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post #1 of 6 (permalink) Old 10-26-2005, 10:49 AM Thread Starter
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Red face Calling Fluid Dynamic Guru's or ME...

So.... I have this project I'm working on and I don't know how to figure out a hose size issue...

You have a fuel nozzle (we'll say 1" in diameter) filling up a fuel tank. For arguments’ sake, lets say the nozzle pressure is 10 psi. How large of hose do you need if you are filling up a second tank via gravity feed from the first tank?

Not the exact project I’m working on, but imagine two tanks in front of the wheel wells of a truck bed. You only one fill neck into the first tank with a small hose connecting the two tanks together (at their bases), and venting is not an issue. What size hose would be used to connect the two together so that when you fill the tanks, it is like filling one large tank?

It is a volume in regard to pressure vs. volume in regard to gravity flow problem.

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post #2 of 6 (permalink) Old 10-26-2005, 01:44 PM
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That's essentially how the twin tanks on big trucks work. When I was driving for a living, if I couldn't get to a station with two hoses per pump (made for big trucks) and the single hose wouldn't reach the other tank, I'd just fill the tank slowly and gravity would mostly fill the other tank. If I had waited around an extra 10-15 minutes, both tanks would have eventually filled.

Here is an idea of what I used to do.

The truck had 2 191 gallon vented tanks. Each tank held 185 useable gallons of diesel fuel.

The pickup and return tubes were in the driver's side tank only.

If I pulled up to a single hose fuel pump with 1/4 tank remaining (meaning that both tanks were at the 1/4 level since they were joined by a 1/2 inch cross-connect line) and put the single, car sized pump into the driver's side tank and held it wide open, filling the driver's side tank as fast as possible and then left, within a half hour, both tanks would be at about 3/4 of a tank.

The math works like this.

Already had .25 tank in each tank for a total of .50 tank.

I added .75 tank to one tank but, it was cross flowing into the other tank the whole time I was pumping fuel.

End result = two .75 tanks of fuel.

Keep in mind that the cross flow hose connects near the bottom of the tanks so the weight (pressure) of the fuel in one tank would force the liquid level to balance out between the two.

On another note, since the two tanks had vented caps, if I filled both tanks to within 2 inches of the top (recommended level by manufacturer) and then parked the truck on a crowned street, the low side would slowly dribble fuel out of the cap vent since the fuel level was constantly trying to balance itself.

Confused yet?

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post #3 of 6 (permalink) Old 10-26-2005, 01:56 PM Thread Starter
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That is exactly what I'm talking about (theory wise at least). What I'm trying to avoid is the "End result = two .75 tanks of fuel" part of it.

I always wondered how those dual tanks were plumbed!

But I've been thinking... if the fill neck of the primary tank is say 2" in diameter, and it doesn’t back up while I’m filling the tank... then all I need to do is connect the second tank with a 2" diameter (or cumulative 2" diameter) and I should be good to go! Just as long as the vent side of the second tank is ok and will "pass" the air exiting.

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post #4 of 6 (permalink) Old 10-26-2005, 02:18 PM
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any size hose would eventaully cause both tanks to be of equal level

but, if the hose is to small, the 1st tank will fill, before the tanks can come to equilibrium. (via limited flow) DD wants to fill the first tank, and as he is filling the 1st, the 2nd is filling at an equil rate, ie. flow in tank 1 = flow from tank 1 to 2. so when he is does filling both tanks are full.

so here's what you do

1. determine flow into tank 1 based upon cross-sectional area of the filler hose and the pressure of the fluid. (if viscousity is an issue it also needs to be addressed, but most likely not with gasoline)

as you fill tank #1, the level will rise, thus increasing the pressure in the connecting hose from #1 to #2.

so what you need to do is determine max force tank 1 can extert, ie. full tank. basically this will be volume x gravity. or 'weight'

flow into tank #2 needs to be equal (or greater) than the flow into tank #1 via filler hose.

now determine dia. based upon cross sectional area based upon your max pressure & flow needed. (remember: pressure is force per area)

eq.

P1 ~ filer pressure = 10psi
CSA1 ~ cross-sectional area of filler hose = pi x (DIA/2)^2 = 0.79
F1 ~ filler flow = ?

P2 ~ presure in hose connecting tanks = ?
CSA2 ~ """ = ?
F2 = F1

gravity= 9.81 m/s = 32.2 ft/s

P2 = [(max volume tank 1) x gravity] / (CSA2)

confused yet? you can find flow based on time & volume, but i'm sorry i can't remember how based upon pressure and CSA, but it should be possible

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post #5 of 6 (permalink) Old 10-26-2005, 02:31 PM Thread Starter
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Quote:
Originally Posted by tomaso12
any size hose would eventaully cause both tanks to be of equal level

but, if the hose is to small, the 1st tank will fill, before the tanks can come to equilibrium. (via limited flow) DD wants to fill the first tank, and as he is filling the 1st, the 2nd is filling at an equil rate, ie. flow in tank 1 = flow from tank 1 to 2. so when he is does filling both tanks are full.

so here's what you do
...

...

Confused yet? you can find flow based on time & volume, but i'm sorry i can't remember how based upon pressure and CSA, but it should be possible
Or just make the hose(s) so stinking big that it's a non-issue.

Like the old saying goes... "If you can't tie a knot... tie a lot"...

But yeah that is what I'm looking for. I'm probably going to go with four or five 1" hoses. I have serious space limitations... width is not an issue, but I only have about 2" of height that I can work with on the original tank.

97 Lincoln Mark VIII LSC (Chip'd, 3.73 T/L... so far... )
97 Ford Aspire (Slow, but getting 36 mpg (f'n Ethenol!! )
84 F250 Dually w/6.9L Diesel (7.3L IDI pending)
73 Mercury Cougar Convertible w/351C 4V (Partially Restored)
69 F100 LWB w/460 Engine
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post #6 of 6 (permalink) Old 10-26-2005, 02:59 PM
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Quote:
Originally Posted by 94 Daily Driven 4.6L
Or just make the hose(s) so stinking big that it's a non-issue.

Like the old saying goes... "If you can't tie a knot... tie a lot"...

But yeah that is what I'm looking for. I'm probably going to go with four or five 1" hoses. I have serious space limitations... width is not an issue, but I only have about 2" of height that I can work with on the original tank.
i was kinda thinking the same thing...but figured i give more a engineering answer

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