# Critical Velocity

Published: January 7, 2019 | Last updated: July 5, 2023

## What Does Critical Velocity Mean?

Critical velocity of a fluid is the velocity at which the liquid flow turns from streamlined to turbulent. When the velocity of a fluid in a pipe is less, the streamlines are straight parallel lines but as the velocity is gradually increased, the streamlines continue to be straight and parallel to the pipe wall until a velocity is reached that causes the streamlines to break and form patterns. Critical velocity disperses the streamlines randomly throughout the pipe.

Gravity flow sewer pipes are gradually sloped to keep the flow non-critical. Excess velocity of flow can cause pipe erosion or scouring due to solid particles present in the flow leading to damage. Pipes damaged by the action of high velocity fluid flow can be rectified using trenchless methods such as cured-in-place pipe (CIPP), sliplining and pipe bursting.

## Trenchlesspedia Explains Critical Velocity

Critical velocity can be calculated using Reynolds number which characterizes the flow as streamlined or turbulent. Reynolds number is a dimensionless variable. It can be calculated using the formula

Vcrit = NRμ/ Dρ

where Vcrit – critical velocity

NR – Reynolds number

μ – coefficient of viscosity or resistance to flow in m2/ sec

D – internal pipe diameter in meter

ρ – density of fluid in kg/m2

If the Reynolds number lies between 0 and 2320, the flow is considered streamlined or laminar. Reynolds number between 2320 and 4000, indicates an unstable flow condition ranging from streamlined to turbulent. Reynolds number above 4000 indicates a turbulent flow meaning the flow velocity is critical.