How do you compute the net suction head available for a centrifugal pump in a suction lift arrangement?

• A. NPSHa = (p_atm/γ) + z_s − h_f_s − (p_v/γ)
• B. NPSHa = (p_atm/γ) − z_s + h_f_s + (p_v/γ)
• C. NPSHa = (p_atm/γ) + z_s − h_f_s + (p_v/γ)
• D. NPSHa = (p_atm/γ) + z_s + h_f_s − (p_v/γ)

Answer: (A)

Net positive suction head available in a suction lift comes from the atmospheric pressure pushing the liquid up the suction column, plus the static suction head provided by raising the liquid to the pump, but it is reduced by friction losses in the suction line and by the liquid’s vapor pressure. This gives:

NPSHa = (p_atm/γ) + z_s − h_f_s − (p_v/γ)

Here p_atm/γ is the atmospheric head, z_s is the static suction head (the lift height the liquid must overcome to reach the pump), h_f_s is the suction-line friction head loss, and p_v/γ is the vapor pressure head of the liquid. The signs reflect that friction losses and vapor pressure decrease the head available at the pump, while the static lift adds to the head available.

If friction or vapor pressure increase, NPSHa decreases, increasing cavitation risk. If z_s increases (pump higher above the surface), NPSHa increases, but this is balanced by the practical effects of a taller suction column.