Cavitation and Water Hammer Phenomena in Fluid Systems

Understanding Cavitation in Fluid Dynamics

Cavitation, sometimes referred to as suction vacuum, is a hydrodynamic effect that occurs when water or another liquid fluid passes at high speed over a sharp edge, producing a fluid decompression due to the conservation of the Bernoulli constant (Bernoulli's Principle). It can reach the vapor pressure of the liquid so that its constituent molecules immediately change to a vapor state, forming bubbles or, more correctly, cavities. The formed bubbles travel to areas of higher pressure and implode (the vapor suddenly returns to a liquid state, abruptly collapsing the bubbles), producing a trail of gas and potentially damaging the metal surface where this phenomenon occurs.

Discharge Cavitation Explained

Discharge cavitation occurs when the pump's discharge pressure is excessively high. This typically happens when a pump operates at less than 10% of its best efficiency point (BEP).

Suction Cavitation Explained

Suction cavitation occurs when the pump suction operates under low-pressure or high-vacuum conditions, causing the liquid to vaporize at the impeller inlet.

Understanding Water Hammer Effects

A water hammer, also known as a fluid hammer, is caused because the fluid is slightly elastic (although in many situations, fluids can be considered incompressible). Consequently, when a valve or faucet at the end of a long pipe is abruptly closed, the fluid particles that have stopped are impacted by those behind them still in motion. This creates a pressure surge (shock wave) that travels through the pipe at a speed that can exceed the speed of sound in the fluid. This pressure surge has two primary effects: it slightly compresses the fluid, reducing its volume, and it slightly expands the pipe. When all the fluid in the pipe has stopped, the momentum ceases, and the compressed fluid tends to expand back. Simultaneously, the slightly expanded pipe tends to contract back to its normal size. Combined, these effects generate another pressure wave traveling in the opposite direction. The fluid attempts to move in the reverse direction, but since the valve is closed, this creates a low-pressure area (depression) relative to the normal pipe pressure. At this reduced pressure, the liquid can vaporize (flash), forming a vapor bubble as the pipe contracts. Upon reaching the other end of the pipe (e.g., a reservoir at atmospheric pressure), if the wave hasn't dissipated, it reflects back. The wave gradually diminishes due to friction, fluid compressibility, and pipe elasticity.