The pump of Prob. P11.28 is scaled up to an 18-in diameter,...

The pump of Prob. P11.28 is scaled up to an 18-in diameter, operating in water at best efficiency at $1760 \mathrm{r} / \mathrm{min}$. The measured NPSH is $16 \mathrm{ft}$, and the friction loss between the inlet and the pump is $22 \mathrm{ft}$. Will it be sufficient to avoid cavitation if the pump inlet is placed 9 ft below the surface of a sea-level reservoir?

Key Concepts

Cavitation

Cavitation is the formation and implosion of vapor bubbles in a flowing liquid when the local pressure drops below the liquid's vapor pressure. This phenomenon can cause significant damage to pump components and reduce efficiency, making it essential to design and operate pumps in such a way that cavitation is avoided.

Net Positive Suction Head (NPSH)

NPSH is a critical parameter in pump design, representing the minimum pressure level needed at the pump suction to keep the liquid from vaporizing. There are two related terms: NPSH Available (NPSHa), which is the actual pressure head offered by the system including static pressure and correcting for losses, and NPSH Required (NPSHr), which is the minimum pressure head needed by the pump to avoid cavitation.

Net Positive Suction Head Available (NPSHa)

NPSHa is the actual available energy head at the pump suction, determined by factors such as the static head (the elevation difference from the liquid surface to the pump), atmospheric pressure and any losses due to friction in the suction piping. Ensuring that NPSHa exceeds the pump's NPSHr is essential for proper pump operation and to prevent cavitation.

Friction Loss in Suction Lines

Friction losses in the suction line are the losses of pressure due to the flow resistance in pipes, fittings, and valves. These losses reduce the effective pressure head available at the pump suction, and must be carefully accounted for in the calculation of NPSHa to ensure that sufficient pressure is maintained to avoid cavitation.

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