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.