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
    Posts: 815
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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
    Posts: 123
    Likes: 3, Points: 18
    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.
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