Can you increase P in water via air pressure? 18

[Mechbob1]

Please help settle a discussion…
As per the picture, can you increase the pressure in the water by increasing the pressure in the air above? It’s a sealed container.
If yes/no, why?
I thought you couldn’t, due to P equaling rho x g x h. But someone else reckons you can, and I’m second guessing myself.
Thanks.

 

[Latexman]

Yes. Consider the pump sprayer.

Why? Open your high school physics book.

Good Luck,
Latexman

 

[Mechbob1]

Good point… I feel daft now.

Then, if I wanted to simulate, for example, 1m of depth I’d just increase the pressure by 1 ATM/14.7psi?
That sound right?

 

[LittleInch]

No.

Pressure of 1m of water is more or less 0.1 bar / 1 ft of water is 0.433 psi.

Pressure from a fluid at a certain depth is P (Pa)= height x density x gravity, so 1 x 1000 x 9.81 = 9810 Pa or 0.0981 bar

Just remember that pressure acts in all directions at the same time in the same place.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Latexman]

From my old PADI scuba training:

Surface 1 atm or 1 bar
10 m 2 atm or 2 bar
20 m 3 atm or 3 bar
30 m 4 atm or 4 bar

Good Luck,
Latexman

 

[bimr]

Mechbob1 (Mechanical)(OP)Please help settle a discussion…
As per the picture, can you increase the pressure in the water by increasing the pressure in the air above? It’s a sealed container.
If yes/no, why?
I thought you couldn’t, due to P equaling rho x g x h. But someone else reckons you can, and I’m second guessing myself.
Thanks.

Yes, you are second guessing yourself, as the increasing pressure comment is in error. Also, don't believe that your question is written clearly. Perhaps you can restate it.

Please help settle a discussion…
As per the picture, can you increase the pressure in the water by increasing the pressure in the air above? It’s a sealed container.
If yes/no, why?

No, you would be increasing the pressure in the sealed container, not in the water. Water is considered to be incompressible material like steel and other materials. If you put a piece of steel in the container instead of the water, would the steel pressure increase? No.

You also have not increased the static pressure either. Static water pressure is the result of the weight of all the water above pushing down on the water below. As you go deeper into a body of water, there is more water above, and therefore a greater weight pushing down. This is the reason water pressure increases with depth. You are not increasing the water depth.

Yes. Consider the pump sprayer.

Why? Open your high school physics book.

Not really correct. You haven't increased either the kinetic energy or static pressure as the container is sealed and the depth has not changed.

A pump sprayer creates flow. A pump does not create pressure, it only creates flow (kinetic energy). Pressure is a measurement of the resistance to flow. Figure 2.

https://www.pumpsandsystems.com/cha...surement of the resistance to flow. Figure 2.

 

[hydtools]

DeltaP causes flow. Without deltaP there can be no flow.

Ted

 

[bimr]

DeltaP causes flow. Without deltaP there can be no flow.

Also incorrect post.

Review the link above, a pump doesn't create "DeltaP", the pump creates flow (kinetic energy), not pressure. Pressure is a measurement of the resistance to flow.

Also, there is no change in the the head (potential energy) due to an water elevation change.

 

[Compositepro]

The only good answer so far is the first one from Latexman. The others are either just wrong or confusing. How can that be from a group of practicing engineers? The pressure vs. flow thing is just a chicken vs. egg argument and not even relevant to the original question.

To be clear, increasing the air pressure above the surface of the water will increase the pressure in the water.

 

[TiCl4]

How can that be from a group of practicing engineers?

A common problem with engineers - missing the forest for the trees. Too much thought put on a tangent idea (using a pump to increase pressure) rather than addressing the OP's question.

OP, the rho*g*h only determines the change in pressure due to height. Usually, this is ignored with vapors due to the very low density.

The pressure at the bottom of the liquid will be the pressure of (1) in your picture, plus rho*g*h of the liquid (2).

If your colleagues don't believe you, have them fill a syringe half-way full and put their finger over the opening. Tell them if they are right and increasing pressure (1) will not increase pressure (2), then they should be able to hold the liquid inside the syringe no matter how hard you push the syringe piston.

Proceed to press the syringe piston and watch the increase in (1) cause an increase in (2), spraying water everywhere. Bonus points if you have them hold it up to their eye for examination during testing.

 

[bimr]

The only good answer so far is the first one from Latexman. The others are either just wrong or confusing. How can that be from a group of practicing engineers? The pressure vs. flow thing is just a chicken vs. egg argument and not even relevant to the original question.

To be clear, increasing the air pressure above the surface of the water will increase the pressure in the water.

Really? I will assume that you never took Hydraulics 101. Otherwise, with that post you would have failed the course.

In fluid dynamics, total pressure refers to the sum of static pressure p, dynamic pressure q, and gravitational head, as expressed by Bernoulli's principle. Have any of these changed for the incompressible water in the sealed container?

"Thus, for many practical purposes water is incompressible with change of pressure."

Hydraulics 101

"A centrifugal pump, in its simplest form, consists of an impeller rotating inside a casing. The impeller imparts kinetic energy to the fluid."

Hydraulics 101

"In fluid dynamics, total pressure refers to the sum of static pressure p, dynamic pressure q, and gravitational head, as expressed by Bernoulli's principle"

Link

Zero change in static water pressure, zero change in kinetic pressure and zero change in gravitational head.

 

[Mechbob1]

Where I (think) I went astray was only including gauge pressure. Ie, pressure (absolute) = gauge (internal P) + Absolute (or, pressure at the top of the fluid).

But, now I read bimr’s post I’m again uncertain!

 

[bimr]

Just remember, water is incompressible with change of pressure. When you open the sealed container from the top, pressure is released from the compressed air, not released from the water.

Hydraulics 101

 

[bimr]

TiCl4 (Chemical),

I will admit the question is not clearly stated, but nonetheless you are also in the forest.

"As per the picture, can you increase the pressure in the water by increasing the pressure in the air above? It’s a sealed container."

Nobody mentioned flipping syringes or spray guns as another poster stated. Also, pumps don't increase pressure, pumps impart kinetic energy. Pressure is a measurement of the resistance to flow.

The answer is Hydraulics 101. Water is incompressible with change of pressure.

 

[Mechbob1]

I’ll rephrase/provide more clarity regarding the exact question in a few hours.

 

[TiCl4]

Bimr, I’m afraid it is YOU who is missing the point. If you arbitrarily increase P1 (by whatever means), then P2 will likewise increase. It doesn’t matter HOW P1 is increased, just that it is increased.

If P1 is 1 atm and the pressure at the bottom of P2 is 1.1 atm, then increasing P1 to 2 atm will increase the bottom of P2 to 2.1 atm.

The syringe was just a fun way to imagine OP’s scenario, as once you have the water and air in the syringe, it acts like a closed system (not exactly, once air or water starts leaking, but close enough).

If your point is that it is sealed, then just heat the air inside to increase. Again, it doesn’t matter how that is accomplished.

Yes, water is almost incompressible. So what? That doesn’t mean you can’t increase the pressure in P2 by increasing P1, just that increasing P1 has almost no effect on the water’s volume (at reasonable pressures).

 

[bimr]

TiCl4 (Chemical)

Really, the question is "can you increase the pressure in the water".

Water is incompressible with change of pressure. Since it is incompressible, the water pressure can't increase or release pressure.

Steel is also incompressible. Are you implying that one can increase the pressure of steel or other incompressible materials?

The pressure in the container is from the compressed air, not the water. Water adds no pressure other than static pressure from the water depth.

 

[TiCl4]

Bimr,

Incompressible means volume does not change with pressure. Your statement that water can’t “release pressure” is nonsensical. Following your logic, I could put a rupture disc on the bottom of the container at P2 + 1 psig, increase P1 to 20,000 psig, and not blow the disc. That’s simple ridiculous. Likewise the concept of pressure is not the same when comparing fluids to solids, so using steel as an example doesn’t make sense either.

Let me be more clear. If you put a pressure gauge on the bottom of the container and increase P1 (either by adding more air to the container or by heating the container) the the pressure gauge will increase in pressure. And yes, all if the increase will cone from increasing P1, but the total pressure at the bottom of P2 will ALWAYS be:

P2 = P1 + rho*g*h

Edit for clarity: P2 is the pressure at the bottom of P2 water phase.

 

[bimr]

TiCl4 (Chemical)

You are not clear. The question is "can you increase the pressure in the water". That is impossible.

Let me be more clear. If you filled the container completely with water, is it possible to increase the pressure. The answer is no. You can't increase the pressure to 20,000 psig because the container is completely full and the water is incompressible. Other than static pressure of course.

Incompressible means volume does not change with pressure. That means also that water can't release pressure if the container was opened, other than static pressure of course.

Your examples are off the mark.

 

[bimr]

Didn't anybody take Hydraulics 101? It's a required course in Civil/Environmental.