Lesson 11 - Control of Fluid Flow 2 SAQ

*Describe the mechanism of cavitation, its cause and the impact on pump operation and integrity.

Response:

Cavitation is the generation and subsequent collapse of gas or vapour bubbles due to localised reductions in pressure within a pump/pipeline system. The cause of cavitation is related to flashing (or vapour generation), which will occur if the pressure on a liquid is reduced to or below a threshold pressure. Threshold pressure differs depending on whether a dissolved gas is present, or if the liquid is gas free and its vapour generated.

Threshold pressure is:

• The vapour pressure if vapour is being generated from the liquid or

• The equilibrium pressure if dissolved gas comes out of solution from the liquid.

Cavitation can impact pump performance and its integrity in the following ways:

• Impaired pump performance can happen when cavitation creates vapour or gas. These can impair the performance of a pump by gas-binding the pump.

• Slugs of unwanted vapour can adversely affect pipeline instrumentation, such as flow meters, which would register values vastly different from the actual flowrate.

• Physical damage to pumps, valves, pipes, and the liquid being transported can occur due to the short-lived but high pressures generated as the cavities re-fill. This can become apparent as:

  • Severe surface pitting and erosion, also known as spalling, which can lead to reduced component life cycle.

  • Permanent loss of performance due to the loss of close tolerance geometry – prone in valves and pumps.