Like all electrical equipment, electromagnetic flowmeters (EMF) must be grounded in accordance with safety regulations, e.g. protective grounding or potential equalization. Firstly, EMF grounding ensures protection against contact and prevents electric shock. Hence in the event of an error there is no hazardous voltage to the conductive parts of the device. Secondly, grounding provides a fixed reference potential to the EMF signal voltage.
This EMF signal voltage is typically about a millivolt or less. The converter can only process such small signals without interference and with maximum resolution provided there is not a great difference between the potential (the voltage) of the medium and the reference potential of the signal processing in the converter. There are several methods to ensure this. In addition to the three classical methods of grounding – grounding via a blank metal pipe, grounding via grounding rings in electrically non conducting pipelines and grounding via grounding electrodes – there is the method of virtual reference, which is done without separate grounding of the medium.
The alternative to classical grounding
In certain applications, the conventional grounding methods of EMFs pose problems, e.g. in lines with cathodic corrosion protection or in galvanisation plants voltage is also present between the electrodes and the earth. When using aggressive media in the application, the grounding rings for conventional grounding must usually be manufactured from expensive special material (Tantalum, Titanium, Nickel, etc.) which amounts to extremely high costs when dealing with large pipe sizes.
With the virtual reference, the EMF sensor can be installed in any type of pipeline without grounding rings or electrodes. The converter's input amplifier records the potentials of the measuring electrodes and a sophisticated method is used to create a voltage which corresponds to the potential of the ungrounded liquid. This voltage is used as the reference potential for signal processing. Thus, there are no interfering potential differences between the reference potential and the voltage on the measurement electrodes.
Cost and safety benefits
The elimination of grounding rings and the simpler installation of the EMFs results in lower costs and help operators reduce systematic failures as faulty grounding is the most common cause of error when commissioning an EMF. There is no risk of electrolytic destruction when there are potential differences in the system. No stray currents flow over the product or grounding lines.
The virtual reference is basically possible from a diameter (nominal width) of DN10 (3/8”) and from a conductivity of ≥ 200 µS/cm. KROHNE invented this method for the virtual referencing of EMFs back in 1998.
Benefits of KROHNE virtual reference
Significantly lower investment costs
No grounding rings or grounding electrodes required
Increased safety by reducing the number of potential leakage points
No grounding rings with additional sealing points or grounding electrodes as potential leakage points
No risk of electrolytic destruction when there are potential differences in the system
No equalising currents
Reference potential is generated in signal converter and is insulated against earth: No current runs through the pipeline, the process liquid or the earth
No equalising current in electrolysis or galvanic systems and no stress on the cathodic protection
Simplified installation, engineering & maintenance
Reduces the risk of wrong installation of grounding rings and gaskets as one of the most common reasons for faulty installations, especially in case of large diameters
Reduced installation time
Right selection of grounding ring material no longer necessary
No chemical compatibility issues
Compatible with Custody Transfer applications: OIML-R49 and MID MI-001 certified