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Old 07-05-2020, 12:35   #16
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Re: Leave a Diesel tank empty or Fill it

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Originally Posted by fred4936 View Post
I always wondered about this myself. Last fall I drained the tank and cleaned it. I left it closed and empty. A week or so ago I took the panels back off to give it a once over. It was dry inside. So I gave it another wipe down with kerosene and buttoned it up. Not sure I will do this every year, but I do like knowing that at least right now the tank and system are clean.
A clean dry tank will most likely never accumulate any water or any other problem, itís the ones that may have several gallons of fuel left in it and may have significant sludge etc on the bottom that may be a problem.
Maine Sail conducted a test to bust the myth of tank condensation and had an empty tank sit for considerable time and subjected to daily heating and night time cooling and after an extended period it never accumulated even a drop of water.
I believe his test to be correct, but may not be applicable to a fuel tank with fuel in it, the difference in my opinion is the fuel, fuel can absorb water, and can when conditions change drop that water out of suspension, so while I agree an empty tank wonít condense water, itís possible for one with fuel to do so, possible doesnít mean will of course, just it could.
However the desiccant filter will prevent any moisture from getting into the tank to start with, or I guess close the vent off, it seems it wonít have enough pressure change to matter.
Closing the vent off would be very simple, just stick a ball valve in line.
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Old 07-05-2020, 12:42   #17
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Re: Leave a Diesel tank empty or Fill it

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but may not be applicable to a fuel tank with fuel in it, the difference in my opinion is the fuel, fuel can absorb water, and can when conditions change drop that water out of suspension,
My thoughts too, particularly in Spring or Autumn with big changes in temperature leading to condensation in the tank. However, I bet a worn o ring seal on the tank filler cap is a much more frequent reason water gets into fuel.

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Old 07-05-2020, 12:52   #18
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Re: Leave a Diesel tank empty or Fill it

The only time I have ever seen water in a Racor it came from the fuel filler O-ring. Took us a while to figure it out too, we were first drawn to the vent but that wasn’t it.

Then I have only gotten water twice in an airplane fuel sample, once in an OH-58, it had it fuel tank vent coming out the bottom of the fuselage right beside the engine deck drains, it was a trick to put the hose over the drain to unblock it, the kid who washed it put the hose over the fuel tank vent.

The second was a Cessna 210, that I found out had a bad fuel cap seal and just like a boat water go in, in the rain.
Idiot owner had it kept outside when there was plenty of room inside of the plant.
The Cessna had bladder tanks, and they sometimes will wrinkle, and a significant amount of water can be retained behind a wrinkle so even though you sump the tanks, there may still be water.

I have never seen water in a tank from condensation, not saying it can’t happen, but in sumping tanks for over 40 years I have never seen it
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Old 07-05-2020, 13:11   #19
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Re: Leave a Diesel tank empty or Fill it

Best to keep diesel fuel tanks full as there is less accumulation of humidity and condensation of water into the fuel, and possible settling to tank bottom. Water in fuel is natural, but accumulation to excessive levels leads to other problems. Algae growth. Empty tank exchanges more air during alternate day / night expansion, leading to accumulation of water in fuel. Keep a Full tank. Treat fuel for long term storage. Buy your fuel while cheap!
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Old 07-05-2020, 13:20   #20
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Re: Leave a Diesel tank empty or Fill it

Formerly Diesel was plain Diesel, today they add alcohol, 5 to 15%, alcohol is highly hydroscopic, hence water in your Diesel tank, another reason is condensation, hot/cold expands and contracts air in the tank - more water in the tank, unless tank is really full & no air is pulled in. The lovely brown livestock grows in water and eats Diesel. In the old days you could store Diesel for years, since they add alcohol, better avoid it. For me: first choice empty tank, second choice full tank, anyway, deep sump with drainage as often as possible is a must for me.
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Old 07-05-2020, 13:29   #21
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Re: Leave a Diesel tank empty or Fill it

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Formerly Diesel was plain Diesel, today they add alcohol, 5 to 15%, alcohol is highly hydroscopic, hence water in your Diesel tank, another reason is condensation, hot/cold expands and contracts air in the tank - more water in the tank, unless tank is really full & no air is pulled in. The lovely brown livestock grows in water and eats Diesel. In the old days you could store Diesel for years, since they add alcohol, better avoid it. For me: first choice empty tank, second choice full tank, anyway, deep sump with drainage as often as possible is a must for me.


Ulsd doesnít have alcohol, E-diesel does, but Iíve never seen it for sale anywhere.
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Old 07-05-2020, 13:46   #22
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Re: Leave a Diesel tank empty or Fill it

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Originally Posted by kmacdonald View Post
Read this: https://pbase.com/mainecruising/fuel_tankt_condensation

Leave it empty. Less weight.

Exactly.


Fuel tank condensation is a myth which has been thoroughly busted by our own MaineSail.


Leave it empty and don't worry about it.


I prefer leaving mine near empty over the winter. That way the fuel I start off with in the summer is fresh and dry.
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Old 07-05-2020, 14:50   #23
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Re: Leave a Diesel tank empty or Fill it

I would drop a cloth flour bag containing desiccant sachets on a string into the dry empty tank, then screw on the lid. The desiccant will absorb the condensation and keep the tank dry.
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Old 07-05-2020, 15:05   #24
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Re: Leave a Diesel tank empty or Fill it

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Originally Posted by Glacier Crews View Post
For a number of reasons, only one may be relevant, we left 200 gal of diesel in our 3rd tank. Five years went by. No treatment, just fuel. Tank full. In year six we finally drained that tank to polish and dispose of fuel, only to discover that the fuel was in perfect condition as was the tank. So we polished it in our onboard West Coast fuel polisher and used it. Used it in 2019 put into tank in 2013 or 14.

Shared information.

I like to remind people that advice Is probably “worth what you paid for it.”

So when I bought this boat, it had 3 full tanks. That diesel had been sitting in the tank for 9 months (on the hard) in SC. We burned almost all of it moving the boat to Texas. No issues and the filters did their job. It did have biocide in it when they were filled previously.
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Old 07-05-2020, 15:09   #25
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Re: Leave a Diesel tank empty or Fill it

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Best to keep diesel fuel tanks full as there is less accumulation of humidity and condensation of water into the fuel, and possible settling to tank bottom. Water in fuel is natural, but accumulation to excessive levels leads to other problems. Algae growth. Empty tank exchanges more air during alternate day / night expansion, leading to accumulation of water in fuel. Keep a Full tank. Treat fuel for long term storage. Buy your fuel while cheap!

Good point on the cheap fuel. Paid $2.19/gal last week in Texas at the fuel dock.
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Old 07-05-2020, 15:14   #26
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Re: Leave a Diesel tank empty or Fill it

I believe both camps are right and it comes down to preference and storage.

I kept 100 gallon tank full for 8 or 9 months, polished it and was good as new.

The next year I emptied to the sump. and tank was good after 7 months.

I did keep an eye on the tank regularly in both cases.

My preference?

I dont know.
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Old 07-05-2020, 15:25   #27
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Re: Leave a Diesel tank empty or Fill it

That's weird because my hull collects a gallon or two *a day* of condensation when there is a large heat differential inside to out. Sure, a closed tank,you can't add water to a closed system. An open system you absolutely can.

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Exactly.


Fuel tank condensation is a myth which has been thoroughly busted by our own MaineSail.


Leave it empty and don't worry about it.


I prefer leaving mine near empty over the winter. That way the fuel I start off with in the summer is fresh and dry.
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Old 07-05-2020, 16:05   #28
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Re: Leave a Diesel tank empty or Fill it

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That's weird because my hull collects a gallon or two *a day* of condensation when there is a large heat differential inside to out. Sure, a closed tank,you can't add water to a closed system. An open system you absolutely can.


Youíre not moving vast quantities of air through the fuel tank.
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Old 07-05-2020, 17:01   #29
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Re: Leave a Diesel tank empty or Fill it

Fill it or you might get condensation which will grow algae
that lives on water surface and feed on the diesel. Biobor
will kill it but will clog your filters if bad enough.
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Old 07-05-2020, 17:22   #30
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Re: Leave a Diesel tank empty or Fill it

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Fill it or you might get condensation which will grow algae
that lives on water surface and feed on the diesel. Biobor
will kill it but will clog your filters if bad enough.
Unless of course the Bio-bor is there to start with, then it wonít even start to grow.
If you can get it clean and dry it wonít ever grow anything.
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