Diesel fuel absorbs a LOT of water before you ever see it...
...If condensation was an issue, I would have seen water and I have not.
My biggest issue is condensation on the outside of the tanks when we get a warm front.
Hmmm...how is it that you think water can be condensing on the outside of the tanks, but not on the inside? Wet air meeting cold metal = condensation, period.
The reason you are not actually be seeing the water is because it has not precipitated out yet. Just like air, diesel fuel will "absorb" water, up to it's saturation point. Like in air, the amount of water the fuel can hold is temperature dependent. Diesel fuel can absorb a LOT of water before it 'condenses' (the more accurate term is 'precipitate') in the bottom of your tank, which will happen at the 'dew point' of the fuel-water mix.
The best write-up I've seen on this is here:
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"Diesel can hold water in solution but it doesn't condense out of solution due to gravity, it condenses out due to a drop in temperature, just like water condenses out of air. Diesel #2 holds about 120 parts per million of water at body temperature but can only hold about 40 ppm at 4 degrees Celsius. If you fill up your tank on a warm summer day, even if the diesel is dry, it'll start absorbing water from the air (via the tank vent) until it reaches its saturation point around 120 ppm.
Let that fuel sit in your tank until it gets close to freezing outside and
the diesel will be able to hold only about a third of the water it could when warm so the water will condense out as a separate liquid phase. It's only after condensation that it drops to the bottom of the tank due to gravity and the difference in density between water and diesel. The 80 ppm difference between diesel's water saturation at body temp and its saturation close to freezing equates to about 0.3 ml of water per gallon of diesel. If your tank is 100 gallons, you're talking about 30 ml of water, which is about an ounce. If you're on a trawler with 1600 gallons of diesel, you're talking about one beer can worth of water in your tanks.
If the tank were sealed and you brought the temp back up, the water would dissolve back into the fuel. However, the tank is vented so it has access to relatively moist air, which will supply water to the fuel at the same time the water at the bottom of the tank is dissolving back into the fuel, so you'll never get it all back into solution and you'll end up with residual water at the bottom of the tank.
By the same reasoning, you absolutely can get condensation on the tank walls from the water in the air. All it takes is warm, moist air coming in contact with the cold walls of the tank. When that happens, the air will cool near the tank wall, lose its ability to hold as much moisture, and it'll condense out on the surface. This can happen if you're out on a hot, humid day and your tank is colder than the surrounding atmosphere due to its close proximity to cold water on the other side of the hull.
Bottom line, if you can arrange to use all your fuel up without giving it a chance to cycle from warm to cold, you won't get much condensation due to water dropping out of the fuel but you're still vulnerable to water condensing out of the air if you can't keep the tank close to the same temp as the air at all times. In practice, it's virtually impossible to assure both conditions so you just have to live with water in your tanks via good water separators to handle emulsified water, filters that are designed to absorb dissolved water, and biocides that'll kill the bugs that want to grow at the water/fuel interface in the tank.
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Keeping diesel tanks full stops condensation T or F? - Fix It Anarchy - Sailing Anarchy Forums