Cavitation - an Introduction

Cavitation occurs in fluid flow systems where the local static pressure is below vapor pressure

Cavitation is a common problem in pumps and control valves - causing serious wear, tear and damage. Under the wrong conditions cavitation reduces component life time dramatically.

What is Cavitation?

Cavitation may occur when local static pressure in a fluid reach a level below the vapor pressure of the liquid at the actual temperature. According the Bernoulli Equation this may happen when the fluid accelerates in a control valve or around a pump impeller.

The vaporization itself does not cause the damage - the damage happens when the vapor almost immediately collapses after evaporation when the velocity is decreased and pressure is increased.

Avoiding Cavitation

Cavitation can in general be avoided by

  • increasing the distance (pressure difference) between the actual local static pressure in the fluid - and the vapor pressure of the fluid at the actual temperature

This can be done by:

  • reengineering components initiating high speed velocities and low static pressures
  • increasing the total or local static pressure in the system
  • reducing the temperature of the fluid

Reengineering of Components Initiating High Speed Velocity and Low Static Pressure

Cavitation and damage can be avoided by using special components designed for the actual rough conditions.

  • conditions with huge pressure drops can - with limitations - be handled by Multi Stage Control Valves
  • challenging pumping conditions - with fluid temperatures close to the vaporization temperature - can be handled with  special pumps - working after other principles than centrifugal pumps

Increasing the Total or Local Pressure in the System

By increasing the total or local pressure in the system the distance between the static pressure and the vaporization pressure is increased and vaporization and cavitation can be avoided.

The ratio between static pressure and the vaporization pressure - an indication of the possibility of vaporization, is often expressed by the Cavitation Number.

Unfortunately it is not always possible to increase total static pressure due to systems classifications or other limitations. Local static pressure in components may be increased by lowering (elevation) the component in the system. Control valves and pumps should in general be positioned in the lowest part of the systems to maximize static head.

This is a common solution for boiler feeding pumps receiving hot condensate (water close to 100 oC) from condensate receivers.

Reducing the Fluid Temperature

The vaporization pressure depends of fluid temperature. Vapor pressure of Water, our most common fluid:

Vapor Pressure
0 0.6
5 0.9
10 1.2
15 1.7
20 2.3
25 3.2
30 4.3
35 5.6
40 7.7
45 9.6
50 12.5
55 15.7
60 20
65 25
70 32.1
75 38.6
80 47.5
85 57.8
90 70
95 84.5
100 101.33

Note! - possible evaporation - and cavitation - increases dramatically with water temperature.

Cavitation can be avoided by locating components to the coldest part of systems. Example - it is common to locate pumps and modulating valves in heating systems in the "cold" return lines before heaters and heatexchangers.

Related Topics

  • Fluid Mechanics - The study of fluids - liquids and gases. Involves various properties of the fluid, such as velocity, pressure, density and temperature, as functions of space and time.
  • Pumps - Piping systems and pumps - centrifugal pumps, displacement pumps - cavitation, viscosity, head and pressure, power consumption and more

Related Documents

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