# Gravity GPM for 1 pipe

1. Dec 10, 2011

### docsmith

Gravity GPM for 1" pipe

I am in the process of buying an 83 gal. fuel transfer tank. The tank is 48"L x 20"H x 20"W. Instead of buying pumps for filling up my 5 gallon gas tanks I was thinking about using gravity flow. The builder can install a 1" fitting at the base of the tank with a 1" hose. The tank is going inside an enclosed trailer so I am limited on how high I can build the tank. I can build the tank 2 feet off of the floor of the trailer plus the 9 inch clearance from the floor of the trailer to the ground level. I will be using a ball valve to turn it off and on. What is the gallons per minute when in use. I understand that the GPM will vary depending on the amount of gallons in the tank. What is the GPM at 100% capacity, 50% capacity and 25% capacity?

Thanks for your time and consideration.

2. Dec 11, 2011

### edgepflow

Re: Gravity GPM for 1" pipe

Welcome to PF.

These formulas are good for estimating.

http://www.lmnoeng.com/Tank/TankTime.htm

Use a "full port" ball valve rather than a "standard port" for improved flow.

Try C = 0.5 in the formulas.

I have these (with some enhancements) programmed at work in MathCAD.

Let me know if you need any help !

3. Dec 11, 2011

### NUCENG

Re: Gravity GPM for 1" pipe

A full solution requires the diimensions of the tank being filled and details about the hose and valve. But as a working solution the following apprach is probably adequate.

To solve this problem use Bernoulli's equation.

P +ρV2/2 + ρgh = HL

P is atmospheric pressure and in this case is a constant and can be ignored. Simplify by assuming Head Loss (flow resistance from hose and valve) is small and can be ignored.

So what you end up with is a term due to elevation head (ρgh) and a term due to flow (ρv2/2).

The elevation head term uses ρ the density of the fuel, gravity and the height of the fuel surface in the tank above the level in the 5 gallon tank. Basically the elevation head is converted to velocity head ρv2/2. The value of v is the velocity in the hose and ρ is still the density of the fuel.

So the equation becomes v = sqrt(2gh) since the density terms cancel out.

Finally the volumetric flow (gpm) can be found by multiplying the velocity v by the area of the hose.

BTW, Please ensure that you vent the tank outside the trailer for safety.