Safeguarding your
process flow control
system
1
Understanding
Flashing and
Cavitation
Cavitation and flashing are major causes of metal erosion in
flow control systems. It occurs when a media goes through
a valve and the resulting pressure drop causes the media to
increase in velocity and turn into a vapor. If it stays in a vapor
state the resulting flow is called flashing and damage to the
flow control system is smooth and shiny. If it returns to a liquid
state, it is called cavitation.
As the vapor returns to the liquid state, it forms liquid microjets
and the pressure in the flow control system cause these
microjets to burst, sending pressure waves of up to 100,000
PSI of force over the valve and piping surfaces. This bursting
is the cause of pitting and rough surfaces in the flow control
system. It also causes noise and vibration which can damage
the flow control system and be harmful to workers safety.
There are several factors that impact the effects of flashing
and cavitation including:
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Intensity caused by greater pressure drops
Materials the valve is made from
Length of exposure
Frequency of flashing and cavitation
Valve size
Design of the valve and trim
A properly designed system using appropriate materials can
add years to the productive life of capital assets and provide
greater efficiency and quality to a process control system.
But, where to start?
2
Flow Control
Considerations
To minimize the effects of turbulence and cavitation on
a flow control system, several steps need to be taken.
First, anything that impedes the flow of the media has
to be considered. This is usually a valve, but could be
changes in the flow direction like a ninety degree bend
in a pipe. As the media flows past the restriction, there
is lower pressure on the outlet side. With this reduction
in pressure comes an increase in media velocity which
can cause the liquid to move to a vapor state. So
making this pressure reduction as low as possible is
key.
Next, the flow past the restriction should be optimized
to limit the amount of turbulence that is caused by the
obstruction. Figure 1 shows the impact of a butterfly
valve and it’s easy to see the downstream impact of the
obstruction.
Figure 1: Butterfly Valve during throttling
Another key consideration is the materials used in the
flow control system. Depending on the intensity and
the duration of cavitation a system is exposed to, the
materials selected are of prime importance. As patents
have expired and flow control vendors compete on
price rather than innovation, it is possible to get subpar
materials into various parts of a flow control system.
3
The Solution
With the valve as the key component of a well
designed and installed flow control system, let’s
consider the attributes that a well-engineered valve
should have.
Zero pressure drop. The valve should have zero
pressure drop between the inlet and the outlet of the
valve. This maintains a constant velocity of flow and
limits the potential for a liquid to turn into a vapor.
Precise flow control. The valve should be able to
manage the flow of media to reduce the introduction of
turbulence. This can be achieved by having surfaces
that are engineered and designed to limit flat points of
contact with the media.
Leak-proof closure. Even small amounts of media can
cause cavitation. The valve should be leak-proof even
under high pressures.
Only one valve on the market today has the right set
of features to achieve these benefits. The Shutter
Valve® from Clarke Industrial Engineering combines
the correct mix of leak-proof closure up to 1,000 PSI,
precision flow control using aeronautically designed
control petals and has zero pressure drop so the
pressure at the valve inlet equals the pressure at the
valve outlet.
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Clarke Industrial Engineering
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North Kingstown, RI 02852
(401) 667-7880
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