02-06-2012, 01:52 PM
PRESSURE AND STRESS DISTRIBUTION ANALYSIS OF CENTRIFUGAL PUMP
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INTRODUCTION
A pump is a machine used to move liquid through a piping system and to raise the
pressure of the liquid. A pump can be further defined as a machine that uses several
energy transformations to increase the pressure of a liquid.
Historical Background
The transfer of liquids against gravity existed from time immemorial. A pump is one such
device that expends energy to raise, transport, or compress liquids. Pumps are used in a
wide range of industrial and residential applications. Pumping equipment is extremely
diverse, varying in type, size, and materials of construction. There have been significant
new developments in the area of pumping equipment since the early 1980s.
Vast tonnages are pumped every year in the form of solid-liquid mixtures, known as
slurries. The application which involves the largest quantities is the dredging industry,
continually maintaining navigation in harbors and rivers, altering coastlines and winning
material for landfill and construction purposes.
Reciprocating pump
In a positive displacement reciprocating pump, discrete volumes of fluid are isolated
between the moving and stationary parts, and moved from the suction to the discharge
branch by direct mechanical action. Hence the flow path is not continuous, as it is with a
rotodynamic pump. Such pumps necessarily give a pulsating flow, although pulsations
can be reduced with multi-cylinder pumps, or by using damping devices
CENTRIFUGAL PUMPS
The hydraulic machines, which convert the mechanical energy into hydraulic energy, are
called pumps. The hydraulic energy is in the form of pressure energy. The mechanical
energy is converted into pressure energy by means of centrifugal force acting on the
fluid; the hydraulic machines are called centrifugal pumps. The flow in a centrifugal
pump is in radial outward direction. Figure 1.4 shows the 3 dimensional model of a
centrifugal pump.
Working Principle of centrifugal pump
The centrifugal pump works on the principle of forced vortex flow, which means that
when a certain mass of liquid is rotated by an external flow, the rise in pressure head of
the rotating liquid takes place. The rise in pressure head at any point of the rotating liquid
is proportional to the square of tangential velocity of the liquid at that point. Thus at the
outlet of the impeller where the radius is more, the rise in pressure head will be more and
the liquid will be discharged at the outlet with high pressure head. Due to high-pressure
head, the liquid can be lifted to a high level. Figure 1.5 shows flow of fluid in a
centrifugal pump.
Radial flow impeller
Most centrifugal pumps are of radial flow. Radial flow impellers impart energy primarily
by centrifugal force. Water enters the hub and flows radically to the periphery. Flow
leaves the impeller at 90 degree angle from the direction it enters the pump.