Centrifugal Pumps
Pumps
Pump is a device, which adds to the energy of a liquid or gas causing an increase in its pressure and perhaps a movement of the fluid.
A Simple Pumping System
A simple pumping system consists of a suction branch, a pump, and a discharge branch. See the figure above.
Liquid flows into the pump under either "GRAVITY & ATMOSPHERIC PRESSURE" (when the liquid to be pumped is above the center line of the pump) or only under "ATMOSPHERIC PRESSURE" (when the liquid to be pumped is below the center line of the pump).
Pump Characteristics
Pump only adds to the energy of the fluid in the system. Energy required to bring the fluid to the pump is an external one and in most practical conditions is provided by the atmospheric pressure.
Atmospheric Pressure Pushing Up Liquid into Pump Suction
Referring figure above, even though liquid on suction side is below the pump center line, still liquid will rise upto the pump center because of external atmospheric pressure acting on the surface of liquid; and no pressure (vacuum) acting on the other side (at pump center).
Types of Pumps
Displacement
Liquid or gas is displaced from suction to the discharge by the mechanical variation of the volume of a chamber or chambers. All displacement pumps are self-priming pumps. These pumps include Reciprocating Pump, Gear Pump,Screw Pump, etc.
Centrifugal
Flow through the pump is induced by the centrifugal force imparted to the liquid by the rotation of an impeller or impellers.
Centrifugal pumps are not self-priming pumps. These pumps must be primed by gravity or by priming equipment external or internal with the pump.
These pumps are basically radial flow, axial flow or mixed flow type.
CENTRIFUGAL PUMP
Sectional view showing parts
Construction and Working
The pump consists of a rotating impeller within a stationary casing. The impeller construction has two discs joined at in between surface by a set of internal curved vanes. Impeller has an eye (opening) at the center and is mounted on shaft, which is driven by a suitable prime mover such as an electric motor, steam engine through crank mechanism, or a turbine.
Opening in the sides of the impeller near shaft, called eye, communicates with the suction branch as shown in the figure below.
Cross section showing eye, shaft,etc.
Assume there is a certain amount of fluid at the eye of the rotating impeller. The fluid will flow radially outwards (because of centrifugal action) along the curved vanes in the impeller, increasing its linear velocity.
The fluid leaves the impeller in a similar manner to sparks shooting from a Catherine wheel. The high velocity fluid is collected in specially shaped casing called volute casing, where some of the kinetic energy of the fluid is converted into pressure energy. Fluid under pressure now leaves the impeller producing a drop in pressure behind it at the eye of the impeller. (Throwing off the water from the eye of the impeller leaves the space with vacuum). This causes the fluid from the suction pipe to flow into the pump under atmospheric pressure and subsequently that fluid also gets discharged like earlier one. This way fluid in the pump acts like a piston moving outward causing drop in pressure behind it. However, if initially there is no liquid at the eye, there will be no pumping action as explained, since there is no vacuum formed at the eye of the impeller. Centrifugal pump therefore is not a self-priming pump. In such case, where normally at the start of the pump the level of the liquid is below the eye of the pump, a self priming unit is normally attached to the pump which helps to create vacuum at the eye of the impeller hence priming the pump. As soon as pump starts taking suction self priming unit is automatically disengaged.
Refer the performance characteristics drawn above.
n-Q - Efficiency Vs Flow Rate and
HP/Q - Horse Power (of the prime mover)
Theoretical Discharge Head Vs Flow Rate (H/Q) plot is a straight line as shown. When there is no flow or discharge valve is shut, loss of head is mainly due to shock and eddy losses. As flow rate increases, frictional losses come into picture and it dominates other losses.
Efficiency Vs Flow Rate plot is well explained down below.
From above graphs it is clear that,
- If the pump discharge head is lesser the flow rate of the liquid is higher and therefore pumping of the liquid is faster.
- Pump if run at normal duty flow rate by maintaining normal duty discharge head the liquid will be pumped utilizing least possible rate of energy by the pump or in other word at this point efficiency of the pump is maximum.
