Introduction to Flow Measurement and Types

Introduction to Flow

• Flow is a fluid in motion.
• Fluids move from high to low pressure.
• Flow rate is the amount of fluid moving through a pipe or channel in a given time.
• Laminar flow is smooth and consistent.
• Turbulent flow is caused by obstructions and promotes mixing.
• Reynolds number predicts whether flow is laminar or turbulent.
• Factors influencing Reynolds number: fluid velocity, density, viscosity, pipe inside diameter.
• Direct flow measurements measure flow directly.
• Indirect flow measurements infer flow from other variables.
• Flow can be measured in absolute quantities, percentages, or volumes.

Positive Displacement Flow Measurement

• Used for accurate volume measurements.
• Devices admit fluid into a chamber, measure volume, and transfer it to another point.
• Common types include piston, oval gear, nutating disk, and rotary vane.

Percentage Flow Rate

• Indicates a flowing process.
• 100 percent represents a specific quantity, such as GPM or liters per minute.
• Flow rates are expressed as percentages of this quantity.

Volumetric Flow Units

• Measure instantaneous flow rate by volume per time unit.
• Measured in GPM (gallons per minute) or CFM (cubic feet per minute).

Mass Flow Units

• Measure instantaneous mass flow per time unit.
• Expressed in pounds per time unit.
• Considers mass density, which is influenced by temperature.

Types of Flow Measurement Devices

• Most common flow elements measure differential pressure, which is related to flow rate.
• Primary flow sensing devices include:

  Orifice Plates

  • Thin metal plates with precise holes (metering holes).
  • Create a pressure drop in the flow.
  • Types: concentric, eccentric, segmental.
  • Allow solids to pass.
  • Mounted in orifice flanges.
  • Changes in flow rate cause changes in differential pressure.

  Venturi Tubes

  • Create a pressure drop using a tapered design.
  • Less permanent pressure loss than orifice plates.
  • Pressure is reduced as the fluid speeds up in the narrower throat section.

  Flow Nozzles

  • Similar to venturi tubes, but with a smaller throat and no recovery cone.
  • Inserted at flange connections or in spool pieces.
  • Allow more flow per unit pressure drop than orifice plates.
  • Suitable for slurries due to sloped inlets.

  Pitot Tubes

  • L-shaped tubes inserted into the pipe, measuring velocity at one point.
  • One end faces the flow, the other measures static pressure.
  • Provide accurate average velocity with high Reynolds numbers.

  Multiport Pitot Tubes

  • Multiple impact points across the pipe to measure velocity.
  • One tube measures static pressure.
  • Averages multiple readings for a single measurement.

  Rotameters

  • Variable area flow tubes.
  • Fluids flow upward, lifting a free-floating indicator plummet.
  • Float position indicates flow rate.

  Magmeters

  • Measure volumetric flow of electrically conductive liquids, slurries, and corrosive materials.
  • No obstruction of flow path, non-invasive.
  • Based on Faraday’s law – electrical potential is generated when a conductor moves through a magnetic field.
  • Electrodes detect the potential.

  Turbine Meters

  • Contain a free-spinning turbine wheel.
  • Revolutions per unit time are proportional to flow rate.
  • A magnetic blade creates pulses as the turbine rotates.
  • Requires calibration to establish a K factor (pulses per unit volume) considering viscosity and specific gravity.

  Mass Flow Meters

  • Most common: Coriolis meters
  • Measurements are independent of temperature, pressure, density, and viscosity.
  • A vibrating tube with velocity sensors measure the frequency change to determine mass flow.

  Differential Pressure (D/P) Transmitters

  • Convert pressure generated by primary elements into a standard output signal.
  • Output signals can represent differential pressure or flow rate.
  • Cell responds to pressure measurements.
  • Flow rate changes are squared in the pressure drop.

  Miscellaneous Flow Meters

  • Vortex flow meters measure fluids but do not handle high viscosity or low flow applications.
  • Target flow meters use a disc to measure force proportional to pressure head.
  • Integral orifice flow meters measure small flows and are permanently installed in a meter run.
  • Ultrasonic flow meters measure velocity by timing an ultrasound beam travel time.
  • Can be inline (intrusive) or clamp-on (non-intrusive), measure temperature differences, and particulate content.

Flow Rate, Total Volume Flow, and Volumetric Flow

• Two types of flow measurement:
  • Flow rate: instantaneous measurement.
  • Total flow: summation of instantaneous flow rates over a time interval or accumulated counts from positive displacement devices.
• Measured in volume or mass units per time interval.
• Units
  • Volumetric flow: GPM (gallons per minute), L/min (liters per minute).
  • Mass flow: lbs/min (pounds per minute), kg/min (kilograms per minute).
  • Total flow: gallons, pounds, cubic feet, liters, kilograms, cubic meters over specified time.

Check Your Knowledge

• To determine Reynolds number you need: pipe inside diameter, fluid viscosity, fluid density, and fluid velocity.
• Increase in absolute viscosity makes Reynolds number smaller.
• Differential flow rates require consideration of flowing fluid temperature.
• Segmental orifice plates are half-moon shaped.
• A rotameter uses a free-floating indicator plummet.
• A Pitot tube measures flow using an L-shaped tube with a static pressure tube.
• An orifice plate measures flow using a metal disc with a drilled hole.
• A flow nozzle measures flow with a tapered inlet device inserted into a flange connection.
• A venturi tube measures flow using a cone-shaped device.
• The % flow rate for a D/P of 28% is 52.9%.
• A turbine meter measures flow using a spinning fan.
• A magmeter measures flow by sensing electric potential.
• A mass flow meter uses a U-tube vibration frequency for flow measurement.
• A D/P transmitter responds to signals from primary devices.
• If flow through an orifice doubles, the differential pressure quadruples.
• Special flanges for mounting orifice plates are called orifice flanges.
• Units associated with mass flow are lbs/min.
• Image Matching
  • Venturi: Cone shaped tube with inlet and outlet.
  • Flow Nozzle: Tapered inlet device inserted into a flange.
  • Multiport Pitot Tube: Tube with multiple sensors measuring velocity.
  • Orifice Plate: Thin plate with a hole.
  • Pitot Tube: L-shaped tube measuring velocity at one point.
  • Mass flow meter: Uses vibration for flow measurement.
  • Rotameter: Variable area flow tube with a floating plummet.
  • Electromagnetic flow meter (magmeter): Uses magnetic field and electrodes to measure flow.
  • Turbine meter: Contains a turbine wheel.
  • D/P transmitter: Transmits signals based on pressure measurements.