NPSH Calculator
Net Positive Suction Head Available (NPSHa) is the absolute pressure at the pump suction port, expressed in feet of the pumped liquid, minus the vapor pressure of the liquid at the...
Formula
Source: Engineering Toolbox, Hydraulic Institute Standards | Last reviewed: June 8, 2026
Examples
0 psi
= 40.8 ft
- P_atm = 14.7
- P_vapor = 0.5
- h_static = 10
- h_friction = 2
- SG = 1
Water at 70°F with 10 ft flooded suction = 40.8 ft NPSHa
0 psi
= 19.4 ft
- P_atm = 14.7
- P_vapor = 11.5
- h_static = 15
- h_friction = 3
- SG = 1
Water at 200°F — reduced NPSHa due to high vapor pressure
0 psi
= 29.9 ft
- P_atm = 12.2
- P_vapor = 1
- h_static = 5
- h_friction = 1
- SG = 1
High altitude (5,000 ft) installation
Where is this used?
Troubleshooting cavitation problems: if NPSHa < NPSHr, the pump will cavitate.
Hot liquid pumping: vapor pressure increases with temperature, reducing NPSHa.
High-altitude installations: lower atmospheric pressure reduces NPSHa.
In boiler feedwater pump (BFP) selection for utility and industrial steam boilers per ASME PTC 4 and the HI 9.6.7 standard, the NPSH calculation is a primary design check that determines the deaerator elevation (typically 30–50 ft above the pump to provide the required NPSHa for the hot water at 220–250°F), the suction piping size (low velocity 3–6 ft/s to minimize friction loss), and the need for a booster pump for the higher-pressure boilers.
The hot water at 250°F has a vapor pressure of 30 psia, requiring significantly more suction head than cold water at the same flow — a 100% BFP speed change for load tracking must maintain the NPSHa above the NPSHr at all operating points, which limits the minimum flow at which the pump can operate without cavitation.
In LNG and cryogenic pump applications per API 685, the NPSH calculation is dominated by the very low temperature of the fluid (LNG at -162°C has essentially zero vapor pressure, so the NPSH is the static head minus the friction loss, with the pump located below the LNG tank to ensure flooded suction).
Cryogenic pump design requires very careful NPSH margin accounting for transient operations (vapor bubble formation during start-up, sudden valve closure, and sloshing in the LNG tank).
Chemical process pump applications with high vapor pressure fluids (butane, propane, VCM, vinyl chloride, light hydrocarbons) require pressurized suction vessels or suction-side booster pumps to maintain NPSHa above NPSHr, with the NPSH calculation accounting for the vapor pressure at the maximum expected operating temperature.
Slurry pump and dredge pump applications (mining, mineral processing, oil sands, dredging) have additional NPSH considerations because the vapor pressure of the slurry is the vapor pressure of the carrier liquid (typically water), but the solids content affects the effective density and the two-phase flow behavior at the pump impeller.
API 610 (centrifugal pumps for petroleum, petrochemical, and natural gas industries) specifies the minimum NPSH margin as 10% of NPSHr or 3 ft, whichever is greater, for hot services, and requires the pump manufacturer to publish the NPSHr curve up to 120% of the design flow.
Industry Standards Referenced
Frequently Asked Questions
What is the difference between NPSHa and NPSHr?
NPSHa (available) is what the system provides — it's calculated from the installation conditions. NPSHr (required) is what the pump needs — it's a pump characteristic provided by the manufacturer. The rule is NPSHa ≥ NPSHr + margin (typically 3 ft).
Why does hot water have a lower NPSHa?
As temperature increases, the fluid's vapor pressure increases. At 200°F, water's vapor pressure is about 11.5 psia, compared to 0.5 psia at 70°F. This higher vapor pressure reduces the margin above cavitation, requiring more static head or a lower pump elevation.
How can I increase NPSHa in an existing system?
Options include: (1) lower the pump elevation relative to the suction source, (2) increase suction pipe diameter to reduce friction loss, (3) cool the fluid to lower vapor pressure, (4) pressurize the suction vessel, or (5) reduce the pump speed (which lowers NPSHr).
Reviewed for accuracy
Reviewed against ANSI/HI 9.6.1 NPSH margin requirements · Last reviewed: June 8, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.