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Classic cavitation in centrifugal pumps primarily occurs when there is no fluid present to fill the pump, leading to the formation of vapor bubbles. This situation typically arises when the pressure at the pump inlet drops below the vapor pressure of the fluid being pumped. When the fluid pressure is insufficient, the pump can create low-pressure areas, causing vaporization of the fluid. The resulting vapor bubbles can collapse violently when they move into higher-pressure areas of the pump, leading to damage and inefficiency.

In this context, the presence of sufficient fluid and optimal pressure conditions is crucial for the appropriate operation of centrifugal pumps. The other factors, while they can influence pump performance, do not directly lead to classic cavitation in the same way that the absence of fluid does.