Chapter 3: Flow Measurement

Basic Flow Equation

where

This equation applies in all cases. If flow is in a pipe, the cross-sectional area can be found in piping handbooks.

Flow is laminar or turbulent, depending on the flow rate and viscosity. This can be predicted by calculating the Reynolds number, which is the ratio of inertial forces to viscous forces:

where:

Reynolds numbers below 2,000 indicate laminar flow; above 4,000, turbulent flow. However, some velocity meters require values above 20,000 to be absolutely certain the flow is truly turbulent and a good average velocity profile is established that can be measured from a single point on the flow profile. Most liquid flows are turbulent, while highly viscous flows like polymers or very low flow rates are laminar.

Typical flow measurements can determine: average velocity, velocity at one point, volume of material flowing, and/or the mass of material. Velocity measurements, in particular, require the flow stream velocity to be relatively consistent across the diameter of the pipe. Less than fully turbulent flow creates lower velocities near the pipe wall.

Fittings, valves anything other than straight, open pipe upstream of the sensor will cause velocity variations across the diameter of the pipe. To achieve uniform flow, different types of flowmeters require...