Brochure

4 | Fisher
®
Cavitation-Control Technologies
Material Damage
Physical damage to a control valve due to uncontrolled
cavitation is a concern because of the high maintenance cost,
inconvenience, unpredictability, and unplanned downtime.
Damage can consist of a mechanical and selective chemical
attack on the material surface.
Typically, cavitation damage is characterized by a very
irregular, pitted, rough appearance. Valve parts with extensive
damage may have large amounts of material missing.
The mechanical attack can occur in two forms: microjet
impingement and shockwave impingement. Microjet
impingement causes erosion of the material surface. Shockwave
impingement causes material deformation and failure.
A chemical attack occurs when the protective, passive oxide
layer is physically removed from the base metal during the
mechanical attack. The base material is left vulnerable to
further chemical attack.
The science of vapor cavity nucleation, growth, collapse, and
rebound is at the core of cavitation damage. Rather than
go through the science, the focus will be on how to reduce
cavitation effects. Further information is available by contacting
your local Emerson Process Management sales office.
There are several factors that determine a material’s
performance in a cavitating situation, including its toughness,
hardness, and corrosion resistance in the application
environment. These factors are discussed in the materials of
design section.
Factors Affecting Cavitation Damage
Cavitation doesn’t always cause damage when it occurs.
The extent of cavitation damage is a function of the
following factors:
n Intensity/magnitude of pressure drop – Greater drops in
pressure increase the potential for damage.
n Materials of construction – Hardened materials reduce
damage. Examples include R30006/CoCr-A, nickel-
chromium-boron alloys, hardened S44004 stainless steel,
hardened S17400 stainless steel, and hardened S41000/
S41600 stainless steel.
n Time of exposure – The longer cavitation occurs in an area,
the more likely it is to result in damage.
n Quantity of flow – Cavitation issues generally scale with
flow rates. A larger flow rate means more fluid is available
to cavitate and there is a greater potential for cavitation
damage.
n Valve/trim design – Fisher control valve trim can be
selected to combat the damaging effects of cavitation using
isolation or elimination techniques
.
n Leakage while closed – If seat leakage occurs when a valve
is closed, the liquid moves quickly from a high-pressure area
to a low-pressure area, which may result in cavitation and
potential damage.
n Fluid – Fluid behavior should be considered in trim selection.
For instance, water in a power plant behaves differently than
crude oil for the same service conditions.
Effects of Damage
This valve plug has sustained
extensive cavitation damage.
It shows a complete loss of
seating surfaces.