The total suction head in feet (meters) of liquid absolute, determined at the suction nozzle, referred to datum, less the vapor pressure of the liquid, is called

• A. TPSH
• B. Suction Head
• C. Net Positive Suction Head (NPSH) (hsv)
• D. Static Head

Answer: (C)

Net Positive Suction Head is the energy margin on the suction side of a pump that tells you how close the liquid is to cavitation. It is defined as the total suction head, measured as an absolute head at the suction nozzle (referenced to a datum) minus the vapor pressure of the liquid at the operating temperature. The difference, expressed in feet or meters of liquid, shows how much positive pressure is available to prevent the liquid from boiling as it is pumped. You compare this available NPSH to the pump’s required NPSH; if the available amount isn’t enough, cavitation will occur. The concept uses subtracting the vapor pressure because that vapor pressure represents what the liquid would need to overcome to start forming vapor bubbles, which is exactly what cavitation is.