Fuel pump questions

[Shannen Durphey]

Orin Eman 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.
>
> > >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.

What about atmospheric pressure?  If I remember correctly, it's around 14 psi at
sea level.  If  a vented tank were connected to a small pipe in a U shape so water
were allowed to flow from the tank through the U, (something like a water
manometer) there should be a difference in height, I would think. Especially if
there were an extreme difference in surface areas between the tank and the pipe.

Although I can't explain it well, I think of the gravity aspect like this:  If the
tank of water were a falling object, the pressure at the outlet pipe corresponds
to the speed a falling object attains.  Just as a 1 pound ball lands at the same
speed as a 50 pound ball,  1 lb of h20 makes the same pressure as 50 lbs.  But the
water would have to be in pipes with the same area exposed to air pressure,
wouldn't it?   Take an extreme example, if a large (vented)tank of water were
connected to a pipe, the opposite end of which terminated roughly 25,000 feet
above the tank, wouldn't the air pressure push the water up the pipe?  Or if the
pipe ended higher up?

Hope I'm not getting too deep.  Just trying to stop the wheels from spinning so
fast.

Shannen