Re: TOTALLY OT: Pumping water with an elevated pump.

On Wed, 21 Nov 2007 07:09:10 -0800 (PST), Clay <physics@xxxxxxxxxxxxx>
wrote:

On Nov 20, 11:00 pm, Eric Jacobsen <eric.jacob...@xxxxxxxx> wrote:
On Tue, 20 Nov 2007 18:51:24 -0800, "Fred Marshall"

<fmarshallx@xxxxxxxxxxxxxxxxxxxx> wrote:
As I recall, boiling can be ignored until one asks "why?" about one of the
practicalities of it.

Exactly the process I went through. The logic got stuck for a while
on lifting a water column strictly by the pressure differential, I
suggested getting around that via priming, but hadn't taken the next
logical step to realize that the conditions at the top of the column
would lead to boiling. As soon as I saw a sort of obscure hint about
cavitation it occured to me that it'd easily go to full-on boil as
soon as there was a cavitation opportunity, and then there'd be
nothing to fully support the column.

Hayward's trick was to eliminate as many cavitation opportunities as
he could, including using a bellows for the pump rather than anything
with parts that would have exposed friction surfaces, and a few other
subtleties that were pretty creative. I think it's impressive that he
went from a 10m theoretical limit to a 17m practical limit...very
cool.

Eric's diagram Figure 3 Case A comes close enough.

You draw a free body diagram of a chunk of water.
Consider a chunk of water at the top of the water column.
The pressure at the bottom is 1 atm. or around 15psia.
The pressure reduces going up the water column until, at the surface the
pressure is zero psia.

There's no such thing as negative pressure in a gas or liquid - in the gross
sense at least. They have no tensile strength. The minimum pressure
(theoretical at that - a perfect vacuum) is zero psia.

And that's where some other interesting stuff comes in. Apparently
negative pressure in liquids has been demonstrated since the middle
1600s, and that's where the arguments for strong tensile strength
comes from. Section 4.1 in Caupin's paper that I previously
mentioned, here:

http://www.lps.ens.fr/~caupin/fichiersPDF/CRPhys_2006_7_1000-1017.pdf

describes the simple experiment to demonstrate negative pressure in
liquids. They even thought to then introduce a bubble into the
column and watch the whole thing collapse, since it's not stable in
that condition.

Hayward's pump realized something like -0.17Mpa suction in the water
to get the 17m of lift.

Now, back to that chunk of water free body diagram. If the pressure on one
side were smaller than the pressure on the other, the water would move.
y'know: f=ma, and it could be thus "pumped". But, the pressure on the top
side can't be smaller than zero. Turn things around and ask "at what height
will the pressure be zero?" Oh! around 32 feet. There doesn't have to be a
void to reach this conclusion. But, it tends to explain how a void would be
formed through some mechanism or other if one tried to pump thereafter.

One might ask: "What happens if a piston pump "pulls" on the water after the
pressure reaches zero psia?" The only thing that happens is that the piston
stops or breaks or a void is formed. If there's a void and no more water to
boil then the void gets larger and the vacuum becomes "better" in terms of
the number of molecules of gas still bouncing around in there. But it's
still not zero psia actually.

Agreed. The boiling will continue in order to maintain some low
pressure, and won't stop until the pressure rises enough to exceed
boiling conditions. It's a stable system from that perspective, and
not intuitive at first glance, at least for me it took a second
glance, at which point it became obvious. ;)

Eric Jacobsen
Minister of Algorithms
Abineau Communicationshttp://www.ericjacobsen.org

Eric,

A lot of shallow wells have a practical limit of around 20 feet.
Boiling is a main problem. But also with small pressure differences,
you don't get much flow. A household well needs to deliver at least 3
gallons per minute with 5 gpm being much nicer. Certainly one may use
a large storage tank, but one wants the well's flow rate to be able to
exceed the sustained pump rate. And irrigation wells have extreme flow
rates.

Cavitation as you mentioned is actually a huge problem. Turbulent flow
in the pipe can lead to vapor locking of the pump. Plus minerals in
the water make it a little heavier than simple fresh water. I.e. salt
water 33 feet is equivalent to frest water at 34 feet.

So the common method for a well over 20 feet deep is either a
submersable or a jet pump.

Somehow I just knew there'd be expertise on this subject in this
crowd. I've not been disappointed. ;)

Capillary action is a neat way to move water up a great height. The
California Redwood, which if I recall correctly, is the tallest living
thing and it tops out at about 350 feet. And of course these trees are
themselves growing at an altitude much higher than sealevel. So this
method works, but how fast is the transport? If you had a drilled well
(4 inch diameter) 300 feet deep, What would be the rate of production
of the well if it will filled with some material that simulated the
fiber structure of a tree? I don't know, but that answer would tell if
this approach is practical (high enough flow rate) or not.

I'm guessing the flow rate is low as well. Without a need for a high
flow rate, trees can use whatever technique just meets their
requirements.

Nevertheless, I've no idea how much water a redwood would pump in a
day. I can imagine on a hot, dry day that the structure of a giant
redwood could be losing a lot of water, but maybe they're really
efficient at minimizing loss as well.

Another thing occurred to me last night, which is that the negative
water pressure supported along the length of the tree probably
contributes to the tendency of trees to explode when hit by lightning.
I've always heard that it's the water vaporizing that does this, but
it's not just vaporizing it's essentially detonating (well, I hope the
gist of the distinction makes sense), since it's already in a
metastable superheated state in the tree. A lightning bolt comes
along and adds energy (and a lot of cavitation) and BOOM!, instant
vaporization. It doesn't even need to be heated to change the state
from liquid to vapor, it just gets triggered.

Eric Jacobsen
Minister of Algorithms
Abineau Communications
http://www.ericjacobsen.org
.

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