Expert: Mike Fulcher - 4/29/2008

Question
QUESTION: Will fuel vapor in my petrol tank condense back into liquid fuel? I presume that this may be dependent on temperature and pressure yet since I assume that the pressure isn’t going to be very great if the temperature is low enough will it condense back? Is this likely to happen at room temperature – if not can I presume that once in vapor form in my tank it will always be in vapor form?

ANSWER: At a fixed temperature in a closed (non-leaking) system, there will always be fuel condensing (with an equivalent amount evaporating) with the net result of zero change in liquid or vapor levels.  The partial pressure of fuel vapor in your tank is directly related to the temperature, and at colder temperatures there will less vapor (pressure wise) occupying the same volume in the tank.  Because the vapor volume is roughly 22 times the volume of an equivalent amount of liquid, unless you have a really big tank any change in liquid level over the normal range of climate temperature will be unnoticeable on your fuel guage.

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QUESTION: Thank you. Do you have any idea what the maximum temperature that petrol can be safely stored at is? Do you know its combustion temperature?

Answer
While I don't have any exact numbers at my disposal, I can suggest a few things.  First, gasoline should only be stored in approved, ventilated containers.  This prevents pressure build up and uncontrolled release of vapor which could result in fire.  These containers should be stored to minimize exposure to high temperature and also provide ventilation again to prevent vapor buildup.