Fuel pump questions

[Orin Eman]

> >> 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. 


> >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.

Assuming that the pipe doesn't create a significant restriction, he is
correct!!!  Pressure depends _entirely_ on the height of a water column,
it doesn't matter if the column is 50' wide (water tower) or 3/8" wide (pipe).

Put symbolically, p (pressure) = d (density) * h (height of column).

In the water tower example, look at the picture at the bottom of the
water tower.  The line going directly into the tank has the pressure
due to the depth of the water in the tank, say w for water.
The line going to the top would have pressure due to the water in the
line which is going to be 'deeper', let's say w + x (x for extra height).

So, pressure in the line going to the top is d * (w + x) and to the bottom
is d * w.  The extra pressure to overcome by going to the top
is d * x.

Consider this.  The pipes going to the tank are joined.  One branch
goes to the bottom, the other to the top.  What happens when you start
pumping?  Do you get anything out of the open pipe at the top?

BTW, I hadn't thought of all this before, and the result is
counter-intuitive, but the physics is correct.  You have the lowest
pressure to overcome if the return is to the bottom of the tank.
This is true even if the source is higher than the tank since the
pressure from column of fluid in the pipe cancels out the pressure
due to the fluid in the tank.  (Again assuming that the pipe
isn't restrictive.)

Orin.