What Does Flow Velocity Mean?
Flow velocity is the distance traveled by a fluid per time i.e. meter/second or feet/minute. Friction along the pipe walls can affect the flow velocity of a fluid. There are two types of velocities that effect pipes namely, minimum velocity or self-cleansing velocity and maximum or non-scouring velocity.
Trenchless rehabilitation methods such as pipe bursting, sliplining and cured-in-place pipe (CIPP) are used for rehabilitating pipes that have been damaged and have lost their optimum flow velocity to adequately convey the fluid they carry.
Trenchlesspedia Explains Flow Velocity
In a sewer pipe, the minimum velocity or self-cleansing velocity is the velocity of the fluid that would prevent any solid particles present in the fluid from settling down or from unsettling deposited particles of a given size. Minimum velocity ensures that free flow isn't obstructed and deposits aren't formed as these could build up and lead to blockage. The formula for calculating minimum velocity is:
Vs = √ (8K/f') (Ss-1)g.d'
where K (constant) - clean inorganic solids = 0.04 and organic solids = 0.06
f' (friction factor) - for sewers = 0.03
Ss - specific gravity of sediments
g - gravity acceleration
d' - diameter of grain
Maximum velocity or non-scouring velocity in sewer pipes is the velocity that would prevent the solid particles present in the flow from scouring the pipe walls. Sewer pipes contain suspended solids that are capable of scouring the pipe walls when the flow velocity is very high.
If the flow velocity is not controlled, it can cause wear and tear of pipe resulting in reduction in flow capacity and service life of the pipe. Controlling this velocity is difficult in places that have steep gradients such as in hilly places where gravity itself will speed up the flow velocity. To counter this problem, drop manholes are constructed along the sewer length where required.