Virtual Reference Grounding for Electromagnetic Flowmeters

Thu, 12/15/2011 - 12:21pm

KrohneElectromagnetic 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 conductive non-lined pipe flange, grounding rings or occasionally 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 discs 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.

Grounding Needed to Ensure Electrical Isolation

The EMF’s basic measuring principle relies on Faraday’s Law of Induction, which requires that the inner wall of the measuring tube be electrically isolated. That is why most EMF measuring tubes for chemical applications are lined with polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA) or polypropylene (PP), or made entirely of ceramic.

EMFs must be grounded in accordance with safety regulations to ensure protection against contact and prevent electric shock. This guarantees that, in the event of an error, there is no hazardous voltage to the conductive parts of the device. In addition, 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.

Grounding Methods

There are several grounding methods available. The three standard grounding methods are the conductive non-lined pipe flange, grounding rings or grounding electrodes. In addition, there is the virtual reference method, which is done without separate grounding of the medium.

Grounding in pipelines that are electrically conductive — non-lined pipe flange

This is the simplest grounding method, used in pipelines that are electrically conductive on the inside (for example blank steel or stainless steel). The liquid in the pipe has the same potential as the grounded pipe. The signal voltage on the electrodes thus has a fixed reference potential.

Grounding in electrically non-conducting pipelines — grounding ring or discs

For ceramic, plastic or concrete pipelines, and for lines isolated on the inside, the product is brought to a known fixed potential, usually by the use of metal grounding rings (grounding discs). The ring is in conductive contact with the product and usually jointly grounded with the sensor.

When assembling the pipeline and the EMF flanges, two additional gaskets are usually necessary. Groundings rings and gaskets must not disturb the flow profile at the measuring point. Careful selection and assembly of grounding rings and gaskets prevent leaks.

This method is technically reliable and has been proven for many decades. Disadvantages include higher costs when special materials are needed for aggressive media or in the case of large pipe sizes. There are some instances in which the cost of the grounding rings exceeds that of the EMF itself.

In addition, when there are electrical potential differences, stray currents occur between the product and the earth via the grounding rings and the grounding cable. The grounding rings can be destroyed as a result of electrochemical reactions with the product. Since they can be expensive, this solution has significant cost implications.

Grounding with grounding electrodes

With this method, the grounding electrode is situated at the invert of the pipe and is in direct contact with the housing that is connected to the functional earth (FE) of the EMF sensor. Often the cost of this additional grounding electrode is less than grounding rings. In the event of electrical potential differences in the plant, these grounding electrodes can be destroyed by electrolytic action, resulting in leakage or destruction of the whole EMF, however.

Abrasive solids in horizontal pipelines can also quickly destroy these grounding electrodes. In some cases, deposits on the grounding electrode can prevent the proper function of the product grounding, thus also inhibiting correct measuring results.

In the case of large EMFs with grounding electrodes, significant deviations also occur when — as is often the case — the EMF was calibrated in an electrically conductive pipeline and then used in an isolated pipeline.

Virtual reference — the alternative to classical grounding methods

With virtual reference (virtual grounding), 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 patented method is used to create a voltage that corresponds to the ungrounded liquid’s potential. 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.

This method has several advantages: For one thing, there is no need for any grounding method to come in contact with the product. The elimination of grounding rings and simpler EMF installation results in lower costs. This advantage should not be underestimated, as faulty grounding is the most common cause of error when commissioning an EMF.

Moreover, there is no risk of electrolytic destruction when there are potential differences in the system, such as when using grounding electrodes. No stray currents flow over the product or grounding lines. Ungrounded use is also possible where voltage and current are applied to the pipeline, such as with electrolytic and galvanic treatment. The virtual reference method can be used on pipes with a diameter (nominal width) of DN10 (3/8-inch) and conductivity of greater than or equal to 200 micro Siemens per centimeter (µS/cm).

Virtual Reference in Practice

Andritz AG, headquartered in Austria, uses EMFs for pickling steel, an acid treatment used to clean off the surface and remove coatings resulting from the production process, giving the raw metal parts a better surface finish. The flow of mixed acids, consisting of hydrofluoric acid, nitric acid and water, is measured. At 90°C and a pressure of 3 BAR (194 F, 43 PSI), the acid flows at a speed of approximately 1.5 meters per second (5 feet per second).

The extremely corrosive fluid would ordinarily have required corrosion-resistant rings made of tantalum. These rings are exceptionally expensive, costing about 1.5 times more than the price of the EMF instrument itself, so the company was interested in using another, less expensive, grounding method.

Andritz opted to use OPTIFLUX 4300 EMFs, in sizes of DN10 to 300 (3/8 to 12 inches), manufactured by KROHNE. The OPTIFLUX 4300 uses virtual reference, a method patented by KROHNE.

According to automation engineer Helmut Platzer, there are many advantages to using virtual reference: “Without this virtual reference electrode, grounding rings would have to be used. The rings must be made of different materials for different media, which would make it easy to confuse them during installation.” He expressed concern that this could result in problems, because the chemical resistance is uncertain. He adds, “Also, these grounding rings can be very expensive, so not using them results in a significant reduction of cost.”

Switching from a built-in reference electrode to a virtual electrode was technically easy. Another factor in favor of virtual reference is that Andritz is using small-sized pipes, where an external reference electrode could not be mounted.

“Virtual reference performed well in every instrument in which we’ve installed it. There have not even been problems with difficult applications, such as the mixture of two acids with different temperatures right before the EMF,” says Platzer. The use of OPTIFLUX EMFs with virtual reference made device installation faster, and it reduces costs in many ways.

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