Can a hose spray higher than the water tower?

[ChemEngrMom]

TL;DR

Can a water hose theoretically spray higher than the water tower that supplies it? In other words, is it possible for the water pressure created by a water tower to push water higher than the tower itself if a small enough diameter pipe were used to constrict the flow enough to push the water up?

As our family drove along the highway, one of my 3 college boys started commenting about the water towers we were seeing. We started a deep discussion about how water towers work and that led to a fascinating discussion about water supply in skyscrapers that are clearly taller than your average water tower. We are aware of the many wooden water towers on the roofs of buildings across the New York City skyline, but we wondered, does the Empire State Building have a water tower on top of it? Do really tall buildings use the same kind of water tower?

That led to all sorts of theories floating around about how tall buildings and big cities deal with water pressure needs.

A friendly argument developed about whether it's possible for the water pressure created by a water tower to push water higher than the tower itself if a small enough diameter pipe were used to constrict the flow enough to push the water up.

Can you settle this for us?

 

[.Scott]

The diameter of the pipe itself would not do it.

First, let's look at the "central problem" here. If the entire water stream was capable of going higher than the water level in the tank, it could be directed at the tank with energy left over. You would be getting energy for free. And since that violates conservation of energy, it can't happen.

But what if only a portion of the water went that high?

 

[phinds]

But what if only a portion of the water went that high?

Doesn't matter. You'd still be getting free energy (just put a little water wheel above the water tower) so it would still be a PMM

 

[.Scott]

I believe a ram pump will accomplish this. I am looking for a link now...

... Looks like this one is good: Ram Pump explained - video

 

[Dale]

Yes, a ram pump will do it. There is no free energy. For a ram pump to get water up above the source requires more water to go down than goes up. The center of mass of all the water still goes down.

 

[Baluncore]

For a steady flow without any mechanism the height of the fountain is limited to be less than the height of the water tower.

When flow and pressure pulses are employed, such as in a ram pump, some of the water can be projected higher than the water tower.

There are also hydraulic flow dividers, also called intensifiers, that can operate continuously like a hydraulic motor driving a pump to increase the pressure of part of the fluid only.

 

[Lnewqban]

That led to all sorts of theories floating around about how tall buildings and big cities deal with water pressure needs.

Welcome!
Pumps at the basement of very tall buildings give water the energy it needs to reach the penthouse.

Please, see:
https://www.cibsejournal.com/cpd/modules/2019-10-wat/

 

[Dale]

Here is an explanation of a ram pump. There is an explanation about the physics at 2:40, the installation starts at about 11:00, and the rest is agricultural stuff that I enjoy.

 

[ChemEngrMom]

Wow. I'm blown away. As a chemical engineer by training my gut was saying conservation of energy, but my sons kept entertaining ideas of the constriction and flow and that was showing some insight that I couldn't put my finger on. The ram pump and your explanation about flow and pressure pulses brought it all into focus! Thank you, you very intelligent people!

 

[A.T.]

Doesn't matter. You'd still be getting free energy (just put a little water wheel above the water tower) so it would still be a PMM

The rest of the water also leaves the tower, but doesn't go up.

 

[A.T.]

Wow. I'm blown away. As a chemical engineer by training my gut was saying conservation of energy,

Simple experiment: If you drop water from a cup into a filled bathtub, the splash can go higher than the drop.

 

[Dale]

As a chemical engineer by training my gut was saying conservation of energy, but my sons kept entertaining ideas of the constriction and flow

It is important to note that simply constricting the entire flow cannot work. You must split the flow and have the part of the flow going down do work on the part going back up. That is key to using the conservation of energy. If you don’t split, but merely constrict, then your gut was right.

 

[Ibix]

You must split the flow and have the part of the flow going down do work on the part going back up.

A related trick is to put a small ball in contact with the top of a basketball and drop them together. The basket ball doesn't bounce very high, but does give some of its energy to the small ball which will easily bounce above the initial drop height (wear goggles!).

Demo:

 

[DaveE]

Here is an explanation of a ram pump. There is an explanation about the physics at 2:40, the installation starts at about 11:00, and the rest is agricultural stuff that I enjoy.

Reminds me of back in the day when I had my first real job. I'd just graduated with lots of book learning about switching power supplies. But as a city kid, I'd never heard of a ram pump. An older engineering mentor was explaining them to me, when I suddenly got it and exclaimed "Oh, you mean, it's just like a boost converter!"