When the pressure of flowing liquid drops to or below the liquid’s vapour pressure, the liquid boils and vapour cavities (bubbles) form locally inside the liquid. If the pressure within the flow path subsequently increases above the vapour pressure, the vapour cavities implode, releasing energy. The formation and sudden collapse of these “bubbles” is called cavitation.
The generation of head in a centrifugal pump does not commence until the liquid enters the vane area and is accelerated towards pump discharge. As the liquid flows between the pump inlet flange and vanes, several points of head loss occur due to:
• Friction in the suction nozzle
• Acceleration losses as the liquid velocity increases from the suction nozzle to the impeller eye
• Shock losses as the liquid contacts the leading edges of the impeller vanes
• The sum of these losses is known as the entry loss. If the suction head minus the entry loss reduce the liquid pressure to or below the vapour pressure, then a condition
for cavitation exists
Some types of installations are more likely to incur cavitation than others. The presence of any of the following conditions, detailed below significantly increase the possibility of low NPSHA values and cavitation:
• High temperature or boiling liquids: This will increase the vapour pressure head
• Volatile liquids: These have a high vapour pressure head
• Suction tank under vacuum: This will reduce the absolute pressure head on the liquid surface
• High suction lift applications
• Circuitous suction pipework: This will lead to increased friction loss
• A high number of fittings in suction pipework: This will increase friction losses
Please consider this in your pump selections and if in doubt, discuss the matter with your pump supplier.
Article courtesy of Kelair Pumps Australia “When Pump Knowledge Matters”. For more information, visit www.kelairpumps.com.au