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Old 29-03-2012, 08:24   #1
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Larger Fuel Tank Vents

Equipment suppliers offer small fuel tank vents/breathers which are usually acceptable for smaller tanks. What do other people use for bigger fuel tanks which get filled at a faster rate and need appropriately larger vents to prevent blow-back. I have a 2000 litre tank and cannot see the usual tiddly vents being adequate. I have not seen anything commercial on show and have sort of resigned myself to having an industrial-looking goose neck. Anyone seen anything useful ?
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Old 29-03-2012, 18:17   #2
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Re: Larger fuel tank vents

Oh good question. Really its not so much the tank size as the fill rate of the fueling nozzle. Assuming a 25 gpm (90 ish lpm) commercial pump rate, a 3/4" (21mm) pipe would vent air at about 4 feet per second (1.25 mps) assuming a tight fill connection with is not bad. This would move .05 CFS of air give or take. A longer vent more then a few feet (meter or two), might want to upsize to 25 mm ish.

Note, There will be blow back at the fill unless the vent is much larger as the air will vent at the easiest location, that is at the lowest mean static pressure drop, which is usually at the fill. The vent hose length adds to the pressure head. So even with a 25mm vent you might still get venting at the fill port, Its just easier for the air to move that way with the low pressures during fill

Of course if the deck fill connection is short and large then a small 1/2" (13mm) vent might be fine. Lots o varibles depending to fill routing and vent routing. Short vent pipes work better. From an engine operating standpoint the 13 mm vent is plenty big enough.

If its a concern you could use two deck fills, one as vent and one for fast filling. That way you don't have a large vent opening to flood the tank with a boarding wave.

Using a fill dip tube, that is a pipe extending down to say within 25-50 mm of the tank bottom would prevent back venting at the fill, But your going to want that bigger 21-25 mm vent hose then....
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Old 30-03-2012, 01:40   #3
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Re: Larger fuel tank vents

Quote:
Originally Posted by sailorchic34 View Post
Oh good question. Really its not so much the tank size as the fill rate of the fueling nozzle. Assuming a 25 gpm (90 ish lpm) commercial pump rate, a 3/4" (21mm) pipe would vent air at about 4 feet per second (1.25 mps) assuming a tight fill connection with is not bad. This would move .05 CFS of air give or take. A longer vent more then a few feet (meter or two), might want to upsize to 25 mm ish.

Note, There will be blow back at the fill unless the vent is much larger as the air will vent at the easiest location, that is at the lowest mean static pressure drop, which is usually at the fill. The vent hose length adds to the pressure head. So even with a 25mm vent you might still get venting at the fill port, Its just easier for the air to move that way with the low pressures during fill

Of course if the deck fill connection is short and large then a small 1/2" (13mm) vent might be fine. Lots o varibles depending to fill routing and vent routing. Short vent pipes work better. From an engine operating standpoint the 13 mm vent is plenty big enough.

If its a concern you could use two deck fills, one as vent and one for fast filling. That way you don't have a large vent opening to flood the tank with a boarding wave.

Using a fill dip tube, that is a pipe extending down to say within 25-50 mm of the tank bottom would prevent back venting at the fill, But your going to want that bigger 21-25 mm vent hose then....
Good logical thinking! Love it. Thanks for that - will go with the submerged fill pipe and two vents of the usual variety placed high in midline.
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