Formula & Engineering Reference
| Symbol | Variable | Unit (SI) |
|---|---|---|
| NPSHa | Net positive suction head available | m |
| hp | Pressure head at the liquid surface | m |
| Z | Static suction head (+ flooded, − lift) | m |
| hv | Vapor pressure head of the liquid | m |
| hf | Suction-side friction loss | m |
| Patm | Atmospheric pressure (elevation or manual) | kPa abs |
| Pgauge | Reservoir gauge pressure | kPa |
| NPSHr | NPSH required, from the pump curve | m |
When the elevation mode is selected, atmospheric pressure comes from the standard barometric formula Patm = 101.325 · (1 − 0.0000225577·h)5.25588. The pump is safe from cavitation when NPSHa exceeds NPSHr by at least your chosen margin.
An open tank at sea level feeds a pump with a flooded suction. The water level sits 2 m above the pump (Z = +2 m), the suction line loses 2 m to friction, the water is at 20°C so its vapor pressure is 2.3 kPa, and the pump curve gives NPSHr = 3 m.
P_atm = 101.325 kPa, so the surface head h_p = 101.325 × 1000 / (1000 × 9.81) = 10.33 m. Vapor head h_v = 2.3 × 1000 / (1000 × 9.81) = 0.23 m.
NPSHa = 10.33 + 2 − 0.23 − 2 = 10.10 m. Margin = 10.10 − 3 = 7.10 m — comfortably clear, so no cavitation worry here.
Flip two inputs and the picture changes fast: put the pump on a 3 m lift (Z = −3) and pump 70°C water (vapor pressure ≈ 31 kPa, h_v ≈ 3.2 m), and NPSHa falls to about 2.1 m — now below NPSHr. Suction lift and hot liquid are the two things that quietly eat a margin.
Reading NPSHr at the wrong flow. NPSHr rises with capacity, so take it from the pump curve at the actual design flow, not at the best efficiency point or some nominal rating. Checking NPSH at one flow and running at another is a classic trap.
Getting the sign of Z wrong. Z is positive when the liquid level is above the pump (flooded suction) and negative when the pump sits above the liquid (suction lift). A flipped sign can turn a failing layout into a passing one on paper.
Forgetting vapor pressure rises with temperature. Cold water barely registers, but near boiling the vapor head can swallow most of the atmospheric head. Always use the vapor pressure at the actual pumping temperature, not a room-temperature default.
Leaving out suction losses. h_f here must be the loss in the suction piping only, computed at the design flow with the real fittings. An undersized or fitting-heavy suction line is one of the most common reasons NPSHa comes up short.
Running with no margin. NPSHa equal to NPSHr is not safe — the published NPSHr is usually the 3% head-drop point, where cavitation has already begun. Keep a margin per ANSI/HI 9.6.1 or your project standard.
Net Positive Suction Head. NPSHa is the suction head the system delivers above the liquid's vapor pressure; NPSHr is the minimum the pump needs to avoid cavitation. The system must always supply more than the pump requires.
NPSHa = h_p + Z − h_v − h_f, in metres of the pumped liquid: surface pressure head, plus static suction head, minus vapor pressure head, minus suction friction loss.
Often 0.5–1 m, or a ratio of about 1.1–1.3, but harder services need more per ANSI/HI 9.6.1. Use your project standard and never sit right on NPSHr.
Suction lift, high friction in a long or undersized suction line, hot liquid with high vapor pressure, and high altitude. Volatile fluids near their boiling point are the hardest.
The pump cavitates: noise, vibration, lost head and flow, and impeller pitting. Raise NPSHa — cut losses, lower the pump, raise the level, enlarge the suction — or pick a pump with lower NPSHr.
NPSH & Cavitation Engineering Guide
3 topics • Pump suction & cavitation referenceMost pump problems that look mechanical are really suction problems. If the liquid arriving at the impeller does not have enough pressure to stay liquid, it boils, and the pump cavitates — losing performance and slowly destroying itself from the inside. NPSH is the bookkeeping that prevents this: a head balance on the suction side that tells you whether the system can keep the liquid above its boiling point all the way to the impeller eye.
This calculator works out the NPSH the system makes available and stacks it against what the pump demands from its curve. The guide explains what each term in the balance means, why the gap between available and required is the number that actually matters, and the handful of levers you have when the margin comes up short.