Cavitation

Overview

Engineering reference data for Cavitation in fluid mechanics.

Key Formulas

Reynolds Number

$Re = \frac{\rho v D}{\mu}$

Ratio of inertial to viscous forces — determines flow regime.

Bernoulli's Equation

$P + \frac{1}{2}\rho v^2 + \rho g h = \text{const}$

Conservation of energy for steady, inviscid, incompressible flow.

Continuity Equation

$A_1 v_1 = A_2 v_2$

Conservation of mass for incompressible flow.

Darcy-Weisbach

$\Delta P = f \frac{L}{D} \frac{\rho v^2}{2}$

Pressure drop due to friction in a pipe.

Variables

Avoiding Cavitation

Cavitation can be avoided by increasing the distance between the actual local static pressure in the fluid and the vapor pressure of the fluid at the actual temperature. This can be achieved through three primary approaches:

Methods to Prevent Cavitation

1. Re-engineering components — Use special components designed for harsh conditions:

   • Multi-stage control valves for conditions with large pressure drops
   • Special pumps (non-centrifugal) for challenging conditions with fluid temperatures near vaporization temperature

2. Increasing the total or local static pressure — This increases the distance between static pressure and vaporization pressure. Local static pressure can be increased by lowering (elevation) the component in the system. Control valves and pumps should generally be positioned in the lowest part of a system to maximize static head.

3. Reducing the fluid temperature — Since vapor pressure increases dramatically with temperature, locating components in the coldest part of systems reduces cavitation risk.

Cavitation Number

The Cavitation Number expresses the ratio between static pressure and vaporization pressure, serving as an indicator of cavitation likelihood:

$\sigma = \frac{p - p_v}{\frac{1}{2}\rho v^2}$

Where:

• $p$ = local static pressure (Pa)
• $p_v$ = vapor pressure of the liquid (Pa)
• $\rho$ = fluid density (kg/m³)
• $v$ = fluid velocity (m/s)

Lower values of $\sigma$ indicate higher cavitation risk.

Water Vapor Pressure Data

Practical Applications

Boiler feed pumps — Common to position pumps receiving hot condensate (~100°C) at the lowest elevation to maximize static head and prevent cavitation.

Heating systems — Pumps and modulating valves are typically located in the cold return lines before heaters and heat exchangers to minimize cavitation risk.

References

• Original Source