Velocity in sewers

VELOCITY IN SEWERS

The flow velocities should be such that suspended particles in sewage should not get silted or cause scouring of the sewage pipe material. The first condition sets the minimum velocity called self cleansing velocity and the second condition sets the maximum velocity called scour velocity. Hence, the velocity of flow in the sewer should lie anywhere between self cleansing velocity and scour velocity.

Silting of sewers can be avoided by generating high velocities that prevent the settling of solids so as to cause automatic self cleansing velocity. This velocity will scour the deposited particles of a given size. The generation of minimum self cleansing velocity in the sewer at least once a day is important.

The smooth interior surface of a sewer pipe gets scoured due to continuous abrasion caused by suspended solids present in sewage.This wear and tear is much more pronounced at velocities higher than what can be tolerated by the pipe material. This will reduce the life span  but also reduce their carrying capacities. Hence, it is necessary to limit the maximum velocity that is produced in a sewer at any time. The limiting or non-scouring velocity depends on the material of the sewer. The problem of controlling high velocities generated in a sewer mainly arises in a hilly areas. Hence in such areas, this can be achieved by:

• limiting the design gradient and adjusting balance head by
• constructing drops through manholes