Fluid Mechanics and Machinery

Md. Mohiuddin

Lecturer

Department of Mechanical Engineering

ME 3219

Centrifugal Pump

What is Centrifugal Pump?

• It is a type of rotodynamic type of pumps in which a dynamic pressure is developed which enables the lifting of liquids from a lower to a higher level.
• The basic principle of a centrifugal pump is that when a certain mass of liquid is made to rotate by an external force, it is thrown away from the central axis of rotation and a centrifugal head is impressed which enables it to rise to a higher level.
• By consistently providing more liquid at the center of rotation, a continuous supply at an elevated level is achieved.
• The name "centrifugal pumps" stems from the fact that their liquid lifting mechanism relies on centrifugal action.

Parts of a Centrifugal Pump

1. Impeller:
1. a wheel with backward curved blades or vanes, is fixed to a shaft driven by an external energy source, typically an electric motor, which provides the necessary rotational motion.
2. They can be classified as closed, semi-open, and open impellers.
3. The closed type contains shrouds on both sides which enhances efficiency by guiding pure, debris-free liquids effectively. One of the shroud is called base and the other one is called crown plate.
4. Semi-open impellers, lacking a crown plate but having a base plate, handle liquids with some debris.
5. Open impellers does not have crown and base plates, excel in pumping liquids with suspended solid matter, resisting clogging when dealing with debris-laden fluids.

Closed Vane

Semi-closed Vane

Open Vane

2. Casing:
1. An airtight chamber encloses the impeller called casing.
3. Suction pipe:
1. Connected to the pump's inlet or the impeller's center (known as the eye) which draws liquid from a suction tank or sump.
2. Equipped with a foot valve and strainer at its lower end, the suction pipe prevents debris like leaves and wooden pieces from entering the pump.
3. Liquid passes through the strainer, then the foot valve—a one-way valve allowing upward flow only—ensuring the liquid moves towards the pump and not back into the sump.

Parts of a Centrifugal Pump

1. Priming:
1. Priming is the initial step in operating a centrifugal pump.
2. Priming involves filling the suction pipe, pump casing, and the portion of the delivery pipe up to the delivery valve with the liquid to be pumped.
3. The goal of priming is to expel all air (or gas or vapor) from the mentioned portions of the pump.
4. Without proper priming, the presence of air pockets may prevent the effective operation of the pump.
5. Centrifugal pump performance relies on generating pressure in the impeller, which is directly proportional to the fluid density in contact with it.
6. Rotating the impeller in the presence of air generates negligible pressure which makes the pump ineffective in lifting liquid.

Working of Centrifugal Pumps

• After priming the pump, the delivery valve remains closed during the initial phase.
• The electric motor is then started to set the impeller in rotation.
• Keeping the delivery valve closed at the start reduces the initial torque required by the motor.
• The rotating impeller within the liquid-filled casing generates a forced vortex.
• This vortex imparts a centrifugal head to the liquid, leading to an increase in pressure throughout the liquid mass.
• Pressure increase at any point is directly proportional to the square of the angular velocity and the distance from the axis of rotation.
• A sufficiently high rotation speed of the impeller significantly raises the pressure in the liquid surrounding it.

Working of Centrifugal Pumps

• While the delivery valve is closed, the rotating impeller agitates the liquid within the casing.
• When the delivery valve is opened, the liquid flows outward radially.
• This flow leaves the impeller vanes at the outer circumference with high velocity and pressure.
• Centrifugal action at the impeller's eye creates a partial vacuum.
• Atmospheric pressure causes liquid from the sump to rush through the suction pipe to the impeller's eye, replacing the discharged liquid.
• The liquid leaving the impeller at high pressure is employed to lift the liquid to the required height through the delivery pipe.

Working of Centrifugal Pumps

• Centrifugal pumps are classified into the following two classes
1. Volute pump.
2. Diffuser or turbine pump

TYPES OF CENTRIFUGAL PUMPS

Volute pump

Diffuser pump

• In a volute pump the impeller is surrounded by a spiral-shaped casing which is known as a volute chamber.
• As shown in the Figure, the shape of the casing is such that the sectional area of flow around the periphery of the impeller gradually increases from the tongue T towards the delivery pipe.
• This increase in the cross-sectional area results in developing a uniform velocity throughout the casing, because as the flow progresses from the tongue T towards the delivery pipe, more and more liquid is added to the stream from the periphery of the impeller. The volute casing may be designed to have the velocity of flow approximately equal to that of the liquid leaving the impeller. If the casing is designed according to this consideration then the loss of energy is considerably reduced, but the conversion of kinetic energy into useful pressure energy will not be possible leading to eddy loss.

Volute Pump

• On the other hand if the casing is so designed that the casing velocity may be kept down to the value of the velocity in the delivery pipe, then there will be a considerable loss of energy due to the difference between the casing velocity and that of the liquid discharged from the impeller.
• As such a compromise design is often used in which the casing is gradually enlarged so that the velocity is gradually reduced, from the velocity of the liquid leaving the impeller to that in the delivery pipe.

Volute Pump

• Diffuser or Turbine Pump. In the diffuser pump, the impeller is surrounded by a series of guide vanes mounted on a ring called the diffuser ring.
• The diffuser ring and the guide vanes are fixed in position.
• The adjacent guide vanes provide gradually enlarged passages for the flow of liquid.
• The liquid after leaving the impeller, passes through these passages of increasing area, wherein the velocity of flow decreases and the pressure increases.
• The guide vanes are so designed that the liquid emerging from the impeller enters these passages without shock.

Diffuser Pump

• This condition may be achieved by making the tangent to the guide vane at the inlet tip coincide with the direction of the absolute velocity of liquid leaving the impeller.
• After passing through the guide vanes the liquid flows into the surrounding casing which may be circular, and concentric with the impeller or it may be volute-shaped like that of the volute pump. However, the common practice is to adopt a circular casing for these pumps.

Diffuser Pump

• The centrifugal pumps may also be classified based on certain other factors as indicated below:
1. Number of impellers per shaft.
1. Single Stage: Only one impeller on the shaft.
2. Multi Stage: More than one impeller on the same shaft.
2. Relative direction of flow of liquid through the impeller.
• Radial flow pump
• Mixed flow pump
• Axial flow pump
3. Number of entrances to the impeller: Single suction/ Double suction
4. Disposition of the shaft: Horizontal or vertical positioning of the shaft
5. Working head: Low head, medium head, high head

Classification

Work done by Reciprocating Pump

Manometric Height

• The manometric head is the head against which a centrifugal pump has to work.

Efficiencies of a Centrifugal Pump

1. Manometric Efficiency
• Is defined as the ratio of the manometric head developed by the pump to the head imparted by the impeller to the liquid.

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• If Q is the volume of liquid delivered per second by the pump and w is the specific weight

Efficiencies of a Centrifugal Pump

2. Volumetric Efficiency:
• the ratio of the quantity of liquid discharged per second from the pump to the quantity passing per second through the impeller.

Efficiencies of a Centrifugal Pump

3. Mechanical Efficiency:
• the ratio of the power actually delivered by the impeller to the power supplied to the shaft by the prime mover or motor..

Efficiencies of a Centrifugal Pump

4. Overall Efficiency:
• the ratio of the power output from the pump to the power input from the prime mover to the shaft to drive the pump.

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• Overall efficiency is also equal to the product of all three efficiencies.

Difference between Centrifugal and Reciprocating Pump

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