Flow Measurement Principles

Flow Measurement Concepts

• Speed of flow is measured in meters per second (m/s) and is mainly used in scientific applications.
• Mass flow rate, a crucial measured quantity, is expressed in kilograms per second (kg/s).
• Volume flow rate is denoted in cubic meters per second (m³/s).
• Density (ρ) is the ratio of mass to volume and is variable; in gases, it depends on temperature (T) and pressure (p), while in liquids, it primarily depends on temperature.

Key Terms in Flow Measurement

• Turn-down ratio is 10:1, indicating the measurement range's lower limit compared to the full scale.
• Rangeability is the ratio of full-scale flow to minimum flow, often leading to non-linear output scales.
• Square root extractors are used to translate flow rates, available in pneumatic and electronic versions.

Measurement Process

• Flow measurements are inferential, relying on pressure drop measurements to estimate flow rates.
• Fluid density affects flow readings, influenced by temperature and pressure conditions.
• An increase in density can lead to a higher indicated flow rate.

Types of Flow Meters

• Head types and area types are commonly categorized flow meters alongside mass, positive displacement, electrical, open channel, and solid material flow metering types.
• Different flow meter types include:
  • Obstruction type (differential pressure / variable area)
  • Inferential (turbine type)
  • Electromagnetic
  • Positive displacement (integrating)
  • Fluid dynamic (vortex shedding)
  • Anemometer
  • Ultrasonic
  • Mass flow meter

Fundamental Principles

• Bernoulli’s Equation describes conservation of energy in fluid flow: ( \frac{1}{2}p + \rho v^2 + \rho gy = \text{constant} ).
• It illustrates the relationship between pressure and velocity in a flowing fluid.

Flow Patterns and Dynamics

• Flow velocity can vary across a pipe and is characterized by the Reynolds number (R).
• The Reynolds number is calculated using the average fluid velocity (V), diameter of the pipe (D), and dynamic viscosity (µ), indicating flow regime (laminar vs. turbulent).