How do vapour bubbles cause noise and damage inside a pump?

In engineering, the phenomenon you are describing is known as cavitation. It occurs when liquid in a pump is accelerated to high velocities, causing the local pressure to drop to the liquid's vapour pressure.12

This process creates vapour bubbles that behave in the following ways:

• Noise and Vibration: As these bubbles move into regions of higher pressure, they collapse suddenly and violently. This rapid growth and collapse cycle generates significant noise, knocking, and vibrations within the pump.12
• Physical Damage: The collapse of these cavities creates extreme localized pressures. When these bubbles collapse near metal surfaces, such as rotating blades, the resulting force pits and erodes the material.12
• Efficiency Loss: Beyond physical damage, cavitation distorts the flow pattern of the liquid, which leads to a marked reduction in the pump's operational efficiency.1

To prevent these issues, engineers must carefully design blade passages and flow paths to maintain pressure levels that avoid the formation of these vapour bubbles.2