Magnetic Flow Meter

Since the mid 1950's, magnetic flow meters have been commercially used to measure the flow of water and a broad range of other electrically-conductive liquids, including corrosives, slurries and sludge in closed pipe systems.

Today, sensor manufacturers offers a full range of magnetic flow meters, including: wafer-style, flanged-style and ceramic flowtubes, with a wide range of liners and electrode materials, as well as insertion and electrodeless designs.

These primary heads are offered with integrally-mounted or remote- (field-) mounted signal converters featuring both pulse and 4-20mA output signal options.

Other than offering relatively "clog-free" obstructionless flow measurement with no moving parts to wear out or pressure drop to overcome, this cost-effective technology also offers several other advantages, including:

• Easy-to-Install setup
• Low Cost
• Reduced Maintenance
• Linear Analog Output
• Insensitivity to Specific Gravity, Viscosity, Pressure, Temperature, etc.
• Ability to measure flow of a broad range of difficult-to-meter fluids, such as corrosives, slurries, and sludges
• Insertion-style probe with exclusive Venturi Tip offers greater economy, especially for large pipe sizes

Applications

• Environmental Engineering / Water and Waste Water Treatment
• Pulp and Paper
• Cellulose
• Pharmaceuticals
• Food and Beverage
• Cosmetics
• Mining
• Chemical Processing
• Agricultural Fertilizer / Fertilizer Batching
• Liquid Feed and Batching
• Heating, Refrigeration, Ventilation and Air Conditioning\

How Magnetic Flow Meters Work

Magnetic flow meters are intended to measure the flow of electrically conductive liquids in full pipes. The pressure drop through the meter is minimal, (equal in magnitude to a piece of pipe of the same diameter and length) making it an excellent choice for low pressure systems. There are no moving parts or obstructions in the fluid stream so the meter is virtually maintenance free and suitable for fluids containing abrasives. Modern pulsed DC excitation eliminates problems with zero shift often found in other designs.

Operating Principle
The detection of flow is accomplished using the principle of electromagnetic induction. As a conductor passes through a magnetic field, a voltage is generated that is proportional to the velocity of the conductor moving through the field. In the case of the magnetic flow meter, the conductor is a conducting liquid. The higher the flow rate the greater the generated voltage.

Faraday's Law of Electromagnetic Induction is the underlying principle of operation for magnetic flow meters. This law states that the magnitude of the voltage induced in a conductive medium moving through a magnetic field and at right angles to the field is directly proportional to the product of the strength of the magnetic flux density (B), the velocity of the medium (V), and the path length (L) between the probes.

E = constant x B x L x V
Where, E = The voltage generated in a conductor
V = The velocity of the moving conductor
B = The strength of the magnetic field
L = The length of the conductor path (The distance between probes)

In other words, the faster a wire is passed through a magnetic field, at right angles to the field, the more voltage will be induced.

Grounding Rings
For process flow measurement reasons most magnetic flow meters must be grounded. Such a grounding system is lacking in pipes upstream and downstream of the primary head which feature a corrosion-resistant internal coating or liner, or are made entirely of plastics material. In such cases, grounding rings must be fitted on both sides of the primary head.

Selection Criteria

When selecting among different models of magnetic flow meters, the engineer should review the physical characteristics of liquids to be metered. These characteristics include:

Conductivity: Even water varies in conductivity, due to the various ions present.

Deionized water and distilled water are not adequately conductive.

Chemical and /or pharmaceutical solutions are often not adequately conductive. The concentration level of the solutions can also affect conductivity. Therefore, this variable should also be considered.

Temperature affects conductivity. Therefore, the temperature of the liquid being considered should also be known.

The key, and only constant rule is that liquid conductivity must be above minimum µ S/cm requirement specified for individual product.

Acids/Caustics: Generic descriptions of chemical solutions may not adequately describe all component substances. Detailed descriptions should be confirmed to select chemically compatible materials of construction.

Velocity: The liquid velocity should be maintained within the specified flow range of the meter for proper operation. However, applications monitoring abrasive slurries, sludge or greases will require special consideration.

Abrasive Slurries: Mildly abrasive slurries can be handled by magnetic flow meters. Flow velocity should be maintained at 6 ft/sec or slower to minimize the risk of abrasion damage. However, velocities should not be allowed to fall below 4 ft/sec to prevent suspended solids from settling out of the flow stream.

Sludge and Greases: Sludges and grease-bearing liquids should flow at higher velocities (approximately 6 ft/sec minimum) to minimize coating of electrodes.

Example

above: Electromagnetic Flow Meter (Primary Head) with mating electronics from Omega Engineering

SPECIFICATIONS

Accuracy:
±0.25% FS or ±0.5% of rate at <1.0 m/s
±0.1% FS or ± 0.5% of rate at 1.0 to 10 m/s
Fluid Electrical Conductivity Minimum: 5 µS/cm
Fluid Temperature: -10 to 120°C (14 to 250°F)
Ambient Temperature: -10 to 60°C (14 to 140°F)
Fluid Pressure: 290 psig
Enclosure: NEMA-4X (IP-67)
corrosion-resistance type
Metering Tube Case: Carbon steel
Electronic Housing: Aluminum alloy
Display: LCD,16-column, 2-row LCD,
alphanumeric with backlight
16 Engineering Programmable Units
Including: %, MPS, GPM, LPM, m^3/s
Output: 4 to 20 mA/dc current with digital I/O provided, 1 open collector
Load Resistance: 0 to 1 k ohm
Capacity: 30 mA dc max. 200 mA
Flange: ANSI 150
Electrode:Hastelloy
Grounding Ring: 316L SS
Wafer: 1⁄2 to 4": Ceramic lining 6 to 8": Teflon PFA lining; no coating for sizes 1 to 4" sizes; all other sizes have phthalic acid resin coating
Flanged: Teflon PFA lining; phthalic acid resin coating
Power Supply: 100 to 240 Vac, 50/60 Hz power supply (24 Vdc option)
Cable Connection Ports: A cable gland with a cap nut is provided for each port
Power Cable Requirements: Dia: 11 to 13 mm (0.43 to 0.51"), 3-wire sheathed (not included)
Cable Gland Material: Nylon 66
Port Holes in the Housing: G(PF) 1⁄2 " female screw
Vibration Resistance: No resonance to the following levels of vibration:
10 to 60 Hz, amplitude 0.07 mm
60 to 150 Hz with acceleration of 9.8 m/s^2
Heat shock resistance for ceramic tube detector:
Heating:∆T ≤150°C/0.5s
Cooling:∆T ≤100°C/0.5s
Note: Avoid constant vibration environments

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