Grounding in Electromagnetic Flow Meters

• When should I use grounding rings or grounding electrodes?
• Should I connect the flow meter to the ground?

The purpose of this article is to address some of the basic questions about the installation and grounding of the electromagnetic flowmeter and to address these same problems in the most complicated situation of the applications of the electrolytic process.

Introduction
Proper installation and grounding on electromagnetic flow meters is important for accurate and reliable measurement performance. The AC or DC currents disperse through the fluid or the instrument, they can produce noise signals that, in turn, can interfere with the relatively low flow signals generated in the current modern DC flow meter. Manufacturers provide a variety of elements (grounding strips, grounding electrodes, grounding rings) and instructions for standard grounding of the flowmeter.

There are applications in which the user cannot or should not make use of the traditional earthing connection to the adjacent piping and to the earthing. These flow measurement applications are often found in electrolytic processes. In this case, the fluid that passes through the flow meter of the flowmeter can have a significantly greater or lesser potential than the ground and a connection to the ground can be detrimental to the performance and even the reliability of the flowmeter. These applications are typically composed of the use of non-conductive or coated tubes and may have acidic or caustic fluxes that may require the use of expensive electrodes and grounding rings such as titanium, platinum or tantalum.

For the remainder of this article, the term "grounding" will be defined as: the arrangement of metallic process materials (pipes, grounding rings, grounding electrodes), cabling (grounding strips, ground wires) and connections to stable references, but not always ground) necessary to achieve satisfactory operation of an electromagnetic flowmeter. As such, it applies to the instrumentation aspect of the ground, not to the "safety ground". However, the "safety ground" issue will also be addressed for these specific applications.

• Principle of Operation of the Electromagnetic Flowmeter
The principle of operation of the electromagnetic flowmeter is based on Faraday's law of electromagnetic induction, which states that a voltage will be induced in a conductor that moves through a magnetic field.

Faraday's Law: E = kBDV

The magnitude of the induced voltage E is directly proportional to the speed of the conductor V, the width of the conductor D and the intensity of the magnetic field B. The secondary (or transmitter) supplies the controlled current to the coils to generate the magnetic field, and amplifies, filters and converts the resulting signal to user outputs, such as 4 - 20 mA, frequency or digital communication information (HART, Profibus). In the modern pulsed Electromagnetic Flowmeter (DC), the signal generated is really very small: of the order of 100 uV per foot per second of flow rate (300 uV per meter per second of speed). Since this signal is so small, designers and users of the electromagnetic flowmeter should take steps to minimize noise and maximize noise rejection. One of these steps is the proper grounding of the system.

• Basic Grounding
Establishing a process is one of the most important details of the installation. Proper grounding of the process ensures that the flow tube and fluid are at the same potential, so that only the induced flow signal is measured. Why is this necessary? To answer that question, let's look at how the magnetometer flow tube and the transmitter are electrically connected (Figure A.). In a typical pulsed alternating current magnetic meter, the flow signal is connected to a differential amplifier that is electrically isolated from the transmitter case. The grounding process provides a stable reference for this differential amplifier. In most applications, the best and most stable reference is grounding itself. By connecting the flowmeter tube, the fluid and the reference to the amplifier to a stable and noise-free reference point, the user is guaranteed to get the best performance from their electromagnetic flowmeter.

FIGURE A - ELECTRICAL CONNECTION BETWEEN METER AND TRANSMITTER TUBE

The grounding arrangement is mainly determined by the type of pipe in which the flowmeter is installed. The recommended grounding arrangements for uncoated, coated and non-conductive conductive tubes are shown in the Figures below

• Grounding rings vs. grounding electrodes
As shown in the previous figures, grounding rings or grounding electrodes are required when the tubing adjacent to the flowmeter does not provide a good electrical connection to the fluid; that is, the tube is coated or made of non-conductive material. Grounding rings or grounding electrodes provide this electrical connection. Grounding electrodes are integrated into the flow tube, so installation is easier and less expensive, particularly when "exotic" materials are needed. Grounding rings provide a greater surface area connection to the process fluid and limit the effects of the conductivity of the adjacent piping, which is important for wafer flow tubes. Therefore, grounding rings are recommended in relation to grounding electrodes in the following situations:
* The conductivity of the fluid is less than 50 uS / cm
* Wafer type flow meters installed in non-conductive or coated piping
* Applications of the electrolytic process (described in the next section).

• Safety grounding
To avoid risks to operating personnel, electrical equipment must always be installed and wired in accordance with the local electrical code. For AC powered equipment, this usually takes the form of connecting the equipment box to the safety ground. This is usually done by connecting the grounded green wire to the grounding terminal provided in the wiring area of ??the flowmeter transmitter. If the transmitter is integrally mounted on the flow tube, it will also automatically connect the flow tube to ground.

Electrolytic process applications
In typical electrolytic processes, flow meters are used to measure the flow of supply fluids in the cell (s). Large DC currents (1000 amps or more) are fed into the cells to conduct the electrolytic process. The resulting liquids and gases can also be monitored by additional flow meters. The electrolytic process can occur in a reactor or in many cells. In the latter case, each cell can have its own magnetic flow meter for measuring raw material flow.

Whatever the layout, the large voltages and currents present can cause currents to flow in unexpected ways. Current interest flows here are generally of two types:
• Current flow in the fluid through the magnetic flowmeter
• Current flow through grounding components
Both types of chain can be present in a typical application of these types. In the first case, the current flow in the fluid that passes through the flow tube generates noise that can interfere with the low level flow signal. Tests at Rosemount indicate that this noise varies with the current level and has components that can easily interfere with the flow signal. The result is generally not an inaccurate flow measurement, but an unstable flow measurement that can make control difficult or impossible. In this situation, the grounding rings provide a way to deflect the current around the fluid in the flowmeter.

Current flow through the grounding components can occur if:
1) multiple meters are used in a system;
2) they are at different potentials;
3) the grounding components for multiple meters are connected to a common point. An example is shown in the figure below. The most common common point can be through the grounded green wire. Situations like this have resulted in high corrosion of the grounding components, up to and including the loss of seal around the grounding electrodes. In addition, the current through grounding components generates noise that can result in an unstable current magnetic meter output.