What is the NPSH formula?

NPSH available = absolute pressure head on the liquid surface + static head of the surface above the pump centerline (negative for a suction lift) − suction line losses − vapor pressure head of the liquid. For cold water at sea level the atmospheric head is about 34.0 ft (10.35 m) and the vapor pressure head is about 0.8 ft (0.24 m) at 68 °F (20 °C).

What is the difference between NPSHa and NPSHr?

NPSHa (available) is a property of the suction system: how far the absolute pressure at the pump inlet sits above the liquid's vapor pressure. NPSHr (required) is a property of the pump, read from its curve at the duty flow. Pump makers publish NPSH3, the NPSH at which the pump's total head has already dropped 3% from cavitation, so NPSHa must exceed NPSHr by a margin, not just equal it.

What does NPSH requerido and NPSH disponible mean?

They are the Spanish terms for the same two quantities. NPSH requerido (NPSHr) is the net positive suction head the pump requires, taken from the manufacturer's curve. NPSH disponible (NPSHa) is the net positive suction head the installation makes available at the pump inlet. The pump avoids damaging cavitation when NPSH disponible exceeds NPSH requerido with an adequate margin.

How much NPSH margin is enough?

ANSI/HI 9.6.1 recommends an NPSHa/NPSH3 margin ratio above 1.0 that depends on the application and the pump's suction energy, higher for high-energy and continuous-duty pumps. A common screening rule is NPSHa at least 3 ft (0.9 m) above NPSHr and a ratio of at least 1.1; confirm the required margin with the pump manufacturer.

How does water temperature affect NPSH available?

Vapor pressure rises steeply with temperature. The vapor pressure head of water is 0.8 ft (0.24 m) at 68 °F (20 °C) but 16.3 ft (4.97 m) at 176 °F (80 °C) and 35.4 ft (10.78 m) at the boiling point. Hot water therefore usually needs a flooded suction rather than a suction lift.

How does elevation affect NPSH available?

Atmospheric pressure falls with altitude, so every foot of NPSHa from the atmosphere shrinks. The atmospheric head of cold water drops from 34.0 ft at sea level to 28.3 ft at 5,000 ft and 23.4 ft at 10,000 ft (U.S. Standard Atmosphere). A suction lift that works at the coast can cavitate in the mountains.

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NPSH Calculator — NPSH Available vs Required

Net positive suction head available (NPSHa) for a pump drawing water from an open sump, a closed tank, or a measured suction-gauge reading. It accounts for site elevation, water temperature, suction lift or flooded suction, and suction-line losses, then checks the result against the pump's NPSH required (NPSHr, NPSH3). SI and US units.

Units

Method

Water temperature°F

Site elevationft above sea level

Barometric pressurepsia (blank = standard atmosphere)

Suction source

Tank gauge pressurepsig above liquid

Liquid level above pump centerline, h_sft (+ flooded, − lift)

Suction line losses, h_fft (friction + fittings + entrance)

Suction gauge readingpsig (− = vacuum)

Gauge height above pump centerlineft

Flow rategpm

Suction pipe inside diameterin (at the gauge)

Pump NPSH_r (NPSH3) at duty flowft, from pump curve

Atmospheric pressure—psia

Absolute pressure head on surface—ft

Velocity head, V²/2g—ft

Vapor pressure / density—psia · lb/ft³

Vapor pressure head, h_vp—ft

NPSH available, NPSH_a—ft

Margin, NPSH_a − NPSH_r—ft

Margin ratio, NPSH_a / NPSH_r——

Check—

Max suction lift for NPSH_a = NPSH_r—ft

Screening check used here: margin ≥ 3 ft (0.9 m) and NPSH_a/NPSH_r ≥ 1.1. ANSI/HI 9.6.1 sets application-specific margin ratios above 1.0 that increase with suction energy; confirm the required margin with the pump manufacturer. Water properties (density and vapor pressure, 32–212 °F / 0–100 °C) come from the site's water properties table.

Design method, from the suction source:

$$ \mathrm{NPSH_a} = \frac{p_{abs,s}}{\rho g} + h_s - h_f - \frac{p_v}{\rho g} $$

Field method, from a suction gauge (Hydraulic Institute test convention):

$$ \mathrm{NPSH_a} = \frac{p_{atm} + p_{g}}{\rho g} + z_g + \frac{V^2}{2g} - \frac{p_v}{\rho g} $$

Barometric pressure from elevation (U.S. Standard Atmosphere 1976, troposphere):

$$ p_{atm} = 101.325\,(1 - 2.25577\times10^{-5}\,z)^{5.25588}\ \text{kPa} \quad (z\text{ in m}) $$

p_abs,s absolute pressure on the liquid surface (atmospheric for an open sump, atmospheric + gauge for a closed tank) · h_s surface elevation above the pump centerline / impeller eye (negative for a lift) · h_f suction-line head loss · p_v vapor pressure at the pumping temperature · p_g suction gauge pressure · z_g gauge height above the pump datum · V suction pipe velocity at the gauge · ρ liquid density · g 9.80665 m/s².

What NPSH means

A centrifugal pump lowers the pressure of the liquid as it enters the impeller eye. If that local pressure falls to the liquid's vapor pressure, vapor bubbles form and then collapse violently as the pressure recovers further along the vane. That is cavitation: noise, vibration, lost head and pitted impellers. NPSH available measures how much absolute pressure head the suction system delivers to the pump inlet above vapor pressure. NPSH required is how much the pump needs at a given flow. Both are in feet or metres of the liquid being pumped.

Pump manufacturers test NPSH required by throttling suction pressure until the pump's total head drops by 3%. That point is NPSH3, and it is what the NPSH_r curve on a pump data sheet shows. At NPSH3 the pump is already cavitating enough to lose 3% of its head. Operating with NPSH_a equal to NPSH_r is therefore not a safe design point; the installation needs margin above it.

Atmospheric pressure head by elevation

Standard-atmosphere pressure and the equivalent head of 68 °F (20 °C) water (ρ = 998.2 kg/m³). This is the most an open sump can contribute before subtracting lift, friction and vapor pressure.

Elevation (ft)	Elevation (m)	p_atm (psia)	p_atm (kPa)	Head (ft)	Head (m)
0	0	14.70	101.33	34.0	10.35
1,000	305	14.17	97.72	32.8	9.98
2,000	610	13.66	94.21	31.6	9.62
3,000	914	13.17	90.81	30.4	9.28
4,000	1,219	12.69	87.51	29.3	8.94
5,000	1,524	12.23	84.31	28.3	8.61
6,000	1,829	11.78	81.20	27.2	8.29
7,000	2,134	11.34	78.19	26.2	7.99
8,000	2,438	10.92	75.26	25.2	7.69
10,000	3,048	10.11	69.68	23.4	7.12

Vapor pressure head of water by temperature

Vapor pressure from the site's water properties table (IAPWS-IF97 steam-table values), converted to head with the density at the same temperature. Between table rows the calculator interpolates vapor pressure log-linearly, which stays within 0.5% of steam tables.

T (°F)	T (°C)	p_v (psia)	p_v (kPa)	h_vp (ft)	h_vp (m)
50	10	0.178	1.23	0.41	0.13
68	20	0.339	2.34	0.78	0.24
86	30	0.615	4.24	1.42	0.43
104	40	1.070	7.38	2.49	0.76
122	50	1.788	12.33	4.18	1.27
140	60	2.889	19.92	6.78	2.07
158	70	4.519	31.16	10.66	3.25
176	80	6.866	47.34	16.30	4.97
194	90	10.167	70.10	24.30	7.41
212	100	14.697	101.33	35.37	10.78

Why hot water and suction lift don't mix

NPSH_a for an open sump at sea level with a 3 m (9.8 ft) suction lift and 0.5 m (1.6 ft) of suction losses, from this calculator. The same system that has over 6 m of NPSH_a with cold water has none left at 90 °C.

T (°C)	T (°F)	NPSH_a (m)	NPSH_a (ft)
10	50	6.71	22.0
20	68	6.61	21.7
30	86	6.44	21.1
40	104	6.16	20.2
50	122	5.69	18.7
60	140	4.94	16.2
70	158	3.82	12.5
80	176	2.16	7.1
90	194	−0.20	−0.7

Worked examples

1. Suction lift from a sump at 1,000 ft elevation (US units)

Given: open wet well at 1,000 ft elevation, water at 80 °F, water surface 12 ft below the pump centerline, suction losses 2.5 ft, pump NPSH_r = 10 ft at the duty point.

Atmosphere: p_atm = 14.17 psia. At 80 °F, ρ = 996.6 kg/m³ (62.21 lb/ft³), so h_atm = 14.17 × 144 / 62.21 = 32.80 ft.

Vapor pressure: p_v = 0.507 psia, so h_vp = 0.507 × 144 / 62.21 = 1.17 ft.

NPSH_a = 32.80 − 12 − 2.5 − 1.17 = 17.13 ft. Margin = 17.13 − 10 = 7.13 ft; ratio = 1.71. The suction lift could grow to 19.1 ft before NPSH_a fell to NPSH_r, but not to the 33.9 ft "theoretical" lift sometimes quoted for sea-level cold water.

2. Hot water with a flooded suction (SI units)

Given: vented hot-water tank at sea level, water at 80 °C, water surface 4.0 m above the pump centerline, suction losses 0.8 m, pump NPSH_r = 3.5 m.

Heads: ρ = 971.8 kg/m³. h_atm = 101.325 × 1000 / (971.8 × 9.80665) = 10.63 m. p_v = 47.34 kPa, so h_vp = 47.34 × 1000 / (971.8 × 9.80665) = 4.97 m.

NPSH_a = 10.63 + 4.0 − 0.8 − 4.97 = 8.86 m. Margin 5.36 m, ratio 2.53. The same pump on a 3 m lift would have 10.63 − 3.0 − 0.8 − 4.97 = 1.86 m, well below NPSH_r: hot water needs a flooded suction.

Field check from a suction gauge

On an operating pump, NPSH_a can be measured instead of estimated. Read the suction gauge as close to the pump inlet as practical, add barometric pressure to get absolute pressure, convert to head, and add the gauge height above the pump datum and the velocity head in the suction pipe at the gauge. Then subtract the vapor pressure head. This is the method the Hydraulic Institute test standards use, and it catches losses that design estimates miss: a partly closed valve, a clogged strainer, air-entraining vortices. Select "Field test" above to use it.

References: ANSI/HI 9.6.1, Rotodynamic Pumps — Guideline for NPSH Margin, Hydraulic Institute. Hydraulic Institute, ANSI/HI 14.6 Rotodynamic Pumps for Hydraulic Performance Acceptance Tests (NPSH3 definition and test method). Karassik, I.J., Messina, J.P., Cooper, P. & Heald, C.C. (2008). Pump Handbook, 4th ed., McGraw-Hill. U.S. Standard Atmosphere, 1976 (NOAA/NASA/USAF). Water properties: see the water properties table and its sources.

Related tools

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Darcy-Weisbach — suction pipe friction loss
Minor loss — entrance, elbow, valve and strainer losses in the suction line
Antoine vapor pressure — vapor pressure of other liquids
Lift station sizing — wet well volume and cycle time
Water properties by temperature
Fan & pump affinity laws — NPSHr scales with speed squared