PUMPS, MULTISTAGE PUMPS, LOSSES IN PUMP

MECHANICAL TIPS AND STUDY MATERIAL

PUMPS

These are devices which are efficient in conversion of mechanical energy into hydraulic energy or also called pressure energy. Basically categorized on fluid flow direction as under:-
  • Axial flow pump
  • Centrifugal pump
  • Mixed flow pump
Let’s start with centrifugal pump in detail.
Centrifugal pump is the name derived because the head generated by the pump is result of centrifugal action. The construction of such pump consists of:-
  • Impeller
  • Casing
Impeller is mounted on the shaft and it lifts the water from low level to required high level. This is done by forced vortex motion which generates dynamic pressure. Due to generation of dynamic pressure the water leaving the impeller has high velocity. This high velocity water then enters static blades called casing. We use Bronze made impeller to handle small capacity but for large water and corrosive situation we use stainless steel and aluminium-bronze impellers. Given below is the figure of impeller.


Casing is the guide ways that directs the fluid flow and also add in raising the static head. Static head is nothing but the vertical distance between the liquid level in sump (lower level) and the delivery tank (high level). These are blades which are fixed around the impeller to act like diffuser and increase the static pressure.

MULTISTAGE PUMPS

Multistage pumps are required when high discharge or high head is required. First of all we have to know what multistage pump is. A multistage pump consists of two or more impellers which may be connected in series or parallel.

Ø  To develop high head – SERIES CONNECTION

When high head is required then number of impellers are connected in series such that water enters into first impeller through suction pipe and the pressure increases and then this increased pressure water enters into second impeller where pressure adds up more. The discharge is same in both the impeller but the head is increased which is greater than either of the one.

Ø  To develop high discharge – PARALLEL CONNECTION

When we require high discharge or flow we connect impellers in parallel. Each pump sucks water from sump and delivers it to common pipe. The common pipe receives large amount of water which is result of sum of water received by each of the pump. Hence the discharge increases. All the pump works under same head in this case.

PUMP LOSSES 

Mainly there are three losses that are encountered in centrifugal pump:-

Ø  Mechanical losses
Ø  Hydraulic losses
Ø  Leakage

Mechanical losses occur mainly due to friction. These are:-

Ø  Friction on the main bearing glands.
Ø  Friction between rotor and the fluid.

Hydraulic losses occur due to many reasons. These are:-

Ø  Loss in suction and delivery pipes.
Ø  Friction loss in casing and vanes and impeller.
Ø  At entry and exit of impeller shock and eddy current losses occur.

Leakage is integral part of centrifugal pumps. There is always some amount of leakages or water slip out and never passes through the delivery pipe due to which some part of energy is also wasted. This is called leakage loss.
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