in plastic piping. It is also ideal for use in chlorine alkali electrolysis plants, where very low voltages and very strong currents cause stray currents on liquids flowing through pipes, which can disturb measurements. Finally, it can be of use in circumstances where fatty substances like emulsions may coat surfaces inside pipes, reducing the conductive connection needed for proper grounding.