vapour pressure is the property of a liquid. This term was defined to understand the changing nature of the liquid. (Instead of understanding the definition, it’s better to learn through practical application.)
This term has its application in designation of tanks
This term usually come across while handling liquid in a tank or while pumping from a lower altitude to a higher one. This liquid property changes, when there is any change in ambient temperature.
Design engineers are very particular about the vapour pressure of the liquid and the ambient conditions at which the liquid is handled, in order to construct the compatible container( pressure vessel) to handle the liquid .
Let us understand using a practical illustration of a house hold LPG gas cylinder. The cooking gas which we use are pressurized to liquid state. This pressure has to be sustained by the material of the pressure cylinder.Usually household LPG cylinder are handled at ambient temperature of 25 to 30 o C. At this temperature the liquid LPG exerts a pressure on the walls of the cylinder. This pressure is the vapour pressure of LPG at the respective ambient temperature. And it comes to be around 8 bar.Thus the pressure vessel will be designed at the optimum thickness to withstand this pressure of 15 to 17 bar with a factor of safety as 2.
Let us assume that we are reducing the storage temperature of LPG liquid, simultaneously the vapour pressure also reduces and at -40oC , the liquid exerts a pressure of 1 atm pressure at the walls of the storage cylinder, ie the vapour pressure of liquid LPG reduces to 1 atm. Here Design engineers are lucky to design a pressure vessel which could withstand the pressure of 2 bar which has drastically reduced from 8bar to just 2 bar. But the most challenging part is to select a material that could withstand such a low temperature of -40oC. There are standard Engineering materials that will withstand such a low temperature of -40oC.
If methane is stored at 25 degrees ambient temperature, then its vapour pressure will be tremendously high. And containing this tremendous amount of pressure requires a huge amount of thickness for designing the closed vessel to contain the vapour at ambient temperature.
The only solution was to reduce the pressure that will be exerted on the walls of the container, which is done by reducing the ambient temperature or storage temperature.
Vapour pressure is also used to understand the criteria of boiling in liquids
Now let’s consider water in this case. The vapour pressure of water at 100OC is 1 atm (1.01325 bar). It means that when water is partially contained in a closed vessel at 100OC, the vapour fills the partial space in the container, and the vapour exerts a pressure of 1 atm (1.01325 bar) on to the walls of the container. Now let’s change the temperature of water from 100OC to ambient temperature of 30OC. Now the vapour pressure of water reduces from 1 atm (1.01325 bar) to 0.042 bar. This is the reason why the water at 30OC never boils when kept at atmospheric pressure of 1.01325 bar. The interesting part is water at 30OC can be made to boil if the external pressure or the atmospheric pressure is reduced beyond 0.042 bar.
Now let’s look at the worst consequences, that will occur if the proper low temperature is not maintained in the tanks ie if the refrigeration system fails. The main purpose of reducing the temperature is to reduce the vapour pressure of the liquid, so as to design the compatible pressure vessel to handle the liquid. When the refrigeration system fails the storage temperature increases, due to which the vapour pressure of the storage liquid will also increase to a tremendous value, if this vapour pressure increases beyond the design pressure of the pressure vessel then there are possibilities for the pressure vessel to explode due to high pressure, which is beyond the design pressure of the pressure vessel.
