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  1. #1
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    Default Venting diesel tank

    Currently I have a poorly installed diesel tank and since it is behind the engine hard up against the cockpit sole it is staying there.

    Previously it took sometime to bleed the fuel system and then I noticed the return from the engine was going back to the filter so I moved it from here and tee-d it in to the vent for the tank. This has made bleeding much quicker but now when I fill up residual diesel is being pushed out of the vent, this has caused problems a couple of times as I have assumed the tank has been filled when in fact it has not! (At least I think that is what is happening).

    So today the brain wave was to move the vent in to the side of the deck filler, below the deck but it has occurred to me this will be fine when running the engine but could cause problems when filling the tank.

    So the question is if I fill slowly will the tank vent OK from the filler when filling up? The tank holds about either 60 or 80 Litres (can not remember sat here) with a good size filler pipe about 40mm internal diameter. The vent pipe is 3/8 reinforced plastic.The alternative is to take the return up to the filler but then the pump will be lifting it at least 3 to 4 feet.

    The deck filler is old fashioned chromed bronze and I was going to drill a hole and tap it for a 3/8 BSP nipple and attach the 3/8 plastic tube. The tank is stainless steel.

    Other ideas welcome.

    Writing this: I guess the only solution is to move the breather pipe from the stern up higher, somewhere on the pushpit I suppose....As I say ideas welcome.

    Further thinking about it whilst watching Frannies feet with daughter: Put a tee on the nipple then take the vent pipe from the tank to the tee and then the other side to the vent in the transom. I think that will work.
    Last edited by stav; 20-02-10 at 20:31. Reason: i Think I have solved the conundrum

  2. #2
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    The vent pipe is primarily to let air in to the tank, as the engine uses fuel, so Teeing into the filler won't work.
    3/8" seems a bit small for a vent. The vent pipe should be above deck level, but must be in a protected position, so that no water can get into it.

  3. #3
    VicS is offline Registered User
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    The vent pipe is primarily to let air in to the tank, as the engine uses fuel, so Teeing into the filler won't work.
    3/8" seems a bit small for a vent.
    Why wont the vent work if positioned in the filler pipe. You could have a vented filler cap!

    It wont if its to allow air out while filling though.

    3/8" is huge for a vent to allow air in while running, maybe bigger that the diameter of the fuel pipes unless this Nich 36 has an unusually huge engine!


    Personally I would retain the vent in its existing position if possible and put the spill-back in via the filler neck if possible.

    If the two have to be combined a tee piece fitted like this with the vent connected to the top and the spill-back to the side arm is the way to do it but if I understand correctly you dont have the space above the tank to do it
    Last edited by VicS; 21-02-10 at 02:09.
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  4. #4
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    Quote Originally Posted by VicS View Post
    Why wont the vent work if positioned in the filler pipe. You could have a vented filler cap!

    It wont if its to allow air out while filling though.

    3/8" is huge for a vent to allow air in while running, maybe bigger that the diameter of the fuel pipes unless this Nich 36 has an unusually huge engine!


    Personally I would retain the vent in its existing position if possible and put the spill-back in via the filler neck if possible.

    If the two have to be combined a tee piece fitted like this with the vent connected to the top and the spill-back to the side arm is the way to do it but if I understand correctly you dont have the space above the tank to do it
    As Vic says, if the sole purpose of the vent was to allow air in as the engine consumes it then a vented cap or a vent into the filler pipe would be fine. However, its primary function is to allow air out of the tank when you are filling it to avoid pressurising the tank ( and in extreme cases rupturing it). A vented filler pipe will not allow this which is why most tanks are vented seperately. The most important issues in the design of an efficient tank vent system are:

    1) The vent must exit from the top of the tank.
    2) There must be sufficient 'run' on all sections of the pipe so that any liquid which gets into it will run back into the tank at all angles of heal.
    3)There ahould be some kind of vent head attached at deck level which will prevent rain or sea water from getting in to the tank
    4) Ideally the vent should be of a large enough cross sectional area to allow air/ fluid out faster than liquid can be put into the tank. This last point is rarely seen on pleasure craft.

    If you can achieve points 1, 2 and 3 then you should have a system which will work.

  5. #5
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    Quote Originally Posted by VicS View Post
    Why wont the vent work if positioned in the filler pipe. You could have a vented filler cap!
    The OP said that he has a "Deck Filler". It would be strange for a deck filler to be vented.

  6. #6
    VicS is offline Registered User
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    Quote Originally Posted by NormanS View Post
    The OP said that he has a "Deck Filler". It would be strange for a deck filler to be vented.
    Sorry I did not mean to suggest that the OP had a vented deck filler. I was questioning the statement that teeing into the filler would not work. I should have perhaps have pointed out that cars for example sometimes have (or used to have) vented filler caps. Portable fuel tanks have vented filler caps too.

    Why won't a vent into the filler work?? Statement made ... challenged ... but no explanation forth coming.

    Also wondered why anyone would think that 3/8 " was a bit small.
    Last edited by VicS; 21-02-10 at 14:33.
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  7. #7
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    Quote Originally Posted by VicS View Post
    Sorry I did not mean to suggest that the OP had a vented deck filler. I was questioning the statement that teeing into the filler would not work. I should have perhaps have pointed out that cars for example sometimes have (or used to have) vented filler caps. Portable fuel tanks have vented filler caps too.

    Why won't a vent into the filler work?? Statement made ... challenged ... but no explanation forth coming.

    Also wondered why anyone would think that 3/8 " was a bit small.
    VicS.
    Further explanation: The vent pipe allows air into the tank as fuel is used. It is also used to let air out of the tank, as fuel is poured in. Obviously, to let air into a tank as the fuel is used, 3/8" is more than adequate. However, when filling the tank, from a pump, tanker lorry, or any other high volume supply, I think you would find that a 3/8" vent pipe might be inadequate, and could lead to air forcing its way up the filling pipe, and therefore to fuel spillage.

    I was responding to the OP's problem. He has a deck filler. That is why I said that leading the vent into the filler would not work. I stand by what I have said. I hadn't realised, that in your mind, the thread had drifted off to cars. I suspect that nor had anyone else.

  8. #8

    Default Just a thought......

    I believe the ISO minimum standard for any approved vent system is 11mm diameter in the EU and 5/8" in the US. That would make any vent line smaller than about 7/16" (11mm) illegal and may subject your marine insurance policy to be canceled or any claim denied should something happen.

  9. #9
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    pheran is offline Registered User
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    I don't know whether it will affect the OP but when a previous boat of mine had its first inspection under the inland waters Boat Safety Scheme some years ago, amongst the requirements for the tank vent were

    a) it had to vent to atmosphere ie not back into the filler down spout
    b) the vent outlet had to be higher than the deck filler
    c) the vent fitting had to be fitted with a flame guard

    Whether or not the OPs boat is subject to a BSS, these provisions seem eminently sensible and are imposed with safety in mind. My boat fell foul of b) and c) but the problems were easily solved. Incidentally, mine was a 100gallon tank and was fitted with a 7/8" breather pipe.
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