Cavitation and flashing occur where there is an orifice in the pipeline because of any kind of obstructions such as valves & flowmeters. Thus such phenomenons can appear in any types of flow arrangements, here we try to explain more about cavitation and flashing.
What causes cavitation & Flashing?
Chocked flow causes cavitation and flashing. The maximum flow rate is called chocked flow, which is due to the additional differential pressure with a fixed stream condition.
In liquids, when the static pressure within the valve falls below the vapor pressure of the liquid, choking happens as a consequence of vaporizing the liquid.
If, as described by the system circumstances of P1 and P2, the real stress drops across the valve is higher than DPmax, then either flash-ing or cavitation may occur.
What is Flashing?
If pressure drops below the vapor pressure of the fluid at the vena contracta (due to the enhanced fluid speed at this stage) bubbles will form in the flow stream.
If pressure at the valve outlet remains below the vapor pressure of the liquid, the bubbles will remain in the downstream system and the process is said to have flashed.
Flashing can cause severe erosion harm to the trimming components of the valve and is distinguished by a soft, polished surface appearance. Flashing harm is normally the most severe at the lowest speed level, normally at or near the seat row of the valve socket and seat ring.
What is cavitation?
If downstream pressure recovery is sufficient to raise the outlet pressure above the vapor pressure of the liquid, the bubbles will collapse, or implode, producing cavitation. The bubble explosion produces sound through the pipe walls.
The bubble collapse close to the solid surface of the valve, pipe result in tearing away the material leaving a rough, cinder like surface.
Cavitation air bubbles always cause damage because they move at high speed. The damage will be even greater if the bubbles experience impulsion (the opposite of the explosion) in the valve wall
High recovery valves tend to be more subject to cavitation since the downstream pressure is more likely to rise above the liquid’s vapor pressure.