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post #1 of 13 Old 04-18-2011 Thread Starter
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Diesel Fuel Tank Fill & Vent

If I use very little diesel fuel (5 gallons per year) is it best to keep the tank (I have a plastic 19 gallon tank) completely full or partially full so the fuel is newer.

Also, on the fuel vent line, is there any benefit to installing a valve on the vent line and close it when the engine is not running so that humid air does not enter the tank and condense leaving water in the fuel?
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post #2 of 13 Old 04-19-2011
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I believe that it is better to keep your tank as full as possible to reduce condensation. I don't think you will gain any reduction of moisture and condensation by installing a valve on the vent. You might want to use "Store 'n Start" fuel treatment to eliminate the water in the fuel.

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post #3 of 13 Old 04-19-2011
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fuel tank vent

It's not generally a good idea to close the vent. Thermal expansion will generate pressure and vacuum cyclically. There are a host of reasons why neither is desirable. Having the tank full will cut down on the amount of air entering and leaving the tank during these cycles and will therefore minimize the amount of water carried with it. You should be fine using a quality fuel stabilizer. As to which one to use seems to be a source of endless debate.

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post #4 of 13 Old 04-19-2011
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"Beliefs" notwithstanding, it doesnt matter if the tank is full or nearly empty. The laws of chemical equilibrium dictate that if fuel is in contact with the atmosphere, the moisture in the air will equilibrate into and ultimately 'saturate' the fuel with water. Simply chemistry - laws of partial pressures.

Closing the vent by valving can be accomplished to prevent the migration and saturation of water into the fuel; but, the tank must be 'rated' for pressure/vacuum swings .... or apply a pressure/vacuum relief valve (and connected to a desiccant chamber), etc. to prevent implosion or overpressure of the tank. Doesnt take all that much more design or 'material thickness' to construct a 'rated' tank.
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post #5 of 13 Old 04-19-2011 Thread Starter
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"Beliefs" notwithstanding, it doesnt matter if the tank is full or nearly empty. The laws of chemical equilibrium dictate that if fuel is in contact with the atmosphere, the moisture in the air will equilibrate into and ultimately 'saturate' the fuel with water. Simply chemistry - laws of partial pressures.

Closing the vent by valving can be accomplished to prevent the migration and saturation of water into the fuel; but, the tank must be 'rated' for pressure/vacuum swings .... or apply a pressure/vacuum relief valve (and connected to a desiccant chamber), etc. to prevent implosion or overpressure of the tank. Doesnt take all that much more design or 'material thickness' to construct a 'rated' tank.
I the desiccant chamber, relief valve, pressure tank ever done on a marine diesel fuel tank?
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post #6 of 13 Old 04-19-2011
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I used to run a desiccant chamber filled with a mix of dyed Silica Gel (turns from blue to pink when water vapor saturated) and plain Silica Gel. Was a PITA to regenerate all the time plus it absorbed oil vapor.

Now, I simply keep the MINIMUM amount of fuel on board as needed and empty the tank for long time 'haul outs'. If I feel the tank oil has become 'saturated', I recirculate the oil through a water ABsorbing filter to 'trim' its saturation level.
Again, the least amount of oil in the tank will 'pick up' the least amount (mass) of water. Condensation only means that the oil it totally saturated with water from the atmosphere, and it makes no difference if the the tank is full, 1/2, or near empty .... its just that a near empty tank will have the least 'absolute' amount of water. As long as the tank is connected to or 'communicates' to the ambient atmosphere, the oil will eventually 'saturate' with water due to the 'Laws of Partial Vapor Pressures' (saturation).
If one had an explosion proof vacuum pump all one would have to do is remove the oil, pull a high vacuum until most of the water 'flashes' out .... a pretty standard method of 'significant' water removal from oils in the 'high tech' and power generation industries.

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post #7 of 13 Old 04-19-2011 Thread Starter
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Quote:
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I used to run a desiccant chamber filled with a mix of dyed Silica Gel (turns from blue to pink when water vapor saturated) and plain Silica Gel. Was a PITA to regenerate all the time plus it absorbed oil vapor.

Now, I simply keep the MINIMUM amount of fuel on board as needed and empty the tank for long time 'haul outs'. If I feel the tank oil has become 'saturated', I recirculate the oil through a water ABsorbing filter to 'trim' its saturation level.
Again, the least amount of oil in the tank will 'pick up' the least amount (mass) of water. Condensation only means that the oil it totally saturated with water from the atmosphere, and it makes no difference if the the tank is full, 1/2, or near empty .... its just that a near empty tank will have the least 'absolute' amount of water. As long as the tank is connected to or 'communicates' to the ambient atmosphere, the oil will eventually 'saturate' with water due to the 'Laws of Partial Vapor Pressures' (saturation).
If one had an explosion proof vacuum pump all one would have to do is remove the oil, pull a high vacuum until most of the water 'flashes' out .... a pretty standard method of 'significant' water removal from oils in the 'high tech' and power generation industries.
Ok, I follow you on this. Could you explain how the water gets to the bottom of the tank? Is the tank bottom water from condensation that forms on the tank walls (walls exposed to air) or is the water that settles in the tank bottom coming out of the fuel?
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post #8 of 13 Old 04-19-2011
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Whats on the bottom is a 'gravimetric settling' (water more dense than oil) of the 'coalesced' water from emulsified and macromolecular water. This doesnt happen unless there is almost FULL saturation of water into the 'above' oil. If the above the water/oil 'interface' is less than fully saturated a lot of the 'bottom' phase will saturate back into the oil (on top). Only when equilibrium conditions (saturation) conditions are met, does 'free water' drop-out or settle and remain to the 'bottom'.

Simple speak when there is free water beneath oil, it usually means that the oil is FULLY saturated with water (3 kinds: free water, emulsified water, or 'molecular sized' water).
Condensation only happens when the oil is at near full saturation with water .... If the phenomenon of 'water condensation' on tank walls was applicable or valid for other than fully saturated conditions ... we wouldnt need wells to get our drinking water as all we'd have to do is put out empty tanks and they'd magically fill up ... and we know this isnt true.
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Ok, follow, Just one questions, say I have 10 gallons diesel in a 20 gallon tank and have 1 inch of water in the bottom of the tank and the diesel is fully saturated. Will any more water collect at the tank bottom (assume the tank is vented to atmosphere)? Say I add 10 gallons of new "dry" fuel, will some of the free water at bottom of tank move into the new "dry" fuel?. Also, when I look into my tank (I removed the tank and completely cleaned it and dryed it then installed back into boat with new fuel filters) at the bottom I see what looks like fine black particles that form swirls. This formed within a week of installing clean tank and new fuel. The black sediment does not seem to cause a problem (and I plan to polish the fuel once a year), but what is it and where did it come from?

Thanks for your very knowledgable input.
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post #10 of 13 Old 04-20-2011
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Cetane degradation?

At the risk of introducing a new angle to this discussion, have I read somewhere that diesel fuel loses some of its potency over time?

Fill a large tank and use only one-fifth of it per season, and that means that if you don't top it off, after several seasons you're left with years-old fuel.

Is that a problem?
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