A flow meter measures the flow rate of a fluid. The most common flow meters are
(i) Differential Pressure Flow meters
(ii) Velocity Flow meters
(iii) Positive Displacement Flow meters
(iv) Mass Flow meters
(v) Open Channel Flow meters
Differential Pressure Flow meters
An obstruction in the flow creates a pressure drop. Calculate flow velocity and the flow rate using Bernoulli’s equation.
(i) The various differential pressure flowmeters are:
(a) Orifice plate
Fig. Orifice Meter
(b) Nozzle
(c) Venturi tube
Fig. Venturimeter
(d) Rotameter
Fig. Rotameter
(ii) Velocity Flow meters
(a) Pitot Tubes
Fig. Pitot Tube
(b) Turbine Flowmeter
Turbine flow meters measure clean, dry gases and liquids of less viscosity. These measures the flow rate of hydrocarbons, chemicals, gases and vapors. Its measurements are highly accurate and precise.
(iii) Positive Displacement Flow meter
(a) Reciprocating piston meters are of single piston and multiple-pistons type
(b) Oval-gear meters
(d) Rotary vane meters
(iv) Mass Flow meters
Mass flow meters measure the mass flow rate directly.
(a) Thermal Flow meter
(b)Coriolis Flow meter
(v) Open Channel Flow meters
(a) Weir and V-notches (b) Sluice Gates
PRACTICAL APPLICATIONS OF FLOW MEASURING METERS 1. Orifices, Venturimeter, Rotameters and Pitot Tubes
These meters use the Bernoulli Equation to calculate the fluid flow rate using pressure difference through obstructions in the flow. These are multi-purpose.
A Venturimeter measures the flow of a fluid in a single state in a larger diameter pipe. Waste treatment plant use it. It can increase the velocity of any type of fluid in a pipe at any particular point.
Orifice meter measures the flow rate of single and multi-phase fluids. Water treatment plants and Natural gas industries use Orifice meter for flow measurement.
Process industries use rotameter for flow measurement. It measures the rate of flow of water or gas.
A pitot tube measure speed of an aircraft or a boat. It measures fluid flows in different industries. A pitot tube is useable in a pipe, duct or channel. It gives accurate flow measurement.
2. Weirs and V-notches
Weirs & V-notches measure flow rates in open channels and rivers.
3. Sluice Gate Sluice gates measure and control flow rates in open channels and rivers.
TURN DOWN RATIO
Turn down ratio (or Rangeability) is the ratio of maximum flow rate to minimum flow rate. This makes the meter of acceptable accuracy.
Orifice plate turn down ratio 3: 1 to 5:1 Orifice meter turn down ratio 4:1
Comparison Between Orifice/Venturimeter and Rotameter Flow Meters