Homemade gas tank venting design considerations

[billyjones454]

I am modifying an aluminum fuel cell to be a gas tank with a fill neck that goes behind the license plate.  It has 2 line ports on the bottom, and 2 vent ports on the top. I will be moving the location of the ports.  However I am not quite sure what the best practices are for venting a gas tank.  I've seen lots of examples of fuel cell vents, and I've seen OEM type gas tanks that have a separate vent line that goes parallel to the fill neck and terminate in the fill neck below the gas cap.  I've seen some vents with hard lines that have several loops.  I've seen fuel cells with rollover valves, but I have not seen those on gas tanks.

 

Ideally I would like to run a separate line from the tank to the fill neck, however, it doesn't seem like that's accomplishing a true vent, if it's behind the gas cap.  Could someone clarify some of the design features for a gas tank setup?  Thanks!

 

I failed to imbed photos.  See photos of my project here: https://imgur.com/a/umu7K2J

[AydinZ71]

Fuel cell and passenger vehicle gas tanks have the same principles. The main difference is the OEM vent system must comply with EPA regulations that dictate where and how the vent Vapor is routed/treated. 
 

Besides the nozzles (feed, return, gauge, etc.), you have a fill port and a vent port. Fuel cell has an a roll-over check-ball in the vent line for safety. Passenger vehicle has all sorts of stuff like carbon canisters, vent connected to the fill port etc. 

 

your main challenge is how to deal with the vented gas. If you don’t care about the smell of gasoline vapor filling your car and garage, you can just put a small breather filter on that fuel cell vent port. 
 

for those who prefer to manage the vented gas like myself, there are many strategies. Some coil a length of tube connected to the vent port. This creates a volume and restriction for the vented gas to occupy. Eventually, the Vapor cools and condenses back into the tank.

 

I have a different strategy. I use two parallel check-valves made of plastic compatible with gasoline. They have a crack pressure less than 1/3 psi. One is for air make-up, the other for gasoline vapor to escape. Therefore, my fuel-cell is actually a closed system that operates +- 1/3psi. It’s not enough pressure to stress the cell (mine has a bladder), but just enough to keep the vapor from escaping under most conditions. On particularly hot days, pressure rises above 1/3 psi and vents. 
 

no more gasoline smell in the car or the garage. 

[billyjones454]

1 hour ago, AydinZ71 said:

Fuel cell and passenger vehicle gas tanks have the same principles. The main difference is the OEM vent system must comply with EPA regulations that dictate where and how the vent Vapor is routed/treated. 
 

Besides the nozzles (feed, return, gauge, etc.), you have a fill port and a vent port. Fuel cell has an a roll-over check-ball in the vent line for safety. Passenger vehicle has all sorts of stuff like carbon canisters, vent connected to the fill port etc. 

 

your main challenge is how to deal with the vented gas. If you don’t care about the smell of gasoline vapor filling your car and garage, you can just put a small breather filter on that fuel cell vent port. 
 

for those who prefer to manage the vented gas like myself, there are many strategies. Some coil a length of tube connected to the vent port. This creates a volume and restriction for the vented gas to occupy. Eventually, the Vapor cools and condenses back into the tank.

 

I have a different strategy. I use two parallel check-valves made of plastic compatible with gasoline. They have a crack pressure less than 1/3 psi. One is for air make-up, the other for gasoline vapor to escape. Therefore, my fuel-cell is actually a closed system that operates +- 1/3psi. It’s not enough pressure to stress the cell (mine has a bladder), but just enough to keep the vapor from escaping under most conditions. On particularly hot days, pressure rises above 1/3 psi and vents. 
 

no more gasoline smell in the car or the garage. 

Thank you for this clarification.  Differing requirements for how the vented gas is handled explains why I've seen several different designs.  I would love to have a managed system like you're describing.  Could you expand on your setup?  It sounds like you have a valve that will let air in as the fuel is used, and another that will vent the vapor out in the event that pressure builds up beyond a certain pressure.  Does that sound accurate?  Can you link to the valves that you use? Do you have them covered or protected from debris or dust in any way?

Thanks again.

[AydinZ71]

Yes, the way you described the check-valves are correct. I bought them off McMaster Carr. They are enclosed plastic with hose barbs on both ends. I’ll have to look them up again, and il post it.

all spring loaded check-valves have a “crack”pressure. It’s the pressure necessary to open the valve in the direction of flow. Just enough pressure to push the spring and allow fluid to flow in the intended direction. This spring creates the back-pressure preventing the valve from being open all the time. I’m just using this tiny amount of necessary pressure to create a “closed” system that operates in a + 0.3psi to -0.3psi range.

 

the plan was to put an air stone on the make-up air (one of two). I just have not gotten around to it. It’s not a big risk, given where the valve is (right on-too of my fuel cell) how Infrequently it opens to let air in, and the downstream fuel filter. 
 

So it goes: vent nozzle on fuel cell -> AN fitting 90 to hose barb -> fuel hose -> tee fitting -> short length of hose on 2/3 ports on the tee -> check valves on those short lengths of hose. Check valves oriented in opposite directions 

 

PS: before doing this, make sure your fuel cell can handle the little bit of pressure that will build up. I did this by closing off all the nozzles and blowing in one until I hit 1/3psi. Yes, the fuel cell was drained and all residual fuel had evaporated. I am an engineer who works in fuel infrastructure (go figure) but just because it works for me I can’t guarantee it will work for you. 

[AydinZ71]

Here is the check valve without hose barbs. Probably a better installation:

 

https://www.mcmaster.com/product/5492K51