Erosion caused by the collapse of small vapour bubbles in pitting areas of low fluid pressure.
It causes pitting or localize LOWT D
orAMAGE
Sharp-edged
MECHANISMSlocal grooving
AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 81
3.16 Cavitation
3.16.1 Description of Damage
a) Cavitation is a form of wear caused by the formation and instantaneous collapse of innumerable tiny vapor
bubbles.
b) The collapsing bubbles exert severe localized impact forces that can result in metal loss. (Figure 3-16-1)
c) The bubbles may contain the vapor phase of the liquid, air, or other gas entrained in the liquid medium.
3.16.2 Affected Materials
Most common materials of construction including copper and brass, cast iron, carbon steel, low-alloy steels, 300
series SS, 400 series SS, and nickel-based alloys can be affected by cavitation, although certain materials have
greater resistance than others.
3.16.3 Critical Factors
a) In a pump, the difference between the actual pressure or head of the liquid available (measured on the
suction side) and the vapor pressure of that liquid is called the net positive suction head (NPSH) available.
The minimum head required to prevent cavitation with a given liquid at a given flow rate is called the NPSH
required. Inadequate NPSH can result in cavitation.
b) Temperatures approaching the boiling point of the liquid are more likely to result in bubble formation than
lower-temperature operation.
c) The presence of solid or abrasive particles is not required for cavitation damage but will accelerate the
damage.
d) Cavitation taking place in a corrosive environment can be accelerated by the corrosive effects of the
environment. This is often referred to as cavitation-corrosion.
3.16.4 Affected Units or Equipment
a) Cavitation is most often observed in pump casings, pump impellers (low-pressure side), and in piping
downstream of orifices or control valves.
b) Damage can also be found in restricted-flow passages or other areas where turbulent flow is subjected to
rapid pressure changes within a localized region. Examples of affected equipment include heat exchanger
tubes, venturis, and seals.
3.16.5 Appearance or Morphology of Damage
a) Cavitation damage generally looks like sharp-edged pitting but may also have a gouged appearance in
rotational components. Damage is typically localized to the cavitation zone. (Figure 3-16-2 to Figure 3-16-5)
b) Cavitating pumps or downstream of control valves may sound like pebbles are tumbling or rattling inside and
are typically accompanied by higher vibrations.
3.16.6 Prevention/Mitigation
a) A mechanical modification or design or operating change is usually required in order to fix a cavitation
problem. Resistance to cavitation damage may not be significantly improved by a material change. However,
wear-resistant alloys and ceramic coatings can help improve cavitation resistance in some situations.
b) Cavitation is best prevented by avoiding conditions that allow the absolute pressure to fall below the vapor
pressure of the liquid. Changing the material may also help. Examples of steps that can be taken include:
82 API RECOMMENDED PRACTICE 571
streamlining the flow path to reduce turbulence;
decreasing fluid velocities;
removing entrained air;
increasing the suction pressure of pumps while reviewing the pump efficiency curve;
altering the fluid properties, perhaps by adding additives;
using hard surfacing or hardfacing; or
using a harder and/or more corrosion-resistant material.
c) When attack is accelerated by the mechanical disruption of protective layers or films on the metal surface,
such as a protective corrosion scale or inhibitor film, changing to a more corrosion-resistant material may be
beneficial. However, changing to a higher hardness version of the same or a similar material in this situation
may not improve cavitation resistance. In addition, excessively hard materials may not be suitable if they lack
the toughness required to withstand the high local pressures and impact (shear loads) of the collapsing
bubbles.
3.16.7 Inspection and Monitoring
a) VT of suspect areas, including use of a boroscope if direct access is not available, can often identify cavitation
damage. Typically, the inspection is performed during plant shutdowns.
b) UT, including manual UT scanning and/or AUT, can be used for measuring remaining thickness at damaged
areas if the damaged area is large enough and smooth enough for UT to be effective. However, since the
damage is normally highly localized, it might be difficult to pinpoint the location of the damage.
It can be difficult to get accurate thickness readings on pump casings or other castings due to their
inherent thickness variability combined with the fact that inside and outside surfaces may not be parallel.
c) RT can be used to quantify thickness loss in affected components if accessibility allows. Pitting location and
depth are measured with quantitative radiographic techniques.
d) Acoustic monitoring of turbulent areas can detect characteristic sound frequencies associated with cavitation.
The technique is a qualitative method to determine damage progression.
e) Other techniques include monitoring of fluid properties to find locations of highly turbulent flow.
3.16.8 Related Mechanisms
Erosion and erosion-corrosion (3.27).
3.16.9 References
1. “Evaluation of Erosion and Cavitation,” Metals Handbook—Corrosion, Volume 13, ASM International,
Materials Park, OH.
2. C.P. Dillon, Corrosion Control in the Chemical Process Industries, Materials Technology Institute (printed by
NACE), MTI Publication No 45, Second Edition, St. Louis, MO, 1994.
3. V.R. Pludek, Design and Corrosion Control, Macmillan Press, 1979.
DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 83
Figure 3-16-1—Mechanism of cavitation damage. As vapor bubbles collapse, microjets
form that result in high forces that damage the equipment.
Figure 3-16-2—Cutaway of a CS butterfly valve with cavitation damage after 2 years
of service due to a high pressure drop across the valve in a hydrocarbon drain line
off a cold low-pressure separator in an atmospheric resid desulfurizing unit.
CAVITATION
Description Appearance
Erosion caused by the collapse Sharp-edged local grooving or
of small vapour bubbles in pitting
areas of low fluid pressure
ru
rum
Inspection: Rotating components can vibrate badly under
cavitation.
VT or UT/RT to identify loss of wall thickness
(LOWT).
Critical factors: Occurs on pump impellers/casings, downstream
of control valves/orifice plates, heat exchangers
and other areas with rapid fluid flow/pressure
variations
Requires a low-pressure area to occur, e.g.
suction side of pump impellers.
FFP/Severity: On rotating components, unbalance/vibration
may be more important than LOWT.
Strong/hard materials can be more prone to
cavitation damage than softer, ductile ones.
References: API 571 (4.2.15). Pitting assessment (from one
side of component only) to API 579 (Sec. 6).
84 API RECOMMENDED PRACTICE 571
Figure 3-16-3—Closer view of the damaged surface of the butterfly valve in Figure 3-16-2.
Figure 3-16-4—Cavitation pitting on the low-pressure side of a stainless steel pump impeller.
DAMAGE MECHANISMS AFFECTING FIXED EQUIPMENT IN THE REFINING INDUSTRY 85
Figure 3-16-5—Pitting caused by cavitation on the water side of a
cast iron cylinder liner in a large engine.
