A pump suction has static head z_s = 2 m, suction line head loss h_f = 3 m, vapor pressure head p_v/ρg = 0.8 m, atmospheric head p_atm/ρg = 10 m. Compute NPSH_available.

• A. 6.2 m
• B. 8.2 m
• C. 10.0 m
• D. 11.2 m

Answer: (B)

NPSH_available is the amount of pressure head (energy) available at the pump suction to prevent cavitation. It comes from the total pressure head on the suction side minus any losses and the vapor pressure head that could cause cavitation. Use this relation: NPSH_available = p_atm/ρg + z_s − h_f − p_v/ρg.

Plugging in the numbers: 10 m (atmospheric) + 2 m (static head) − 3 m (suction line losses) − 0.8 m (vapor head) = 8.2 m. So the pump has 8.2 meters of NPSH available. This value should be compared to the pump’s required NPSH to ensure cavitation is avoided. The other numbers would result from misapplying signs or omitting a term, such as forgetting to subtract h_f or adding instead of subtracting p_v/ρg.