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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 don’t
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 don’t 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. |
