ELECTROMAGNETIC FLOWMETERS

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Electromagnetic flowmeters (magmeters) can handle most liquids and slurries, including corrosives, providing that the material being metered is electrically conductive. Most industrial and municipal water and waste liquids can be measured by these meters. Liquids with suspended solids and certain waste flows which are often impossible to meter otherwise, are dependably measured with electromagnetic flowmeters.

1. Operating Principles: Electromagnetic flowmeters operate on Faraday’s law of electromagnetic induction, which states that the voltage induced across a conductor, that moves at right angles through a magnetic field, is proportional to the velocity of that conductor. The liquid serves as the conductor; the magnetic field is created by energized coils outside the flow tube (Figure 6-8). The amount of voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage, which is measured by the secondary element. Forward and reverse flow measurements are possible with no pressure drop.

METER DESIGNS: Electromagnetic flowmeters are available in full-bore and Insertion type configurations. Neither type has moving parts.

FIGURE 6-8. Electromagnetic Meter

FIGURE 6-9. Full-Bore Electromagnetic Meter

1. Full-Bore Meters: The full-bore electromagnetic flowmeter (Figure 6-9) is an inline device with an inside diameter identical to the system. It is inserted between two flanges. These meters offer the same resistance to flow as a comparable length of pipe. The meter section of pipe is constructed of, or lined with, a nonmagnetic material. This prevents the metal tube from short-circuiting the conducting path of the induced magnetic field through the fluid from one electrode to the other. The electrodes for the liquid excitation are attached to, or implanted in, the pipe wall.

2. Insertion Meter: Insertion type electromagnetic flowmeters have the field-developing magnet and the electrodes for energizing the fluid combined in a single probe (Figure 6-10). The reduced size and weight make the installation simpler and maintenance cost lower.

RECOMMENDED APPLICATIONS: Electromagnetic flowmeters are available for liquid q easuring in pipe sizes greater than 0.1 inch. The flowmeters are recommended for measuring liquids of a conductive nature [a conductivity of 5 microsiemens (micromhos) per centimeter will meet the threshold requirements for most electromagnetic flowmeters]. Liquids may be clean, viscous, corrosive, or contain solids and may contain fibrous or abrasive material.

LIMITATIONS: Limitations that may preclude the use of an electromagnetic flowmeter for a specific purpose include the following:

• Temperature limit is 360°F (it is usually a consideration of the flow tube lining or the insulation of the magnetic coils).
• Pressure limit is commonly the same as the supporting system.
• Cost, initial and operational, may exceed project funding or negatively affect the project Savings-to-Investment Ratio (SIR).
• Flowmeter may require installation in a bypass line to avoid magnetic fields present in an existing pipe configuration.
• Data are inaccurate less than full-flowing pipe.

FIGURE 6-10. Insertion Electromagnetic Meter

INSTALLATION: Both configurations of the electromagnetic flowmeter, full-bore type and insertion type, are discussed.

1. Full-Bore Meter Installation: The full-bore electromagnetic flowmeter inserts into a pipeline between two flanges and is secured with bolts. To obtain accurate measurements, the following conditions must exist:

• The meter is subject to electromagnetic fields and must be shielded from large electric motors, transformers, communications equipment, and other large electrical devices to avoid electromagnetic disturbances.
• The meter must be positioned so that the pipe is full flowing approaching and exiting the meter.
• The location of the full-bore electromagnetic flowmeter in the system is important. Whenever possible, it is preferable to locate the primary element in a horizontal line. To ensure accurate flow measurement, the fluid must enter the primary element with a fully developed velocity profile, free from swirls or vortices. Such a condition is best achieved by the use of adequate lengths of straight pipe, both preceding and following the primary element. The minimum recommended lengths of piping are shown in Figure 6-5. The diagram in Figure 6-5 that corresponds closest to the piping arrangement for the meter location should be used to determine the required lengths of straight pipe on the inlet and outlet. These lengths are necessary to limit piping configuration errors to less than ±0.5%.

1.1 Meter Grounding: A grounding system must be provided for the meter and the fluid. Detailed instructions for grounding systems are usually supplied by the equipment manufacturer. Some grounding considerations are as follows:

• For conductive piping, the third wire ground to the power supply and a ground tie to the piping flanges is typically all that is required.
  
• For nonconductive or lined piping systems, a protective grounding orifice must be used to provide access to the potential of the liquid being measured.
  

2. Insertion Meter Installation: Insertion electromagnetic flowmeters are available as hot tap or permanent types. Both types require installation of a tee or welded fitting on the pipeline for access. Insertion meters are generally restricted to lower temperature and pressure conditions. Installation of an insertion electromagnetic flowmeter requires consideration of the following items!

• The meter is subject to electromagnetic fields and must be shielded from large electric motors, transformers, communications equipment, and other large electrical devices to avoid electromagnetic disturbances.
• The meter must be positioned so that the pipe is full flowing approaching and exiting the meter.
• Location of the insertion electromagnetic flowmeter in the system is important. Whenever possible, it is preferable to locate the primary element in a horizontal line. To ensure accurate flow measurement, fluid should enter the primary element with a fully developed velocity profile, free from swirls or vortices. Such a condition is best achieved by use of adequate lengths of straight pipe, both preceding and following the primary element. Minimum recommended lengths of piping are shown in Figure 6-5. The diagram in Figure 6-5 that corresponds closest to the piping arrangement for a q eter location should be used to determine required lengths of straight pipe on the inlet and outlet. These lengths are necessary to limit piping configuration errors to less than —+0.5%.

MINIMUM AND MAXIMUM FLOW RATES: Minimum and maximum flow rates must be established to maintain accurate flow measurements. Wear to the flow tube, buildup of coatings, and deposition of solids can be reduced if recommended flow rates are maintained.

1. Minimum Flow Rate: The minimum flow rate is usually determined by the rate that produces the lowest acceptable voltage requirement of the signal transmitted. If the minimum, rate causes excessive sedimentation, the diameter of the tube may have to be decreased to increase velocity through the meter. This provides a self-cleaning action.

2. Maximum Flow Rate: The maximum recommended flow rate is arbitrary, but is based on wear patterns experienced with different flow tube materials interfacing with a specific fluid. A maximum recommended flow rate of 30 feet per second is common.

ACCURACY AND RELIABILITY: Although it is possible to calibrate electromagnetic flowmeters to measure flow within +0.25%, a more representative accuracy is ±1.0% of its rated capacity. The downturn of electromagnetic flowmeters can be up to 40:1. If recommended maximum flow conditions are not exceeded (causing excessive flow tube lining wear), and hydraulic shocks (water hammer) are avoided, electromagnetic flowmeters are very reliable. Electrode corrosion greater than 0.002 inches per year is excessive and indicates a need for investigation to determine the cause.

MAINTENANCE: Since these are obstructionless instruments, the maintenance required is small. At least twice a year, remove and inspect the pressure sensors of the secondary element. Where there is a possibility of coatings accumulating, periodic cleaning is necessary because the coating will insulate the liquid from the electrodes and impair operation. All data transmission sensors and processors should be checked and diagnosed every six months for correct input and output.

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