Fuel pump questions

[Christopher J. Bruno]

amcknig at creighteld.k12.az.us wrote:
> 
> The return line pressure is based on the vertical distance the fuel
> is pumped, not the weight of fuel in the tank.  So there would be
> more pressure if the fuel were returned to the top of the tank .
> Most gas tanks are lower than the high point of the system so the
> return line is down hill.
> 
> Bob McKnight phx az

Not really...

Think of it this way, there is higher pressure at the bottom of the
tank simply by the weight of the fuel.  (This is the same reason
that as you swim below the surface of a pool or ocean, etc. you can
feel higher pressures as you go deeper).  To pump INTO the bottom
of the tank when it is full, you would need to overcome this pressure.

The equation for the pressure of a fluid in a container is simply:

p = y*z 
where:
	p = pressure
	y = specific weight of the fluid
	z = depth within container

For water y = 62.4 lb/ft^3.  If we assume fuel is approximately
water and if the gas tank had a total height of 1.5 feet and was filled
to the top, then the pressure at the return line if the return line
was at the bottom of the tank would be:

	p = 62.4*1.5 = 94.6 lb/ft^2 = 0.65 psi.

However, if you had to pump the fuel up 1.5 ft into empty volume
then you would incur the same pressure.  So, if the fuel return
line started level with the bottom of the tank, then you would incur
the same pressure by pumping up to the top of the tank where there
is no fuel as you would as pumping to the bottom of the tank when the
tank is full.  However, since it is unlikely that the tank is always
full and that the reutrn line starts level with the bottom of the
tank then this is not necessarily valid. In fact the return line 
probably starts above the bottom of the tank in which case you might
as well just place the return line whereever it is easiest since
you won't incur that much pressure by placing at either the top or
the bottom.

But now I'm just rambling...

-Chris
84 S-10 with 350 soon to be TPI
95 SHO
95 VFR