Each of the following criteria should be considered. Note that the maximum value for a particular criterion or the extreme process conditions are often used as the design conditions.
• Normal and extreme operating conditions
• Minimum and maximum flow rates
• Physical properties of the fluid, such as viscosity and entrained solid particles or air
• Pressure and temperature
• Dynamic properties of the fluid, characterized by the Reynold’s number (Re)
The accuracies of many flow meters can be affected by the velocity profile of the fluid, which is measured by a nondimensional number called “Reynold’s number” (Re). Reynold’s number is the ratio of the inertia forces to viscous forces, and is defined by the following equation:
Up to approximately Reynold’s number 2,000, the flow is called laminar, viscous, or streamline flow. The flow profile in this regime is not affected by the wall roughness of the pipe. Above 10,000, the flow is called fully developed turbulent. The region between 2,000 and 10,000, where the flow is shifting from laminar to turbulent, is not clearly defined and is called transitional. Within the transition regime, the flow profile exhibits a flat parabolic geometry and is called “plug flow.”
Generally speaking, the velocity profile in the transition between laminar and turbulent flow regimes can be unstable and difficult to predict, as the flow may exhibit properties of both laminar and turbulent regions or oscillate between them.