I want to redesign the tank vent system

Discussion in 'Powerboats' started by sdowney717, Dec 6, 2010.

  1. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    (Please excuse the rough drawings)
    Currently, I think it is a problem with E10 and also overfilling issues
    spurting fuel out the vent is easy if your not careful.
    I am thinking a new much smaller exterior vent higher up would mean less moisture able to enter the tank.

    First picture shows the current system. Air easily flows in and out contaminating E10 with water and phase separating etc...
    Also filling up fuel vent is lower than fuel fill, so it can spurt out easily.

    [​IMG]

    new design idea
    Drill and tap a fitting into the upper bronze fill pipe for the vent line to attach
    Add a tee in the line and send a new small 1/4 inch copper vent line up higher than the fuel fill venting to the outside of the boat.
    Or loop it up high and then bring the loop back down and vent out the same exterior vent.

    So would this work ok when filling the tank without causing filling issues?
    Where should the new large vent attachment be located in the fuel fill pipe, higher or lower? I was thinking up high close to the underside of the boat deck.

    The tank fill is a good quality bronze Perko system and the pipe screw cap seals well to the bronze pipe. The pipe can be pulled out from underneath the boat deck to easily drill a hole for a fitting.

    I was thinking the fuel fill nozzle from the fuel pump would fit down in the tank fill pipe and the tank vent would vent the out rushing air on top of the nozzle and away.

    If you let it fill too far, fuel will push out that line perhaps give you some warning the tank is overfilled. It might even kick off the fuel fill nozzle?
    Question is how far down the bronze tank pipe to put the vent fitting?
    [​IMG]
     
  2. mark775

    mark775 Guest

    Are the tanks already made?
     
  3. sdowney717
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    sdowney717 Senior Member

    yes, the boat is from 1970, an Egg Harbor 37' sedan cruiser.
     
  4. Ike
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    Ike Senior Member

    Why not just replace the fuel fill fitting with one that is a combination fill and vent. Then you just run the vent line to the fill. These are readily available on the market. Plus no drilling, tapping or teeing off lines (most leaks in fuels systems are at fittings. the fewer the fittings the lower the chance of a leak) If fuel burps up the vent, it simply goes back down the fill. Problem solved. All you have to do is install the fitting and repair the hole where the old vent was.

    By the way if your vent is burping you may have one or a combination of several problems. The first is the obvious, over filling.Most tanks are built with a 5% ullage, that is, extra space for heat expansion. Many boat owners think this is usable space. It is not.

    Problem 2. Vent lines that extend too deep into the tank. They should extend a little , that's how the manufacturer creates the ullage. But sometines they extend too far.

    problem 3. The vent is at the wrong end of the tank. the vent should be at the high end of the tank, or.......

    Problem 4: is the boat sitting level on it's waterline. If it's down at the bow this may make the high end of the tank actually lower than the fill end.Tanks are usually installed so that when the boat is sitting level on it's water line the vent is at the high end of the tank. Check it out.

    Problem 5. Constrictions in the vent line can cause excess pressure usually resulting in the tank never filling, or burping up the fill line.
     
  5. Ike
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    Ike Senior Member

    Oh yeah, I forgot to add that the vent on the outside of the boat should be placed higher than the fill, and there should be a loop at the top end of the vent line before it connects to the fitting. This prevents water from entering the tank through the vent fitting. If you use a combo fitting you don't need the loop. Water can't get in because the cap is sealed, and the fuel cap has a bypass in it to keep the vent open to the atmosphere.

    As for tripping the pump nozzle, ABYC is working on some solutions to this. You also should be aware that the EPA is going to require charcoal canisters in the vent line very soon and this raises a few problems.

    If the canister gets wet from fuel it ruins it. How to prevent that?

    Fuel systems on boats have to be open to the atmosphere to prevent pressure in the fuel system. Otherwise if you get a leak it empties the whole tank into the boat. So how does the new canister keep from creating pressure?

    How long are these canisters going to last and how much will a replacement cost. Are they going to be fire resistant like Type A hose?

    And the beat goes on.......
     
  6. sdowney717
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    sdowney717 Senior Member

    I am also thinking of adding a metal gasoline can paper filter in the bottom tee line that would function as the vent. The paper filter would pass air slowly in and out and I think would retard more moisture exchange between tank and outside air than just a small long vent line.

    I dont like plastic fills.
    I can get a similar effect by adding a vent return into my current bronze fill pipe. Although cap wont be vented as it is on those combo fill-vents.
    I did notice on the bronze Perko screw cap, (It says Perko cast into the inside of the cap), Perko drilled a 1/8 inch hole at about a 60 degree angle up thru the lower outer edge of cap which intersected with the open center. This was plugged with a solder plug and was designed as a vent, if you drilled it out.

    I thought about using that as a vent BUT, I like the idea of a vent opening up much higher off the deck, away from water.

    I was thinking of running a small 1/4 copper tube up the cabin side and exiting high up above the salon windows and covered with a clam shell vent.
    I would likely pack some stainless mesh in the end as a flame arrestor.
    So from the lower copper Tee, run the small 1/4 inch vent line up high to a 1/2 inch section of copper pipe filled with some mesh and that end exits out the side of the cabin high up.
    I think I can hide this tube on the inside of the salon between the windows. There is a wooden channel formed there which the builder used to run some 12V wiring for cabin lights inside.
    NOW, question is, would you ever smell fumes coming from a vent located on the cabin side high up, high up like at head level as you walked along the outside deck? Or would any fumes ever be smelled inside the cabin?
    Currently a fuel vent line the exits below the outside deck, any fumes would simply roll out along the surface of the water. I though about a long high loop up, Uturn and then back down to the current outer hull vent, BUT I dont want to start a siphon action. If the tank was overfilled, you could start a siphon action which would lower the fuel till the level hits the top most level of the tank inside.

    The current vent looks like a pipe is screwed into the top of the tank, so I dont think it extends down at all into the fuel tank.
     
  7. sdowney717
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    sdowney717 Senior Member

    I think I will simply do a test hookup using just the tank vent tube to the new small long vent and see how that works before doing any more mods.

    http://www.daremarina.com/Page.aspx/pageId/54890/Contact-Your-Congressmen-about-E10.aspx

    [​IMG]

    my tank, after I added 30 gallons of E10 to MTBE gas which likely had some water.
    That is phase separated fuel the bottom portion.

    [​IMG]

    On the Dare Marina site, this comment I dont like to hear.
     
  8. Ike
    Joined: Apr 2006
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    Ike Senior Member

    A paper filter would not work for two reasons. Anything in the fuel system has to be capable of passing a 2 1/2 minute fire test. If the filter creates any back pressure in the line it cannot exceed 80% of what the tank was tested at which usually no higher than 3 psi. So 2.4 psi would be the max pressure in the vent line.

    Sec. 183.520 Fuel tank vent systems.
    (a) Each fuel tank must have a vent system that prevents pressure in the tank from exceeding 80 percent of the pressure marked on the tank label under Sec. 183.514(b)(5).
    (b) Each vent must:
    (1) Have a flame arrester that can be cleaned unless the vent is itself a flame arrestor; and
    (2) Not allow a fuel overflow at the rate of up to two gallons per minute to enter the boat.

    I am not visuallizing where you want to run the vent line. But it can't run through any compartment with electrical equipment, unless that equipment is ignition protected (won't cause any sparks such as when a switch is thrown or an electric motor starts up, or a circuit breaker trips) or the space is ventilated to the atmosphere.

    Flame arrestor screens are always copper at least 20 mesh. (20 openings per square inch)

    The loop doesn't need to be that high, only a few inches, 3 at the most, This will not create a siphon.
     
  9. sdowney717
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    sdowney717 Senior Member

    The paper filter is inside a metal steel can just like a metal can of a fuel - water separator.
    If you have worked on fuel filters, you can easily blow threw them, they pass air or water or fuel with no real restriction. My idea was maybe putting that in the vent line would help keep the air exchange down with the air in the tank.
    I watched a video on you tube where a marine mechanic took E10 fuel and put it in front of a fan. The simple act of outside air passing over the fuel caused it absorb water and phase separate in less than an hour.
    Air flows past the boat, that air causes free air exchange with the fuel tank, the larger the vent, the more air exchange in the tank. Anything that can slow down the air movement into and out of the tank is going to help.
    ALL the vent needs to do is allow a tiny amount of air in or out of the tank, it does not need to be that large 5/8 inch tube which it currently has.
    It only needs a large vent when you fill the tank.

    http://www.youtube.com/watch?v=xzFzYoxxA10
     
  10. sdowney717
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    sdowney717 Senior Member

    There wont be any pressure in the tank.
    But I think if you had a vent setup where it would be closed until pressure rose to 2psi or if the tank developed a 2psi negative pressure would be a good idea to ELIMINATE air exchanges from minor temp changes in and out of the tank. EPA may say do that to also prevent venting of hydrocarbons unless absolutely necessary. Right now the tanks are wide open. This can be done using 2 spring loaded one way valves in the vent, both normally closed, one opens out, one opens in.

    The vent line I am thinking of adding would go up the cabin side. It would be soft copper 1/4 inch tubing. It would go up high and then exit the cabin out to the outside at head level way above the water and way above the fuel fill.

    There is a wooden channel inside the salon which the builder ran some 12 volt wiring for lights. If I passed the tube threw that it would be totally hidden.
    Some vent tubes serve as their own flame arrestor so I dont know if the mesh is even needed, then just run the 1/4 inch tube up and out with the clam shell cover still on it.
    The current long copper tube on the boat has no mesh, it just exits the side of the boat with a clam shell cover over it.

    [​IMG]

    any thoughts on how the following works out?
    Part of how a flame arrestor works is it cools down the flame until it falls below the combustion temperature.
    metal slats, screens, mesh quench the flame, a long run of metal tubing might do the same thing.
    a engine flame arrestor can be aluminum, copper or stainless steel. The ones I have seen for boat tank vents look like a SS screen.
    http://www.uscgboating.org/regulations/boatbuilder_s_handbook/fuel_standards_partd.aspx
     
  11. michael pierzga
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    michael pierzga Senior Member

    The fuel tank on a boat needs a BIG vent when high speed fueling and a small vent for everyday use. This is how large yachts are set up. The BIG vent is vented thru the side of the boat and is only opened when taking on fuel...then closed. Its a simple TEE arrangement with a valve. The small vent is high up...discreet and typically an inverted " J " with flame arestsor. I don't know what this vent line you show returning to the fuel fill accomplishes other than blowing fumes and an overfull fueling mistake back into the operator face . Not a bad idea...I hate it when careless people overfill tanks and pollute the sea.
     
  12. sdowney717
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    sdowney717 Senior Member

    How high up is high for the small vent?
    What diameter compared to the size of the vent?

    running the vent back to the fuel fill is the same thing those new plastic combo fill vents do.
    Yes, dont want to drip any fuel into the water for several reasons.
    Part of my idea is to make it very hard to burp out any fuel into the water.
    The other is to try and keep the fuel drier.

    Leaving the big vent on the boat side even with a valve open when filling makes a chance you will spill fuel.
    I dont want to buy a racor LG-100 which does stop that, it still leaves the big vent wide open to the moist air.

    When filling, I also wrap an old towel around the fill in case their is a small amount of spitting out the fuel fill.

    When you fill your car to the top, it never spits back at you, and automatically shuts off the flow.
    Boat system should be at least that good. NO Excuses, IMO should be allowed for a system that is such an old design and cant cope well with todays environmental laws and E10 fuels. Sure it works but it requires great care to prevent spills. Whole system should be made much more user friendly with less risk of a spill. Accidents happen and this is just a preemptive attempt on my part to manage risk.

    If you look at how they do this on a car, they run the tank vent back to the fill nozzle.
    And the vapors are captured by a charcoal cannister.
    The fill cap is sealed and does have a pressure safety valve release built in, incase the pressure rises or falls below certain points.
     
  13. michael pierzga
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    michael pierzga Senior Member

    The only way a fuel fill burps, is if the air in the tank cant escape as fast as the fuel fills...or you over fill your tank...filling a tank above 90 percent is bad practice. On a motorboat the transom is a good place for vents. Its dry back there. The inverted "J" in the small vent can be a upward loop formed in the vent line.. inside the boat... to keen water from entering the vent that is located in the transom. I dont know much about alcohol fuels other than they dont like to be exposed to air.

    Your vent into the fuel fill will work....just be careful that you can also vent air at the same time you fill. ..
     
  14. sdowney717
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    sdowney717 Senior Member

    yes, the vent inlet into the fuel fill tube has to be higher then the nozzle end dispensing fuel into the tank, but not so high as to spit out easily onto the deck.

    90% fill rule is a very good idea. If the fuel heats up it can expand and a large tank could be like adding 5 gallons of fuel if it heats up. If your going to fuel and then go boating, your going to run the tank down., so then maybe ok to go 95%.
    My current vent, today I looked and it is 1/2 inch copper pipe. At the tank there is a 90 degree fitting where the copper tube has a flare nut attachment, it then runs over to the side of the hull, turns 90 degree and goes up about a foot, then turns 90 degree and slowly slopes back down and exits the boat about 3 feet after it bends at the inner hull with another 90 degree on the very end. The exit level is about midway the total height of the fill fuel tube.
    Means the exit is below the very top of the fill screw cap by about 7 inches.
     

  15. Submarine Tom

    Submarine Tom Previous Member

    Wraping a towel around the inlet when filling blocks displaced air in the tank from escaping past the nozzle while fueling. It also inhibits the sound when the fuel begins to rush up the fill tube increasing the chances of an over-fill. Both encourage fuel spattering out the vent. There should be no venting on the tank of less than 1/2" inside diameter. I would not encourage you to vent inside your cabin. Have you tried slowing your filling rate to reduce your problems? Why is the vent at the opposite end of the tank? It should be beside the fill tube. I suspect your tanks vented end is lower and fuel is reaching it well before the tank is "full". This would drive fuel up and out the vent.

    -Tom
     
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