3.16 Cavitation

Question 1

is a form of wear caused by the formation and instantaneous collapse of innumerable tiny vapor bubbles.

Answer 1

Cavitation

Question 2

Affected Materials

Answer 2

Copper and brass, cast iron, carbon steel, low-alloy steels, 300 series SS, 400 series SS, and nickel-based alloys

Question 3

A critical factor in a pump, is the difference between the _________________________ and the __________________ of that liquid. Is called the net positive suction head (NPSH) available.

Answer 3

between the actual pressure on the suction side ; vapor pressure

Question 4

Críticals factors are:

Answer 4

a) Pressure diferencial.
b) Temperatures approaching the boiling point of the liquid
c) The presence of solid or abrasive particles
d) Corrosive environment (cavitation-corrosion)

Question 5

Cavitation is most often observed in

Answer 5

• pump casings
• pump impellers (low-pressure side), and in
• piping downstream of orifices or control valves.
• heat exchanger tubes, venturis, and seals. restricted-flow passages or other areas where turbulent flow is subjected to rapid pressure changes within a localized region.

Question 6

Damage is typically ____________to the cavitation zone. generally looks like _________________ but may also have a gouged appearance in rotational components

Answer 6

localized ; sharp-edged pitting

Question 7

Cavitating pumps or downstream of control valves may sound like ____________________ inside and are typically accompanied by _______________.

Answer 7

pebbles are pumbling or rattling ; higher vibrations

Question 8

___________________________ is usually required in order to fix a cavitation problem. Resistance to cavitation damage may not be significantly improved by a material change. However, __________________ can help improve cavitation resistance in some situations.

Answer 8

A mechanical modification or design or operating change ; wear-resistant alloys and ceramic coatings

Question 9

Cavitation is best prevented by avoiding conditions that allow the absolute pressure to fall below the vapor pressure of the liquid. Examples of steps that can be taken include:

Answer 9

1. streamlining the flow path to reduce turbulence;
2. decreasing fluid velocities;
3. removing entrained air;
4. increasing the suction pressure of pumps while reviewing the pump efficiency curve;
5. altering the fluid properties, perhaps by adding additives;
6. using hard surfacing or hardfacing; or
7. using a harder and/or more corrosion-resistant material.

Question 10

Inspeciton and monitoring

Answer 10

1. VT of suspect areas, including use of a boroscope.
2. UT, including manual UT scanning and/or AUT. However, since the damage is normally highly localized, it might be difficult to pinpoint the location of the damage.
3. RT.
4. Acoustic monitoring.
5. monitoring of fluid properties to find locations of highly turbulent flow.