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Flowmeter Comparison

  • Reynolds number measures the ratio between inertial forces and viscous forces. It is normally used in fluid dynamics and heat transfer applications. For instrumentation it is used as an indicator whether the flow is turbulent of laminar
  • ReD = pVD/m where
    • ReD = Reynolds Number
    • P = density
    • V = velocity
    • D = diameter
    • M = viscosity
  • As a general rule, the higher the flow, the higher the Reynolds number or turbulence. The higher the viscosity, the lower the Reynolds number and flow laminar
  • There are 4 types of flowmeters
    • DP type – Orifice, Venturi, Flow Nozzle, and VA Flowmeter
    • PD (Positive Displacement) Type
    • Velocity Type
    • Mass Type
  • Things to consider when selecting a flow meter
    • Liquid or Gas?
    • Clean or Dirty
    • Accuracy
    • Turndown ratio
    • Erosive or corrosive
    • Viscosity
    • Flow Rate
    • Laminar or turbulent
    • Temperature
    • Single phase or multiphase
  • Venturi Type Flowmeter and Flow Nozzle type
    • Advantage
      • Low pressure drop. Venturi has the lowest pressure drop
      • Proven
    • Disadvantage
      • Expensive
      • Longer installation length
      • Low turndown ratio (3:1)
      • Like orifice plate, needs long straight length requirement
    • When to Avoid
      • High Turndown ratio applications
      • Needs to be used at highly viscous fluids
      • Highly corrosive or erosive
      • High viscosity or laminar flow. This is because the flow pattern of laminar flow is not evenly distributed. It is only distributed in the middle
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    • Flow Nozzle (low pressure drop, but not as low as venturi)
    • clip_image004
    • Venturi Flowmeter
  • Pitot Tube
    • clip_image006
  • Averaging Pitot Tube (Annubar)
    • clip_image008

 

  • Positive Displacement Meter
    • Directly measures volume
    • Types
      • Oval Gear Meter
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      • Nutating Disc
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      • Rotating Impeller
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      • Rotating Lobe
        • clip_image016
    • Advantages
      • Good accuracy
      • Good turn down ratio
      • Can be used in high viscosity fluids
      • Directly measure volume
    • Disadvantages
      • Has moving parts, requires high maintenance. Needs to lubricate
      • High pressure drop
      • Gas and bubble gas reduce accuracy, therefore would require air eliminator
      • Expensive for large diameters
      • Cannot withstand turbulent flow or high Reynolds number since turbulent flow
    • When not to use
      • Unless to be used for metering, avoid!. Why? Because it has high DP and requires high maintenance
  • Vortex Flowmeter
    • When liquid flow is obstructed, it creates vortices. The frequency of these vortices created is proportional to flow
    • Vortex meter size should not be more than 8”. This is because more than 8” it is not economic since it’s too large
    • Advantage
      • Little maintenance
      • Good accuracy
    • Disadvantage
      • Low to medium pressure drop
      • Requires low Reynolds number < 20,000. Therefore long straight pipe length is required
    • Types
      • When not to us
      • Corrosive applications
  • Turbine Flowmeter
    • Advantage
      • High turndown ratio(10:1)
      • High accuracy
      • Fast response
    • Disadvantage
      • Has moving parts, requires high maintenance
      • Not suitable for viscous fluid
      • Requires straight length requirement
      • Affected by gas blow by. If there is gas, it will damage the rotors
    • When not to use
      • For steam application
      • For liquid with dual phase (which has gas), do not use turbine meter as the presence of gas will cause the turbine to spin faster. This will effect the accurac
      • Cannot use when there is pulsating flow such as when monitoring reciprocating transmitter. Therefore requires straight length requirement

 

  • Ultrasonic Flowmeter
    • Advantage
      • No pressure loss
      • No moving parts
      • Can detect zero flow
    • Disadvantage
      • Expensive
      • Complex
      • Accuracy susceptible to noise
    • When not to use
      • Near a noisy location like situated next to a control valve
      • Vibrating pipe
  • Magnetic Flowmeter
    • Advantage
      • No pressure loss
      • Withstands high corrosion
      • High Rangeability (40:1)
      • Good accuracy (0.5 to 1%)
    • Disadvantage
      • Meter must always be full (should be vertical or gravitated installation)
      • Cannot measure gasses
      • Liquid needs to be conductive (1-5 uS/cm)
    • When not to use
      • Liquid cannot be full and have bubbles
      • Liquid is not conductive
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