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Do you top up diesel tanks for winter to avoid condensation?


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As previously noted I have had 2800 litres of fuel in my tanks for the last 5 years (due to family problems), Having eventually 'got some cruising in this Summer' with no starting, running, or fuel problems I remain in the "keep your tanks full" camp.

 

From the link :

I guess what I'm saying is that it doesn't really matter as you can't effectively stop the water from getting into the tank. If you keep the tank level low, you'll get more condensation from the air due to a greater area of cold, exposed tank wall but the water dissolved in the relatively small amount of fuel won't be as big a contributor to the problem. If you keep the tank level high, you'll get more water out of the fuel when the temp drops but you won't have as much exposed tank wall to condense water out of the air. If I had to pick one, I'd probably keep the tank full to minimize the tank wall area that'd allow condensation from the air every time the dew point drops below the tank temperature. Over the course of months, this could happen quite a number of times. Theoretically, even a full tank of diesel could draw water from the air directly into the fuel but in practice I doubt that the fuel can equilibrate with the moisture in the air very quickly via a vent line so the majority of the water in the tank would come from the initial fuel fill.

 

As always, the otherside of the argument using 'sound logic' will be expounded so we are no nearer a conclusion.

 

Cassette or pump-out ?

Edited by Alan de Enfield
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If you seal the air vent surely there can only be a finite amount of moisture in the trapped air, and after the initial condensation, the water would sink to the bottom of the tank and thereafter be unable to condense again, so theoretically, the air above the diesel should be “dry”.

Over to the scientists to disprove this theory.

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9 hours ago, jenevers said:

If you seal the air vent surely there can only be a finite amount of moisture in the trapped air, and after the initial condensation, the water would sink to the bottom of the tank and thereafter be unable to condense again, so theoretically, the air above the diesel should be “dry”.

Over to the scientists to disprove this theory.

That has always been my theory.

Also I bet more water gets into fuel tanks through leaky seals on fillers and air vents.

  • Greenie 1
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I've always been rather sceptical of the need to keep the tank full, as I suspect that the number of air changes in the ullage over the course of a year is fairly small. The driving force for exchange is the slow variation in external air pressure as weather fronts go past, and probably more importantly the rapid fluctuation in pressure due to wind. A few years ago I did a study on the exchange rate in a vented nuclear reactor containment. I can't remember the details but the annual number of volume exchanges was quite small. In a fuel tank, the smaller the ullage the less water vapour will be drawn in. As it doesn't hurt to keep the tank full I try do so, but don't get obsessive about it.

 

 

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1 hour ago, Onewheeler said:

I've always been rather sceptical of the need to keep the tank full, as I suspect that the number of air changes in the ullage over the course of a year is fairly small. The driving force for exchange is the slow variation in external air pressure as weather fronts go past, and probably more importantly the rapid fluctuation in pressure due to wind. A few years ago I did a study on the exchange rate in a vented nuclear reactor containment. I can't remember the details but the annual number of volume exchanges was quite small. In a fuel tank, the smaller the ullage the less water vapour will be drawn in. As it doesn't hurt to keep the tank full I try do so, but don't get obsessive about it.

 

Surely the biggest driver is daily temperature change. During the day, the air in the top of the tank expands, and some is expelled through the breather. In the late afternoon and evening as the temperature drops, the air in the tank contracts, drawing in more air from outside. Since, on most boats  the breather is relatively low down ( I.e. deck rather than cabin roof level), the surrounding air is relatively humid, and this is drawn into the tank. As it cools further the relative humidity rises until the dew point is reached at which point some water condenses and drops to the bottom of the tank.

Repeat daily.

The smaller the volume of air above the fuel, the less air is drawn in each time, and with it less water. So keeping the tank as full as possible is best.

39 minutes ago, jenevers said:

I'm going to block the breather on my diesel tank with masking tape and remove it next summer.

Which will have s*d all impact, as it will be unable to resist the daily changes in tank air pressure with temperature.

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Or light a little bonfire around the breather as the sun goes down to dry humid air trying to get into the tank. Or aim a fan heater at it.  I have made a little shelter over mine in case rain tries to get in, a little plastic pot about the size of an egg cup glued on top with a gap at the bottom so it can still breath. A long flexible tube could be fixed to the breathers tank hole with the breather unit fixed at the other end which can be brought indoors when the sun goes down where it should be nice and try, providing you don't keep having showers every five minutes or boiling big stews and things. Use a pressure cooker adjusted so the pressure relief valve doesn't quite blow off and take it outside to open it so its steam blows away on the wind into someone elses breather.

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I just remove any water from the bottom of the tank in the spring. I always brim the tank in winter and remove any any water from the bottom of the tank in spring. I have never got more than an egg cup full out. 

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Yes, I top up the tank before winter.

I've not had any water-in-fuel problems (so far) and there is no primary fuel filter or strainer- just the standard Beta screw-on job.

 

On a previous boat I didn't top up the tanks, and the Vetus engine had primary and secondary filters. I had nasties in the diesel, which may have been condensation, but equally, it may have been because of the stupid diesel filler cap design. It was flush with the deck so that there was always a puddle of water sitting on top of it.  

 

While I can afford the diesel I'm going on taking the precaution.

 

 

5 hours ago, 1agos said:

I just remove any water from the bottom of the tank in the spring. I always brim the tank in winter and remove any any water from the bottom of the tank in spring. I have never got more than an egg cup full out. 

 

Seems to suggest that brimming the tank works, and it's a waste of time getting the egg cup out (unless you want a boiled egg, that is).

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3 minutes ago, Bod said:

Heavy haulage have been doing this for decades, so I think there must be something in it. 

This is the sort of real world based evidence I like: based on this validation, I feel fully justified in going for a full fat boy "Trucker's'" breakfast in the morning! :D

 

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