Explain about Target flow meter?


#1

Target flow meter

Construction:

  • The target meter measures flow by measuring the force on a target (or disc), centered in the pipe at right angles to the direction of fluid flow.
  • The fluid flow develops a force on the target which is proportional to the square of the flow.
  • A target meter consists of a target (or disc) which is mounted on a force bar (or beam) passing through a flexible seal, and is positioned in the centre of and perpendicular to the flowing stream.
  • The device may be installed directly in the flow line, thus eliminating the need for pressure-tap connections.

Working:

  • The flowing fluid while passing through the pipe, develops a force on the target which is proportional to velocity head (the square of the flow).

  • The force bar transmits this force to a force transducer (either electronic or pneumatic) to measure the force which is proportional to the square of the flow.

  • The relationship between the flow rate and force is expressed by the equation Q = k √F

  • Where Q= flow rate, K= a known coefficient, F= force

  • The target meters are available in sizes from 12.5 to 203 mm pipe diameter, and an accuracy of about ±1/2% with proper calibration. The targets (or discs) are available with diameters of 0.6 to 0.8 times pipe diameter.

Another variation of target meter is also available which is known as drag body flow meter, as shown in Fig.

It also uses the same operating principle where the force is given by the equation,

F = CdA ρ (V 2/2g)

where,
F=force, Cd= drag coefficient, A= sensor area,
ρ = fluid density, (V 2/2g= velocity head

• This force is detected and transmitted by a strain gauge bridge circuitry, into a voltage signal which is proportional to the square of the flow rate.

• The accuracy of this device ranges from ±1/2 to ±3% depending on the calibration used.

Advantages:

  • They are useful for difficult measurements such as slurries, polymer bearing and sediment-bearing
  • materials corrosive mixtures, etc.
  • They provide good accuracy when calibrated for specific streams.
  • Their repeatability is good.
  • They are good for relatively high temperatures and pressures.

Disadvantages:

  • In-line mounting required in these flowmeters.
  • They have a limited calibration data.
  • In case of target flowmeters no-flow conditions must exist for zeroing

Applications:

Target meters are applied in a number of fields for measurement of liquids, vapours and gases. They are especially useful for measuring heavy viscous, dirty or corrosive fluids.

The force-balance type (i.e. target type) can handle pressures upto 1,500 psig and temperatures to 398 °C, and the strain gauge type (i.e. drag body type) can handle pressures upto 5000 psig and temperature to 315 °C.