Fuel pump questions

[Joe Boucher]

Squash 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
> 
> So you mean to tell me that you can just as easily pump water up a 3/8"
> pipe 75' straight up and into the bottom of a water tower and that 3/8"
> line will have LESS pressure on it than the same line going to the top
> of the water tower and dripping in through the top?  I've been wrong
> before, but I really don't think this is right.
> 
> If you put the fuel return line into the bottom of the tank, wouldn't
> that be just like pinching the line and creating more pressure?  Or the
> same as using a smaller fuel line diameter?
> 
> Please do explain to me as i don't really understand this i guess...
> 
> --
> Andy Quaas
> p.s. !!!!NO FLAMES INTENDED!!!!

Potential Energy = g x h
g = gravity
h = height
Throw in a density of the fluid with the P.E. and you have pressure, if
I remember right.

I don't have my fluids book here with me so I can't put a bunch of neat
looking symbols and equations but the undergraduate type calculations
for fluid flow only take into consideration the height or column and not
the width of the tube or pipe.  When you starting talking about flow
then the resistance of the tubing comes into play.  This is dependent on
a host of factors including the tube size.  A one inch diameter tube 100
feet high will have the same pressure at the bottom as a tube 100 feet
wide and 100 feet high.  Or at least real close.  The total weight of
the whole system is different but the pressure at the bottom of each is
the same.

I'm a little fuzzed at the moment or I would come up with an electrical
analogy.  Well, pressure and voltage are analogous but I can't think of
a height analogy.

Joe Boucher
'70 RS/SS Camaro  '81 TBI Suburban