Introduction: Centrifugal pumps are the most widely used of all the turbo machine (or rotodynamic) pumps. This type of pumps uses the centrifugal force created by an impeller which spins at high speed inside the pump casing.
Principle: Its principle work on Centrifugal force.
CONTRUCTION DETAILS OF A CENTRIFUGAL PUMP: Centrifugal pump is classified as the following:-
1. Stationary components
2. Rotating components
1. Stationary components of the centrifugal pump are the following:
a) Casing: – It is an air tight passage surrounding the impeller. It is designed in such a way that the kinetic energy of the water discharged at the outlet of the impeller is converted into pressure energy before the water leaves the casing and enters the delivery pipe. Types of casing:-
· Volute casing: – It is spiral type of casing in which area of flow increase gradually. The increase in area of flow decreases the velocity of flow and increases the pressure of water.
· Vortex casing: – if a circular chamber is introduced between casing and the impeller, the casing is known as vortex casing.
· Casing with guide blades: – the impeller is surrounded by a series of guide blades mounted on a ring know as diffuser.
b) Suction pipe: – a pipe whose one ends is connected to the inlet of the pump and other end dip into water in a sump.
c) Delivery pipe: – a pipe whose one end is connected to the outlet of the pump and other end is involved in delivering the water at a required height.
2. Rotating component of the centrifugal pump is Impeller.
Impeller: – It is the main rotating part that provides the centrifugal acceleration to the fluid. Classification of impeller:
a) Based on direction of flow:
· Axial-flow: – the fluid maintains significant axial-flow direction components from the inlet to outlet of the rotor.
· Radial-flow: – the flow across the blades involves a substantial radial-flow component at the rotor inlet, outlet and both.
· Mixed-flow: – there may be significant axial and radial flow velocity components for the flow through the rotor row.
b) Based on suction type:
· Single suction: – liquid inlet on one side.
· Double suction: – liquid inlet to the impeller symmetrically from both sides.
c) Based on mechanical construction:
· Closed: – shrouds or sidewall is enclosing the vanes.
· Open: – no shrouds or wall to enclose the vanes.
· Semi-open or vortex type.
Working: Water is drawn into the pump from the source of supply through a short length of pipe (suction pipe). Impeller rotates; it spins the liquid sitting in the cavities between the vanes outwards and provides centrifugal acceleration with the kinetic energy.
This kinetic energy of a liquid coming out an impeller is harnessed by creating a resistance to flow. The first resistance is created by the pump volute (casing) that catches the liquid and shows it down.
In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to BERNOULLI’S PRINCIPAL.
SPECIFIC SPEED: – speed of an imaginary pump geometrically similar in every respect to the actual pump and capable of delivering unit quantity against a unit head. It is denoted by NS:-
NS = N (Q)1/2/(H)3/4
N: – pump speed in r.p.m
Q: – discharge in m3/sec
H: – head per stage in mete
Tabulated form of specific speed in a centrifugal pump:
Specific speed (in r.p.m)
EFFICIENCIES OF CENTRIFUGAL PUMPS:-
Mechanical efficiencies: – It is ratio of the impeller power to the shaft power.
Hydraulic efficiencies: – It is ratio of the manometric head to the Euler head.
Volumetric efficiencies:- It is ratio of the actual to the theoretical discharge.
Overall efficiencies: – It is ratio of the water power to the shaft power.