Magnetic Flow Meters

Electromagnetic flow meters, also called mag meters or magnetic meters. Magnetic Flow Meters are volumetric flow meters.

Mag meter is ideal for wastewater applications, or any dirty liquid which is conductive or water based.

Accuracy over a wide flow range can be as good as ± 0.5% of flow rate or better.

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Electromagnetic Flowmeter Technical Guide

Introduction to Magnetic Flow Meters Technology. How They Work & What Is It? Their applicaitons and use.

What are Electromagnetic Flow Meters?

A magnetic flow meter (mag flow meter) is a volumetric flow meter. Electromagnetic Flow Meters do not have any moving parts. Magnetic flow meter is ideal for any dirty liquid which is conductive or water based, like: wastewater applications. Where low-pressure drop and low maintenance are required, Magnetic flowmeters are also idel. Magnetic flow meters are often selected and specified for applications, because they are obstruction-less, cost-effective for aggressive chemicals and slurries. Magnetic flow meters provide highly accurate volumetric flow measurement. Accuracy over a wide flow range can be as good as ± 0.5% of flow rate or better. Magnetic flow meters can be compact type, remote type, and insertion type. A wide range of liner materials, electrode options, and line sizes accommodate a wide variety of process application.

How does a magnetic flow meter work?

Magnetic flow meters use the principle of Faraday’s Law of Electromagnetic Induction to measure the flow rate of liquid in a pipe. In the magnetic flowmeter pipe parts, a magnetic field is generated, and channeled into the liquid flowing through the pipe. Faraday’s Law states that the voltage generated is proportional to the movement of the flowing liquid. A conductor moving through a magnetic field produces an electric signal within the conductor. And the singal is proportional to the velocity of the water moving through the field. As fluid flows through the magnetic field, conductive particles in the fluid create changes. This variation is used to measure and calculate the velocity of water flow through the pipe. When the fluid moves faster, more voltage is generated. The electronic transmitter processes the voltage signal to determine liquid flow.

What is the principle of magnetic flow meter? - Faraday's Formula

Faraday’s Formula
E is proportional to V x B x D where:

E = The voltage generated in a conductor
V = The velocity of the conductor
B = The magnetic field strength
D = The length of the conductor

To apply this principle to flow measurement with a magnetic flow meter. First is to state that the fluid being measured must be electrically conductive for the Faraday principle to apply.

As applied to the design of magnetic flow meters, Faraday’s Law indicates that:

the signal voltage (E) is dependent on V, B, D.

V is the average liquid velocity ;

B is the magnetic field strength ;

D is the length of the conductor ( which in this instance is the distance between the electrodes).

Advantages and Disadvantages of Magnetic flow meters

Advantages of using Electromagnetic Flow Meters

HIGH PRECISION AND STABILITY
SELF CHECKING
BIDIRECTIONAL MEASUREMENT
NO PRESSURE LOSS
NO MOVING PARTS
CAN BE SANITARY USES
LARGE LINE SIZES AVAILABLE
DIRTY LIQUIDS AND EVEN SLURRIES OK

Disadvantages of using Magnetic Flow Meters

ELECTRODES CAN BECOME COATED
CAN NOT BE USED TO MEASURE LIQUID MEDIA WITH LOW OR NO CONDUCTIVITY
WILL BE AFFECTED BY STRONG MAGNETIC FIELDS AROUND
SITLL CAN’T MEASURE HIGH TEMPERATURE AND HIGH PRESSURE FLUID

How to Use Magnetic Flowmeters

The electromagnetic flowmeter consists of two parts: a sensor and a converter.

Magnetic flow meters work Based on Faraday’s law of electromagnetic induction. Magnetic flow meters measure the velocity of conductive.

Magnetic flow meters measure the volumetric flow of conductive liquid with conductivity greater than 5μS/cm.

It is an inductive meter for measuring the volumetric flow of conductive media.

Measuring the volume flow of general conductive liquids,

it can also be used to measure the volumetric flow rate of strong corrosive liquids. Such as strong acid and alkali, and uniform liquid-solid two-phase suspension liquids such as mud, slurry and pulp. Widely used in liquids in pipes, such as water, acids, caustic, and slurries .

Magnetic flow meters do not require much upstream and downstream straight runs. Magnetic flow meters can be installed in relatively short meter runs. Magnetic flow meters typically require:

5-10 diameters of upstream straight run and 5-2 diameters of downstream straight run,

measured from the plane of the magnetic flow meter electrodes.

What is a mag meter used for?

The construction of magnetic flow meters is that the only wetted parts are liner and electrodes. Both the wetted parts can be made from materials that can withstand corrosion. So, magnetic flow meters can measure many corrosive liquids and abrasive slurries.

Magnetic flow meter liners and electrodes can be constructed of materials that do not contaminate the liquid. So Magnetic flow meters can be applied in sanitary applications.

Dirty liquids

Applications for dirty liquids are found in:

the water, wastewater, mining, mineral processing, power, pulp and paper, and chemical industries.

Highly corrosive liquids

With proper attention to materials of construction,

the flow of highly corrosive liquids (such as acid and caustic) and abrasive slurries can be measured.

Corrosive liquid applications are commonly found in the chemical industry processes,

and in chemical feed systems used in most industries.

Slurry

Slurry applications are commonly found in the mining, mineral processing, and wastewater industries.

For slurry service, be sure to size magnetic flow meters to operate above the velocity,

at which solids settle (typically 1.5 – 2 ft/sec),

in order to avoid filling the pipe with solids that can affect the measurement and potentially stop the flow

Liquid is feed using gravity

Magnetic flowmeters are often used where the liquid is fed using gravity. Be sure that the orientation of the flowmeter is such that the flowmeter is completely filled with liquid. Failure to ensure the flowmeter is completely filled with liquid can affect the flow measurement.

Vacuum service

Be especially careful when operating magnetic flowmeters in vacuum service. Because some magnetic flowmeter liners can collapse and be sucked into the pipeline in vacuum service. Note that vacuum conditions can occur in pipes that seemingly are not exposed to vacuum service. Such as pipes in which a gas can condense (often under abnormal conditions). Similarly, excessive temperature in magnetic flowmeters

(even briefly under abnormal conditions) can result in permanent flowmeter damage.

Industries that Use Magnetic Flow meters

Magnetic flow meters are well suited for a variety of applications across a range of industries including:

Pulp and paper;

Metals and mining;

Water and wastewater;

Food and beverage;

Chemical;

Petrochemical;

Oil and gas.

Application Cautions for Magnetic Flowmeters

1. Do not operate a magnetic flowmeter near its electrical conductivity limit. Because the flowmeter can turn off.

2. Provide an allowance for changing composition and operating conditions. This can change the electrical conductivity of the liquid.

3. In typical applications, magnetic flowmeters are sized. So that the velocity at maximum flow is approximately 2-3 meters per second.

4. Gravity fed pipes may require a larger magnetic flowmeter to reduce the pressure drop. So as to allow the required amount of liquid to pass through the magnetic flowmeter without backing up the piping system. Operating at the same flow rate in the larger flowmeter will result in a lower liquid velocity as compared to the smaller flowmeter.

5. For slurry service, be sure to size magnetic flowmeters to operate above the velocity at which solids settle (typically 1 ft/sec). To avoid filling the pipe with solids that can affect the measurement and potentially stop flow.

6. Magnetic flowmeters for abrasive service are usually sized to operate at low velocity (typically below 3 ft/sec) to reduce wear.

Why does a magnetic flow meter need grounding?

The electromagnetic flowmeter signal is weak, only 2.5~8mv at full scale. Only a few microvolts when the flow rate is very small, and slight disturbance will affect the measurement accuracy.

So, the transmitter casing, shielded wire, measuring conduit and transmitter of the pipes. They must be grounded, and the grounding points should be set separately. They must not be connected to common ground lines such as motors and electrical appliances or to the upper and lower water pipes.

The converter part has been grounded through the cable, so do not ground again to avoid interference caused by different ground potentials.

Magnetic Flow Meters Selection

The key questions which need to be answered before selecting a magnetic flow meter are:

Is the fluid conductive or water based?
Is the fluid or slurry abrasive?
Do you require an integral display or remote display?
Do you require an analog output?
What is the minimum and maximum flow rate for the flow meter?
What is the minimum and maximum process pressure?
What is the minimum and maximum process temperature?
Is the fluid chemically compatible with the flow meter wetted parts?
What is the size of the pipe?
Is the pipe always full?

Insertion Magnetic Flow Meters

Insertion Magnetic Flow Meter, also called Insertion type Electromagnetic flow meter. Insertion style flow meter, measures the flow velocity of conductive liquids in large diameter pipes.

Insertion magnetic flow meter, with easy installation, is suitable for use with conductive fluids,

including water, raw sewage, wastewater, clarified water, RAS, and WAS. Available process connections include hot-tapped, DIN and NPT threads

Installation Considerations of Magnetic Flow Meters

Electromagnetic flowmeter installation specification requirements precautions:

1. Where there is no vibration or strong electromagnetic field (such as large motor and inverter);

2. As far as possible vertical installation. Horizontal installation needs to be installed in a low place. Can not install the pipe apex. Ensure full pipe. Prevent air bubbles;

3. Straight pipe requirements, it is best to ensure 5 times the diameter of the first 10;

4. When welding, please pay attention not to connect the instrument to prevent damage to the flowmeter;

5. The shell should be grounded as far as possible. The shielded wire should be grounded at one end, and the point can not be shared with the strong electricity;

6. If it is convenient for maintenance, the installation location needs to be selected to meet the maintenance requirements. At the same time, the side pipe can be installed, and the fluid moves along the side pipe when the fault occurs;

7. To avoid direct sunlight, high temperature, will degauss;

8. Other requirements should be considered, the signal wiring should not be with the strong electric line.