Fuel Tank Vent Trap

[Witzend]

I posted this over on the C310 site, but not getting any comments.

I'm wondering how others will view the effectiveness of this design?

Here is how I plan to improve my fuel quality. I'm convinced that I'm getting water in my fuel tank from the poorly designed vent. The vent in our C310 goes strait down to the tank. This is a poor design, it should first go up before it goes down to back drain any liquid water that may invade. There is also no provision to remove water vapor.

In my design there are (2) separate traps, one for liquid (a clear bottle that can be easily monitored) and one for vapor that will migrate through the vent saturating the fuel. I plan to use hygroscopic substances that are used to remove water from the air, called desiccants. These are readily available in packets that can be opened and poured into the hold. The traps will mount directly under the vent thru-hull in the aft port locker.

These questions become obvious and for me a pretty tough thing to answer, because of these variables:
How often will I need to change the desiccant? If not could it swell and block or restrict the air flow.
Will I have enough surface area in the desiccant trap to be effective?
How much air will be traveling through the trap over time?
How much fuel you're consuming over a season?
The difference in air temp causing a pressure differential turning over the free air?

As next season unfolds I will be monitoring the condition of the desiccant and will probable change my design based on my observations.

 

[Roger Long]

I think you need a much bigger desiccant container and there is already an off the shelf item discussed here earlier to do this job. The water trap is a good idea.

http://forums.sbo.sailboatowners.com/showthread.php?t=124473

We'll probably hear more from Rich H next time he checks in.

 

[Steve Dion]

Considering how many boats are running around with the exact same setup as your boat originally had, I do not see why you need something like this. Do you know that you are getting moisture in your fuel tank? Could it be the o-ring on the fuel filler cap?

When you have the proper fuel filtration system in your engine you should not have any problem with fuel quality.

 

[Witzend]

Yes, I have checked the O ring and it is good, and yes I keep getting water in the bottom of my Rayco from an unknown source. I obtain my fuel from a truck stop figuring that they have a good fuel turnover.

 

[RichH]

These questions become obvious and for me a pretty tough thing to answer, because of these variables:
How often will I need to change the desiccant?
Desiccant will need 'regeneration' when 'saturated' - for 'visual' indications also mix 'dyed' silica gel into the desiccant bed; the dyed component will turn from blue to pink when saturated. Regeneration of Silica Gel will typically be by putting into an oven at ~300 deg. F for several hours.
Common desiccants such a Silica Gel is based on a percent weight of water vapor per percent mass of gel ... typically silica gel is considered 'saturated' when it picks up 30% of its weight by water vapor. However, if the tank vent is open to the atmosphere the saturation cann occur rapidly ... dpending on the the tank 'dynamics' AND how 'dry' the contents are (already 'saturated' fuel) - the less amount of fuel in the tank the less the 'drive' of moisture into the tank. More/already saturated, less amount of the mass of fuel will have less 'equilibrium drive', etc.

If not could it swell and block or restrict the air flow.
Silica Gel - It wont swell but it will fracture and turn to a powder if in contact with 'free' or liquid water. In this degraded condition it can/may block air flow.

Will I have enough surface area in the desiccant trap to be effective?
All depends on the RH outside the tank, the RH inside the tank ... as the water vapor (and oil vapor) will be coming from these two sources. The desiccation will occur as a 'frontal passage' (isotere) of moisture from both 'ends' of the desiccant container ... one side of the frontal zone(s) in the desiccant will be dry and the other side of the 'front' will be saturated. To keep the 'front' under control, long or 'high aspect' ratio (L/D) containers such as 'fat pipes' seem to be best.

How much air will be traveling through the trap over time?
My desiccant trap for a 100 gal tank is 8' dia and 24" long ... and will last about -2-3 months before the opposing 'fronts' meet. The SIMPLE answer is if you keep the MINIMUM amount of fuel in the tank, just enough for what you actually NEED plus a 'reserve' amount for contingencies ... you will have less an absolute amount of moisture 'equilibrating' into the tank oil - much simpler and easier than depending on a moisture/vapor trap !!!!!!! "Condensation" only occurs after the tank fuel is fully saturated with water vapor ... to keep the saturation amount low, simply keep the tank as 'empty' as reasonably possible. "Empty tanks do not 'condense' large amounts of water vapor. "Fresh" fuel from a 'high-turnover' source such a high volume truck-stop will be relatively 'dry'. Small (low turnover) marinas typically will be dispensing already water saturated fuel.

How much fuel you're consuming over a season?
When Im using fuel (about 1000 gal/yr.), the desiccant trap is disconnected. Its not really how much fuel that youre using but rather how much 'mass' of fuel is being stored and how long the fuel is being stored. As stated above the least amount of fuel in the tank will 'take-up' the least amount of water!!!

The difference in air temp causing a pressure differential turning over the free air? Such follows simple 'perfect' gas laws: PV=RT ..... which for simplicity in atmospheric conditions can be reduced to: Volume of air is directly proportional to the Temperature differences between inside to outside ... the simplistic answer for a small boat tank is- not much at all!!! The mass transfer of vapor into oil is more dependent on how 'wet' the atmosphere is and how 'dry' the fuel is .... as always, the least amount of 'fresh' (dehydrated) fuel is best.

Data for various desiccants: http://www.sorbentsystems.com/desiccants_charts.html

My deeply involved professional engineering, etc. experience with such situations/applications leads me always to the simple and sometimes very profound things before depending on 'advanced technology' as a solution:

1. Keep the minimum amount of fuel in the tank... just the amount you need, plus some reserve.
2. Empty the tank when the boat is long-term stored. Take the oil home and burn it in your oil-burner.
3. Buy only 'fresh' fuel (truckstop or high-turnover marina that caters to watermen, etc.)
4. Make SURE that the O-ring in the 'deck fill' is there and is not 'distorted', etc.
5. Make sure that the atmospheric tank vent has a U bend to prevent aspiration of 'free' water.
particle/crud considerations
6. Inspect/clean the tank internals as regular maintenance item. Clean either with chemical means or by mechanicial scrubbing of the internals.
7. Use an enzymatic stabilizer to prevent/retard fungal growth.

These 7 items will prevent 99.9% of all 'fuel problems'. Then, and only then, if you still have 'water problems' consider to add a 'moisture/vapor trap'. :-D

 

[Witzend]

I'm aware of the product and price. The size of the container may need to be larger, I agree. Not sure of the size on the H2O Out product.

 

[Steve Dion]

Have you considered draining your tank completely. Then you can either dispose of the old fuel or use a Baja filter to eliminate the water. This would give you a "clean" starting point.

I must admit that I have not seen any sign of water in my tank so this has not been an issue for us. I did install a dual filter system which allows me to switch between one of two filters on the boat in case of an emergency or for general maintenance. Just turn a couple of valves and I can switch from our "A" filter to the "B" filter.

I did this because I originally had an issue with crud in the tank.

 

[RichH]

Witzend ---

Just a thought....

Do you operate you AIR CONDITIONER all the time in summer ???????

If so, I would recommend that you isolate or insulate the tank from the COLD AC air.

 

[Witzend]

RichH, thanks for your in-depth response.
Your trap is 8" X 24" wow that enormous! " long or 'high aspect' ratio (L/D) containers such as 'fat pipes' seem to be best" So why the large Dia. container?
'When Im using fuel (about 1000 gal/yr.), the desiccant trap is disconnected'.
You only pass vapor thru it when not running the engine, do I have this right?
Why are you doing this, are you afraid that the desiccant will get into the tank?
The desiccant I intend to use come from paper packets(of which I have hunderds of them) and are small crystals that when wet expand, not fracture.
I believe there will still be condensation on the wall of the tank regardless of whether there is fuel in the tank or not, so why not just pump the tank out in the spring?

 

[Ed Schenck]

These threads always amaze me. Are people really having that much trouble with their fuel supplies? Ten years with two different engines and never a water problem. Same old aluminum tank and one old Racor filter/water separator. On the trip from Lake Erie to Tampa fuel came from all different sources. We changed the Racor fuel filter maybe three times and never drained any water. The boat sat on its cradle for seven winters with half a tank or less of fuel. I cleaned the tank once in 2000. So what's going on that all these expensive and complex fuel systems are required?

 

[Witzend]

I hear ya Edd!, that is why I don't want to spend a lot of money here, and design my own, and see if it solves the problem, if not, I'm not out much. I clearly have a problem, I keep seeing water in my Rayco, and have run out of ideas of where it's coming from, not a lot mind you (table spoon every couple of months, but still I'm concerned.

 

[Witzend]

Rich, we did for a short period 2 times this summer, but on this boat the tank is at the very aft and is isolated and not in a cooled area.

 

[RichH]

Why are you doing this, are you afraid that the desiccant will get into the tank?
**** No not at all. The 'draw down' of my engine is only 1 gal./hour max. = 231 cu. in. /hour which is insignificant volume exchange on a small time basis .... and my return line is pumping back 'warm' oil which elevates the tank temp - which is good for 'dewatering'. I have a 'conical screen' at the ends of the desiccant chamber to retain it.

The desiccant I intend to use come from paper packets(of which I have hunderds of them) and are small crystals that when wet expand, not fracture.
**** .... dont know what this is? as most industrial desiccants are zeolites or silica gel and are dimensionally stable when saturated. Silica Gel is Cheap, even from a 'craft store' where they use it for drying flowers, etc. If your compound expands, then its probably an ABsorbent not an ADsorbent ... very different animals as most ABsorbents are more difficult to 'regenerate'.

I believe there will still be condensation on the wall of the tank regardless of whether there is fuel in the tank or not, so why not just pump the tank out in the spring?
**** The amount of condensation will be minimal with an empty or near empty tank due to the ability of the empty tank to quickly change/vary its thermal mass ... hence LESS 'condensation' because the LESS mass of oil allows a quick temp. change ... probably the reason that 'empty' tanks dont automatically fill themselves with condensate. If you have a mostly full tank and leave it for a long time, subject it to a long sequence of thermal wings ... and with the thermal mass of oil always out of phase and lagging behind the ambient/atmoshperic swing ... its going to start filling itself with WATER because the water that 'settles' to the bottom removes itself from the oil mixture above (changing the equilibrium conditions) ... and the almost full or full tank becomes a chemical 'water pump'.

Why let the oil contaminate/saturate itself by moisture uptake over time? Better to prevent than to 'remedy'.
In industry (power generation and super-high tech), on such (but 'purity' oils, etc.) to 'remedy' they pull a 'severe' ~ 29" hg. vacuum on it to 'flash' the water out, then run the oil through water absorbing filters containing absorbent 'starches'. The 'starch' is essentially the same compound used in absorbent baby diapers ... and water absorbing gasoline filters. The problem is with such ABsorbents is that they usually 'swell'; ADsorbents are usually dimensionally stable and dont decrease the gaseous flow when they 'saturate'.

My suggestion is to do the simple things first, then when you have good control there, then if you continue to have problem, consider 'vapor traps', etc.

Ed Schenk - Every damn tankage will respond and behave differently, depending on location, thermal swings and 'lags', relative humidity, and maintenance of internal tank condition, etc. etc. etc..
Ive been inside of million gallon storage tanks; one will have a poor water uptake performance, self generates large amount of particles, etc. and an identical tank right next to it will remain clean, and with relatively low water vapor uptake and will do so for years. So maybe with your own tanks .... you're just 'lucky'. ;-)

 

[higgs]

Yes, I have checked the O ring and it is good, and yes I keep getting water in the bottom of my Rayco from an unknown source. I obtain my fuel from a truck stop figuring that they have a good fuel turnover.

I would try a different source. Maybe that stations tanks have a water problem - they may turn over the diesel, but it still has water in it.

 

[Steve Dion]

Another option is to install an onboard fuel polishing system.

http://www.boatfix.com/shop4/store/viewItem.asp?idProduct=11189

 

[Roger Long]

Another option is to install an onboard fuel polishing system.
http://www.boatfix.com/shop4/store/viewItem.asp?idProduct=11189

Hey Rich, how does this thing work? Is it snake oil like the magnetic gizmos or is it for real?

 

[Witzend]

Rich, I understand now that you're not concerned with the desiccant getting into the tank, (I only made that assumption based on your description " It wont swell but it will fracture and turn to a powder if in contact with 'free' or liquid water."), but why do you bother disconnecting your system from the vent when operating your engine?
These are Mil-D-3464 Unit Paks. it is a white irregular crystalline substance that will swell up when water is added, so they are and absorbent.
I'm going to pump my tank next time I'm at the boat, which I have already done a number of time before and never found anything. In the spring I will install my widget with a larger hold and see what happens.

 

[Don12364]

http://forums.hunter.sailboatowners.com/showthread.php?p=668819&highlight=fuel vent response #6 describes how the fuel siphons OUT. Water can also siphon IN in the same way if the tank is not full.
The attached diagram shows how I overcame this.
I am of the opinion that the ½ to 1 gallon of water necessary to cause the engine to stop or to cause diesel bug could never be the result of condensation and is purchased with the fuel.
http://forums.hunter.sailboatowners.com/showthread.php?p=263964&highlight=Condensation
Posting #3 relates as does:-
http://forums.hunter.sailboatowners.com/showthread.php?p=608262&highlight=moisture
Posting 17 which is my attempt at an analysis of what actually occurs.

To add to the above when the tank cools at night it is always warmer than the incoming air so there will be no tendency to condense. The following morning, as it warms up the air inside will support more water and this is exhaled.

If the current old wives tales were true then every empty vessel and every fuel tank, cars, trucks etc. would be gradually filling up with water.
We know this is not so.

 

[Steve Dion]

Roger:

It is just a pump. You still need valves & filters to put in the line. The nice thing about these units is that you can also add a time so they run at pre-determined time for pre-determined periods.

This is not like the Algae-x. I have a hard time believing that those things actually do anything.

 

[RichH]

Hey Rich, how does this thing work? Is it snake oil like the magnetic gizmos or is it for real?

Ive been retired and away from these folks for several years now; but, my conjecture is a 'water shedding' hydrophobic membrane (PTFE, etc) that coalesces (making larger and larger water, etc. droplets from minute droplets/emulsion until large enough to 'gravity settle') into a small sump. Most filters below 10µM will 'coalesce' if you run them 'backwards' (outside to inside). Very interesting, especially the pricing (usual 30-40% margin for such). Of course such must be protected by a filter system (such as the one you have) and thats not included in the price! but if this is 'continuous duty' then the resident particle burden as well as emulsion levels will be quite low. Neat!!!!!!!!