How do you compute NPSHa from system data and what data do you need?

• A. NPSHa = (p_atm/γ) + z_s − h_f_s − (p_v/γ); need atmospheric pressure, suction static head, suction line losses, and vapor pressure head.
• B. NPSHa = (p_v/γ) + z_s − p_atm/γ − h_f_s
• 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)

NPSHa represents the pressure head available at the pump suction to avoid cavitation. To get it from system data you combine the head contributed by atmosphere, the suction static head, and then subtract the losses in the suction line and the vapor-pressure head of the liquid. The correct expression is:

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

Here’s what each term means:

• p_atm/γ is the atmospheric pressure converted to a head of liquid; with the open surface, this provides pressure at the suction.

• z_s is the suction static head, the vertical distance from the liquid surface to the pump suction centerline. If the liquid surface is above the pump, this adds head.

• h_f_s represents friction losses in the suction piping; these reduce the pressure available at the pump, so you subtract them.

• p_v/γ is the vapor-pressure head of the liquid at the operating temperature; higher vapor pressure lowers the available suction head, so you subtract it.

Data you need: atmospheric pressure, suction static head, suction-line friction losses, and the liquid’s vapor pressure head (at the operating temperature). This form correctly accounts for how each factor either adds to or reduces the pressure head at the pump suction.