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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 04-27-2011
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How Full Is Your Tank?

So, for as long as I can remember I have followed the mantra of keeping your diesel fuel tank as full as possible so as to prevent the possibility of condensation occurring in the empty portion and rhus contributing unwanted water to the fuel. Today, I met a man who has been cleaning diesel tanks on boats for 35 years and he tells me that I should carry "only as much fuel as I need" due to the short shelf life of diesel fuel and how it begins to "asphaltize" after about 100 days. I'm curious about your thoughts and opinions on this. Personally, I KNOW that I have used 10 year old diesel in the past, so the 100 day thing doesn't pass muster with me.

Mike
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Old 04-27-2011
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Diesel, the product, is classed as a "light fuel oil" so AFAIK it isn't going to "asphaltise". It might slowly lose it's "bang" as the lighter fractions vaporise off, but given that it's flash point is over 100degF, that isn't going to happen in a hurry unless you live in the tropics.

There is an algae that grows on the diesel/water boundary that is black in colour and can wreak havoc with diesel fuel systems if not addressed in some way - perhaps that's what he is referring to.
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Old 04-28-2011
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I keep mine full to minimize condensation .I have had diesel trucks,boats, & tractors many yrs.Have never had an asphalt problem,did have an a@#hole problem when the trk driver put it in the tank they were supposed to clean.marc
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Old 04-28-2011
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I keep it as full as I can, but never lose sleep over it. It can't be full all the time. I add a biocide and stabilizer at each fill up which runs a few dollars worth per season. The Racor filter should remove any minor amount of condensation, assuming it isn't just blended in with the fuel and burned anyway.

Truth is, I'm more interested in having ample fuel to use for the genset underway than I am about condensation.
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Old 04-28-2011
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Letting the tank level drop too low can result in having to bleed the fuel system. I never let mine drop below 1/2 during the season. And as noted use biocide and stabilizer.
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Old 04-28-2011
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I cruise about eight months out of the year an my two tanks hold 55 gallons each. My typical refill is near fifty gallons,- about 30 in one tank and 20 in the other to top off. My tanks, then, are usually 3/4 full. Take care and joy, Aythya crew
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Old 04-28-2011
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I filled mine almost two years ago I think. They are now about 1/6 full. There is never water in racor, and the engine runs great. When I changed filters recently the fuel looked good.
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Old 04-28-2011
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Keeping a tank full in order to somehow prevent 'condensation is foolhardy at best and at worst (especialy if the fuel isnt being used quickly) can lead to fuel degradation, water ad/absortion, enhanced biological growth (fouling).

Water 'uptakes' into fuel by 'chemical equilibrium' because the 'new' fuel is essentially dry and the atmosphere is moist. This 'mass transfer' will continue until the oil is fully saturated with water ... and then the water will begin to 'settle down' due to gravity. It makes NO difference if the tank is full, half empty or nearly empty ... as a long as there is unsaturated fuel in the tank and the tank is in contact with the moist atmosphere the water will 'enter' the oil. The AMOUNT of oil in the tank governs the AMOUNT of water the oil 'uptakes' .... simple chemistry (- law of partial pressures).
Once a tank with oil saturates and the water begins to 'drop out' at the bottom to form a distinct 'layer' of free water with the saturated mixture on top .... you now have a 'chemical pump', allowing the water to drop out and if the 'ambient conditions' are 'right', more water will 'equilibrate' into the oil and only to later 'drop out' at the bottom and cause more 'equilibrium imbalance' to allow more 'water uptake' ... ad nauseum.

The amount of 'saturation' is dependent on the AMOUNT of oil in the tank. Empty tanks do not magically 'fill up with water, do they?'. If they did we wouldnt need to drill water wells as all we'd have to do is put out empty tanks to get all the water we need ... and we know this isnt true.

'Condensation' ONLY means that the oil is already fully or nearly already saturated with water. The LEAST amount of fuel in the tank will 'uptake' the LEAST amount of water.

Rx
So, keep only enough fuel in the tank for your relative immediate needs plus some reserve; buy your fuel only from 'high turnover/fresh' (dry oil) sources, if possible; drain/empty the tank when long term storing; avoid buying your oil from low-turnover sources - marinas where the fuel sits for long time (water saturated) before pumping.
Why 'store' an unnecessary amount of oil if 'chemistry' tells you that it will soon 'saturate' with WATER ??????
Better to keep a minimum quantity that can only 'uptake' a minimum amount of water; buy 'fresh' (dry) when and as needed rather than 'storing' it only to have it saturate all by itself.

Last edited by RichH; 04-28-2011 at 01:12 PM.
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Old 04-28-2011
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Quote:
Originally Posted by Hartley18 View Post
Diesel, the product, is classed as a "light fuel oil" so AFAIK it isn't going to "asphaltise". It might slowly lose it's "bang" as the lighter fractions vaporise off, but given that it's flash point is over 100degF, that isn't going to happen in a hurry unless you live in the tropics.

There is an algae that grows on the diesel/water boundary that is black in colour and can wreak havoc with diesel fuel systems if not addressed in some way - perhaps that's what he is referring to.
Your fuel polisher is correct.

The primary biological contamination in fuel oil (that is stored and/or exposed in any way to the atmosphere) is commonly called 'kerosene fungus' (Cladosporium Resinae, et al) fungal species that uses the oil as its nutrient source and whose 'metabolism' produces 'resins' - chiefly 'asphaltines', etc. which are the 'black sticky stuff' that adheres to tank walls, etc. .... the probable primary cause of 'choked filters'. Algae need 'sunlight' and WATER to propagate, the fungals dont need sunlight and can extract the 'emulsified' and molecular water direct from the (apparently 'dry') oil.
The fungals produce spores and cell fragments which 'agglomerate' .... 'growing' into larger and larger and large particles; and these particles are 'soft and deformable' and under increasing pressure across a filter can simply 'extrude through' a filter (@ high ∆P) and simply 'agglomerate' or 'coalesce' on the downstream side of the filter. Such particles do not 'burn' very well and are readily available to 'deposit' and form 'coke' on the hot surfaces of the exhaust system (like the injection nozzle) ..... probably THE source of 'choked' water injection elbows.

The only way to maintain a fuel tank is to get inside and MECHANICALLY scrub it; then to keep it clean you need to constantly 'recirculate polish' to remove these 'particles' .. AS THEY 'FORM'.
If you develop a thick mat of cells and their 'resins', etc. on your tank walls .... and these dead cells begin to 'decompose' and the mat or 'massive colony' (calcyx) breaks loose, you will need 'cases' of filters in stock to keep your engine running.

Your fuel polisher KNOWS what he's talking about.
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Old 04-28-2011
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Quote:
Originally Posted by RichH View Post
The only way to maintain a fuel tank is to get inside and MECHANICALLY scrub it; Your fuel polisher KNOWS what he's talking about.
Thanks for the replies. Yes, this is what he's doing. He's going to empty the tank, completely scrub & polish the inside, cut two inspection plates (one on each side of the baffle) and add a backup dipstick plug so I can drop a dowel in to check the fuel level if needed. For a 35 gallon tank, is it helpful to add a recirculating polishing system or is the Racor enough?

Mike
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