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codensation in half full fuel tank

1312 Views 9 Replies 5 Participants Last post by  tommays

usually obsessive about keeping ca. 100 L diesel tank full.

by necessity has been half full for about 1 mo.

high humidity area, temps typ cycle between days of 15-20 deg C and nights 8-12 deg C this time of year. no idea what the dew point is.

how much water would you estimate has accumulated in the tank due to condensation?
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Depends on how much external air could get in.

50 liters of fuel, assuming the same density as water, would be 0.05 cubic meters of air. At sea level with standard pressure 1 cubic meter of air saturated at 100% capacity will contain 17.3 grams of water at 20c and 7.84g at 8c. Assuming your fuel changes temperature at the same rate and that no water evaporates from the fuel you could get about 9.5g per cycle. Assume again that the outside humidity is 100% and your dry air (after condensing) in the tank gets completely replaced each daily cycle.

30*10g = 300g or 300ml of water. In reality this won't happen, it won't even come close. If your vent line is long, your daily temp cycle won't even exchange any significant amount of air with the outside, which means you might have one condensation cycle but no more water after the first 10ml (in 50l) get condensated. That is less than the amount of water naturally dissolved in diesel.

In reality your fuel temp might change 5c and that would be only 5g water.
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That is an excellent back of the envelope upper estimate.

i'd give you a rep point if only i knew how.

how much water would you estimate has accumulated in the tank due to condensation?
That really depends on how often you warm up the engine and how full you keep the tank. Most condensation in diesel tanks is a result of the hot fuel cooling off and the hot and humid air in the unused area of the tank then condensing as it cools. The fuel gets hot due to the return line from the injector pump and, after running a while, sooner or later heats up the whole tank. That's the way it used to be anyway, and still is on auto/truck diesels. Not sure if the marine industry has found a way to avoid the return line or not. You can bet though the increase in water is due to you not keeping the tank full. Always best to fill up upon return to the dock and not departure.
How about putting a water separator in the vent line and never worry again.
I just looked at the numbers and realized that I used 1 cubic meter for the volume of air in your tank - in reality you will have 9.5g x 0.05 cubic meters per cycle at most, or .475g; using .5g at 30 days that will be 15g or 15ml, a small shot glass holds 20ml.
In my real world i have to be careful filling my 26 gallon powerboat tank in the COOL fall because it will push out about 1 gallon of fuel on the first warm day of spring :(

My 2 gallon emergency can will suck down to about 2/3 of its size during temp changes while secured by its strap

Sorry its 36 gallons :)
The formula for diesel volume expansion is 0.00046 per degree fahrenheit. So 26 gallons will expand by a .6 of a gallon when going from cold 40F to hot 100F. Any spill sucks, especially on teak :(
I wonder--

Will the air temprature really change the temp of the fuel that much?

Most sailboat tanks are at or below the water line and thus surrounded by water that would would keep the fuel at a temp about the same as the water, not the air.

I am guessing that the water temp does not change more than a few degrees between night and day and it probably won't change the fuel temp half of that.

I agree that the formulas are correct IF the fuel temp changes 60F but I find it hard to believe it would In a sailboat in the water.
Around this neck of the woods most of the time on a warm day you can see the water vapor in the air :) which makes taking photos a ***** about 75% of the time because there is so much humidity haze

NOT that i don't believe there plenty of water from the pump also
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