Cavitation destroys pumps. When the pressure at the pump suction drops below the vapor pressure of the liquid, tiny vapor bubbles form and then collapse violently as pressure recovers inside the impeller. The result is pitting damage to impeller vanes, reduced flow and head, excessive noise and vibration, and eventual pump failure. Net Positive Suction Head (NPSH) is the engineering tool for preventing this. It quantifies the pressure margin between the liquid's actual condition and its boiling point at the pump suction.
Every pump installation must ensure that NPSHa (available from the system) exceeds NPSHr (required by the pump) by an adequate margin. This guide covers the NPSHa formula, the factors that reduce suction pressure, and the practical design decisions that keep pumps running cavitation-free.
The NPSHa Formula
NPSHa = H_atm + H_static − H_friction − H_vapor. In words: start with atmospheric pressure head, add or subtract the static elevation difference between the liquid surface and the pump centerline (positive if the liquid is above the pump, negative if below), subtract all friction losses in the suction piping, and subtract the vapor pressure head of the liquid at the pumping temperature.
At sea level, atmospheric pressure provides about 33.9 feet of water head. If the liquid surface is 5 feet above the pump (flooded suction), static head is +5 feet. If suction friction losses total 2 feet and the liquid is water at 68 °F (vapor pressure head = 0.78 feet), then NPSHa = 33.9 + 5 − 2 − 0.78 = 36.1 feet. Most centrifugal pumps in this size range require 5–15 feet of NPSHr, so 36.1 feet provides a comfortable margin.
NPSH Available Calculator
Calculate Net Positive Suction Head Available to prevent pump cavitation.
Altitude and Atmospheric Pressure
Atmospheric pressure decreases with altitude, directly reducing NPSHa. At sea level, atmospheric pressure provides 33.9 feet of water head. At 5,000 feet elevation, it drops to about 28.2 feet, a loss of 5.7 feet. At 10,000 feet, only 23.1 feet. A pump installation that works fine in Houston may cavitate in Denver if altitude is not accounted for.
Closed or pressurized systems (like boiler feed or process loops) replace atmospheric pressure with the vessel pressure in the NPSHa formula. A deaerator at 5 psig provides 5 × 2.31 + 33.9 = 45.5 feet of pressure head at sea level (minus vapor pressure, which is significant because the water is hot). In these systems, altitude has no effect because the vessel pressure is independent of atmospheric conditions.
Temperature and Vapor Pressure
Vapor pressure rises exponentially with temperature. Water at 68 °F has a vapor pressure of 0.34 psia (0.78 feet head). At 150 °F, it jumps to 3.72 psia (8.59 feet). At 212 °F (boiling at sea level), vapor pressure equals atmospheric pressure and NPSHa from atmospheric head alone is zero. Hot water pumps, boiler feed pumps, and condensate return pumps operate with very thin NPSH margins and must be carefully designed.
Non-water liquids have their own vapor pressure curves. Light hydrocarbons like gasoline or propane have high vapor pressures and require pressurized suction vessels to maintain adequate NPSHa. Viscous liquids like glycol solutions have low vapor pressures but create high friction losses in the suction piping, which also reduces NPSHa. Always use the actual fluid properties at the actual pumping temperature, not water tables.
NPSH Available Calculator
Calculate Net Positive Suction Head Available to prevent pump cavitation.
Suction Piping Best Practices
Suction piping design has more impact on pump reliability than almost any other factor. Keep suction pipes short and straight, one or two sizes larger than the pump suction nozzle, and sloped continuously upward to the pump (no high spots that trap air). Use eccentric reducers at the pump suction with the flat side on top to prevent air pockets.
Avoid suction strainers with fine mesh unless absolutely necessary. They create friction loss that increases as they foul. If a strainer is required, size it for less than 0.5 feet of clean loss and install a differential pressure gauge to monitor fouling. Never install a valve directly at the pump suction flange; provide at least 5 pipe diameters of straight pipe between any valve or fitting and the pump suction. Turbulence from close-coupled fittings disrupts the flow entering the impeller eye and can cause localized cavitation even when bulk NPSHa appears adequate.