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.

 

[bimr]

The OP is talking about static pressure.

That is your assumption that is not stated in the original question.

My first post stated that the original question was not stated clearly and should be revised. Why don't you wait until the OP revises the question.

 

[danschwind]

Holy shit. I saw this post yesterday and thought it would end at Latexman's second post. Enter here today and 40+ messages!!!

Btw you can have pressure with no flow

Good luck to all!

Daniel
Rio de Janeiro - Brazil

 

[Latexman]

Unclear questions bring out the pedant in us.

Good Luck,
Latexman

 

[GBTorpenhow]

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.

bimr, You absolutely can increase the pressure in a closed volume containing only liquid. This is a very common method of pressure testing piping and vessels using water.

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"

Look carefully at this quote. The problem is that you are confusing gravitational head with static pressure. Based on your displayed field that isn't surprising because of the use of the term 'hydrostatic pressure', probably frequently shortened to 'static pressure', to mean gravitational head (rho*g*z).

Atmospheric pressure can be neglected in most examples involving water exposed to the atmosphere because the water static pressure is significantly higher than atmospheric pressure.

No, this is not why atmospheric pressure can be neglected. It can usually be neglected because we tend to deal in gauge pressures where atmospheric pressure is by definition zero. From your Hydraulics 101 link:

p-p_a = rho*g*h
and almost always we measure pressure as gauge pressure, relative to the atmosphere, as shown in Figure 2-4, so that we usually just write
p = rho*g*h (2.6)

In the above, the subscript a refers to atmospheric pressure because the system is open to atmosphere, so the pressure above the liquid column is always atmospheric. But in a closed system p_a can be something higher than atmospheric pressure and can't just be eliminated. Eqn 2.6 as presented in your link is dealing with a very specific subset of Bernoulli where there is 0 velocity and 0 static (gauge) pressure. In this case, the total pressure does reduce to just the gravitational head.

In a closed system eqn 2.6 above becomes p = p_a + rho*g*h or (total pressure) = (static pressure) + (gravity head).

 

[1503-44]

Latexman is absolutely correct.

It's also why you have to equalise the pressure in your ears almost every foot between 0 and 10ft depth, then not so often and even less as you reach lower depths. BTW I think I still have the record for highest score on the written PADI Diver College Instructor exam. I know, because 100% is the max.

Water has a measurable bulk modulus, so yes, it compresses as well.

https://www.engineeringtoolbox.com/bulk-modulus-elasticity-d_585.html

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Joe591]

Of course you can. The only reason why you wouldn't want to do that is because air pressure can be a bit dangerous, it also depends on how much air pressure you are talking about. 1 bar might still be okay, but can still hurt you if you're not careful. If it's anything more than that I'll rather move away from doing it at all. It also depends on what your setup is and how well your setup is suited to pressure.

If you want to avoid air pressure you can use a hand pump to force more water in than the container can take. Water is actually compressible. We routinely pressure test boilers by filling them to the top with water and forcing some more water in with a handpump until the pressure gauge reaches the required pressure. A lot of people will tell you that water is not compressible, but it's actually just a very, very, very stiff.

Obviously the highest pressure will be at the bottom of the water because that's where all the weight is.

 

[Latexman]

Folks,

I think since the OP was not worded clearly, bimr took it literally that the container was sealed. Therefore, they could not add more air to the container. It's not the way I took it, but it's a perfectly understandable basis. bimr, is that right?

Good Luck,
Latexman

 

[1503-44]

Both the air and water pressure in a sealed container could be increased by heating.

If there any remaining doubts, see any typical pump NPSH calculation taking water supply from a pressurized tank. The calculation done at the point of entry to the pump inlet, well away from the tank, indicates that the water's pressure includes the tank's pressure, not only water density x height of the water column.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Latexman]

And, that's why we should not be limited by the way someone writes their question. Think outside the box, or in this case, the sealed container!

Good Luck,
Latexman

 

[IRstuff]

Sure, note that a hyperbaric chamber essentially does what the OP describes.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[Mechbob1]

Apologies for the delay.

I feel I've made several mistakes here:

1. Posed an unclear question (noted, will try and not let it happen again). This likely mislead bimr - apologies.
2. Made a silly assumption at the start, forgetting about P above the surface of the water.

For context, work is attempting to simulate items' reactions at 1m of water depth, ideally using the minimum amount of water. Also, they're cautious about having a 'pressure cooker' type of device. This is despite the container still being somewhat 'pressurised', as per the attached image (current solution).

We were discussing the idea of just pressurising the sealed container, rather than having a tube of water.

Anyway, thanks for all the responses. For what it's worth, I learned from it.

Regards, Bob.

 

[1503-44]

You might add a valve to the lid to allow any air to escape as you fill the tube and the pot. Close it when water starts coming out of the valve and continue to fill the tube to the 1m height. Compressed air eliminated.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[LittleInch]

Or regulate the pressure in the air space to 70mbar = 0.7m

Many industrial systems use a low pressure gas blanket system of a similar pressure.

The tube is quite a simple system though. If you have a bigger tank at the end compared to the tube it would be easier to fill and maintain the right pressure.

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

 

[Frederic Martin]

bimr,

Your lack of understanding basic physics is shocking to be honest.

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.

Water is indeed compressible, not at lot, but it is compressible. That being said, the (in)compressibility is completely irrelevant to whether the pressure in the water will increase or not if the air pressure is increased above it.
And it also doesn't matter if you replace the water with a piece of steel. Will the steel "feel" the increased air pressure? Of course it does, it is called material stress due to an externally applied force (pressure times area). The pressure inside the steel increases, due to a balancing effect between the internal forces and the externally applied force (being the air pressure above it). If the steel or the water would not "feel" the pressure increase, it would mean the whole system would be out of mechanical equilibrium and the bottom of the container wouldn't feel this pressure increase (and that doesn't make much sense, doesn't it)?

If you add compressed air to the container half filled with water, the air is on top of the water and that causes the pressure to increase on the bottom. From the compressed air not the water.

It really doesn't matter where the initial increase of pressure is coming from. The increase in pressure will spread throughout the complete container.

https://www.eng-tips.com/faqs.cfm?fid=376

 

[pierreick]

Compositepro, Frederic martin,
Why do you need to be rude ?
The forum is a place for sharing not fighting .
My view
Pierre

 

[Frederic Martin]

@pierreick (Chemical)

Hi Pierre,

I don't see how I have been rude, at least compared to the rudeness in the posts of bimr. I count 18 posts from bimr in this thread, all of them wrong. As far as I understood, eng-tips is not an opinion forum, but a forum that should be based on science. Everybody can be wrong (me included), but persevere in stating wrong statements 18 times in one thread, requires a less diplomatic reply, wouldn't you think?

https://www.eng-tips.com/faqs.cfm?fid=376

 

[GD2]

Air over water is very commonly used for testing relief valves in shops. This way, you minimize the volume of air in the test bench, which obviously is more hazardous than water volume.

GDD
Canada

 

[Mechbob1]

You might add a valve to the lid to allow any air to escape as you fill the tube and the pot. Close it when water starts coming out of the valve and continue to fill the tube to the 1m height. Compressed air eliminated.

Thanks - a great idea.
The only downside I see is that it's another area where the system can 'leak', but it's likely to be a small surface and simple to seal.

 

[1503-44]

You have very low pressure. You might get 2 drops at worst.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

 

[Joe591]

If you are worried about leaks from the little air purging valve then maybe just get a little screwed plug that you can screw into it with thread tape. It helps. It probably won't leak though