What is Valve Cavitation? (Animation)

What is Valve Cavitation? (Animation)

Metal erosion is a costly problem
it attacks pumps, valves, piping, and other expensive equipment. No matter what type
of liquid is being used there’s always the risk of damage from flashing and
cavitation. Wherever there’s a restriction in flow, the decrease in
pressure can cause a fluid to reach its vapor point. Flashing occurs when the
fluid vaporizes and remains a vapor. Metal erosion caused by flashing appear
smooth and shiny. Cavitation occurs when the fluid vaporizes then returns to a
liquid state as the pressure increases downline. The metal damage caused by
cavitation is rough and irregular due to pitting of the surface. Cavitation is a
major source of damage in control valves and other components. Cavitation occurs
as a liquid passes through a restriction such as a valve. The restriction causes
the liquids velocity to increase and it’s pressure to decrease. The point of
maximum velocity and minimum pressure is called the vena contracta.
Vapor bubbles form in the liquid when the pressure falls to near the level of
the liquids vapor pressure. When the pressure recovers downstream, the vapor
bubbles implode and return to liquid form. After the initial bubble formation and collapse, the bubble may reform and
collapse a second time. Liquid micro-jets form when the
recovering pressure makes an indentation in a bubble, then the micro-jet bursts
through the bubble. These implosions can also cause local
pressure waves of up to 100,000 pounds per square inch.
The combination of pressure waves and micro-jets can cause severe damage to
the valve plug, seat, and body when they’re located near the material
surface. Cavitation can also cause unacceptable
noise and vibration that reduce efficiency or lead to loss of process
control. Even though cavitation occurs, it doesn’t always cause damage. The extent
of cavitation damage is affected by the following factors. The intensity of the
cavitation. Greater drops in pressure increase the potential for damage.
The materials used in the construction of the area where the cavitation occurs.
Hardened materials reduce damage. The length of exposure to cavitation. The more frequently that cavitation occurs in an area the more likely it is
to sustain damage. Valve size. Increasing the valve size tends to make the effects of cavitation worse. The design of the valve and trim in the
area of cavitation. High recovery ball and butterfly valves are more
susceptible to cavitation damage. If there is leakage occurring when a valve
is closed. Fluid leakage moves fluid from a high pressure area to a low pressure
area which results in cavitation and potential damage. Cavitation damage can be reduced by altering or accommodating any of these
factors. Cavitation damage can often be reduced
or prevented by using the proper components. Emerson offers these
solutions. Valve liners are constructed of hardened materials to protect the
valve body. Pressure staging keeps the pressure from
falling to near a liquids vapor pressure which helps prevent vapor bubbles from
forming. Pressure drops are staged by the use of
expanding nozzles or expanding flow areas. For example, in the Cavitrol III valve trim nozzles have holes shaped to manage flow separation. Multiple small holes shift the noise to higher frequencies. Cavitation is isolated by directing fluid into the center of the valve using
a flow down orientation so bubbles employed away from the valve components. In addition, the Cavitrol IV valve trim has expanding flow areas that
decrease pressure as the fluid flows through. Limits changes in pressure at
each step to prevent fluid from reaching the vaporization point and prevents
leakage. Fisher control valve technology from
Emerson Process Management provides customers around the world with leading
critical valve solutions. You can count on Fisher products backed by its 125
years of valve development and manufacturing experience to solve your
critical valve needs.

9 Replies to “What is Valve Cavitation? (Animation)”

  1. I keep having problem with my home water pump. I bought 4 water pumps within 10 years. 3 of them are same hitachi brand, and it keep on overheat and break down. Last year i bought another pump but different brand and now it started making a sound like having gravels in it, and i really dont know what happen until i watch this video, cavitation. But i still not understand, what should i do?
    My water pump was installed just beside water tank and it lay on flat ground as the water tank too. My water pump and tank seperated by roughly 1 ft pipe connecting the 2. Is it too near? How does pipe size can cause cavitation? Please help advice.

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