Hydraulic Machines MCQ with answer and solution

Concept

Principle and necessity of centrifugal pump priming

Formula

P = ρ × g × H

Step-by-step solution

1. A centrifugal pump generates head by imparting kinetic energy to fluid, developing pressure P = ρ × g × H. 2. Because the density of air is only about 1/800 of water, an air-filled impeller develops negligible suction pressure. 3. Priming is the process of completely filling the pump casing and suction pipe with liquid to evacuate all entrapped air. 4. This allows the impeller to develop sufficient suction to lift water from the sump. Answer: C

Explanation

Centrifugal pumps create head through centrifugal action (H = v² / 2g), with the resulting pressure rise proportional to fluid density (P = ρ × g × H). Because air density is roughly 800 times lower than water, an air-filled casing develops negligible pressure that cannot draw liquid from the sump; priming expels all air and fills the pump casing and suction line with liquid.

Why the other options are wrong

A) While an unprimed pump fails to pump, priming is a startup requirement rather than an efficiency optimization. B) Cavitation is controlled by providing adequate Net Positive Suction Head (NPSH), not priming. D) Impeller bearings are external and lubricated with oil or grease.

Shortcut

Priming = Evacuates air from pump casing and suction line so liquid head can develop.

Common mistake

Believing priming is meant to prevent cavitation or lubricate bearings.