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Old 10-11-2016, 21:54   #46
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Re: "Worth it" to top off diesel for the winter?

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Originally Posted by Suijin View Post
You're going to get proponents of "top them up" and "leave them empty" and the truth of the matter is either route is not going to make that much of a difference if you're going to use the boat next spring and burn through a few tanks of fuel.

One of the best marine diesel mechanics here in Annapolis recommends leaving your tank as empty as possible over the winter. His reasoning is that the amount of condensation that might accumulate is negligible, while the longer fuel sits around in your tank, the more potentially bad fuel you have. He advocates simply avoiding letting fuel sit...filling your tank only as much as you're going to use in a few months, instead of carrying around 100 gallons that takes a year or more to use.

Mainsail, who is fairly well respected in this community, did a test of an empty tank over the winter and found the amount of condensation developed to be negligible as well. That does not mean that he endorses leaving your tank empty. I'm not sure what his specific recommendation is on that.

If you end up with water in your fuel, your primary is going to catch it. If you end up with bugs in your fuel, a biocide and your primary is going to catch it. Choose your poison.
The issue with that experiment, is that the tank had zero fuel in it. Fuel vapour expands and contracts a lot more than air. Fuel absorbs and traps moisture. Many have asked that experiment be performed again, but with the tank 1/4 full. I expect the results will be way different.
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Old 10-11-2016, 23:12   #47
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Re: "Worth it" to top off diesel for the winter?

Yes, there would be a slight bit of condensation, but the amount would be too small to measure.

If you are really concerned about water in your fuel, install a small sump on the bottom of your tank with a drain valve. You'll soon work out where your water is coming from.

Most of the time it's either a bad batch of fuel from the pump, or a damaged o-ring on your fuel filler cap.
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Old 11-11-2016, 00:18   #48
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Re: "Worth it" to top off diesel for the winter?

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Originally Posted by Celestialsailor View Post
Maybe you should...I just emptied 2 gallons of water out of my 50 gallon tank and spent 3 hours dismantling the Racor and cleaning it.
Why don't you do the math, and see how much warm, saturated air would have to pass through your tank, hitting tank walls or fuel which is colder than the dew point, to deposit 2 gallons of water in there?

You will see that a thousand years of "breathing" could never put 2 gallons of water in your tank. It is objectively impossible. The water came from somewhere else.
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Old 11-11-2016, 00:55   #49
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Re: "Worth it" to top off diesel for the winter?

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Originally Posted by ramblinrod View Post
. . . Fuel vapour expands and contracts a lot more than air. ..
Do you have a source for this statement? Where did you get such an idea?

And do diesel tanks at room temperature even have significant amounts of fuel vapor in them?

I don't mean to argumentative, but this subject, for some reason, attracts lots of wild made-up stuff which then gets spread.

Diesel fuel typically has vapor pressure of 0.4mm Hg at 20C. That means it will only produce 530 parts per million in air, at that temperature (you need Dalton's Law of Partial Pressures for this). That means that a 500 liter tank half-full of diesel fuel, will contain 132cc of diesel fuel vapor vs. 249,868cc of air assuming it has the same density.

And this 132cc of diesel vapor is supposed to cause enough breathing to put x gallons of water in your tank, whereas a tank full of pure air will not?


I will be glad to corrected if I'm wrong, but until I am, I'm classifying this theory under "wild made-up stuff".
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Old 11-11-2016, 01:19   #50
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Re: "Worth it" to top off diesel for the winter?

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Originally Posted by ramblinrod View Post
The issue is that the fuel (and especially fuel vapours) expand and contract with temperature, so air is being repeatedly pushed out of and sucked back into the tank with each temperature cycle, typically, warm in the day, air is pushed out, cool at night, air is sucked back in.

The greater the volume of air/fuel vapour and wider the temp swing, the greater the air change. do that every day for six months on a 1/4 full tank and you'll have a lot more than couple drops of water in there.

. . .
Have you ever seen any numbers on this alleged mechanism?

You've got one thing right -- any "breathing" which takes place, will bring in cool air at night. It will not bring in warm air in the day time. And that is the crucial fact, because condensation occurs when you cool off warm, moist air, to a temperature below its dew point. No condensation will occur if you bring cool air into a tank, even if its very moist air, and you don't cool it off. So to condense anything out of cool night air, you will have to make it even cooler. But you are in the wrong phase of the temperature cycle to do that.



But on top of that -- even if there were some way for the fuel tank to be cooler than the cool night air -- and there is not, but suspending disbelief -- the amounts of water which can be condensed out of the volumes of air which could be breathed in are far too small.

Here is a table of water vapor capacity of air at various temperatures:

http://web.gccaz.edu/~lnewman/gph111...20of%20air.pdf

At 20C, it's 15g of water per kg of air. At 15C, it's 10g.

Air changes its density by about 1.625% between 15C and 20C. So 250 liters of air in a half-empty 500 liter fuel tank will "breathe" 4 liters of air. 4 liters of air at 20C weighs 4.82 grams. At total saturation, it can contain 0.0723 grams of water, of which 0.0241 grams of water will condense out if you cool it to 15C.

So if you do this 180 times over half a year, you will get 4.338 grams of liquid water in the fuel, assuming none of it evaporates back out.

Except that even this is completely impossible -- because the tank breathes in, as you said, then it's cold, not when it's warm. Besides that, you don't have 100% humidity every day.

Also the daily thermal cycle is overstated because the boat has thermal mass and doesn't cycle to the extent as the ambient air. My fuel tank (I measured it) never changes temperature by more than 2C.



All this shows that it is completely impossible to get a significant amount of water in your fuel, from tank "breathing" and condensation.
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Old 11-11-2016, 02:53   #51
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Re: "Worth it" to top off diesel for the winter?

The most important thing you can do to keep water out of the tank is to not put water into the tank. The second most important thing is to make sure the fill pipe cover is completely sealed. The fill pipe is how 99.9% of water gets into tanks of fuel stored over the winter. Snow and ice cause water to stand over the fill pipe and then water drains into the tank through the sun damaged seal.

The "breathing" tank theory can be completely disproved by a simple experiment. The experiment has been done multiple times and no water was ever found in a nearly empty tank left to breathe. If you don't believe that then do the experiment yourself and publish the results.

The amount of fuel in the tank has no effect on how much water "condenses" into the fuel. This is an old wive's tale repeated numerous times on the Internet.
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Old 11-11-2016, 03:05   #52
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Re: "Worth it" to top off diesel for the winter?

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Originally Posted by OrangeCrush View Post
As OP I'm obviously not the authority here, but it seems to me that an empty tank could breathe in and out endlessly without any condensation. The condensation would occur because as the boat heats up, the thermal mass of a large volume of diesel lags behind. At one point, the air is warmer than the surface of the fuel and condensation forms. Sorta like advection fog, or how on a humid day the toilet tank in a bathroom can have condensation on it in the morning. Is this incorrect?


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Yes, it will "breathe" but the calculations already assumed that.

The problem is it's a negligible amount that goes in and out each day and of that, it's a negligible percentage of the available moisture that could condense.

This is much different from a bathroom which may have a volume equivalent to a several thousand gallon tank and has vents constantly supply new moisture laden air. On top of that, in a bathroom, you will typically see a temperature difference on the order of 20F as 50F water is introduced into a 70F room. With a boat, it's a gradual process over hours and you don't get the huge temperature variations.
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Old 11-11-2016, 03:14   #53
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Re: "Worth it" to top off diesel for the winter?

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Maybe you should...I just emptied 2 gallons of water out of my 50 gallon tank and spent 3 hours dismantling the Racor and cleaning it.
I keep asking someone show me the calculations showing condensation can generate a measurable quantity of water...let alone 2 gallons.

On a previous boat, it had 1/2 gal of water in an 18 gal tank. I replaced the 0-ring and it never came back. If I were you, I would spend some time figuring out where the water is coming from because there is no way you got 2 gal from condensation.

This is like Cotemar's example of water in his oil fill. Zero evidence that it I condensation related.
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Old 11-11-2016, 04:32   #54
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Re: "Worth it" to top off diesel for the winter?

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Why don't you do the math, and see how much warm, saturated air would have to pass through your tank, hitting tank walls or fuel which is colder than the dew point, to deposit 2 gallons of water in there?

You will see that a thousand years of "breathing" could never put 2 gallons of water in your tank. It is objectively impossible. The water came from somewhere else.
Sorry...I think my math is just fine...
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Old 11-11-2016, 04:38   #55
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Re: "Worth it" to top off diesel for the winter?

Well, let me put a vote in for a dryer on the vent line, or top them off. Two anecdotal personal experiences. The first a few years back and in the fall. Lots of humidity, rain and temperature changes. A customer of mine took his steel scuba cylinders to hydro to save a few bucks. He put them in his trunk and a week later brought them to me for visual and fill. I almost had to condemn one of the as there was probably a spoon full of condensate in the cylinder and moist walls. Did the hydro facility not dry them? doubtful. He had no valve in them. Virgin steel tanks rust very quickly.

Second, well take a look. This can was inside the cabin with fill valve loose. It took years to build up enough to destroy the tank, but .... Cabin Heater | Sailing Vessel Vigah
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Old 11-11-2016, 05:31   #56
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Re: "Worth it" to top off diesel for the winter?

Believe it or not but sometimes the source of water is the fuel itself, fuel has water in suspension, sometimes if it has a lot of water in it letting it sit or the fuel cool, the water comes out of suspension.
In the Military we had to test the aircraft fuel for water, if it exceed the standard we had to recirculate the fuel through a drier until it met the standard, this comes from if you have too much water it can come out of suspension and either freeze or of course cause a flame out as water doesn't burn
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Old 11-11-2016, 06:00   #57
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Re: "Worth it" to top off diesel for the winter?

Although I do leave my tank topped whenever feasible, the more salient issue for condensation is temperature. If you leave a small box heater in the cabin with the engine access open, the fuel is unlikely to get below the dew point of the ambient air. Admittedly, places in the icee tundra will take a little more thought; but if you can keep your fuel in the 10' C range that would go a long way. Here on the gulf coast, it would be more important to boost that value to 15+ since we tend to get very warm moist spring air with corresponding high dew point. My little box fan also has the advantage of keeping condensation in and outside the engine at a minimum.
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Old 11-11-2016, 06:04   #58
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Re: "Worth it" to top off diesel for the winter?

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Maybe you should...I just emptied 2 gallons of water out of my 50 gallon tank and spent 3 hours dismantling the Racor and cleaning it.
Which only proves that you had two gallons of water in your tank. It doesn't provide any insight as to how it got there. It's borderline anecdotal information.
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Old 11-11-2016, 06:20   #59
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Re: "Worth it" to top off diesel for the winter?

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Sorry...I think my math is just fine...
Cool, could you share it with us? If yours is right and mine is wrong, I will be very glad to learn something.
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Old 11-11-2016, 06:35   #60
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Re: "Worth it" to top off diesel for the winter?

So far several people have pointed out the math and science that it is impossible to get any substantial amount of water from condensation.


That goes up against people talking about their grandpappy told them to do it, there's water in the fuel and they have no clue where it came from so it must be condensation, the crankcase is full of water (still not clear how that is relevant to the discussion), they do it to save the little children, etc... but oddly, they never provide any calculations to support their theory, just broad generalities that air contains water and it could condense out.


Please if you are going to claim it happens, please someone provide some calculations to support your position.
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