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Old 06-22-2008
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Myth of Condensation in Fuel Tanks

What does everyone think about this?

The Myth of Condensation in Fuel Tanks
by David Pascoe


--------------------------------------------------------------------------------

Frequently we hear it said that the cause of water in fuel tanks is due to condensation. I have long doubted this assertion but the issue has come up so frequently that I was finally motivated to try prove to the point. The basis of my belief or assumption is that:

There isn't enough air volume within a tank to hold much vapor.

On average, tanks are half full, further reducing volume

The amount of water vapor in air is very small, even at 100% humidity

Conditions aren't right to cause condensation in a fuel tank

Research produced the following values for the maximum amount of liquid water in air at the following temperatures:

30C/86F 30 grams/cubic meter
20C/50F 17 grams/cubic meter
10C/13F 9 grams/cubic meter

There are 28 grams per ounce, so 30 grams = 1.07 oz; 17 grams = 0.6428 oz.

A cubic meter equals 264 gallons of liquid volume, therefore:

A 200 gallon tank = 0.76 cubic meter.

At 86F, an empty 200 gallon tank could contain 22.8 grams of water vapor, or 0.81 oz.

At 50F, an empty 200 gallon tank could contain 12.92 grams of water vapor, or 0.46 oz.

Note that this is the maximum amount of water vapor that a completely empty tank could contain, in neither case a full ounce of water.

In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm.

Aluminum is second only to copper for rapid heat transfer properties; it will therefore adjust to atmospheric temperature changes quickly. Gasoline and diesel fuel, like water absorb [sic] heat and cold slowly. Thus one might expect to see sweating on the outside of a tank as the day warms up from cold mornings, but do we? Well, I can say that after 35 years of inspecting boats, I've rarely seen tanks sweating. Note: Sweating may be likely to occur with boats in very cold waters when warm days are encountered.

This issue first came up a number of years ago over a question of whether internal engine rusting could be due to condensation caused by sudden temperature changes as from day to night and vice versa. Since that time, inspection of hundreds of engines showed that rust only occurs on the underside of valve covers due to water contamination of the oil. Very few engines have rusty undersides of valve covers, thereby proving the point that ice cold engine blocks in the morning donít sweat at it warms up during the day. If that is true, then how could it be true that fuel tanks sweat?

My answer is that they donít and these calculations prove it. My initial assumptions were correct. You do not need to store or lay up your boat with full fuel tanks. If you are getting water in your fuel, it is getting there some other way.

Contaminated Fuel

Years ago we had serious fuel contamination problems due to underground steel storage tanks that rusted and leaked. Today all tanks are fiberglass, so this no longer happens (that I know of). However, those underground tanks do have fill plates on the ground surface (usually the parking lot) that can leak just like your boat deck plate. As the marina pumps its tanks nearly dry before the next fuel delivery, those who buy fuel from the near empty tank are the ones that are going to get the water (because it's pumped from the bottom of the tank). This despite the fact that the dock fuel pump has a water separating filter. I've opened the panels on occasion and have found the sight bowls completely filled with water, so at this point the water is being passed on to the customer. Next time you buy diesel, ask to see the filter at the pump! You have to remove the lower pump panel to see it.

However, it is important to note if you're getting water from your fuel retailer, chances are that it won't be a small amount. Most likely it will be a lot and your filters will fill up and engines crap out post-haste.

Leaking Fill Caps

By far the most common cause of contaminated boat tanks are deck fill caps that leak. Most of these things are stupidly mounted flat on decks which may puddle with water. The cap has a tiny little O-ring that is supposed to seal and keep water out. DOES IT? I wouldn't depend on one of these things unless I could prove that it doesn't leak. Check the condition of the o-ring and weather it is sealing.

One way to check positively is to clean the o-ring seat thoroughly; next apply some black or any color paint to the o-ring and screw the cap in place, tight. Then remove it and see if the paint has been completely transferred to the ring seat. If not, you now know where the problem is.

Another problem is the simple failure to seat the cap fully after refueling. This actually happens a lot, so check to see if the cap is loose.

The Tank Vent

Improperly located fuel tank vent fittings are one of the top causes of water getting into tanks. When this is the cause, if you are a salt water boater, then it will be salt water in your tank. A fuel tank vent fitting on the side of the hull should be angled down and aftward. If angled in any other direction, you've got a problem that needs fixing. Watch out for deteriorated plastic and zinc alloy fittings; some of these things deteriorate incredibly fast.

The vent line should have a riser loop on the inside. That is, it travels upward first, then downward. If not, that is another potential problem.

Check the Fuel Gauge Sender

One final possibility is the fuel gauge sender plate on top of the fuel tank. These are often made of steel or have steel screws that can rust away, a situation I've seen several times. Is water puddling on the tank top? Test all screws with a screw driver to make sure they are securely seated.


Posted February 21, 2004
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Old 06-22-2008
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LOL @ "myth" It's not a myth, it's basic physics.
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Old 06-22-2008
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Very interesting and informative...I disagree to some extent...OK the air volume is small but how many times does that volume change over through venting? also where does he think all that white smoke ( Steam ) come from every morning we all start our cars?...yep...condensation...lastly every valve cover I have ever pulled off was pretty well coated with a film of oil...not very conducive to rusting period..Just some thoughts off the top of my head..
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Old 06-22-2008
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Quote:
In order to condense water out of the atmosphere a surface must be much colder than the air. The problem for the condensation in tank theory is; how do we end up with a fuel tank that is much colder than the air? One way would be to have a very cold day that suddenly warms up dramatically, but when does this ever happen? The weather can turn cold very fast, but does not suddenly get very warm.

Apparently Mr. Pascoe has not spent enough time in Maine in the winter where we can see temp swings inside boats of more than 40-50 degrees from night time lows in the single digits to the am sun hitting the shrink wrap, or dark colored Awlgriped top sides, and warming the boats interior to 40 degrees in a mater of an hour or two.... That said I am not a real believer that physical water appears in tanks when they are left half full etc...

Quote:
At 86F, an empty 200 gallon tank could contain 22.8 grams of water vapor, or 0.81 oz.
Yes or .81oz EVERY DAY THE CONDITIONS ARE RIGHT FOR CONDENSATION!!!!!!!! Someone remind him there are 365 days per year and tanks in boats should be expected to last well into the 30 year mark..?? Even if your tank could only hold .01 oz of water vapor over time it ADDS UP!!!!!


Seems to contradict his point....
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Last edited by Maine Sail; 08-28-2013 at 02:23 PM.
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Old 06-22-2008
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Also, if the boat is stored on the water, the level of water vapor available for condensing goes way up.
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Old 06-22-2008
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Stillraining...actually water is a product of burning gasoline. It condenses in the cold exhaust system before the exhaust system heats up. As the system heats we see it exit as steam.


I believe it's fairly common for fuel to have some water in it when we buy it. I've doubted to condensation theory for a while.
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Old 06-22-2008
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As for having water in fuel when you buy it, you are correct. Back in the late 80s I worked for an auto dealership service dept. We did a test of exon, mobile, shell and 7-11 gas. Filled a shot glass with fuel from each (no we did not drink it) and poured it out on the concrete and set on fire. The idea or bet i should say was to see which would leave the largest wet spot when finished burning. Supposidly this was from water in the fuel.
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Old 06-22-2008
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Fuel (all hydrocarbon fuels - even LPG) contain water in differing amounts. I dunno what it is for Diesel, but for Aviation Kerosene the limit is 30ppm. The refiners don't worry about it too much because, in small quantities water can actually aid combustion - it's when there is too much it can be a problem. Two things...

1. Entrained water doesn't mix with oil and so settles out over time (several hours), causing the "condensation" you see in the bottom of the tank. ie. The water has "condensed" out of the fuel.

2. Most hydrocarbon fuels significantly change in volume with changes in temperature, so if you buy fuel at, say 70degF and fill your tank on a 60degF day, the fuel volume will decrease gradually drawing a small amount of air in through the vent. Any moisture in the air will then settle on the tank walls and add to the "condensation" at the bottom of the tank.

The way to avoid issues with water in your fuel?? You have two options: Either pump it out before you go or recirculate it though your fuel system on a regular basis..

Quote:
Originally Posted by halekai36 View Post
Yes or .81oz EVERY DAY THE CONDITIONS ARE RIGHT FOR CONDENSATION!!!!!!!! Someone remind him there are 365 days per year and tanks in boats should be expected to last well into the 30 year mark..?? Even if your tank could only hold .01 oz of water vapor over time it ADDS UP!!!!!
Halekai, there's a small problem with your premise - an empty tank doesn't have any fuel in it.. If what you're saying is correct, over time an empty tank would magically fill itself with water - in practise it doesn't.
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Last edited by Classic30; 06-22-2008 at 10:28 PM.
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Old 06-22-2008
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Cameron...the empty tank allows the moisture to evaporate. Under a nice layer of diesel it does not evaporate.
Pascoe has other interesting theories that contradict every other expert.
In this case, he is simply dead wrong as actual experience affirms for many of us.
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Halekai, there's a small problem with your premise - an empty tank doesn't have any fuel in it.. If what you're saying is correct, over time an empty tank would magically fill itself with water - in practise it doesn't.
Not my premise! It was Pascoe who said that and I merely quoted it! Of course it won't condensate much with no fuel to slow the temperature change..

FWIW I drain my tank in the winter. I also remove my wet exhaust hose and plug the exhaust and air intake.. Overkill? Maybe but I have yet to ever rebuild or even repair the internals, fuel injection pump or injectors of a diesel engine and never had a single drop of physical water in my tanks, so, I think I'll keep doing it..
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Last edited by Maine Sail; 08-28-2013 at 02:21 PM.
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