Type a line (water 60 °C flooded 2 m friction 0.5 m NPSHr 3 m; water 20 C suction lift 4 m friction 1 m) or fill in the fields: the line and the fields stay in step.
Enter the liquid and its temperature, the tank pressure, the flooded height or the suction lift and the friction head of the suction line, and the calculator gives NPSHa, the net positive suction head available, in metres and feet of the liquid pumped, each term with its sign, and the margin and the ratio when the NPSHr of the pump is entered. The vapour pressure of water comes from the IAPWS-IF97 equation and the density of fresh water from a table; the working is shown with your numbers, the terms are drawn to scale, and the result is a line you can copy or add to a request.
NPSHa and its comparison with the NPSHr entered, nothing more: no NPSHr of any pump, no recommended margins or ratios, no pipe losses from a flow and no pump selection. The NPSHr and the margin guidance come from the pump maker; for liquids other than water the vapour pressure and the density are entered.
NPSHa = (p - pv) / (rho g) + hs - hf. The surface pressure p is absolute: the atmosphere for an open tank, the gauge pressure plus the atmosphere for a closed one. The vapour pressure pv comes off because below it the liquid boils. hs is plus for a flooded suction and minus for a suction lift, hf is the friction of the suction line. Water at 60 °C, flooded 2 m, friction 0.5 m: 10.51 - 2.07 + 2 - 0.5 = 9.94 m.
The vapour pressure of water comes from IAPWS-IF97 Eq. (30) with the coefficients n1 to n10, as in the steam saturation calculator: 0.0234 bar(a) at 20 °C, 0.1995 bar(a) at 60 °C, 0.7018 bar(a) at 90 °C. The density of fresh water comes from the table below, on a straight line between points. For sea water the density is entered, for other liquids the vapour pressure and the density; without a density 1000 kg/m3 is taken as an assumption.
The NPSHr comes from the pump maker, from tests, often as NPSH3: the NPSH at which the head falls by 3 % at a given flow. The calculator shows the margin NPSHa - NPSHr and the ratio NPSHa / NPSHr; when NPSHa is below NPSHr the pump cavitates. It gives no recommended margin: compare the result with the NPSHr curve and the guidance of the pump maker.
| Water temperature | Vapour pressure | Density | Atmosphere less vapour, m |
|---|---|---|---|
| 0 °C | 0.006112 bar(a) = 0.6112 kPa | 999.8 kg/m3 | 10.27 m |
| 10 °C | 0.01228 bar(a) = 1.228 kPa | 999.7 kg/m3 | 10.21 m |
| 20 °C | 0.02339 bar(a) = 2.339 kPa | 998.2 kg/m3 | 10.11 m |
| 30 °C | 0.04247 bar(a) = 4.247 kPa | 995.6 kg/m3 | 9.94 m |
| 40 °C | 0.07384 bar(a) = 7.384 kPa | 992.2 kg/m3 | 9.65 m |
| 50 °C | 0.1235 bar(a) = 12.35 kPa | 988.0 kg/m3 | 9.18 m |
| 60 °C | 0.1995 bar(a) = 19.95 kPa | 983.2 kg/m3 | 8.44 m |
| 70 °C | 0.312 bar(a) = 31.2 kPa | 977.8 kg/m3 | 7.31 m |
| 80 °C | 0.4741 bar(a) = 47.41 kPa | 971.8 kg/m3 | 5.66 m |
| 90 °C | 0.7018 bar(a) = 70.18 kPa | 965.3 kg/m3 | 3.29 m |
The last column is the NPSHa of an open tank at the standard atmosphere with the level at the pump and no friction: (101325 Pa - pv) / (rho g). A flooded height adds to it, a suction lift and the friction come off it. At 100 °C the vapour pressure of water is about that of the atmosphere, and water in an open tank boils.
The NPSHa formula is the head balance at the suction: the absolute pressure at the liquid surface less the vapour pressure, as heads, plus or minus the static height, less the friction of the suction line; two independent pump references give it in this form, and both say the pump cavitates when NPSHa is below NPSHr. NPSHr as NPSH3 (a 3 % head drop) follows the definition of the Hydraulic Institute and a second, independent reference. The vapour pressure of water is IAPWS-IF97 Eq. (30) (IAPWS release R7-97), with the coefficients checked against a second published implementation and the verification values of the release. The fresh water density at 0.101325 MPa comes from two independent tables that agree to within 0.05 kg/m3 at every point. The factors are exact: g = 9.80665 m/s2, 1 ft = 0.3048 m, 1 bar = 100 kPa, 1 psi = 6.894757293168 kPa, the standard atmosphere 101325 Pa (NIST SP 811 and the SI brochure).
NPSHa = (p - pv) / (rho g) + hs - hf: p is the absolute pressure at the liquid surface, pv the vapour pressure of the liquid at its temperature, rho its density and g 9.80665 m/s2, so the first term is a head in metres of the liquid; hs is the static height from the liquid surface to the pump, plus when the level is above the pump (a flooded suction) and minus when it is below (a suction lift); hf is the friction head of the suction line from the surface to the pump at the duty flow. Water at 60 °C in an open tank, flooded 2 m, friction 0.5 m: the atmosphere gives 101325 / (983.2 x 9.80665) = 10.51 m, the vapour pressure 0.19946 bar(a) gives 2.07 m, so NPSHa = 10.51 - 2.07 + 2 - 0.5 = 9.94 m.
NPSHa belongs to the installation: the tank, the level, the liquid and the suction line. NPSHr belongs to the pump and comes from the maker, from tests: it is often given as NPSH3, the NPSH at which the head (the first-stage head of a multistage pump) falls by 3 % at a given flow, and it rises with the flow. NPSHa has to exceed NPSHr at the duty flow, or the pump cavitates. The calculator shows the margin NPSHa - NPSHr and the ratio NPSHa / NPSHr; it gives no recommended margin, so compare the result with the NPSHr curve of the pump maker and the margin guidance of the maker.
By a word with the height: flooded 2 m, or 2 m above the pump, for a liquid level above the pump; suction lift 4 m, or 4 m below the pump, for a level below it. The height runs from the liquid surface to the pump. A flooded suction adds the height to NPSHa and a suction lift takes it away, so the same 4 m makes a difference of 8 m between the two. The friction is entered as a head (friction 0.5 m) or a pressure drop (friction 0.05 bar), which the density turns into a head.
For water, the vapour pressure from the saturation-pressure equation of IAPWS-IF97, the industrial formulation for water and steam, at the temperature entered: 0.0234 bar(a) at 20 °C, 0.1995 bar(a) at 60 °C, 0.7018 bar(a) at 90 °C. The density of fresh water comes from a table at 0 to 100 °C, on a straight line between the 10 °C points. For sea water the density has to be entered, and for any other liquid the vapour pressure and the density; when no density is given, 1000 kg/m3 is taken and labelled as an assumption to check.
Hot water has a high vapour pressure: at 90 °C in an open tank it takes 7.41 m of the 10.70 m that the atmosphere gives. When the vapour pressure is above the pressure at the surface, the liquid boils there, and the calculator says so instead of giving a figure: water near 100 °C in an open tank, for example. For a closed tank enter its pressure, gauge or absolute. For a liquid at its boiling point, such as water in a tank under its own steam, add the word saturated: the surface pressure then equals the vapour pressure, and NPSHa is the static height less the friction.
Heights in m or ft (1 ft = 0.3048 m exactly) and mm; pressures in bar, mbar (hPa), kPa, MPa, psi (6.894757293168 kPa) or kgf/cm2 (98.0665 kPa), gauge or absolute, an unmarked pressure read as gauge; the atmospheric pressure 1.01325 bar(a) unless another is entered (atm 0.98 bar); temperatures in °C, K or °F; densities in kg/m3, kg/l or g/cm3. NPSHa comes in metres and feet of the liquid pumped.
The calculator gives NPSHa and its comparison with NPSHr only. When a pump, an impeller, a mechanical seal or another pump part is needed, send the line from the calculator with a photograph of the pump nameplate, and Vessel Core checks the details before an offer goes back.
