Aluminum tanks (at least any that are light enough to be used in a small a/c) will collapse if the vent is blocked and fuel is pumped out at the pressure differential that a typical fuel pump can achieve. 14psi (atmospheric over a vacuum) is an incredible amount of force, when spread across a 6-8" square area.

I have three different pumps in my shop designed as vacuum pumps. All are piston type, one a oil bath, the other dry pumps. No one single stage vacuum pump in my shop will exceed about 24" Hg or about 12 psi. Go to Facet-Purolator's website, and their cube pumps will do a dry lift of 36" to 48" of fuel dependent upon model chosen. Lifting gasoline 36" is a touch less than 1 psi of suction. Their DuraLift pumps, designed specifically to self-prime with pumps well above the tank can lift 120" of fuel, that is a touch above 3 psi of suction. I suspect that these common pumps drawing from the tank are not capable of the vacuum RV7 is suggesting as these numbers are way lower than the 14 psi mentioned.

I am be concerned that tanks be designed so that they will not dump fuel on people and engines and electrical systems. This means we should preclude rupture. I never said that you have to preclude tank yielding or collapse, although rupture is easier to prevent if the tank does not yield or collapse.

A typical 1/4" vent line that is 4-6 feet long (common on 2 seat RVs) will supply enough back pressure that a high-flow transfer pump can create over 10 psi of *internal* tank pressure if the tank is already full, *even if the vent isn't blocked*. Again, 10 psi will destroy a typical a/c tank.

I would sure like to see a citation on this one too.

Pressure drop in the tank due to fuel coming out and air drag to fill in is easily calculated and second year engineering students get taught this in the introductory fluids class. Just to get some numbers, let's say this typical homebuilt has a takeoff fuel flow of 15 gph. 15 gallon/hour * 231 in^3/gallon * 1 Hour/3600 seconds, that is 0.96 cubic inches of gas leaving the tank every second. A 1/4" ID tube has a cross section area of .049 In^2, so the air flow in is at 19.6 inches per second, or about 1.6 ft/s. Go to

__Introduction to Fluid Mechanics__ by Fox and MacDonald. I always ran this easier in metric, so rho is 1.225 kg.m^3, mu is 1.8e-5 N*s/m^2, V is 0.50 m/s, D is 0.00635m, L is 1.83m. I get Re number of 215, so the flow in the tube is deeply laminar - we do not need to worry over tube roughness. Friction factor comes out at 0.297, head loss is 10.6 m. Air is low density stuff, pressure drop in the tube is 128 Pa. One atmosphere is 101,600 Pa, so the head loss and resulting suction is a tiny fraction of atmospheric pressure 0.018 psi. Nope, our suction in the tank due to fuel being pulled out and friction on the air in the vent line is pretty darned close to zero when using the standard engineering methods.

Some might be concerned about recovering ram pressure in our tanks from the vent line. At 60 knots, 100% pressure recovery would add 0.002378 slug/ft^3/2*(101.3 ft/s)^2 = 12.2 lb/ft^2 = 0.085 psi. At 240 knots, it is still only 1.36 psi.

I would not worry over pressure in the tank in either direction while the vents are working. But if your vent is solidly blocked, your pumps can do things to the tank. I would sure want to make sure that the tank does not rupture under those circumstances and spill fuel where it can be easily lit or asphyxiate the crew.

Billski