Myth of Condensation in Fuel Tanks

[RAGTIMEDON]

I think we all agree ( except possibly Mr Pascoe) that there is some water vapor condensing in our fuel tanks. It may be very small, but since it sinks under the fuel and therefore does not evaporate (like the morning dew) it will accumulate. However, I have recently read that one operator of a large, well known marina on Lake Michigan, suggests that there is more water in your diesel fuel fron deck fill leakage than any other source. Most of us top off our tanks while winterizing our boats, but how many of us carefully inspect the sealing o-ring on the deck plate? I just checked mine and found it was cracked badly enough that I really don't need the vent connection! My fuel fill plate is on a horizontal surface, and while it will not hold a puddle of water, I am sure there has been some rain that has gotten in there. Since the current flood conditions prevent me from sailing, next weekend will be spent replacing o-rings and pumping fuel from the bottom of tanks for water removal and filtration, on my boat and a couple others I care for! Have you checked your o-ring lately? By the way, my water tank filler also needs a new o-ring - the only good one was on the cap of the holding tank!

[KeelHaulin]

I think the calculation is incorrect also; the problem is that a vented tank will always allow more water vapor in to balance with the humidity of the outside air. The condensation accumulates because as said before (condensed water drips and then sinks beneath the diesel or gas).

But I also agree with the premise of his article - much more water can get in via a leaking deck fill or vent that gets submerged while heeled. Check to be sure that your tank vent is not "going under" and check the o-rings on your deck fills to be sure they are properly sealing.

[GaryHLucas]

I believe Mr. Pascoe is not correct. I used to build a keyboard box that was 6" x 3" x 9" for a machine used in greenhouses, which are hot and high humidity. It was connected to another electrical box by a 25 conductor cable. We had problems with water getting in all the time. We then went to a box with a submersible rating. To test it we put the boxes in a bucket of water with a brick holding them under for week at a time. They were always completely dry after this test. We then put them out to customers. Aftera month or so every single one had at least a teaspoon of water in it! The other enclosure though was completely dry. We finally figured it out. The other enclosure had some electronics in that produced a little heat. It was always 10F warmer than the greenhouse. The keyboard though had no heat source. During the day it got real hot in the sun. The hot air was forced out through the strands of the cable, and every nigh the cooler damp air got sucked back in, to condense in box!

Drilling a drain hole in the bottom of the submersible enclosure kept it MUCH dryer! A little heat would have completely soved the problem, but we had no spare wires to do that.

[bobmcgov]

I put a new gas tank on my motorcycle -- old one rotted thru from (yes) condensation moisture. I fogged the new tank before I mounted it. But for one reason and another, the bike lived thus in storage for two years. When I opened the stopcock on the bottom after that two years, a half cup of water came out. This is a new tank, never seen gasoline, so the water didn't come from fuel. And we live in a desert -- typically less than 25% relative humidity. It is dry, dry, dry, and the dew point is a loooong way down there.

If not condensation, where did that water come from? Stored indoors, so it isn't rain. Very selective vandals? Mouse urine? Well, Mr. Pascoe?

[rbarnes]

I disagree.
Arizona is a relatively dry state (10% or less most of the year) and hot as well (115 degrees at times) and yet........... enough condensation accumulates in the sumps of airplane tanks to bring them down if not properly drained. If it is a myth, it is a killer of a myth !!!!!
My boat in the middle of the desert gets condensation in its 3 gallon tank.

[motion300]

I have rarely seen water in a tank that did not contain a large quanity og dirt and junk This tell me it came from the supplier and not handled properly

[RichH]

As a physical chemist and mechanical engineer, who has for nearly 40 years worked deeply in fluid and thermal sciences, the ultrapurification of fluids (including exotic 'oils', dielectric oils, etc. etc. for the electric power generation, semiconductor, ultrapure chemical, bio-pharma industries, etc. ), etc. etc. etc. .... Mr. Pascoe is essentially correct, not totally correct but close. Sorry to state this; but, most of you who somehow 'intuitively' oppose Mr. Pascoes statement are not correct.

1. Most oil made from cracking or distillation are almost completely dehydrated by the heat of the process - water content essentially nil.
2. When in CONTACT WITH **ATMOSPHERIC** AIR, water (as vapor and as free gaseous molecules) enters the mass of oils, etc. due to ***vapor pressure equilibrium drive (partial pressure of the water vs. the partial pressure of the oil)***.
3. Water in oil exists in various 'states':
a. molecular water (azeotropic mixture) ... insensible without using 'instrumentation', etc.
b. emulsified water (water that is held 'in suspension') ...insensible without using instrumentation, etc.
c. free water ... sensible water that can be observed *without* instrumentation, etc. In typical oil mixtures, free water is the only 'phase' of water able to 'gravimetrically settle out'.

Water that is 'condensing' on tank walls, etc. is the result of the ~'end stage' of the equilibrium changes to the oil being SATURATED with water ---- the oil is now becoming **fully SATURATED** with water (molecular water, emulsified water, AND free water). The same physical laws of vapor pressure 'equilibrium' which is temperature dependent can reverse the equilibrium direction so that the relatively warm oil, etc. begins to lose it water content ... and the water (as vapor) begins to condense on adjacent cooler surfaces. The greater the water saturation of the oil the greater the equilibrium drive now in the 'opposite direction' if the adjacent surfaces, etc. are cooler (and according to the partial pressure of the water, etc.) ... and the water vapor in the oil NOW condenses on the cooler tank walls ... only to gravimetrically fall/slide back down into fluid until it settles on the bottom.
If you took the same amount of oil and poured into an open 'pan' and exposed it to atmosphere .... it would 'pick-up' essentially the SAME amount of water as if it were in a 'tank' exposed to atmosphere ... and there would be NO evidence of 'condensation' !!!!

The water that gravimetrically settles to the bottom no longer becomes a 'true' part of the mixture (a 'liquid-liquid phase boundary' forms between the oil and free water) .. and this influences to the sum of the partial pressures 'above' the phase boundary ... but leaving the oil (+water mixture) above the boundary free to re-equilibrate and accept vapor migration from additional (humid) atmospheric air in contact with the oil surface. .... and now what is occurring (again by equilibrium) could be described as a 'pump with out moving parts' being operated solely by physical & chemical 'equilibrium'.

Water that is 'condensing' on the walls of a tank is primarily a SYMPTOM that the oil is SATURATED with water - chemical equilibrium (partial pressure equilibrium) moved the water vapor in the atmosphere into the oil.

Dont want 'water' in your oil, .... keep the MINIMUM amount of oil in your tank, put a desiccant trap (silica gel, activated alumina, etc.) on the vent line .... or if you have a 'vacuum rated' tank simply close the vent valve when the engine is off; the minimum mass/volume of oil will 'pick-up' the minimum amount of water vapor ... so DONT keep your tank 'topped-off' especially if you're not using the oil quickly !!!!! Its the MASS of oil in the tank and the sum of partial pressures (equilibrium) of oil AND water vapor that is causing the 'transfer' .... not a tank wall !!!!! Condensation is a SYMPTOM that oil is ALREADY becoming fully saturated with water.

In 'industry', ultrapure oils that have become saturated with water are typically placed into a vacuum chamber, a strong vacuum is applied (to change the partial pressures which reverses the equilibrium) ... and THEN, the oil is run through water absorbing 'filters' (filters that contain the starch hydroxymethylcellulose (the same stuff in "pampers") to achieve less than 1 ppm water content. If water saturation is your problem you can buy filters that contain a water absorbing starch (for gasoline fuels) from the 'typical' suppliers.

Question? If your 'intuitive' condensation hypothesis are correct, then why don't EMPTY tanks (exposed to atmospheric air) automatically fill up with water ????? :-)

BTW- My 'fuel system' includes a constant recirculation filtration stage (including a free water 'knock-out' trap), the tank VENT line includes a bio-blocking filter to retard **fungal spores**, and the vent includes a desiccant chamber to remove incoming water vapor down to -40 deg. dewpoint ....
I gots no water, gots no biological fouling, gets my 'filters' free but hardly ever change them, .... gots NO 'fuel problems' in a 100 gallon black iron fuel tank ... and I never have to down into the bilge during a heavy seastate to change out filters .... and 'power-puke' at the same time. My system also has a 2 gallon 'day tank' that stores filtered oil ... if there is ever a 'problem' I can simply open the 'weir valve' and vent valve on the day tank and let the oil 'drain by gravity' down to the engine .... for about ~2-3 hours run time - time enough to 'motor-away' from any danger, etc.

Water enters oil/fuel by *chemical equilibrium* (the sum of partial vapor pressures of the 'constituents'); the MORE oil, the MORE water. Condensation is a *symptom* that liquid is now FULLY SATURATED with water. Empty tanks (with atmospheric vents) dont automatically fill with water.

:-)

[bobmcgov]

Originally Posted by RichH

Question? If your 'intuitive' condensation hypothesis are correct, then why don't EMPTY tanks (exposed to atmospheric air) automatically fill up with water ????? :-)

Empty tanks (with atmospheric vents) dont automatically fill with water.
:-)

Rich: read my post above. I admire your knowledge of oil/water phase boundary behaviors ... but tank side condensation can take place in parallel to what you describe.

Good science has to explain the observed phenomena. I removed better than half a cup of water from a small, vented motorcycle tank that had never contained gasoline or oil, stored indoors, in a desert. Pls explain how it got there.

Cyclical heating and cooling of a metal vessel in the presence of water-bearing air will result in the accumulation of water condensate in the tank. Solar stills work on this principle.

[pdqaltair]

E-10 exaggerates this, and the following link explains:

http://www.epa.gov/otaq/regs/fuels/rfg/waterphs.pdf

True condensation, without fuel - common sense tells us this is rare (tanks do not fill). Water accumulation due to saturation followed by temperature swing - common, though not as common as leaking filler caps.

I am a chemical engineer working around refineries, and the effect is real.

[Guzzi]

Anyone ever had to drain the fuel tanks on an airplane of the water that condenses overnight?
They have drains in them just for that purpose. Evaporation of the fuel causes cooling. The sun warms the surface of the wing and you have cool inside/warm outside, voila! Condensation. Fill a glass with water and add ice. What happens? Condensation forms on the glass.

E-10 makes matters worse, alcohol is hydroscopic-it attracts water, making the condensation problem worse, as the water doesn't separate out as in pure gas. Water and alky mix, but don't burn well.

Ragtimedon-I was a guest aboard your beautiful Endeavor one afternoon when you and your wife gave us a tour.
I'm a friend of Tom, the guy down the dock with the Helms 27.
Might have a boat after next week...gotta find it first.

Jon