Flow Meter Selection Criteria

What is Flow?

Flow is an indication of how fast a substance moves through a conduit from one place to another. Flow rate can also be used to determine the distance a substance moves over a period of time. Flow rate is usually expressed as

• Volume flow rate
• Mass flow rate

Volume Flow Rate represents the volume of fluid that passes a measurement point over a period of time. An example measurement unit is kg per hour. The volume flow rate can be calculated if the average flow velocity and inside pipe diameter are known. 

The calculation is based on the formula 

         Q = A x V

         where

                  Q = volumetric flow rate

                  A = cross-sectional area of the pipe

                  V = average flow velocity (flow rate)

Mass Flow Rate represents the amount of mass that passes a specific point over a period of time. 

Mass flow rates are used to measure the weight or mass of         a substance flowing through a process operation. If the volumetric flow rate and density are known, the     calculation is based on the formula

         W = Q x r

         where

                  W = mass flow rate

                  Q = volumetric flow rate

                  r = density (r = density “rho” )

Types of Flow ?

Laminar Flow: Streamlined flow of a fluid where viscous forces are more significant than inertial forces, generally below a Reynolds number of 2000. 

Turbulent Flow: When forces due to inertia are more significant than forces due to viscosity. This typically occurs with a Reynolds number in excess of 4000. 

Types of flow meters: -

1. Difference pressure flow meter a. Orifice plate, b. Venturi tube, c. Pitot tube and d. Nozzle 

1. Variable area flowmeter 

a. Rotameter 

b. Movable vane meter

 c. Weir, flume

1.  Positive displacement flowmeter 

• Tri-rotor type PD meter

          • Birotor PD meter, 

          • Piston type PD meter 

    • Oval gear PD meter, 

    • Nutating disk type PD meter 

    • Sliding-vane type PD meter

                • Roots PD meter 

• The CVM meter 

• Diaphragm meter 

1. Turbine flowmeter 

1. Electromagnetic flowmeter 

1. Ultrasonic flowmeter 

a. Doppler 

b. Transit- Time

1. Coriolis (Mass) flowmeter 

1. Vortex Shedding Meter 

Which factors to be considered for flow meter selection: -

1. Type of Fluid – Liquid, Gas , Steam 
2. Viscosity & Flow Profile – Laminar or Turbulent
3. Maximum to Minimum flow ratio
4. Maximum process pressure & Pressure loss allowed
5. Maximum process temperature
6. Accuracy / Linearity / Repeatability requirements
7. Process Connection
8. Type of Output required 

Flow meter selection criteria

Selection Guideline Table 

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