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Comment by wishawa | Hacker News Reader

Pumps might only be able to suck water up 26 inches, but there is no such limit on how high they can push water.

I believe the suck-height limit is more than 26 inches. From memory (it's been a long time) the theoretical maximim for 1 atmosphere is 10m. Practically, because of inefficiencies a lot less.

Annecdotally I have a well-point (pump at top) that lifts water 2 to 3 metres. And I have an agricultural pump that sucks about 4m,and then pushes water up a hill the next 50 vertical metres or so. And its a pretty small pump.

I think you are out by a factor of 12, it's around 26 feet. Or maybe you are thinking of a column of mercury in a vacuum which is about 26 inches.

>there is no such limit on how high they can push water.

Of course there's a limit. If there were no limit we'd have infinite free power generation.

You’re missing the fundamental difference in the power vs distance curve. In the case of sucking water, it flatlines and no amount of additional power can move the water another inch once you’ve exceeded the pulling power of a perfect vacuum.

Without looking up the actual equation, I would guess that this is never true (or at least practically never) when pushing water, because you’re not limited by atmospheric pressure.

When pushing you have to provide enough energy to move all the water in front of you, but if you do provide that energy the water will move. At some point I suppose the pressure will get so high the water will solidify in the pipe though?

>You’re missing the fundamental difference in the power vs distance curve.

You're missing the fundamentals of physics.

>At some point I suppose the pressure will get so high the water will solidify in the pipe though?

Before (way way way before) the water solidifies you will reach the maximum pressure that a given pump can deliver.

If you think the little 1/2 horse sump pump you can buy at home Depot will push a column of water up to the stratosphere then I have a bridge to sell you.

Who said anything about 1/2 a horsepower? I’m talking about the most powerful pump in the universe, both (known and unknown). How far up an unbreakable column can that pump push the water? And then, what if you make it just a little more powerful, can the water hypothetically be pushed higher?

If there's a limit on how much positive pressure a pump can generate to push water above itself, then what approximately is the value of that limit, and how would a pump capable of exceeding that limit allow you to construct a perpetual motion machine?

The limit depends on the pump. If you're pushing water uphill the weight of the water will eventually equal the maximum pressure that the pump can deliver and you won't be able to push any further.

This is basic high school physics.

You've completely misunderstood this this conversation. The limit on the height from which a pump can suck water is a limit that applies to all pumps, no matter how powerful. There's no comparable limit for pumps pushing water upward. Yes, each particular pump will have its own limits, but there's no comparable overarching fundamental limit.

Thank you, you’ve explained this better than I could

Isn't pushing water up doing some work? And then it coming down can do same or less work? Just because pump can push without limit, it doesn't mean it's free.

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