Which statement best describes cavitation risk assessment in pumps?

• A. NPSHA and NPSHR are not related to cavitation risk.
• B. Cavitation risk is determined solely by discharge pressure.
• C. Cavitation risk is determined solely by pump speed.
• D. Cavitation risk depends on the relationship between NPSHA and NPSHR.

Answer: (D)

Cavitation risk in pumps is all about the suction side balance: bubbles form when the pressure at the impeller suction drops below the liquid’s vapor pressure. The system can provide a certain amount of suction head, NPSHA (net positive suction head available), which depends on static head, velocity head, suction losses, and the liquid’s vapor pressure. The pump also requires a certain amount of suction head, NPSHR (net positive suction head required), at a given flow and speed, which comes from the pump design and operating point. Cavitation risk arises when NPSHA falls short of NPSHR; if NPSHA is greater than NPSHR, the pressure stays high enough to prevent vapor formation, and cavitation is unlikely. Conversely, if they are equal or NPSHA is less, cavitation bubbles can form and collapse, causing noise, wear, and efficiency loss.

So, the core idea is the relationship between NPSHA and NPSHR. The other ideas—focusing only on discharge pressure or only on pump speed—don’t capture the full picture because they ignore suction conditions and the pump’s required head, which are the deciding factors for cavitation risk.