gas tank pressure question

taddthewadd · Mar 4, 2010

I am building a gas tank and here is the question.

I am building it with a petcoc on the bottom and two nipples on the top. One nipple is to fill the tank and is capped and the other nipple is a vent. My question is if I put a check valve (one way valve that lets the air in but won't let the gas/air escape) on the vent line will pressure build up in the tank to the point of it exploding?

I know the vent line is important so that when fuel leaves the tank air comes into the tank to replace the volume that was once the fuel. The check valve seems the like a good way to have the vent line but to keep fuel from spilling out of the vent line.

What do you guys think, is the use of the check valve dangerous? The tank is made of PVC. Thanks in advance guys.

civlized · Mar 4, 2010

Most caps are vented, thus eliminating the need for an extra vent. Are you making your own cap or buying a non vented one? If not, you should not need a vent. The check valve sounds like a bad idea to me, but I'm not a tank builder. If it is a spring loaded check valve, it requires a certain negative pressure before it will open. I don't know, but it seems like it could starve your engine for gas, maybe, before it opens. It would require a very sensitive spring.

professor · Mar 4, 2010

Very BAD idea.
Imagine the bike sitting in the sun for a while, pressure builds up and splits the tank, it blows fuel onto someone passing by, they sue you, the EPA comes knocking on your door............ Or, you go to start your motor and after you open the petcock, the engine floods instantly. Your tank held though.
One time at work, i was tossing some PVC pipe into the dumpseter but it was too long. So I thought- smash it on the ground and it will break in half. Boy- did it break - into a zillion bits that I had to pick up.
What I am saying is the stuff is fragile when a sharp blow hits it. Do you really want a tank made of it? Any kind of metal would be better. And put a vent or vent cap on it- a very tiny hole will do!

2door · Mar 4, 2010

I agree with Professor in that if the tank cannot vent it can, and will build up pressure if left in a warm environment and the petcock is closed. Opening the petcock will flood the engine. The float/needle and seat in small engines with gravity flow fuel systems are not designed to hold back much over atmospheric pressure. Increasing tank pressure much over that will result in severe flooding and leaks. Vent the tank for both positive and negative pressure. I wouldn't be too concerned with explosive pressures; something else would probably leak and lower pressure before the tank ruptured.
Tom

taddthewadd · Mar 5, 2010

Thanks for the advice. The cap is not vented that is why I made the vent line. Instead of putting a check valve on it I will use someone elses idea of putting a fuel filter in the vent line so air can go in and out but if fuel tries to slosh out the fuel filter will absorb the fuel and it will drain back into the tank. Also the fuel filter in the vent line will also act as an air filter to some extent for air going into the tank.

civlized · Mar 5, 2010

I am just wondering, what would be the issue with drilling a very small hole in the cap? I am interested in seeing your creation. Do you plan on posting some pics along the way or when it's done?

taddthewadd · Mar 5, 2010

I will post some picks soon. Here is where I did something very different. Instead of haveing a screw on gas cap I just put a 3/16 nipple. I put a short piece of fuel tubing on it and cap it by threading a 6mm screw into the end of the tube. To fill the tank I will use a bulb with a nipple on the end of it. I will remove the screw and plug the bulb into the fuel line and squeez the fuel in.

customshop05 · Mar 6, 2010

civlized said:
I am just wondering, what would be the issue with drilling a very small hole in the cap? I am interested in seeing your creation. Do you plan on posting some pics along the way or when it's done?

Fuel leaks all over when going over bumps and corners during slosh. usually not a good idea.

K.i.p · Mar 7, 2010

Wha-?
Wow looks like we need a global recall on more than a centuries worth of lawn mowers, motor vehicles, generators, and home and garden equipment. Call me a rebel, 1/16" vent hole in my cap, done deal.

taddthewadd · Mar 7, 2010

The tank is tucked under the back part of my seat. I ended up putting a 3/16 nipple at the top and attached a 6" piece of fuel line to it and coiled the fuel line, zip tied it and tucked it under the seat. This will allow it to vent and prevent fuel leakage.

corgi1 · Mar 7, 2010

I see on some motorcycle pic's a tube from the gas cap area leading to the handle bar and possibly zip tied

K.i.p · Mar 7, 2010

I think that is more typical of off road bikes built for really rough tracks, trails, or terrain. I can understand that, nobody wants a lap full of gasoline. I don't get the impression there are very many builds here being designed for that type of punishment.
*edit, I accept I may well be wrong about that

taddthewadd · Mar 7, 2010

I am planing on riding mine in town but also riding off road. I will probably ride up a fire road and down some single track too. I bought two sprockets, a 36 for town and a 54 for off road. I have two chains so it is easy to swap. My sprockets are mounted to the hub with the adapter instead of to the spokes. So fuel would probably slosh out of a little hole so that is why I put some tubing and looped it a few times.

K.i.p · Mar 8, 2010

....and so I am wrong,... tadd so in your example the tube is practical. I need to be quiet and read more. My bad.

taddthewadd · Mar 8, 2010

No problem bro, I can't wait to get this thing out on the trails. I should have this bike done by the end of the month.

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