Theory of flow through pipes, valves and fittings
Theory of flow through valves and fittings
Flashing and cavitation
Flow in pipes and valves theory - full content
Flashing or cavitation inside a valve can have a significant influence on valve capacity. Flashing and cavitation can reduce
the flow through valve in many liquid services. Also, damage can be made to the valve as well as to the piping system.
The effect is represented by the change from liquid to vapor state of fluid, resulting in the velocity increase downstream
from the valve.
As liquid passes through the restriction area inside the valve flow stream is contracted. The smallest cross section area
of stream is just downstream of the actual physical restriction at a point called vena contracta. At that point the velocity
is at its maximum and pressure at the minimum.
As the fluid exits the valve, away from vena contracta, velocity decrease and pressure increase, so the critical point
for flashing and cavitation is at the point where the pressure is smallest which is in vena contracta. If pressure at
vena contracta drops bellows the vapor pressure of the fluid, due to increased velocity at this point, bubbles will
form in the flow stream.
If pressure downstream of the vena contracta increase above the vapor pressure, bubbles will collapse or implode producing
cavitation. Cavitation releases energy and produces a noise. If cavitation occurs close to solid surfaces, the energy released
gradually wears the material leaving the rough surface. Cavitation can also damage the downstream pipeline, if at that place
the pressure rises above the vapor pressure and bubbles collapse.
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