Hydraulic Machines MCQ with answer and solution

Concept

A centrifugal pump is primed (filled with liquid) before starting so that air or gas trapped inside does not stop it from working (gas or air binding).

Step-by-step solution

1. Gas binding (air binding) happens when air or gas is trapped in the pump casing and suction pipe. 2. The impeller then rotates in air, and the head it develops in air is very small. 3. So the pump cannot lift water. 4. To remove this air, the casing and suction pipe are filled with water before starting. This is called priming. 5. So the answer is (D) primed.

Explanation

Pressure head developed by a pump is proportional to the density of the fluid. Air density is about 1/800 of water, so a pump full of air develops almost no head. Priming removes the air and fills the pump with water, so gas binding is avoided.

Why the other options are wrong

A: Heating does not remove trapped air and can cause vaporisation and cavitation. B: Elevating the pump increases suction lift and makes air entry and cavitation worse. C: 'Charged' is not the standard term for removing air from a pump.

Shortcut

Air or gas binding is solved by priming.

Common mistake

Confusing priming (removing air) with methods used to prevent cavitation.