Centrifugal pumps are most commonly used for pumping sewage, because these pumps can be easily installed in pits and sumps, and can easily transport the suspended matter present in the sewage. A centrifugal pump consists of a revolving wheel called impeller which is enclosed in an air-tight casing to which suction pipe and delivery pipe or rising main are connected.
The impellers of centrifugal pumps have backward curved vanes which are either open or have shrouds. Open impellers have no shrouds. Semi-open impellers have only a back shroud. Closed impellers have both the front and the back shrouds. For pumping sewage either open or semi-open type impellers are commonly used.
The clearance between the vanes of the impeller is kept large enough to allow any solid matter entering the pump to pass out with the liquid so that the pump does not get clogged. As such for handling sewage with large-size solids, the impellers are usually designed with fewer vanes. The pumps with fewer vanes in the impeller or having large clearance between the vanes are called non-clog pumps. However, pumps with fewer vanes in the impeller are less efficient.
A spiral-shaped casing called a volute casing is provided around the impeller. At the inlet to the pump at the center of the casing a suction pipe is connected, the lower end of which dips into the liquid in the tank or sump from which the liquid is to be pumped or lifted up.
At the outlet of the pump a delivery pipe or rising main is connected which delivers the liquid to the required height. Just near the outlet of the pump on the delivery pipe or rising main a delivery valve is provided. A delivery valve is a sluice valve or gate valve which is provided in order to control the flow of liquid from the pump into the delivery pipe or rising main
The impeller is mounted on a shaft that may have its axis either horizontal or vertical. The shaft is coupled to an external source of energy (usually an electric motor) which imparts the required energy to the impeller thereby making it rotate. When the impeller rotates in the casing full of liquid to be pumped, a forced vortex is produced which imparts a centrifugal head to the liquid and thus results in an increase of pressure throughout the liquid mass.
At the center of the impeller (/3/) due to the centrifugal action, a partial vacuum is created. This causes the liquid from the sump, which is at atmospheric pressure, to rush through the suction pipe to the eye of the impeller thereby replacing the liquid which is being discharged from the entire circumference of the impeller. The high pressure of the liquid leaving the impeller is utilized in lifting the liquid to the required height.
Pumps for sewage pumping are generally of all cast iron construction. If the sewage is corrosive then the stainless steel construction may have to be adopted. Also, where the sewage would contain abrasive solids, the pumps constructed of abrasion-resistant material or with elastomer lining may be used.