Liquid Flow Measurement Notes

LIQUID FLOW MEASUREMENT Notes

Introduction

  • A flow meter measures the flow rate or quantity of fluid in a conduit.
  • Types of flow measuring devices include:
    1. Differential pressure flow meters
    2. Variable area flow meters
    3. Positive displacement flow meters
    4. Magnetic flow meters
    5. Ultrasonic flow meters
    6. Turbine flow meters
    7. Vortex flow meters
    8. Coriolis flow meters

1. Differential Pressure Flow Meters

  • Measure flow by creating a differential pressure.
  • Types include:
    • Orifice Plates
    • Venturi Tubes
    • Flow Nozzles
    • Pitot Tubes
    • Dall Tubes
i. Orifice Plates
  • Inserted into the pipeline; pressure drop measured.
  • Types:
    1. Concentric
    2. Eccentric
    3. Segmental
    4. Quadrant Edge
Pressure Taps
  • Small holes to measure pressure.
  • Types include D, D/2, and flange taps.
Advantages & Disadvantages of Orifice Plates
  • Some benefits are ease of installation; drawbacks include pressure loss.
ii. Venturi Tubes
  • Comprises a convergent section, throat, and divergent outlet.
  • Measures flow velocity through pressure differentials.
Advantages & Disadvantages of Venturi Tubes
  • Advantages: High flow rates, low pressure loss; Disadvantages: High cost, inspection difficulty below certain sizes.
iii. Flow Nozzles
  • Based on Bernoulli’s principle; used for steam and erosive fluids.
iv. Pitot Tubes
  • Measures flow velocity using pressure difference.
v. Dall Tubes
  • Modified Venturi tube with a pressure drop measurement.

2. VARIABLE AREA FLOW METERS

  • Adjust restriction size to maintain constant pressure differential.
  • Types:
    • Rotameters: Float rises with flow rate.
    • Cylinder and Piston Type: Measures pressure differential to gauge flow.

3. POSITIVE DISPLACEMENT METERS

  • Measure flow quantity by separating flow into known increments.
  • Types:
    • Nutating Disc Type: Used in residential water service.
    • Rotating Vane Type: Counts revolutions of a rotating impeller.
    • Reciprocating Piston Type: Common in petroleum industries.
    • Lobed Impeller Type: Fluid trapped between impellers for measurement.

4. ELECTROMAGNETIC FLOW METERS

  • Measures flow based on voltage generated by conductive liquid in a magnetic field.
    • Voltage (u)(u) is proportional to flow velocity (V)(V).

5. ULTRASONIC FLOW METERS

  • Measure flow using ultrasonic oscillations:
    • Time Difference Type: Measures time variance of ultrasonic waves for flow rate.
    • Doppler Flow Meter: Requires gas bubbles/solids for frequency shift measurement.

6. TURBINE FLOW METERS

  • Multi-blade rotor causes frequency generation, representing flow rate.

7. VORTEX FLOW METERS

  • Types:
    • Swirlmeter: Uses rotational frequency to measure flow rate.
    • Vortex Shedding Meter: Measures velocities based on shed vortices.
    • Fluidic Oscillator: Uses jet flipping for flow measurement.

8. CORIOLIS FLOW METERS

  • Measures mass flow by detecting tube twist caused by fluid motion.