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Variable Area Flowmeters

  • Variable area flow meters measure flow by measuring the displacement of a float/piston/vane through kinetic energy of flowing stream.
  • The VA flowmeter is called so due to the nature of the chamber where the area inside the chamber will be increasing towards downstream flow. The increasing area is used to reduce the opening between the float and the tube/pipe so that the pressure drop will reduce as the float goes up. Therefore the position of the float can be directly related to flowrate
  • The most common type of VA flowmeter is the rotameter. Rotameters are only used for small flow rates and must be installed vertically. On normal conditions, the float will be resting at the bottom of the meter chamber. When fluid comes in, buoyancy plus the increased pressure (due to pressure drop in the Varying Area chamber) will push the float upwards until equilibrium with gravitational forces is met.
  • The VA flowmeter works on a balance force principle where :-
  • The up trust force of liquid is, F = k x Density x Flow2 x A = K x Density x Flow2
  • The weight of the float, W = mg
  • The buoyancy force is, F = (Volume)(Density)g = K x density
  • From here it is derived that: -
    • F = k / (root(Density )), Provided that everything else
    • This formula is used usually when once wants to buy a new rotameter. The vendor specifies his range of rotameters available in water flow (for liquid) and air flow(for gas). To convert your actual process flow to the equivalent flow, the following equations must be used
  • For Liquid
    • Water Volume rate = (Process Flow)(Process Density)(2.65) / root{[(Float Density)-(Process Density)](Process Density)}
  • For Gas
    • Volume rate = (Process Flow in Standard Flow)(Process Density in Standard Flow)(10.34) / root{[(Float Density)-(Process Density)](Process Density in actual flow.
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