Can pump water up but can't pull water up. Explain.

As I understand it, water (or any liquid) can only be pulled up so high by a pump no matter how powerful. A way to overcome this is to pump the liquid up from the bottom.
Is there a simple explanation as to why this is true?

The explanation is that ALL pumps work due to something pushing.

In the case of a pump trying to suck water up, the best that any pump can do is to decrease the suction pressure down to zero, i.e. vacuum. It’s the surrounding air pressure that then forces the liquid up, and the highest that one atmosphere of air pressure can force a column of water up against a vacuum is 33.9 feet. This is the highest that any pump can theoretically “suck” water.

If you put a pump at the bottom, and force water out the discharge side, there is no theoretical limit to how high you can pump.

What if you put it on a treadmill?

Pumps that pull water up work by reducing the air pressure above the water, sucking it into the pipe. The pump works by a difference in pressure - usually between the air and the vacuum created by the pump. The greatest difference occurs when the pump removes all the air from the pipe, reducing the pressure to zero. This maximum difference limits the height of the column of water that such a pump can support.

As an example, suppose you’re using this type of pump to raise water from a well, and that the air pressure is 14.5 pounds per square inch. If you put a long pipe into the water and removed all the air from it, the difference in pressure between the inside and outside of the pipe would be 14.5 PSI. The water would flow up the pipe until the pressure created by its weight was equal to the same value (14.5 PSI). If my calculations are correct, the height of the column of water would be about 418.7 inches, or 34 feet, 10.7 inches. The height of the column of water cannot go higher using this type of pump under these conditions because it’s not possible to reduce the pressure to less than zero.

A pump that pushes water in from the bottom doesn’t necessarily have such a limitation. There will be some sort of limit - no pump can support an infinitely high column of water - but a pump that doesn’t depend on atmospheric pressure won’t be limited by atmospheric pressure.

Actually, it is possible to pull water up a very long distance. Trees do it. They can build up enormous negative pressure, which is possible because water molecules stick to each other the way the molecules in a rope do (although obviously not as well).

You might need a very thin, ie capillary, pipe (trees do it was channels that are as wide as cells) and you also need a pump that sticks to the water. If the water doesn’t stick to the pump itself, you’ll get that vacuum bubble again.

Snipers blow out your brains. Duck!


Isn’t there a correction factor in there for elevation above sea level?

A small pitcher pump, the one with leather cup and check(shallow well sink mount),
Is there a limit to what this pump will lift ? The reason I ask, I have one on my well at the Cabin, and I can hardly pull any water with it. I have water at 14ft. in the pipe, and i can pour water down the pipe as fast as i can upturn 5 gal pails. (pipe is 1 1/4"). w/screen. I was set up to blast the screen clear with compressed air, but water would freely flow down and out the screen.I did do a blast just out of frustration, as it is all i can do to pull water out of this well :confused:
The neighbor 200 ft away can pump his with the same pump like nobody’s business!

Yes there would be a correction factor, but I don’t know what it is. Air pressure decreases as you go up in altitude. Therefore there would be less of a pressure differential between a perfect vacuum inside the pipe and the outside of the pipe.
If you can’t draw with your pump and your neighbor can, it looks to me like your pump needs attention. Either the leather is leaking, or the non-return valve is leaking.

No its working to the best of its ability. Its such a hard pull I have to use both hands on the handle. Its just plane hard to pump, and then just a small amount of water per stroke of the pump.

Its acting like, if you will. That a vent would need to be drilled right next to this well. I know that sounds crazy, but this has been frustrating.
I want a pitcher pump on the counter by the sink, and its not going good.
This is a remote cabin, No power very peaceful. That is until a neighbor starts his generator, Then i wish there was power :mad:

Have you checked the leather cup? Sounds like it isn’t making a good seal.

You can actually pull considerable negative pressure on a liquid, if it is pure enough. That is to say, liquids do actually have tensile strength. What they have almost none of is notch strength or resistance to tearing.

IIRC the tensile strength of water is a few hundred PSI, so you could suck water up perhaps a hundred feet or more.

Trouble is, real water typically has particles and bubbles in it, and they will propagate a tear in the water with almost no tensile stress, and the water will part. But pure enough water will be strong.

This effect is useful in ultrasonication. Tiny enough regions in water happen to have no particles in them and can feel considerable stress, then something opens somewhere with explosive force. This mechanism lets ultrasonic cleaners do their job.

This makes absolutely no sense. What is keeping your 100’ column of water “stuck” to the lifting mechanism? Superglue?

Hydrogen bonding. The summation of a very large number of very small magnetic attractions. It gets more complicated from there.

Ellucidate. Hydrogen bonding would seem to keep water attracted to water. That doesn’t mean the water is attracted to the walls of the column.

From the article:

Certainly hydrogen bonding occurs in water, but there’s no reason to think that it only occurs in water.

There is no reason to believe it happens regularly among all sorts of substances, either. After all, you don’t see water just hanging on to things all over the place, now do you? :wink:

>After all, you don’t see water just hanging on to things all over the place, now do you?

Sure, I do. It’s been raining. Everything I can see outdoors has water hanging on to it.

Spray water at a candle, and you can even see it hanging on wax.

I think that van der Waals attractions are also important.

water-to-water is cohesion, water-to-non-water is called adhesion. both bonds can be very strong.

come on guys… this is junior high physics here

Temperature also influances how high you can suck the water.

Once you drop below the teperature dependant vapor pressure of the water, you get only vapor out the top. This is a gross manifestation of cavitation.

Note that even ice has a measureable vapor pressure, where it sublimates, so the lift will always be a bit less than available due to atmospheric pressure.

Yeah, but you don’t see 100’ of water hanging…