Condensation debate data point

[Roger Long]

During the great tank condensation debate, I promised to come back and report on my tank. It's been sitting all winter about 1/3 full in a shed up here in Maine. I'm in the boat almost every day and there have been days when it looked like someone had hosed down the whole interior it was so running with condensation. On very cold days, it has sometimes looked like that scene from Doctor Zhivago. Condensation is certainly real and there has been plenty inside my boat.

I determined today with a level that my sounding tube is in the low corner of the tank the way the boat is sitting on the blocks. I put clear plastic tube on a rigid tube, stuck it in the tank and connected it to a spare fuel pump. The end of the tube was square and flat against the tank bottom so it was sampling about the bottom 1/128 th of an inch.

I pumped about a gallon out and into a water separating funnel. It came out looking perfectly clean as it went through the clear tube and I could not find a single drop of water or speck of sediment in the funnel.

Tank was cleaned and polished in 2005 and I ran my onboard polishing system whenever the engine was on last summer.

 

[Ross]

Good report! It should put to rest much of the speculation concerning this. I have always used automotive diesel pumped first into portable cans at the filling station and then poured through two water separator screens and into the tank. I have a water separating filter that has never shown any water. Fuel hasn't been a problem for me.

 

[tommays]

Well

Right NOW the humidty is pretty LOW here way down south in many places it might be close to 100 % on bad days in summer

 

[Stu Jackson]

Water & Fuel Tanks

I concur with Ross. I use marina/waterside fuel. We usually keep our tank above half full, but many times have left it for extended periods at half or less full. While we don't have the severe weather experienced by Roger, we do have regular condensation with the 40 degree differences between night and day temperatures here in The Bay Area, clearly exhibited by condensation on the boats every morning, all year round, almost. We have never had water issues in our fuel tank or Racor. No reports from fellow skippers here, either.

 

[Bill Roosa]

Laws of physics confirmed

Well it is as I always suspected, if there is no water in the tank there can't be any condensation forming in the tank. Since diesel fumes fill the tank above the liquid fuel there can only be 2 sources of water getting into a tank. Water introduced during the filling process, AKA water in the fuel, or humid air entering the tank vent. Since Roger and others are taking measures to limit the water entering with the fuel we can conclude that it is not entering through the tank vent either.
In my days in flight training I routinely found water in the gas tank. I believe that the wing tanks where subject to night time cooling and the vapor contracted and drew in moist air which condensed in the tank. Boat tanks are not exposed to this kind of cooling and diesel fumes expand and contract less than gasoline so you get a lot less humid air entering the tank vent.

 

[kloudie1]

Some thumbnail thoughts.. These are rough numbers, but if a tank has a void space of 15 gallons of 100% humidity air, and it cools from 30F to 10F, 0.101cc of water will condense.. If that same void space at 100% humidity is at 80F and cools to 60F, the water condensed will be 0.686cc almost 7 times as much.. I don’t think that condensation in the cooler weather would be much of a problem.. this is per temperature cycle. This is temperature in the void in the tank, so in the summer, this temperature probably doesn’t swing quite that much because of the mass of the diesel and the moderating effect of the tank being inside the boat. Also, a poly tank would not swing as much in temperature as a metal tank would. Great info, Roger, methinks that most water comes in with the newly added diesel.
Just thoughts over a glass of merlot!..

 

[Stu Jackson]

Merlot is a very good indicator

1. ...0.101cc of water will condense.. .ill be 0.686cc...This is temperature in the void in the tank, so in the summer, this temperature probably doesn’t swing quite that much because of the mass of the diesel and the moderating effect of the tank being inside the boat.
2. Also, a poly tank would not swing as much in temperature as a metal tank would.
3. methinks that most water comes in with the newly added diesel.
4. Just thoughts over a glass of merlot!..

Claude,

Very interesting observations. Thanks.

1. Not having my handy dandy conversation tables available right at hand, how many cc in a nominal 20 gallon tank? Just an engineer askin'

2. Poly tanks would perhaps not change as rapidly in temperature as would metal tanks, but over a reasonable amount of time, say overnight, the temperature would drop to that of a metal tank, and its surrounding air temperature; keel tanks would meet the water temperature. Consider for a moment an insulated wall or a pipe. The insulation is to prevent heat transfer. Eventually though, once the heating medium is turned off, any difference between inside and outside will be reached: the insulation would only extend the amount of time the heat transfer would take and the differences between them reduced to zero. Like, turn your home heat off overnight, and it does get cold inside, eventually reaching the outside temperature. Just an engineer respondin'

3. Water comes in from both your suggestion as well as poorly sealed or not intact fuel filler caps - the plastic ones, which we use BTW, are notorious for slight cracks that let water in through the depressions for the jib winch handle holes. I check and replace mine as required. Just an engineer notin'

4. Merlot? Didn't think that would affect diesel fuel at all, wouldn't put it in a tank (what a waste!), but you wouldn't see it in the Racor bowl anyway!!! because it's almost the same color as the fuel. Just an engineer checkin'

 

[Roger Long]

Iin my days in flight training I routinely found water in the gas ank.

That was the cheap fuel caps mandated by an FAA approval process that makes it impractical to bring better ones to market. It was my experience checking tanks for water nearly daily on an aircraft I managed and where I knew the tanks were tight that first made me question the tank condensation conventional wisdom. The only time I found water in nearly a decade was when a cap was loose in a heavy rain.

 

[Dave Groshong]

Re: Merlot is a very good indicator

handy dandy calculator says 75,686ml in 20 gallons

750ml in Merlot

 

[Stu Jackson]

More conversions required, Dave

ml to cc :dance:

almost time for some cabernet here on west (left) coast time...

 

[catsailor]

It is not condensation - it is absoption

In my work I have researched this topic (chemical engineer) and I agree with the first observation; condensation is very rare, and for all of the reasons stated. I won't flog a dead horse. There is certainly physical evidence in the form of rust on the roof of every stationary above ground tank (I also do API inspections), but that takes very little water.

However, absorption is a different animal. In the case of e-10 gasoline, the amount of vapor moving in and out of an unrestricted vent is significant, perhaps most of the head space each day in the summer. E-10 commonly has a boiling point of 110 to 120F. Because the alcohol absorbs the water without requiring condensation, the fuel can become saturated in a few months. A large temperature drop will cause fallout. Read the EPA's own memo. Not a problem for car because we drive frequently and the cap venting is controled through carbon.

ULS diesel does not readily absorb water; however, B-5 (bio-diesel) does, through much the same mechanism as e-10. A temperature drop can cause drop-out, though less dramatic by far.

For 98% of the water-in-fuel issues, there is a rain water leak somewhere in the tale; the marina, a fuel cap, or somewhere.

 

[Dave Groshong]

Re: More conversions required, Dave

hmmmmm, guess it's been many moons since chemistry class, I thought a cc=ml

 

[RichH]

OK Rodger lets have the base data:

What was the hydration (% water) in the original fuel? weight % to the nearest 10th will be OK. What was the hydration of the fuel at the time of test conclusion?
What was the accumulated temperature cycle time degrees F or C vs. hours total time to stturation (or not)?

Comment: Wintertime has the absolute humidity of the atmosphere at it lowest for the entire year. Water uptake of fuels in winter is not the same as when the absolute humidity of summer is much greater. Suggest you repeat your test in August and make the comparison. :-D

best regards
RichH

 

[Roger Long]

OK Rodger lets have the base data: What was the hydration (% water) in the original fuel? ...

Very funny. I was just speaking to the conventional wisdom that, if you don't fill your tank for winter storage, you'll inevitibly find it full of water in the spring, the bottom corroded out, you'll come down with hives, and your pecker will shrivel.

Summer may be a different matter but I've got my polishing system running then.

My storage situation is actually interesting. I've learned that the luxury of having warm, inside space does come with a downside. Since the boat is sitting in a plastic shed on a concrete parking lot (presumably without a vapor barrier) and rain and melt runs in under the shed edges, it is very humid inside compared to a shrink wrapped boat which is isolated from ground moisture. The shed often gets up to 50 - 60 degrees at deck level while the puddles of water on the floor are still frozen. The solar heating results in large temperature cycles as the sun goes up and down and the amount of moisture and condensation inside the boat is truly impressive. If the boat is ever going to spend a winter when I'm not doing major work on it, I'm not going to put it in the shed but shrink wrap it so it stays drier.

 

[jibes138]

More real world experience

I drive a deisel pick up truck (F250). I rigged the back of the bed with a rack that holds four scuba tanks, all my scuba weights (for winer traction) and two 5 gallon jugs of deisel fuel. The fuel was because sometimes it is hard to find deisel at gas stations. I rotate the fuel in the tanks to make sure it doesn't get too old. After pouring fuel from the tanks into my truck tank I have never found the bottom of the jugs to be free of water or stuff growing in the fuel. I always clean these jugs by pouring the residue into a clear glass jar and now have quite a collection of water and bacteria contaminated fuel. Since these jugs are sealed with a cap the fuel is not exposed to regular air other than whats trapped in the jug. I hypothesize that the fuel as purchased has a significant amount of water and contamination aleardy in it. So water in your tanks is more likely coming from introduction with the fuel than from condensation effects, either way it is good practice to pump your tanks run the fuel through a separator/filter and burn it in your deisel truck where it will be consumed in about a week (30 gallons) then fill the boat with fresh deisel. If you don't drive a deisel you should. It gives you a place to get rid of your boat fuel.

 

[Roger Long]

If you don't drive a deisel you should. It gives you a place to get rid of your boat fuel.

Up here in the frozen north, you can also pour it into your heating oil tank. The prices aren't that much different.

I filled two clean 5 gallon jugs with road diesel last fall when the price bottomed because I thought it might be back up to 5 bucks again and hadn't thought yet of trying this experiment. They've been sitting on the floor of my shed. It will be interesting to see what look like when I put them in the boat. Since the floor temperature is fairly stable and quite cold, I expect they'll look as good as the day I filled them. Sitting in the sunlight in the summer has got to be much more bug friendly.

 

[Don12364]

Hi Roger,
Some time ago I posted my calculations on this topic and concluded that we buy the water at diesel prices!
See as below:-

Some time back I did a calculation for my H376 tank.
Worse case scenario:-Tank empty - volume 30 US Gals = 114 litres. Ambient air always 100% relative humidity.
Day temperature 20°C = 68°F - Night temperature 5°C = 41°F.Assume air inside tank is always 100% R Humidity. Ignore barometric pressure changes as these do not occur as frequently as the day/night cycle and may actually reduce the day/night effect.
Water contained in the empty tank at 20°C is 2 grammes = 2 ml = 0.12 cu inch of which 1.2 g will condense on the tank walls when cooled - but only whenever fuel in the tank is used up.
Now when tank temp drops to 5°C it 'breathes in, 6 litres of air = 0.2 cu ft or two human lungs full.
Cooling this air to 5°C releases 0.062 grammes =0.062 ml of water = 0.004 cu in. (from hygrometry tables).
Assume this happens 365 times a year on the day/night cycle. 
Total water released = 22.6 grammes = 22.6 ml = 1.4 cubic inches or 0.006 US gallons per year.
But: We know the air is NOT always saturated, the tank not always empty, the day/night temperature does not always change by as much as 15°C = 27°F and the tank is located at water level so is mainly held at the water temperature by thermal conduction through the hull.
So how does the water get in there?
Answer - WE BUY IT FROM THE FILLING STATION - I don't believe in fairies!
On my 376 I have had water problems on two occasions in the last ten years. On both occasions I drew off all the diesel and it contained between half and one gallon of water. ie. enough to reach the pickup tube. On each occasion I had refuelled from a supplier which had its tanks in the water. Once from a fuelling barge and the other from a marina. Tanks rot at the bottom so, diesel being lighter than water, the water enters rather than the diesel leaking out. IMHO that's how.

A pal was district manager of all filling stations (road) for the UK's largest fuel supplier. I asked him why don't trucks get the same problem? He replied "They do. There is a standing instruction to filling stations to clean out their tanks every three months. A good gas station might do it once in two years. A bad one - never.
Another standing instruction is not to sell any diesel for 3 hours after filling their tanks. Nobody bothers about this".

Consider this too:-
When night falls the air outside the tank cools and drops its dew.
The air inside the tank also reduces in temperature but more slowly than the outside air due to the warmth retained by the hull.
So - the air drawn in has already lost its moisture and warms up a little inside the tank. This slight warming would enable it to hold more water; thus it is no longer at 100% RH so no condensation.
Next day the tank warms up and the cycle repeats.
IMHO All this guff about condensation stopping engines is hogwash as the amount of water needed to reach the dip tube is measured in gallons.

 

[kloudie1]

Water In Diesel

Yes, any significant water that gets into the tank is from the fuel filler! Either through a hole in it or it is poured in with the diesel. Ya hit it on the head, Donalex.
Poly Tank comment.. In order for the condensation to take place, heat must be removed from the water vapor. This happens at the walls of the tank.. The thick poly is an insulator compared to Al or steel. The amount of condensation will be limited by the heat transfer rate of the wall. Lots of complex doo doo going on at the wall during condensation.. Ultimately, it will come to equilibrium but often, the time cycle will not be long enough, especially when you consider the moderating effect of the hull in the water, and the mass of the diesel inside.
MAN Roger.. I am glad I don’t have to store my boat! I’d hate to have those effects (the bottom corroded out, you'll come down with hives, and your pecker will shrivel.) from forgetting to fill my tank !!
I got all excited once and designed a desiccant dryer for my fuel vent with checks in it so that it could inhale through the desiccant and exhale directly overboard.. Then I seriously did the calculations and found exactly what Donalex and some other guys did.. that condensation is real, but not as much of a problem a it is perceived to be. I did this while helping a friend replace the bottom of his aluminum tank.. His water, we found, was from a hole in the plastic fuel fill cap which is located on the cockpit floor in his ’78 Hunter (Cherubini) 33.

 

[Roger Long]

Yes, any significant water that gets into the tank is from the fuel filler!

The worst thing about the condensation myth is not that it's wrong (although we all know that nothing must be more urgently dealt with than someone being wrong on the Internet) but that it creates a false sense of security in those that keep their tanks filled up. Water is so likely to come in the marina hose that it's like TSA taking nail clippers away from little old ladies and not doing much about real threats. You should assume that water will get into your fuel and be able to deal with it.

I've read posts by those who religiously stop at the fuel dock to top off after every outing. They are just increasing their probability of picking up water as well as being a PITA to the attendents and those behind them when they go through the whole docking drill for a couple of gallons. With an onboard polishing system, that's the last time you want to buy fuel. If you do get water, it will be sitting in the tank.

 

[ITMaster]

hmmmmm, guess it's been many moons since chemistry class, I thought a cc=ml

They do Dave..