Vapour Pressure of Water at 20 °C

Water boils whenever the local absolute pressure falls to its saturation vapour pressure, and that pressure is a steep function of temperature: 872 Pa at 5 °C, 2339 at 20 °C, 7384 at 40 °C, 47.4 kPa at 80 °C and 101.325 kPa at 100 °C. Roughly, it doubles every 10 °C in the ordinary range. Nothing about boiling requires 100 °C — that is simply where the curve crosses one atmosphere.

This is the term that decides whether a pump cavitates. NPSH available subtracts vapour pressure from the absolute pressure at the suction, so hot water eats the margin fast: pumping 80 °C condensate leaves only about 54 kPa of atmospheric pressure to work with before the suction lift and friction are even counted, which is why boiler feed pumps are flooded-suction and why condensate pumps have such generous inlet piping. Vapour pressure also governs the vacuum side of siphons and priming lines, and sets the practical suction-lift limit of any pump well below the theoretical 10.3 m of water column.