What NPSHa means
Liquids boil when their absolute pressure drops to the vapor pressure. A centrifugal pump lowers the pressure the most at the eye of the impeller, so the suction side must deliver the liquid with enough absolute head above its own vapor pressure to keep it liquid all the way into the impeller. That buffer is the net positive suction head available (NPSHa) — the absolute suction head the liquid carries from the free surface to the impeller eye, minus the vapor-pressure head:
Evaluated from the suction source, the standard accounting form is:
$$NPSH_a \;=\; H_{atm} \;+\; H_{surf} \;+\; H_{static} \;-\; H_{f} \;-\; H_{vap}$$where \(H_{atm}\) is atmospheric head, \(H_{surf}\) the tank surface (gauge) pressure head, \(H_{static}\) the liquid level relative to the pump centerline, \(H_f\) the suction-line friction loss, and \(H_{vap}\) the vapor-pressure head at the liquid temperature. Every term is a head of the pumped liquid, which is why each pressure is divided by \(\rho g\) (i.e., by SG in the field units below).
The formula, term by term
Atmosphere. The only free energy source on most suction systems:
$$H_{atm}(\text{ft}) = \frac{2.3103\,p_{atm}(\text{psi})}{SG}, \qquad H_{atm}(\text{m}) = \frac{0.101972\,p_{atm}(\text{kPa})}{SG}$$At sea level, 14.7 psi (101.3 kPa) supports 33.9 ft (10.3 m) of water at SG 1.00 — the ceiling for any suction lift. Altitude reduces it per the standard atmosphere (the tool applies the ICAO formula).
Surface pressure. A blanketed or pressurized tank adds \(H_{surf}=p_{gauge}/\rho g\); a vacuum application makes it negative.
Static level. \(H_{static}=z\) is positive for flooded suction (surface above the pump) and negative for suction lift. The theoretical lift limit at sea level is the 33.9 ft atmospheric head; practical lifts are far lower once friction and vapor pressure are paid.
Suction friction. Everything between the surface and the pump inlet is subtracted (Darcy–Weisbach pipe loss plus fitting losses):
$$H_{f} \;=\; f\,\frac{L}{D}\,\frac{V^{2}}{2g} \;+\; \sum K\,\frac{V^{2}}{2g}$$Vapor pressure. \(H_{vap}=p_{v}(T)/\rho g\) from saturation (steam-table) data. It is negligible for cold water — 2.34 kPa abs at 20 °C ≈ 0.24 m (0.78 ft) — but at 100 °C the vapor pressure reaches 101.3 kPa abs ≈ 10.3 m (33.9 ft), consuming nearly the entire atmospheric head. This is why hot-water pumps are almost always pressurized (boiler feed pumps take saturated water at the drum pressure plus static head).
Margin criteria behind the traffic lights
Vendor NPSHr is measured at the 3% head-drop point (NPSH₃) on the certified curve; real head breakdown and erosion can begin earlier, which is why an installed margin is mandatory. Common US practice is a ratio of 1.1–1.3, or a flat +3 ft (0.9 m) over NPSHr — both checks run in this tool:
| NPSHa/NPSHr | Flat margin | Status | Interpretation |
|---|---|---|---|
| ≥ 1.3 | NPSHa ≥ NPSHr + 3 ft | 🟢 Adequate | Comfortable margin; cavitation unlikely across the duty range. |
| 1.1 – 1.3 | NPSHr < NPSHa < NPSHr + 3 ft | 🟡 Thin | Acceptable only for clean, well-defined service — prefer ≥ 1.3× or +3 ft (0.9 m). |
| < 1.1 | NPSHa < NPSHr | 🔴 Cavitation risk | Head drop, noise, vibration, impeller erosion. Fix the system or re-select the pump. |
A third watch always runs: above 60 °C the vapor-pressure head's share of the absolute suction head grows steeply, so re-check NPSHa at the maximum expected process temperature.
Thoma σ and suction specific speed (Nss)
The Thoma cavitation parameter normalizes NPSHa by the stage head (referenced at BEP):
$$\sigma \;=\; \frac{NPSH_a}{H_{bep}}$$An installation runs cavitation-free while its operating σ stays above the pump's critical σ — the value at which head begins to fall (the 3% criterion of the works test). Required σ grows with specific speed, which motivates the suction specific speed:
$$N_{ss} \;=\; \frac{N\,\sqrt{Q}}{NPSH_r^{3/4}}$$in US units \(N\) (rpm), \(Q\) (gpm), \(NPSH_r\) (ft) at BEP; the metric \(n_{ss}\) variant uses m³/s and m. Lower Nss means the impeller needs less suction head — easier installations:
| Nss (US units) | Rating | Meaning |
|---|---|---|
| ≤ 8 000 | 🟢 | Conservative hydraulic design — wide cavitation margin. |
| 8 000 – 11 000 | 🟡 | Normal commercial range; watch installed margin. |
| > 11 000 | 🔴 | Cavitation-prone high-suction-speed design; expect margin penalties. |
| ≈ 27 000 | special | Inducer-equipped pumps reach this region by design. |
Worked example — 120 °F water from an open tank
These are the tool's US-mode default values, so the page opens on this case. Sea level, open tank (0 psig), liquid level 10 ft above the pump centerline, 4 ft of suction friction, vendor NPSHr 16 ft:
| Step | Term | Value |
|---|---|---|
| 1 | Water at 120 °F (48.9 °C) → pv = 1.70 psi (11.74 kPa abs), SG = 0.989 (saturation table) | — |
| 2 | Hatm = 2.3103 × 14.7 / 0.989 | +34.3 ft |
| 3 | Hsurf (open tank) | 0 ft |
| 4 | Hstatic (flooded suction) | +10.0 ft |
| 5 | Hf (suction friction) | −4.0 ft |
| 6 | Hvap = 2.3103 × 1.70 / 0.989 | −4.0 ft |
| = | NPSHa (absolute suction head 40.3 ft ≈ 17.3 psia, minus vapor head) | ≈ 36.3 ft |
| Margin ratio 36.3 / 16 — also above NPSHr + 3 ft = 19 ft | 🟢 2.27 — adequate |
Now heat the same tank to 212 °F (100 °C): the vapor head jumps to ≈ 35.4 ft (33.9 ft even at SG 1.00) and NPSHa collapses to ≈ 6 ft — margin 0.38, a hard red. The tank would have to be pressurized or the level raised dramatically; that is the classic hot-water cavitation trap the third traffic light warns about.