Magnetic flow meter | Mag FT | Magnetic FT

A magnetic flow meter, also called a mag flow meter or magmeter, is a volumetric flow meter that measures liquids or liquid solutions having a minimum conductivity of 5 µS/cm. Magnetic flow meters will generally not work with hydrocarbons, distilled water, non-aqueous solutions and non-conductive solutions.

Magnetic flow meters works on Faraday’s law . Magnetic flow meter consists of a non-magnetic flux tube, liner, electrode and electromagnet to generate a magnetic field. In a magnetic flow meter, a magnetic field is generated and flows into the liquid flowing through the pipe.

Magnetic flow meters are traditionally the first type of flow meter to be considered for highly corrosive applications and for applications involving the measurement of erosive slurries. These meters used Faraday’s Law of Electromagnetic Induction to make flux measurements.

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Magnetic flow meter working Principle :-

The liquid flowing through the pipe is the conductor, and the electromagnetic coils surrounding the flow tube generate the magnetic field. The signal is sensed by two electrodes mounted on opposite sides of the flow tube. The flow signal at the electrodes is directly proportional to the flow velocity, the magnetic field intensity, and the distance between the electrodes.

Magnetic flow meters do not require much upstream and downstream straight runs, so they can be installed in relatively any straight runs.

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In other word –

When a conductor passes through a magnetic field of a given field strength, the voltage involved Conductor that is proportional to the respective velocities between the conductor and the magnetic field. This concept is used in electric generators. In the case of a magnetic flow meter, an electrically conductive flowing liquid acts as the conductor. The induced voltage is given by the equation,

E = C B L V                                                            (1)

or

V = E / C B L                                                         (2)

Where,

E = Induced voltage in volts                                         C = Dimensional constant

B = Magnetic field in weber/m2                                  L = Length of conductor (fluid) m

V = Velocity of conductor (fluid) in m/s

The equation of continuity to convert a velocity measurement to volumetric flow rate is given as

Q = V A                                                 (3)

Where,

Q = Volumetric flow rate                                                              V = Fluid velocity

A = Cross sectional area of the flow meter

Now, putting the value of V  from equation (3) the volumetric flow rate can be written as,

Q = E A / C B L                                    (4)

Since, for a given size of flow meter, A, C, B, and L become constants, the Q can be written as,

Q = K E                                                  (5)

Where,

K = A / C B L  = a constant

Therefore, the induced voltage is directly proportional and linear with volumetric flow rate.

magnetic flowmeter

Magnetic Flow meter Parts

Flow Tube: The tube is made of non conducting & non magnetic alloy & is insulated by glass lining from flowing liquid to prevent the short circuiting of e.m.f. between the Electrodes.

Electrodes: S.S or Pt. Electrodes are located diametrically opposite to each other with their Axis perpendicular to both the magnetic field & the tube Axis.

Source of Magnetic field: A magnetic field is generated by electromagnets formed by winding to saddle shaped copper coils on the laminated iron core.

Mag Flow meter Advantage

The main advantages of using mag meters are that they offer a barrier-free design with no moving parts that eliminates flow obstruction, requires little maintenance and can be used over very large line sizes. may be used. Accuracy can be as good as ±0.5% over a wide flow range Mag meter barrier free flow path and no moving parts Mag meters perform extremely well in many municipal and processing applications and are the meter of choice for measuring conductive fluids such as water or slurry. Magmeters also have the advantage of being able to measure both very low flow and very high volume flow rates. Some other advantages are following-

  • It can handle slurries and greasy materials
  • It can handle corrosive fluids
  • It has very low pressure drop
  • It is totally obstruction less
  • It is available in several construction material.
  • It is available in large pipe sizes and capacities.
  • It is capable of handling extremely low flows (with minimum size , less than 3.175 mm inside diameter) and very high volume flow rate (with sizes as large as 3.04 meter offered)
  • It can be used as bi-directional meter.
  • Measurements unaffected by viscosity, density, temperature and pressure

Mag Flowmeter Disadvantage

Magnetic flowmeters are only effective on conductive liquids, and materials such as immiscible hydrocarbons and gases cannot be measured. However, the magnetic materials themselves can also present problems, as hydrodynamic effects can alter the normal flow pattern and disturb the velocity rate high enough to interfere with operation. Depending on their size and capacity, magnetic flowmeters can be relatively bulky, and expensive ones with high corrosion and abrasion resistance. Some other disadvantages are following- 

  • It is relatively expensive.
  • It works only with fluids which are adequate electrical conductor.
  • It relatively heavy, especially in large sizes.
  • It must be full at all times.
  • It must be explosion proof when installed in hazardous electrical areas.

Magnetic Flow Meter Selection

Some important parameter which need to be considered before selecting a magnetic flow meter

  • Fluid should be conductive.
  • Minimum conductivity should be 5 µS/cm.
  • Is the fluid or slurry abrasive?
  • What is Minimum and maximum flow rate for the flow meter?
  • What is Minimum and maximum process pressure?
  • What is Minimum and maximum process temperature?
  • Is the fluid chemically compatible with the flow meter wetted parts?
  • Size of the pipe?
  • pipe should be always full?
  • Requirement of display is integral display or remote display?
  • Do you require an analog output?

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