Q21Basic Mechanical Engineering
Question
With a suitable sketch explain the working of centrifugal pump.
Answer
A centrifugal pump uses a high-speed rotating impeller enclosed in a volute casing to impart kinetic energy to water, which is then converted into high pressure to lift the fluid.
Working Principle of a Centrifugal Pump: Centrifugal pumps are the most widely used pumps in the world for handling large volumes of liquids. They operate on the principle of forced vortex flow. When a mass of liquid is rotated by an external torque, its pressure head increases radially outward due to centrifugal force. This massive pressure difference literally throws the water out of the pump.
Key Components: - Impeller: A rapidly spinning wheel with backward-curved blades, driven directly by an electric motor. - Volute Casing: A specially designed, snail-shaped airtight chamber surrounding the impeller. The cross-sectional area of this casing gradually increases towards the delivery pipe. - Suction Pipe & Foot Valve: The pipe dipping into the water source. The foot valve is a one-way valve at the bottom ensuring water only goes up, and a strainer prevents debris entry.
The Working Mechanism:
- 1. Priming: Before starting, the entire casing and suction pipe MUST be manually filled with water to completely expel all air. If the pump is full of air, it cannot generate enough suction.
- 2. Kinetic Energy Impartation: When the electric motor is turned on, the impeller spins at high speed (e.g., 3000 RPM). The water trapped between the blades is violently rotated. The centrifugal force hurls this water radially outward towards the casing at an extremely high velocity. This creates a powerful vacuum (low pressure) at the center 'eye' of the impeller.
- 3. Suction Lift: Because a vacuum is created at the eye, the atmospheric pressure resting on the surface of the sump water forces a continuous stream of new water rapidly up the suction pipe and into the center of the spinning impeller.
- 4. Pressure Conversion: As the high-velocity water is flung outward off the impeller, it enters the volute casing. Because the area of the volute casing gradually expands, the velocity of the water is forced to drop. According to Bernoulli's principle, this sudden decrease in kinetic energy (velocity) is converted directly into a massive increase in pressure energy. This high-pressure water is then forced up the delivery pipe to its destination.