Split Venting

[Sean B]

As long as we're rehashing the same stuff again here, I'd like to hear these guys kick this idea around: fuel polishing for FG gasoline tanks.

If the ethanol is taking on so much water that causes it to phase separate from the fuel, and it is suspected that this separated ethanol phase is what is eating the tanks....then what if you set up a system to simply keep the fuel and the ethanol solution nicely mixed up.

A simple pump to keep the fuel in motion while the boat is sitting would do it. A lot of bigger diesel boats have "fuel polishing systems," that constantly do just that, albeit as a side effect, while they endlessly pump the fuel out of the tank, through filters, and back in the tank while the boat is not in use.

Nobody I've heard of does this with gasoline tanks, but there is no reason not to. A simple pump to stir the tank constantly, hooked to shore power, would be an easy upgrade. You could draw from the bottom from the existing pickup tube, through the pump and back through a fitting on the top that would shoot the output sideways in the tank, setting up a circular motion, like a livewell.

Set up an additional filter or two on the stir line, and you're getting the ethanol dissolved tank crud out before it hits your engine line filters, and have thus created a simple fuel polishing system. Make the filter a racor and maybe you'll catch some water too.

Just a thought, not sure if it's a good one or not

[CaptPatrick]

Sean,

Sorry, but no cigar on this one... Once alcohol and water combine, you're stuck with the mix. In small amounts the water just burns along with the rest of the fuel.

Most alcohols and water are infinitely miscible meaning that they will not separate into layers. In phase separation the combined alcohol and water become so heavy that the mix drops to the bottom of the tank. Since neither water nor alcohol combine with gasoline, (alcohol can only coexists as a solution with gasoline), the water saturated alcohol is too heavy to ever go back into solution.

There is no way, short of distillation, that can even remove some alcohol from the water. Even then, you'd never get a full separation and you'd only succeed in reducing the alcohol concentration which is like throwing the baby out with the bath.

Recirculating fuel through filters, (fuel polishing), only removes the trash but does nothing in the way of changing the chemical components of the fuel.

Phase separation will remain the final kiss of death to all ethanol blends.

Likewise to a degree, any disolved esters from the alcohol attacking the fiberglass resin will just pass through the filters...

So, unfortunantly, the only viable answer to tank degradation remains replacing the tank. There is no answer to recovering a load of bad fuel once phase separation occurs.

One way to try avoiding phase separation, provided of course that it hasn't already occured prior to putting the fuel into your tank, would be to develop a closed system venting like is in use on modern automotive equipment.

Br,

Patrick

[Peter]

I have been thinking about the venting problem with gasoline tanks on boats and I have concluded that it is somehow necessary to separate the air going into the tank vent, from the air going out of the tank vent.
Right now one vent line serves all. When the fuel heats up in the day, even at the mooring, vapors need to get out of the tank to prevent dangerous pressure build up. Most boats have a little vent on the topsides someplace that allows this to take place.
Then when the fuel coolsinn the evening, or when the engines are run the air comes in through the same vent, along with any moisture.

So how about a couple of vents with check valves....one that allows dangerous fuel vapors to escape overboard, as they do now,... and a separate vent, also with a check valve, that allows air to get into the tank to replace fuel as it is burned off. This second "intake" vent could draw from a higer, dryer place, and there wouldn't be too much worry about fuel vapors coming out of it and subsequently collecting in low areas like the cockpit or bilge.
I'm picturing a spot like under the side panel of a fly bridge or something of that sort.

I wonder if the regs prohibit something like this? I wouldn't be surprised because we haven't needed it until now, and so up to this point any tank vent should be only where the outgoing vapors can escape safely and easily.
I don't think I've ever heard of or seen a dual path venting like the one I propose.

[CaptPatrick]

Peter,

I've been mulling over the same thoughts for about a week...

The check valves I've been able to find, that have a fairly low pressure release & are fuel/solvent safe, aren't readily available and right pricey.

I'm thinking about a manual ball valve for the main vent when filling the tank and then a some sort of a dissicant chamber for normal pressure relief. Remember that positive pressure isn't the only factor: When the fuel cools & when the engines use fuel, you are creating a partial vacuum, so a check valve is no good. You need two way air flow for basic pressure relief.

Br,

Patrick

[Peter]

You are absolutely right, Capt' Pat. The tank needs to breath in both directions...
That is why I was thinking on the lines of two vents...each only one-way. One to vent gas fumes overboard, and the other to let dry, fresh air in.


What comes out when the tank heats up or when you are filling it up is a potentially explosive mixture of air and gasoline vapors, hence we vent 'em overboard where they can't collect in a low or contained spot like a bilge or cockpit.

BUT what comes in as you cool down, or burn up fuel only has to be air. The drier, the better in our current situation.

That is why I was thinking on the lines of two vents...each only one-way. One to vent gas fumes overboard, and the other to let dry, fresh air in.

The check valves could be dirt simple. I think there is a sort of rubber valve in the squeeze bulbs on outboard tanks that is about the level of sophistication I am thinking about. Or maybe a really simple setup with a ball-in-cone sort of arrangement. The check valves have to be dirt simple, because they should be be no-maintenance 100% reliable.

[frank price]

Patrick , Peter,
I like the way you guys think. I too have been wondering why we couldn't keep the vents venting to somewhere else but out into the atmosphere but I was thinking more about the vapor recovery systems on cars. I am a little ashamed to say that I don't know exactly how the system on a car works, other than the charcoal and vacuum or how much modification to adapt it and how much sevice it would need over time to be safe on a boat but even if we had a vent hose that ran through a desicant canister that needed servicing , like an air supply to a spray booth, wouldn't that be something that could work? On my spray booth I have two big cans filled with desicant and a little pink "eye" that changes color when the desicant needs to be changed. I think I may try something like this on my boat this winter.
Frank

[CaptPatrick]

Here's a sketch that I did a few days ago that might be plausable... The outflow vent remains outboard for gasoline applications while the intake vent would be positioned inside the cockpit, probably behind the side panels.

Intake air would have to pass through the desiccant chamber then drawn into the vent tubing. A drainable moisture trap would be below the desiccant chamber which should allow excess water to drain away even if the desiccant becomes too saturated to any longer be effective as a dryer.



Br,

Patrick

[Sean B]

I wonder if for the air-feed side, you couldn't set up a system to keep the tank very slightly pressurized, say 1 psi over ambient pressure, and keep it that way automatically as the fuel level drops by feeding it from a separate compressed air tank. The air tank would have to hold the equivelent uncompressed volume of several (fuel) tankfulls of air to make it convenient enough. The purpose being, you can carry around a tank of DRY compressed air to refill the fuel tank with, and vent it back out via. Pat's ball valve when you re-fill the fuel tank. After taking on fuel you shut off the ball valve and re-start the pressurization system. Closed system, no water.

The pressure control mechanism probably wouldn't have to be any more complicated than a scuba regulator, and maybe a scuba tank would feed it. Hopefully a FG tank could take a slight amount of pressure.

A bad side effect would be that a tank leak would be helped along slightly by the small pressure. But perhaps there could also be a warning buzzer if the slight pressure in the fuel tank drops to ambient pressure. Shouldn't be much trouble with elevation/ambient air pressure changes as, by definition, we all tend to operate at sea level. As for the pressure change resulting from a temperature change in the fuel tank's air, it could be calculated what that could be given a temperature differential. If significant then a set pressure relief valve would be in order, maybe there should be one anyway.

That is a system that could be manufactured, boxed and sold too.

Just brainstorming here, still trying to get that cigar

[CaptPatrick]

Sean,

Hang in there, there's a cigar with your name on the wrapper... But go for simplicicty, low cost, low maintenance, and no need for higher math. ;^)

Br,

Patrick

[Peter]

I agree with Capt' Pat here. Apply the KISS method;

Keep It Simple, Stupid
(I'm not implying that Sean's idea is stupid. It is actually pretty clever because canisters of dry air are readily available around the waterfront as scuba tanks. That is just the expression with the acronym KISS.)

and that is because you have to not only make the thing fool proof, but when dealing with the dangers of gasoline it has to be

Damn Fool Proof.

Here is my next thought on the whole thing..... Consider a box a couple of inches on each side, mounted to the inside of the hull in say, the cockpit area. The tank vent comes up into the box. The box itself has two very simple flapper valves, on lets air in from the cockpit side, the other lets gas fumes out overboard.
This would be pretty simple to make and install, but has no provision for a dessicant canister.

I kind of like the concept of the dessicant canister, but I don't know much about them. What happens to the airflow when the canister is done? Does it block the flow, or does it just not get the air as dry as one would like?
If the flow can get blocked there has to be a pressure relief bypass, because someone is for sure going to ignore the maintenance.

[CaptPatrick]

Peter,

The desiccant would probably be round silica gel granules which are about 1/8" in diameter. The canister would hold around 4 or 5 cu in of desiccant & air would pass freely through the material.

It would be hard to say, at this point, what the saturation life of the desiccant would be, but possibly as long as three months before it would need to be replaced or reactivated. Reactivation is a bit of a bitch and replacement would probably be a preferred option.

I'm thinking that having the tube coming in buried into the granules would keep moisture captured within the media. Were the media to become totally non-absorbent most liquid would drain off to the water trap. The first signs of moisture in the trap would alert the owner that it's time to change the desiccant.

Br,

Patrick

[Sean B]

We've used dessiants in the cabin, sold as moisture removers. I think they come with about 8 ounces of dessicant in them, similar to what you described: 1/8" white balls. They are packaged in a plastic tub sitting on a smaller tray inside with holes in it, set above the bottom of the tub. In open air they last about a week, and there is probably a half cup of water caught in the bottom of the tub, plus whatever the granules themselves absorb. Remember this is in Florida, walking around in soupy air all the time, probably last a lot longer up north. Also in a closed system I would expect them to last a lot longer too. Could do an experiment with one, weigh it first, pass 100 gallons of air through it, weight it after and see how much water it catches. Then wait 2 weeks and do it again, see if it is as efficient. I imagine that the Achilles' heel of that system is going to be that the efficiency of the dessicant will drop dramatically after it's first exposure to moist air, but you never know until you try

[frank price]

To keep it simple I was thinking of retaining the single vent hose , adding the desicant and also a fuel water separater on the atmosphere side. I originally thought a separater made for a compressed air system would work but after looking at one I realized there is a lot of plastic pieces that can't be made to withstand gas vapors. After giving it some more thought I realized that a regular fuel /water separater , even a fairly cheap one like the standard mercruiser ones, would probably work. That way the filter would keep free air flow to a minimum , yet let the tank breath when it needs to, hopefully extending the life of the desicant. Also, the desicant chamber on my spray booth doesn't have a drain, only two water traps with automatic drains located upstream mounted directly to the desicant tank. So far , every time I've replaced the desicant because the little "eye" turned colors, the desicant looks like new with no water present and it takes about a year of use for that to need replacing.
I will be the ginny pig up here in the north. It may not work well in the south because of the moisture but with the filter keeping the air flow to a minimum , it might last longer than you think.
Frank

[Rawleigh]

How about the desiccant "snakes" that go inline to the airhose? They are threaded on each end and would be easy to atttach and not take up nuch space. They also have the color changining eye to add to it. $17.95 for the snake and $18.95 for the eye.
http://www.ecompressedair.com/pointofus ... .shtml#cha

Yes, it has brass fittings, and yes, it is made of hose, but we are talking about vapor here not liquid ethanol, right? Paint booths have a lot of nasty solvents running arond in them too! If you think it is a problem, then buy one and soak it in gasahol to see what happens.

[Bruce]

I have two large floor safes which use dessicant because of our hugh humidity down here.
Even though they are under AC and are sealed well for fire protection, as well as you'll get a tank sealed, as soon as the door is opened a wave of humidity(water) enters. The same as when you would open the fill valve and fill the tank.
Those dessicant boxes in the safe, real large, need drying out(in oven) every two weeks to remain effective.

It would be dangerous to put dessicant that was subject to gas fumes in an oven to dry out.

Besides unless the mixing distributor, tank delivery truck and finaly the retail outlet did the same thing, the mositure would already be in the fuel.

The idea would be to remove the moisture once the fuel arrives in your tank.
In the HVAC field, we use vaccum pumps down to 30 inches to boil the moisture out of the system.
Don't know if something like that would work.

[Peter]

Bruce,

The problem with using a vacuum pump is that the vapor pressure of the fuel is lower than that of water. The fuel would boil off first leaving the water behind.
One could then condese the dried fuel back to liquid and use it, (just like a still) but such an operation is not the sort of thing your going to keep on board for day to day use

Peter

[Bruce]

Peter,
Nothin wrong with a still on a boat is there?