Test: Centrifugal Pump - Civil Engineering (CE) MCQ

Centrifugal pump:

• It works on the principle of the forced vortex, which means that when a certain mass of liquid is rotated with the external torque, the rise in pressure head of the rotating fluid takes place.
• Due to this high pressure, the liquid can be lifted to a higher level.
• The action of the centrifugal pump is the reverse of radially inward flow reaction turbine ie radially outward flow, which takes water from the tailrace and delivers it to the headrace.

Some terms related to centrifugal pump:
Suction lift:

• It exists when the source of supply is below the centerline of the pump.
• It is the distance between the centerline of the pump to the free level of the liquid to be pumped.

Suction head:

• It exists when the source of supply is below the centerline of the pump (refer to the fig above).
• It is the vertical distance between the centerline of the pump to the free level of the liquid to be pumped.

Static discharge head:

• It is the vertical distance between the centerline and the point of free discharge or the surface of the liquid in the discharge tank.

Total static head:

• The vertical distance between the level of source of supply and the point of free discharge or the free surface of the discharge liquid

Concept:
Pump: 
The function of a pump is to convert mechanical energy into hydraulic energy.
It is the heart of any hydraulic system because it generates the force necessary to move the load. Mechanical energy is delivered to the pump using a prime mover such as an electric motor.
The hydraulic pump takes hydraulic fluid (mostly some oil) from the storage tank and delivers it to the rest of the hydraulic circuit. Pumps can be broadly listed under two categories:

1. Non-positive displacement pumps and
2. Positive displacement pumps.

Given, 
Let, the number of teeth = 12 per gear.
Thus, for each 360o rotation of each gear (n = 1 rotation), 12 × (0.350 cm³) of fluid is pumped.
Since there are two gears, the total volume of fluid pumped per rotation is,
 2 × (12) × (0.350 cm³) = 8.40 cm³.

If a centrifugal pump consists of two or more impellers, the pump is called a multistage centrifugal pump. The impeller may be mounted on the same shaft or on different shafts.
A multistage pump is having the following functions:
To produce high heads:

• For developing a high head, a number of impellers are mounted in series or on the same shaft.

N = Number of identical impellers mounted on the same shaft
H_m = Head developed by each impeller
Total head developed = N × H_m
The discharge passing through each impeller is the same.
To discharge a large quantity of liquid:

• For obtaining a high discharge, the pumps should be connected in parallel.

N = Number of identical pumps arranged in parallel
Q = Discharge from one pump
Total discharge = N × Q
Each of the pumps is working against the same head.

Centrifugal Pump:
Centrifugal pumps are used to transport fluids by the conversion of rotational kinetic energy to the hydrodynamic energy of the fluid flow. The rotational energy typically comes from an engine or electric motor.

• The impeller in the centrifugal pumps is immersed in water and imparts a centrifugal force that moves out the water particles readily.
• The Impellers are known to have backward curved vanes that have less than 90 degrees of blade angle. The backward curved blades are the most preferred types as they have better stabilizing power consumption characteristics than the forward and radial types which are less preferred.

• The centrifugal pump acts as a reverse of an inward radial flow reaction turbine.
• This means that the flow in centrifugal pumps is in the radial outward direction.

• 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 torque, the rise in pressure head of the rotating liquid takes place.
• In a centrifugal pump casing, the flow of water leaving the impeller is a free vortex. 

Centrifugal pump works on the principle of force vortex. Where external torque is provided to the impeller by the means of the prime mover.

• In a pump, there are two important parts, first is the impeller which creates velocity through rotation. And the second is the casing which converts this velocity into pressure by the change in the area.
• When fluid is inside the impeller, then the speed of the fluid experience a rotational motion because of the torque provided by the prime mover, and flow is characterized as a forced vortex flow. When the fluid comes out from the rotating impeller at that time also fluid has a vortex motion because of the inertia of the fluid. But since the external torque is absent outside the casing therefore it becomes free vortex flow.

Closed impellers (Two-sides shrouded):

• In the closed or shrouded impellers, the vanes are covered with shrouds (side plates) on both sides
• The back shroud is mounted into the shaft and the front shroud is coupled by the vanes
• This ensures full capacity operation with high efficiency for a prolonged running period
• This type of impeller is meant to pump only clear water, hot water and acids

Semi-open impeller (One-side shrouded):

• It has a plate (shroud) only on the backside
• The design is adapted to industrial pump problems which require a rugged pump to handle liquids containing fibrous material such as paper pulp, sugar molasses and sewage water etc.

Open impeller:

• In open impeller, no shroud or plate is provided on either side i.e. the vanes are open on both sides
• Such pumps are used where the pump has a very rough duty to perform i.e. to handle abrasive liquids such as a mixture of water, sand, pebbles and clay, wherein the solid contents may be as high as 25%.

Thus, centrifugal pumps dealing with water containing slurry and sewage​ have an open impeller.

Centrifugal pump is a turbomachinery. Turbomachines are machines that transfer energy between a rotor and a fluid, including both turbines and compressors. It is a mechanical device.

The main function of nozzle is to vary the pressure of fluid passing through the nozzle. It is done by opening and shutting the sets of nozzles. Thus, its main function is to regulate pressure of the fluid.

Centrifugal pumps transfer energy from rotor to fluid. The primary objective of a centrifugal pump is to transfer energy. Centrifugal pump is a turbomachinery.