A correctly mounted and grounded magnetic flow meter is essential for accurate and reliable measurement performance. The current will produce a noisy signal is stray AC or DC through the fluid or instrument, which will interfere with the relatively low flow signals produced by today’s modern pulsed DC magnetic meters.
The manufacturer provides a variety of components and directions for the standard earthing of the magnetometer. These components include grounding straps, earthing electrodes, earthing rings, gaskets, and protectors.
The user can’t and shouldn’t use the traditional grounding connection under some process conditions. Electrolytic processes and cathodic protection applications should often encounter in these flow measurements.
In such cases, the magnetometer sensor may pass through the fluid at a potential significantly higher than Earth or pass through the liquid at a potential lower than Earth. The connection to Earth is detrimental to the reliability of the magnetometer. We usually make it more complex when using non-conductive or lined pipelines, and it may have acid or caustic flow.
Magnetic flowmeter Working Principle.
The working principle of magnetic flowmeters is based on Faraday’s law of electromagnetic induction, which states that it is the voltage induced in a conductor moving through a magnetic field.
Law of Faraday: E = kBDV
The magnitude of the induced voltage E is proportional to the velocity V of the conductor, the width D of the conductor, and the strength B of the magnetic field.
To ensure safety for the operator, electrical equipment should permanently be installed and wired following local electrical codes. For AC-powered equipment, we usually take the form of connecting the equipment’s housing to Earth.
We usually connect the grounded green wire to the magnetometer transmitter and then to the grounding lug provided in the wiring area of the transmitter. Then, an integrated transmitter installation on the sensor, and the sensor is automatically earthed.
Under the standard conditions, the magnetic flowmeter’s adjacent piping will connect to Earth ground. The transmitter usually connects to the Earth’s environment by power.
Establishing the Earth’s ground connections is essential, and there is a very low impedance between them. However, there is a very high impedance between these earth-ground connections, resulting in a high current by the coil shield wire.
Basic Process Grounding
It is vital to establish a ground process, one of the most installation details. An electromagnetic flowmeter should have the correct process earth, which ensures that the sensor and the fluid are at the same electrical potential. Therefore the induced flow signal of the electromagnetic flowmeter is measured. Why is this necessary? To answer this question, let’s connect electrically between the magnetic flowmeter sensor and the transmitter.
A differential amplifier will be connected to the flow signal in a classical pulsed DC magnetometer electrically isolated from the transmitter housing. The grounding process offers a steady reference for this differential amplifier.
In the majority of applications, the best and most stable reference is the Earth itself. Therefore, we ensure that users get the best performance from their magnetometer by taking the reference point of the magnetometer sensor, fluid, and amplifier and then connecting it to a stable, noise-free reference point.
FIG. 1. There is an essential electrical connection between the sensor and the transmitter.
The earthing arrangement is mainly decided by the type of pipe the magnetometer is mounted.
Figure 2-4 shows the proposed earthing arrangements for conductive non-lined, conductive-lined, and non-conductive pipes.
Grounding rings and grounding electrodes
The previous diagram shows that a grounding ring or electrode is necessary when the pipe adjacent to the magnetometer does not provide a good electrical connection to the fluid. The pipe is lined or made of a non-conductive material.
The grounding ring or ground electrode offers the necessary electrical connection to the process fluid. This makes the grounding electrode an integral part of the sensor because of its easy installation and low cost.
The grounding ring offers a larger surface area for connection to the process fluid. In addition, it limits the influence of the electrical conductivity of adjacent pipes, which is vital for wafer-based sensors. Therefore, we suggest grounding rings instead of grounding electrodes in the following cases.
Fluid electrical conductivity < 100uS/cm.
Wafer-type sensors mounted in non-conductive pipes or inlined pipes.
Applications of Electrolytic Process
In a classic electrolytic process, a magnetometer is used to measure the flow rate of the feed liquid into the cell. A high direct current flow (1000 amps or more) is sent into the cell to drive the electrolytic process. The resulting liquid and gas can also be measured by additional flow meters.
The electrolysis process might occur in a single reactor or many units. Each team could have its magnetic gauge to measure the feedstock flow in the latter case. Whatever the arrangement, the large voltages and high currents may result in currents flowing in unexpected ways. The current flow of interest here is often of two kinds.
- Current flow in the fluid through the magnetic flowmeter.
- Current flowing through an earthed component.
Both current types may be present in a classic application of these types. When the flow of current in the liquid passing by the sensor creates noise, it may disturb the low-level flow signal. Testing has shown that this noise changes with the current level and has elements that interfere with the flow signal.
The result is not an accurate flow measurement but an uncertain flow measurement that can make control difficult or impossible. In this case, the grounding ring offers a path to divert the current around the fluid in the magnetometer.
A current may flow through the earth element in the following cases
- more than one magnetometer is used in a system.
- They are at various electrical potentials.
- The earthing elements of several magnetometers are tied to a common point. The most common issue is probably the green wire through the Earth. Such a situation has led to a high degree of corrosion of the earthing element, even including loss of sealing around the earthing electrode. In addition, the current through the grounding element can make noise and lead to unbalanced output from the magnetometer.
Suggestions for the application of electrolytic processes
- To be isolated from the ground. The magnetometer should use an electrical connection to the process in these cases. Because the magnetometer is not earthed, an isolated DC supply (~24 VDC) should be used to avert safety problems. If the electromagnetic flowmeter does not have an isolated DC supply, the transmitter will be installed remotely from the sensor. We always follow all applicable national and local safety regulations.
- We use a ground ring or liner protector rather than a ground electrode. A grounding ring will require more surface exposure to the process; it will produce less noise and provide a more stable reference for the amplifier if the current flows. We use two grounding rings, giving a path to divert the wind around the fluid in the magnetometer.
Correct earthing of the magnetometer is one of the essential mounting details. Earthing arrangements for classic applications are discussed, including advice on using earthing electrodes with earthing rings.
Different earthing arrangements are required when magnetometers are mounted in electrolytic processes and cathodic protection applications, as traditional earthing connections can cause problems. Therefore, electrolytic processes and cathodic protective applications bring special safety considerations to all facilities, not only magnetic flow meters.