Myth of Condensation in Fuel Tanks - Page 3 - SailNet Community

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  #21  
Old 06-23-2008
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I'll let Mr. Pascoe buy the draft beer at the local sports bar.
The glass (or fuel tank) sweats like a pig looking at the knife. And thats just a 12 ouncer. Now, multiply that by oh, 40- 50 gallons of fuel stored in cool underground tanks, plus the air exchange cause by usage, and general re. humidity, and his math goes flying out the window.

And he obviously hasn't seen thecubans hair on a west palm beach monday morning...
current conditions here,
ambiant temp, 73, dew point 70, rel. humidity, 90%

conditions in Las Vegas?
ambiant temp, 89, dew point 29, rel. humidity, 12%

myth? my skinny white butt.
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Old 06-23-2008
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Pascoe's entire premise is incorrect because he talks / calculates as if the fuel tank was a closed system; i.e., he completely ignores the FACT that the tank is vented and outside air (latent with humidity) will circulate through.

That's the problem when the chalk board meets the real world, facts keep getting in the way.
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not a likely source for water in the fuel but: aluminium can be porous
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Old 06-23-2008
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Originally Posted by Bullofbarney View Post
not a likely source for water in the fuel but: aluminium can be porous
Quick someone let Anheuser-Busch know that their cans are letting in water! Hence the watery flavor I guess..
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Old 06-23-2008
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Not only is the fuel system vented, it is also designed to act as a pump...as the air heats up, it will pick up more moisture, since the relative humidity drops....then as it cools down, it will dump the moisture out...where ever it can... repeat this over the heating and cooling cycle of day, especially up north, where the night temperatures can be 20-30 degrees cooler than the daytime temps... you're gonna get water collecting. Of course, as pointed out previously, with a nice layer of diesel sitting on top of the collected water, it ain't gonna get a chance to evaporate and go back into the air.
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I wish it didn't condense, but it does.
When I open the top of the tank, you can see it on the walls.

Don't forget to factor in radiated heat. If the water is cold, and with it the hull, the tank will radiate heat to the cooler body. Even in Scotland, where humidity is very rarely high, you still see it on the walls of the tank.

It is an unwelcome mystery.
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I think Pascoe gets the basic premise wrong when he writes "In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm."
The problem is at the interface. It is the outside air that gets cold and this chills the top of the tank and thus lowers the water the air in the tank can hold giving condensation which in due course settles to the bottom of the tank.
You see this with fog where the sea or land is warmer than the air but the air is cold enough that it cannot hold the moisture it previously did. It persists through lack of air movement a condition which applies in a boat giving condensation.
In essence he fails to distinguish the outside air and inside air when he asks how the metal can be colder than the air particularly as it contains fuel.
The air in the tank may be warmer than the outside air but colder than it was so there is some temperature gradient, with the metal being colder than the inside air and condensation from the inside air having reduced water holding capacity. The less full the tank the greater the temperature drop and the greater the air volume able to condense water. Over time the few cc of water will add up.
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Quote:
Originally Posted by chris_gee View Post
I think Pascoe gets the basic premise wrong when he writes "In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm."
The problem is at the interface. It is the outside air that gets cold and this chills the top of the tank and thus lowers the water the air in the tank can hold giving condensation which in due course settles to the bottom of the tank.
You see this with fog where the sea or land is warmer than the air but the air is cold enough that it cannot hold the moisture it previously did. It persists through lack of air movement a condition which applies in a boat giving condensation.
In essence he fails to distinguish the outside air and inside air when he asks how the metal can be colder than the air particularly as it contains fuel.
The air in the tank may be warmer than the outside air but colder than it was so there is some temperature gradient, with the metal being colder than the inside air and condensation from the inside air having reduced water holding capacity. The less full the tank the greater the temperature drop and the greater the air volume able to condense water. Over time the few cc of water will add up.

Good points and Its not just that...Have you ever put your hand on your tank after motoring for say one hour...All diesels have a return line from the injector pump that is returning very warm fuel back to the tank all the while your running your engine...This fuel is at least 80 to 90 degrees from my experience..

It dosent take long to bring up your tank to this temperature..
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You fellas mean to tell me that you don't have the water vapor filter on your fuel vents?? You really need to get one for your fuel tanks.
You can get those WV filters at the same place you can get the 'Cloud spliter' for your sextant that is used on heavily overcasted days.
Any quesions?
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Quote:
Originally Posted by cardiacpaul View Post
I'll let Mr. Pascoe buy the draft beer at the local sports bar.
The glass (or fuel tank) sweats like a pig looking at the knife. And thats just a 12 ouncer. Now, multiply that by oh, 40- 50 gallons of fuel stored in cool underground tanks, plus the air exchange cause by usage, and general re. humidity, and his math goes flying out the window.

And he obviously hasn't seen thecubans hair on a west palm beach monday morning...
current conditions here,
ambiant temp, 73, dew point 70, rel. humidity, 90%

conditions in Las Vegas?
ambiant temp, 89, dew point 29, rel. humidity, 12%

myth? my skinny white butt.
You aways give a answer I can understand. thank you, Paul
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