fuel tank fill and vent question

[bleedblue94]

hi, i recently did a restore on an older boat that used a dedicated fuel fill and a separate vent for the tank. while doing the rebuild i switched to an attwood combination fill and vent to prevent spills when fueling. i noticed the cap for this seals the vent and the tank fill at all times except for when the cap is open, ie when i am fueling (it does NOT allow the vent to be open at all times). im wondering if this is causing excess pressure on my plastic tank? when i open the cap i get the sound of pressure equalizing. should i switch back to a dedicated vent that remains open at all times? should i T off the line and install an open vent at a higher position than the fill so it vents the tank at all times, but also have the line go to the combination fill/vent so any "burps" while fueling will most likely go the path of least resistance (aka to the lower vent which would be located in the fill) and recycle back down the fill?

thanks

 

[UncleWillie]

Re: fuel tank fill and vent question

The new fill/vent caps vent IN to prevent the tank collapsing as the fuel is used.
If your gas in the tank evaporates faster than your burn it, the tank never vents.
There is no reason to vent the pressure off the tank.
It is designed that way.
Is is not just "Fumes" you are venting, it is your hard earned Gas getting blown to the wind and a source of air pollution.

 

[bleedblue94]

Re: fuel tank fill and vent question

The new fill/vent caps vent IN to prevent the tank collapsing as the fuel is used.
If your gas in the tank evaporates faster than your burn it, the tank never vents.
There is no reason to vent the pressure off the tank.
It is designed that way.
Is is not just "Fumes" you are venting, it is your hard earned Gas getting blown to the wind and a source of air pollution.

thanks, im not talking about when its in use. im referring to the time that it is not in use. if i have it out of the water and not in use for a week or so, the tanks have expanded from increase pressure due to heat. im concerned of THAT pressure being trapped. im NOT referring to the pressure shifts from the motor using up fuel

 

[UncleWillie]

Re: fuel tank fill and vent question

thanks, I'm not talking about when its in use. I'm referring to the time that it is not in use. if i have it out of the water and not in use for a week or so, the tanks have expanded from increase pressure due to heat. I'm concerned of THAT pressure being trapped. I'm NOT referring to the pressure shifts from the motor using up fuel

I am referring to THAT pressure also.
You do not want to vent the tank OUT, you are wasting gas and polluting.
It is only a cosmetic issue.
It WILL Blow up like a balloon!
When I said, "It is Designed that Way." I really mean it.
See Questions #3 and #5.
They only need to breath IN. Except when you take the cap off to Fill the tanks.

People have been venting tanks for decades with no apparent issues. and will continue to do so.
That does not make it the correct thing to do!

I keep a spare two gallon can of gas in the back of my SUV all summer in the hot backing sun.
The can is sealed or the cab would be full of gas fumes.
It holds up just fine, and it is a 20 year old "Old School" can.

A tank sitting for a week with the vent open will be venting fuel for the whole week.
Where do all those fumes go?
Use any logic you want, money wasted, Environmental Pollution, Fire safety, there is no valid reason to open the vent to relieve the pressure.

Try to find any reports of un-vented gas cans exploding and dumping gas all over the place...... Bueller?

 

[bleedblue94]

Re: fuel tank fill and vent question

I am referring to THAT pressure also.
You do not want to vent the tank OUT, you are wasting gas and polluting.
It is only a cosmetic issue.
It WILL Blow up like a balloon!
When I said, "It is Designed that Way." I really mean it.
See Questions #3 and #5.
They only need to breath IN. Except when you take the cap off to Fill the tanks.

People have been venting tanks for decades with no apparent issues. and will continue to do so.
That does not make it the correct thing to do!

I keep a spare two gallon can of gas in the back of my SUV all summer in the hot backing sun.
The can is sealed or the cab would be full of gas fumes.
It holds up just fine, and it is a 20 year old "Old School" can.

A tank sitting for a week with the vent open will be venting fuel for the whole week.
Where do all those fumes go?
Use any logic you want, money wasted, Environmental Pollution, Fire safety, there is no valid reason to open the vent to relieve the pressure.

Try to find any reports of un-vented gas cans exploding and dumping gas all over the place...... Bueller?

ok thanks. that alleviates my fears. i wasnt sure how much pressure a 40ga or so tank would create. i know the poly tanks have a threshold for failure and just wanted to make sure i wasnt inviting problems

thanks