A centrifugal pump is a continuous acting pump that accelerates liquid by rotating it outward in an impeller to a surrounding container. The impeller is basically a rotating disk that has vanes attached.
Arrows indicate the direction of rotation and the direction of flow. Because this shape offers the best flow characteristics, the vanes of the impeller are curved inward. Because of its simplicity and low cost, this type of pump is the most popular in buildings.
This article will describe the various types of a centrifugal pump, their construction, performance, efficiency characteristics, application in buildings, maintenance, and installation.
Pump Types and Nomenclature
The types of centrifugal pumps used in buildings are often confusing because such pumps are identified in a number of different ways, according to (a) the internal design, (b) single-suction versus double-suction configuration, (c) the shape of the impeller and its operating characteristics, (d) the casing design, (e) the type of connection between the motor and pump, (f) the position of the pump in relation to the water being pumped, and (g) the number of stages of the pump.
Construction of centrifugal pumps
Centrifugal pumps used for most building services are built with cast-iron casings, bronze impellers, and bronze small parts. Stainless steel impellers and stainless-steel small parts also are common. Cast-iron impellers may be used, but the life of a cast-iron impeller is shorter than that of a bronze or stainless-steel impeller.
Shafts, seals, and bearings
The shaft used to drive the impeller of the pump enters the casing through an opening that must be sealed to prevent leakage around the shaft (i.e., the seal must prevent liquid from leaving and air from entering). Two types of seals are used: soft fiber packing and mechanical face seals. Where packing is used, the shaft enters the opening through a stuffing box.
The liquid is prevented from leaking out by filling this opening with a soft fiber packing. The packing material, which is relatively inexpensive, can usually be replaced without disassembling the pump. However, the packing will leak about 60 drops per minute and requires periodic adjustment. Mechanical seals are commonly used instead of packing because they are reliable, have a good life expectancy, are practically leak-free, and do not require periodic adjustment.