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What is saturated vapour pressure?
The saturated vapour pressure of a liquid is the pressure at which its liquid and vapour phases are in equilibrium at a given temperature: it is the pressure at which the liquid boils. It rises quickly with temperature. For water it is 2,339 Pa at 20 °C and 101,325 Pa (1 atm) at 100 °C. If the pressure in a circuit falls below this value, for example at a pump suction, the liquid vaporises: this is cavitation, which the NPSH calculation is designed to avoid.
Saturated vapour pressure formula (Clausius-Clapeyron)
- Psat: saturated vapour pressure at temperature T, in the unit of P0
- P0: known pressure, for example 101,325 Pa, at the boiling temperature T0 in kelvins (water: 373.15 K)
- M: molar mass, in kg/mol (water: 0.018); Lv: latent heat of vaporisation, in J/kg (water: 2.26·10⁶)
- R: ideal gas constant, 8.314 J/(mol·K); T: temperature, in kelvins (°C + 273.15)
This formula assumes an ideal vapour and a constant Lv: for water it is accurate to a few per cent between 50 and 150 °C. For pump suction calculations, the measured values of the table below are preferred.
Saturated vapour pressure table for water
| Temperature (°C) | Psat (Pa) | Psat (bar) | Equivalent (m of water) |
|---|---|---|---|
| 0 | 611 | 0.0061 | 0.062 |
| 5 | 872 | 0.0087 | 0.089 |
| 10 | 1,228 | 0.0123 | 0.125 |
| 15 | 1,705 | 0.0171 | 0.174 |
| 20 | 2,339 | 0.0234 | 0.239 |
| 25 | 3,169 | 0.0317 | 0.324 |
| 30 | 4,246 | 0.0425 | 0.435 |
| 40 | 7,384 | 0.0738 | 0.759 |
| 50 | 12,350 | 0.1235 | 1.274 |
| 60 | 19,940 | 0.1994 | 2.067 |
| 70 | 31,190 | 0.3119 | 3.252 |
| 80 | 47,390 | 0.4739 | 4.971 |
| 90 | 70,140 | 0.7014 | 7.407 |
| 100 | 101,325 | 1.0133 | 10.78 |
Detailed explanations and calculation in Mecaflux
The saturation vapor pressure is the pressure at which fluid passes from the gaseous state to the liquid state (or liquid to gas) at a given temperature.
If the temperature of the fluid increases, the pressure at which the fluid changes from liquid to gas (saturated vapor pressure) increases.Thus, a liquid such as water can turn into vapor at ambient pressure by supplying heat, but it is possible to make this change, without changing the temperature, by lowering the ambient pressure below the saturated vapor pressure.
When a liquid is sucked into a duct, it creates a depression, If this pressure drop lowers the fluid pressure below the saturation vapor pressure, the liquid begins to boil. (Steam production), with hydraulic , this phenomenon is called cavitation..
Cavitation is the formation of vapor bubbles due to pressure drop. In forming the bubbles increases the volume of fluid present in the low pressure zone, which has the effect of increasing pressure in some places where the gas bubble condenses violently imploding. Shocks created by the bursting bubbles destroy the walls of the organs in contact with the fluid. A pump with cavitation, wears quickly.
Calculating the saturated vapor pressure
The approximate calculation of the saturation vapor pressure can be done using a formula outcome of the Clapeyron equation, by assuming - among other things - that the vapor behaves as an ideal gas and the enthalpy of vaporization does not vary with temperature in the range considered.
with :
-
T0 : boiling temperature of the substance at a pressure P0 , in Kelvin
-
Psat : saturation vapor pressure, in the same unit as P0
-
M : molecular weight of the substance in kg/mol
-
Lv : latent heat of vaporization of the substance in J/kg
-
R : Gas constant, equal to 8,31447 in J/K/mol
-
T : vapor temperature in K
For water, eg:
-
M = 0,018 kg/mol
-
Lv = 2,26×106 J/kg
-
P0 = 1013 mbar
-
T0 = 373 K
With MECAFLUX standard
if you choose as fluid, water, among the 120 fluids proposed in the editor fluid viscosity, saturation vapor pressure is automatically set according to the temperature input
detail of the setting area of the water temperature, the saturated vapor pressure of the water appears to temperature change.
All fluids in the editor viscosity are not filled with saturated steam, you can also use the tool to calculate saturation vapor pressure:
Frequently asked questions
What is the saturated vapour pressure of water at 20 °C?
2,339 Pa, i.e. 0.023 bar or 0.24 m of water column. At 50 °C it reaches 12,350 Pa, at 80 °C 47,390 Pa and at 100 °C 101,325 Pa, the normal atmospheric pressure: this is why water boils at 100 °C at sea level.
What is the link between saturated vapour pressure and cavitation?
If the pressure of the liquid falls below its saturated vapour pressure, it vaporises locally: bubbles form and then implode in higher-pressure zones, eroding pump impellers and propellers. The NPSH available of a pump is calculated by subtracting Psat from the suction pressure.
Why does water boil at a lower temperature at altitude?
Because atmospheric pressure decreases with altitude: at 3,000 m it is about 70,000 Pa, the saturated vapour pressure of water at 90 °C. Boiling starts as soon as Psat reaches the ambient pressure.
How do I check the cavitation risk of my own pump from the vapour pressure?
In Mecaflux Standard, the fluid editor gives the saturated vapour pressure of the liquid at the temperature of your installation, and the pumps tab subtracts it from the suction pressure to compute the available NPSH and display the cavitation alert; you see at once the effect of a higher temperature or an excessive suction lift. For a cavitating pump or a hot installation to design, Mecaflux carries out the study of your project, or offers training on the software applied to your own installation.