Fuel Expansion

[Nate]

Yup, that's posted on the first page of this thread

 

[cojhl2]

But air can still compress a lot, unlike a fluid.



steved

Very true,, but where is the pressure gonna come from to compress it. If you are filled to the top, where is the added head??



No matter how you work this issue,, if the tank is full to the top and the fuel expands it will run out the top. .

 

[steved]

Very true,, but where is the pressure gonna come from to compress it. If you are filled to the top, where is the added head??



No matter how you work this issue,, if the tank is full to the top and the fuel expands it will run out the top. .

THERMAL EXPANSION?????



By itself, it wouldn't expand other than due to vapor pressure. But add heat, and almost ANYTHING expands. Thermal expansion.



steved

 

[brods]

Very true,, but where is the pressure gonna come from to compress it. If you are filled to the top, where is the added head??



No matter how you work this issue,, if the tank is full to the top and the fuel expands it will run out the top. .

I think the short answer is thermal expansion adds the head, the relief valve inside the fuel cap allows some extra head to build and compress the air before fuel comes out.



Hope that helps.

 

[WalterRyan]

The best answer is don't even worry about calculating expansion. This is from the USDOT web site.



(c)(12) Overfill restriction. A liquid fuel tank manufactured on or after January 1, 1973, must be designed and constructed so that —



(c)(12)(i) The tank cannot be filled, in a normal filling operation, with a quantity of fuel that exceeds 95% of the tank's liquid capacity; and



(c)(12)(ii) When the tank is filled, normal expansion of the fuel will not cause fuel spillage.



What I did with my 50 gallon in bed tank was weld a 3/4' long piece of large tubing to the bottom of the tank top around the filler. This creates an air space to prevent expansion and satisfies DOT requirements. I know DOT is not a major problem for private vehicles but the rules do still apply and if you dump several gallons in the wrong place it will become an issue.

Also notice this requirement is also why the factory tank has the air pocket at the top that people fix with the "vent mod".



Walt

 

[Nate]

I'm sure what you mean about that 3/4" piece of tubing. . can you explain a bit more?



Thanks!

 

[cojhl2]

Aha!!! gotit,, sealed cap!!

 

[boatpuller]

I'm sure what you mean about that 3/4" piece of tubing. . can you explain a bit more?



Thanks!

I beleive he's saying he extended the fill 3/4" down below the top of the tank so that when you fill the tank there will be a 3/4" airspace on top of the fuel in the tank allowing room for any expansion.

 

[Nate]

I wonder if that would work?

 

[steved]

I wonder if that would work?

You would lose some volume of your tank, but it would give you the needed expansion volume.



steved

 

[WalterRyan]

Boatpuller is correct a 3/4' long piece of 3' diameter tubing welded on the inside of the tank around the fill hole. Yes you do lose capacity, the 5% required by the DOT. Just multiply the height of the tank by . 05 and make the tube that long. It does not need to be welded all around just tacked well enough to stay on. When you fill the tank to the bottom of the tube the fuel will come up in the tube before it fills the last 3/4" of the tank. It has worked for 2 years perfectly so far.

 

[Nate]

Just to clarify a ' is foot a " is inch.



So I'm guessing you mean inches and not feet, correct??



If it's as simple as dropping the pipe for the fill neck down 3/4" I can do that. The fill neck is just going to be a piece of 2" pipe probally.



61"x10"x. 75" is roughly 2 gallons, so I'd have to in theory put the pipe down a bit more for a 50 gallon tank.



(5% of 50 would be 2. 5 gallons)

 

[WalterRyan]

Oops. Yep that is inch not foot. Apparently my tank building skills are better than my proofreading skills. I did get it right the second time though. I checked and it is about 1 inch long, roughly 2 3/4 gallon. My tank is 64 X 19 X 10 (I thought it was 9 but it is 10) that is over 52 gallons so I am just short of 50 gallons.

 

[WalterRyan]

I tried to get a picture of the inside of the filler but even with a flash it was too dark to see anything.

 

[Nate]

That's pretty much what mine is going to look like.



The reason I was only making it 61" is that I was going to put angle iron on the sides to secure it to the bed. I figure 50 gallons ~400lbs plus probably another 75-100lbs for the tank so it needs a fair amount of clamping to the bed to prevent it from coming out in the event of a rollover or something.



Yours is just attached with the 2 small tabs on the front?



Any reason for the 2 fill caps, and that brass fitting on the left, is that for the vent??



Sorry for all the questions!

 

[WalterRyan]

No problem with the questions. There are 2 fillers so it can be filled from either side. That way I don't have to worry about which side the pump is on. Also when traveling I have found that some places they only have diesel at the truck lane or to get the truck with a 29' 5th wheel thru the normal pumps in front of the station. The truck pumps have two pumps with one slaved to the other so I can start one nozzle on the right side of the auxiliary tank and put the other in the main tank to fill both at once. The fitting on the left is the pickup. I just got a brass street elbow and soldered a piece of 1/2" copper tube in the center of the male end. This was cut off close to the bottom of the tank. There is a Carter pump mounted under the bed with a hose going into the factory fill neck about 3" below the cap. I have a switch in the cab I turn on to transfer fuel.



Walt

 

[WalterRyan]

Forgot, there are 2 tabs at the back and 2 on the front. Just pieces of angle iron welded to the tank and drilled to bolt thru the bed floor. I put the bolts close enough to the tank side of the angle that the hex can not turn and I just tighten the nuts from the bottom.