ToolnozaNPSHa and NPSHr answer different questions
NPSH means net positive suction head. Available NPSH describes the suction system’s head above the liquid’s vapor-pressure head. Required NPSH comes from the pump’s performance data at its actual operating flow and conditions. Do not treat a pump’s NPSHr as a value the system calculator can invent.
Available NPSH formula
For a reservoir with negligible surface velocity: NPSHa = (Psurface,abs − Pvapor,abs) / (ρg) + z − hL. Pressures must be absolute, in Pa, density in kg/m³, g in m/s² and elevation z and suction loss hL in meters. Toolnoza accepts pressure in kPa and converts it to Pa.
Use positive z when the liquid surface is above the pump datum and negative z for a suction lift. Use vapor pressure at the actual liquid temperature. Do not enter gauge pressure as absolute pressure.
Worked example: water reservoir above the pump
Take surface absolute pressure 101.325 kPa, vapor pressure 2.34 kPa, density 998 kg/m³, surface elevation +2 m and suction loss 1 m. With g = 9.80665, NPSHa = (101325 − 2340) / (998 × 9.80665) + 2 − 1 ≈ 11.1139 m.
If pump data gives NPSHr = 3 m and you enter an additional target margin of 1 m, actual margin is 8.1139 m and surplus above that target is 7.1139 m. Those are illustrative inputs, not a universal margin recommendation.
Why suction conditions matter
Increasing suction loss or lowering the liquid level reduces available NPSH in this model. A change in temperature requires updating vapor pressure and density; changing flow can change both pipe losses and pump NPSHr. Review minimum liquid level and relevant operating cases rather than using a single convenient snapshot.
Use the Water Pump NPSH Calculator
Enter surface absolute pressure, absolute vapor pressure, density, signed elevation, suction loss, pump NPSHr and the manufacturer-approved target margin. A negative margin difference means the entered target is not met. Use Pipe Pressure Drop Calculator for a separate suction-loss estimate when its assumptions fit.
Interpret the comparison carefully
Meeting the entered arithmetic target does not certify cavitation-free operation. Check the pump curve, operating point, liquid conditions, transients and manufacturer instructions. Pump total head and NPSH describe different aspects of the system; do not substitute one for the other.