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# Cavitation

Cavitation is a common problem in pumps and control valves - causing serious wear, tear and damage. Under the wrong conditions cavitation reduces components 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 a 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 after evaporation collapses when velocity decreases and pressure increases.

### 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:

• re-engineering 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

### Re-engineering 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 a system to maximize the static head.

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

### Reducing the Fluid Temperature

Vaporization pressure depends of fluid temperature. The vapor pressure for water - our most common fluid - is indicated below:

Cavitation - Water Vapor Pressure vs. Temperature
Temperature
(oC)

Vapor Pressure
(kPa, kN/m2)

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! - be aware that evaporation pressure - and possible 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 heat-exchangers.

## Related Topics

### • Fluid Mechanics

The study of fluids - liquids and gases. Involving velocity, pressure, density and temperature as functions of space and time.

### • Pumps

Design of pumping systems and pipelines. With centrifugal pumps, displacement pumps, cavitation, fluid viscosity, head and pressure, power consumption and more.

## Related Documents

### Boiler Feed Water - Chemistry and Impurity Limits

ABMA recommended feed water chemistry limits for steam boilers.

### Boiling Fluids - Max Suction Flow Velocities

Recommended max suction flow velocity when pumping boiling fluids.

### Cavitation Number

An introduction and definition of the Cavitation Number.

### Condensate Pumping

High temperatures and danger of impeller cavitation is the major challenge for condensate pumping in steam systems.

### Control Valves - Cavitation

Control valves and cavitation, application ratio and multi stage control valves.

### Feed Pumps in Steam Systems - Suction Lift Head vs. Temperature

Cavitation of impellers increases with water temperatures.

### Feed Water Treatment to Avoid Corrosion

Make-up water to steam boilers should be treated with oxygen scavengers to avoid serious corrosion problems.

### Light Oil Suction - Flow Velocities

Recommended max. suction flow velocities when pumping light oils.

### Pumps - Suction Specific Speed

Suction Specific Speed can be used to determine stable and reliable operations for pumps with max efficiency without cavitation.

### Valves - Typical Operating Temperatures

Operating temperatures for typical categories of valves - ball valves, butterfly valves and more.

### Viscous Liquids - Max. Suction Flow Velocities

Recommended max. pump suction flow velocity for viscous fluids.

### Water - Max. Suction Flow Velocities vs. Pipe Size

Recommended max. water flow velocities on suction sides of pumps.

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