Vaporization pressure in a fuel can

[Anders L]

A normal behavior of a plastic jerry can used for gasoline is that it expands by the pressure caused by vaporization of the gas.

Which option below will be the best alternative to reduce the pressure caused by vaporization in a jerry can during storage? :

- Fill up the can as much as possible

- Fill up to let say 80% of the total volume, leaving some air at the top

Regards Anders

 

[tduell]

I think the vapour pressure depends only on the temperature, so my guess is that the fluid level won't have any effect.

Cheers,
Terry

 

[jbriggs444]

If one treats the can as being perfectly rigid, that is correct -- the fluid level will not have any effect. The increase in pressure will be exactly equal to the vapor pressure of the fuel.

If the can is non-rigid then leaving a lesser amount of head room means that a small expansion in the can results in a (relatively) larger expansion in the head room. So the expected increase in pressure due to vaporization of the fuel is countered by a slight expansion in the can and the pressure only increases slightly.

So... in the real world my answer would be to fill the can nearly full up.

However... if the coefficient of thermal expansion of the fuel is different than the coefficient of thermal expansion of the can and if the can is very rigid and if the temperature rises or falls then a completely full can could be subject to very large pressures. (e.g. consider what happens to a glass bottle completely full of water in the freezer). Jerry cans are not that rigid. Still, it can be unwise to fill a rigid container completely.

 

[tduell]

I think that even in the non rigid container case the pressure will always settle at the vapour pressure for the temperature.

Cheers,
Terry

 

[jbriggs444]

Not if there is any air in the head space.

 

[tduell]

OK.
Could you please provide the "why is it so"?

Cheers,
Terry

 

[jbriggs444]

The pressure in the head space is the sum of the air pressure plus the vapor pressure. When you initially tighten the lid on the can the head space is full of air at atmospheric pressure. Fuel eventually evaporates so that the partial pressure of fuel vapor in the head space is equal to the vapor pressure of the fuel at the current temperature. But the air is still there, so the resulting pressure will be strictly higher than the vapor pressure alone.