# Hole where water cannot escape at certain pressure?

1. Jan 16, 2009

### j7on

Hi eveyrone!

I am completely new to this forum but didnt know where else to turn for a "problem" of mine..

I am trying to design a new kind of waterpump and still in the teory part of the design, i would be interested to know how i can calculate the "thickness" of water at a certain pressure?

How small must a hole be that water cannot escape at certain pressure?

Does it matter if it is a direct hole or a gap that is exactly as wide as the hole(do many same sized holes require the same amount of pressure as only one)?

How much does water temperature affect outcome?

Hard to explain, but then again, i am no physist...;)

2. Jan 16, 2009

### Dr Lots-o'watts

If I understand correctly, you are looking for a relation between water pressure and rate of escape through a fixed-diameter hole. Then, by decreasing the pressure, you want to reach a rate of escape that you can consider to be negligible. The pressure causing this threshold rate is your unknown.

By decreasing hole diameter, you will lower the rate, but not "prevent" it completely from escaping. Ultimately, water will evaporate a molecule at a time for as long as its dryer outside.

As for temperature, staying clear of the freezing level is probably your main concern for now.

3. Jan 17, 2009

### j7on

Yes, i guess, i mean the problem is quite simple, i just dont know how to calculate it.

Imagine a syringe with a large needle hole, there is quite little pressure to apply to push water throught it, then, decrease the size of the hole until you reach your desired pressure but water is still not coming out(apply even more pressure and it will ofcourse).

I would just like to know how i can count how thick water is at some pressure.

Example: How small must a hole be that water cannot go throught it at a 10bar pressure(but will at more)?

Now, the second question, if the hole would be like 1/50th mm, will i have the same result if there is a 1/50th mm gap instead of hole or will water sip out before i reach 10bar?

Damn, the hardest thing is to explain(and English is not my first(or even second) language so.. bare with me)

4. Jan 17, 2009

### Staff: Mentor

Dr Lots o Watts (cool name) is correct: there is no such relationship. Water will come out of a syringe at any pressure, at a velocity determined by Bernoulli's equation.
Velocity is pressure dependent only. The area of the hole times the velocity gives you flow rate.

5. Jan 17, 2009

### Staff: Mentor

Sorry, guys - I got a spyware/malware attack that coincided with my posting in this thread and deleted it temporarily as a precaution....it looks ok, so I restored it.

6. Jan 17, 2009

### rcgldr

At some point of diminishing hole diameter, it would seem that friction, viscosity and skin effect of the water would effective prevent low pressure flow (except for the evaporation effect), or reduced the flow to insignificant, depending on the definition of insigificant.

7. Jan 18, 2009

### LURCH

Is there a reason why a simple pressure valve would be undisirable?

8. Jan 18, 2009

### Staff: Mentor

Surface tension will do it at very low pressure, sure.... not sure if that's what the OP was going for, though.

9. Jan 18, 2009

### LURCH

As a compramise between the pricisely sized hole and the pressure valve, you could try a surface made fo rubber or some other very elastic material, riddled with holes that are hld closed by compression. At sufficient pressures, the water would force its way through the holes.

10. Jan 18, 2009

### Loren Booda

What limiting properties does an osmotic membrane and solution have that would cause water to travel one way?

11. Jan 18, 2009

### Emreth

12. Jan 20, 2009

### QuantumPion

This is actually how some pumps are designed, they use what is called a Labyrinth seal. I know that the shafts of nuclear reactor coolant pumps are designed this way (very large 2250 PSI >8000 HP pumps).

13. Jan 26, 2009

### j7on

Well i was thinking of using a ceramic ball inside a ceramic tube(ceramic cuz it does not wear out and leaves no taste trace cuz there will be water inside).

Caeramic balls & tubes can be made with very small precision tolerances, a hydraulic shaft underneath would push the ball creating pressure on top where the water is, now is it even possible to make a clearance that is so small that water can reach temporary pressures(1 min. max.) of 10-20bars and still be able to move freely(return down with just gravity)?

???

14. Jan 27, 2009

### QuantumPion

It sounds like you are trying to make a positive displacement water pump, using a piston like a car engine. You might want to google some manufacturer web pages for similar pumps and see what seal designs they use.

15. Jan 27, 2009

### j7on

Its just that, there isnt any, i am thinking if the gap between the ball & the tube be so small thus creating a natural seal for some time without the need for an actual seal.