Fuel limit vent valves to maintain air space, ullage space in the fuel tank

Discussion in 'Powerboats' started by sdowney717, Jun 1, 2017.

  1. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    I am redoing my old 1970 style fuel system to be modernized.

    I am switching to the VOPR, vapor and over pressure relief fuel fill and caps.
    The vent at 1PSI pressure and open under vacuum at less than .25 psi suction.

    I have bought 2 ICV (inlet control valves), Perko 0635DP0 that fit into the fill pipe.
    I will have the (sealed and vented both ) VOPR Perko fills.

    The other component is the ullage valve, known as the fill limit vent valve, FLVV, which goes into the vent line which returns to the vented fill and cap. The purpose is to maintain an air space in the tank of around 5%. Perko makes screw in 5/8 valves which are pricey.

    Example for a 3/8 space, I have also seen Perko 1 1/8 space valves.
    PERKO 0586T00038 THREADED 3/8" FILL LIMIT VENT VALVE MARINE BOAT | eBay http://www.ebay.com/itm/PERKO-0586T00038-THREADED-3-8-FILL-LIMIT-VENT-VALVE-MARINE-BOAT-/191999155600?hash=item2cb40a9d90:g:x9IAAOSwYIxX99e1&vxp=mtr

    It seems some work by stopping the flow of venting tank air by way of a float valve which cause fuel to back up in the fill hose so then the fuel nozzle shuts off the pump.

    Example:Our Offerings https://bluskies.us/offerings

    Now it seems to me, you can achieve the same effect by putting a small pipe extension directed down into the tank where the vent hose fitting is attached.
    On my metal monel tanks, I have a brass elbow fitting screwed in there, so a short section of copper pipe could be brazed to the brass elbow fitting.
    I would think for my 150 gallon tanks, about 1 inch is sufficient. So when the fuel in the tank rises to the level of the protruding tube sitting there at the vent, fuel will be forced up to the fill nozzle, the tank wont be filling up with fuel anymore and gas present at the fuel nozzle will shut off the fuel.

    Does that make sense to you?
    While It would work since then it is working like your car which backs fuel up the vent line in the same way, to shut off the pump nozzle, for some reason they don't make it that way, I just don't know for sure. It does not splash fuel filling the car tank working like that.
  2. IronPrice
    Joined: Jul 2017
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    Location: NZ

    IronPrice Senior Member

    I think that system would work for shut off during filling, but it won't stop vapour escaping out the vent pipe.
  3. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    In addition, many boats with the new system have carbon canisters in the vent line to scrub the vapor. Any liquid destroys the carbon in the canisters. So, a valve to prevent liquid from rising in the vent line.

  4. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    updating my experience. This VOPR fuel cap is wonderful. I have had zero trouble with fuel, no problems leaving 40 gallons in the 150 gallon tank over the winter, the fuel stays dry. The engines run great on the old fuel from last year. Every boat should be fitted with this, you don't need charcoal canister, that will expose the fuel to humid air.

    All you need is the VOPR fuel fill with integrated vent tube. And the Attwood fuel demand valve in the fuel tank outlet. Perhaps the fuel fill valve to keep fuel from welling back up when your filling the tank is useful. The ullage valve you don't need but is potentially beneficial if you were to overfill your tank.

    All through last fall, winter, spring, my tank has been basically closed off from the air by the VOPR cap. I know because everytime I opened the cap I heard no whoosh, made me wonder if it had broken. But it just had not been hot enough to pressure the tank with fuel vapors. So this summer with mid 90 days, when I open the tank, I hear the 'whoosh', like you hear on your car fuel cap when removing the cap. So it is working like I had hoped.

    The VOPR cap allows a 1 pis pressure to build in the tank before it vents to air, and has caused no trouble to the tank. The gasoline is preserved, so the E10 wont suck water from the air corroding your fuel system like it does in the old conventional wide open vent system most everyone has.

    Here is the Perko catalog with the part number for vpr caps fuel fills

    The carbon canister is a bad idea. It leaves the tank vent open to the air. Defeats the whole idea of a sealed fuel system. Which, IMO, just leaves the whole problem of fuel mixing with humid air. The EPA likes it though I am sure!
    Last edited: Jul 14, 2018
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