Define Net Positive Suction Head (NPSH) and why insufficient NPSH leads to cavitation in marine pumps.

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The situation tests understanding of why a pump must have enough suction head to keep the liquid from vaporizing at the inlet. Net Positive Suction Head (NPSH) is the suction head available to the pump—the pressure head at the suction, expressed as a height of liquid, that must be enough to keep the liquid from reaching its vapor pressure. This available head works against the pump’s demand for suction, which is the NPSH required by the pump at a given flow and speed.

Cavitation occurs when the suction head available is not enough to prevent vapor formation. If the NPSH available is less than the NPSH required, the pressure at the impeller suction drops to or below the liquid’s vapor pressure, causing tiny vapor bubbles to form. These bubbles collapse as they move into higher-pressure regions inside the pump, creating shock waves that erode surfaces, reduce performance, and cause noise and vibration. In marine pumps, factors like suction depth, piping losses, valve restrictions, and seawater temperature affect NPSH available, while pump design and operating point determine NPSH required. Keeping NPSH available greater than NPSH required prevents cavitation.