Pressure question

[scottyp123]

I think this might be the right forum to ask this question, its one that has been bugging me for a while now.

I'm an electrician and we all know the analogy between water and electricity, pressure = voltage, flow = current etc. and in the whole its a pretty good description, but something doesn't quite add up for me.

This is what I know about water, it cant be compressed like a gas can so you cant fit extra water inside a volume by compressing it, a water pump doesn't generate pressure as such like a generator generates voltage, it just creates flow, all the pressure that is built up in water is from something resisting its flow. Am I correct so far.

So if you take a pumped supply of water and feed it into a length of copper pipe with a closed valve on the end the pressure will build up in that pipe if the valve is closed, if you open that valve a little bit water will come out under pressure until the valve is opened all the way where it will just become free flowing again.

So this is the bit I don't understand, if you close the end valve but also insert another valve say half way down, start the pump so the water is under pressure inside the pipe and then shut the middle valve you have trapped a section of water under pressure between two valves. You can turn the pump off and the trapped water will still be under pressure and if you then open the end valve water will shoot out for a second confirming it was still under pressure.

So if there is no longer any pushing force against the water and water can't be compressed so it can fit more into the tube what is giving the trapped water its pressure, is it simply because the copper pipe has expanded enough to let slightly more water in or is some other dark force at work?

I was reminded of this question a couple of days ago whilst putting the jet was away, machine was off and I was trying to get the high pressure hose off at the machine but it was tough to turn, obviously I hadn't took the pressure off with the trigger and when it did come out it hit me in the face.

[Mave]

Hi Scott, couple of comments. You can get all kinds of pump- some deliver flow, some deliver pressure. For example it's not unusual to use a pressure raising pump upstream of a fixed displacement pump to make sure it's always fed enough pressure to operate within its design range.
Nevertheless, assuming a fixed (ish) displacement pump, your understanding is correct - it's just expansion of the copper bits that allows you to trap pressure.
Also worth nothing that tap water can contain air mixed in with it, so it's not totally incompressible.

Edited by Mave on Tuesday 12th May 14:24

[scottyp123]

Mave said:

Hi Scott, couple of comments. You can get all kinds of pump- some deliver flow, some deliver pressure. For example it's not unusual to use a pressure raising pump upstream of a fixed displacement pump to make sure it's always fed enough pressure to operate within its design range.
Nevertheless, assuming a fixed (ish) displacement pump, your understanding is correct - it's just expansion of the copper bits that allows you to trap pressure.
Also worth nothing that tap water can contain air mixed in with it, so it's not totally incompressible.

Edited by Mave on Tuesday 12th May 14:24

Thanks, that's what I thought must be the only possible explanation.

[anonymous-user]

The phrase "you can't compress water" is physically, as you might expect, wrong.

That phrase is broadly a lay-persons description of water in COMPARISON to air, where yes, water is much, much "stiffer". But water is far from uncompressable, even in it's absolutely pure state (ie with no disolved air), and just like solids, that compressibility is the inverse of that compounds elastic modulus.

Pure water has an adiabatic conpressibilty of 0.4477 GPa-1 at 25 °C.

By the way, here's a list of what we know about water:

http://www1.lsbu.ac.uk/php-cgiwrap/water/pfp.php3?...

it's really pretty staggering exactly how many physical characteristics of this compound have been explored and defined, but i guess it's a pretty important part of our life here on Planet Water, woops, sorry, Planet Earth..... ;-)

[mike_knott]

Just a couple of points to reinforce or clarify some of the above. I hope it makes sense!

scottyp123 said:

This is what I know about water, it cant be compressed like a gas can so you cant fit extra water inside a volume by compressing it, a water pump doesn't generate pressure as such like a generator generates voltage, it just creates flow, all the pressure that is built up in water is from something resisting its flow. Am I correct so far.

Not quite. Water can be compressed slightly, and pumps generate both pressure AND flow. (Strictly speaking, they generate volumetric flow and 'head,' which you can read as pressure, even though it isn't). The pump performs work on the water which increases its pressure and this causes it to flow, if there is an open path. If there isn't, it just increases in pressure, without flowing.

scottyp123 said:

So if you take a pumped supply of water and feed it into a length of copper pipe with a closed valve on the end the pressure will build up in that pipe if the valve is closed, if you open that valve a little bit water will come out under pressure until the valve is opened all the way where it will just become free flowing again.

Yes, if you pump water into a closed pipe you pressurise the water (and also compress it very slightly and cause the copper pipe to expand very slightly). When you open the pipe again, the pressure near the pump will be higher than the pressure at the other end of the pips, so the water flows. (There is a pressure drop along the pipe due to friction.) But note that when the valve is fully open, the work done by the pump still pressurises the water.

scottyp123 said:

So this is the bit I don't understand, if you close the end valve but also insert another valve say half way down, start the pump so the water is under pressure inside the pipe and then shut the middle valve you have trapped a section of water under pressure between two valves. You can turn the pump off and the trapped water will still be under pressure and if you then open the end valve water will shoot out for a second confirming it was still under pressure.

Yes, because you have pressurised the water slightly and also expanded the pipe slightly as in the previous case. It is the contraction of the pipe and decompression of the water which causes the flow.

scottyp123 said:

So if there is no longer any pushing force against the water and water can't be compressed so it can fit more into the tube what is giving the trapped water its pressure, is it simply because the copper pipe has expanded enough to let slightly more water in or is some other dark force at work?

Yes, the expansion of the copper pipe, the decompression of the water, and (possibly) the weight of the water (the 'static head') will all cause the water to flow.

Note that the energy stored within a compressed liquid is tiny. Another example: Take a pipe, fill it with water (no gas!) and close valves at both ends. Heat the water by a mere 1degC. The pressure inside the pipe rises by 2-4 bar (4 times atmospheric pressure, for a 1degC temperature rise!). Now, when you open the valve, a small drop of water will squirt out, and the flow of that drop will reduce the pressure down to atmospheric and the water will sit there without flowing. This case is similar to yours, but the cause of the flow was pressure from warming the water rather than pumping it. The end result is the same.

See here (last three lines; 2 to 4.4 bar increase for a 1degC temperature rise) for proof of the amount of pressure rise: https://www.eng-tips.com/faqs.cfm?fid=1339

Mike...

[Ayahuasca]

Right, on this water/pressure/valves thing.

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

[mike_knott]

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

[colin_p]

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

Assuming the water was flowing and then suddenly wasn't, would it not get rather a bit hotter and thus the pressure would increase?

[Ayahuasca]

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

OK, but where is the 15,000 PSI ‘stored’ ? The water cannot compress (much). The pipe cannot expand. As soon as the middle valve is closed there is nothing pressing the water in.

[Kawasicki]

Ayahuasca said:

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

OK, but where is the 15,000 PSI ‘stored’ ? The water cannot compress (much). The pipe cannot expand. As soon as the middle valve is closed there is nothing pressing the water in.

The pressure energy is stored in the elastic deformation of all materials under load.

[Ayahuasca]

Kawasicki said:

Ayahuasca said:

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

OK, but where is the 15,000 PSI ‘stored’ ? The water cannot compress (much). The pipe cannot expand. As soon as the middle valve is closed there is nothing pressing the water in.

The pressure energy is stored in the elastic deformation of all materials under load.

It’s an imaginary diamond pipe. It is not elastically deforming.

[Kawasicki]

Ayahuasca said:

Kawasicki said:

Ayahuasca said:

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

OK, but where is the 15,000 PSI ‘stored’ ? The water cannot compress (much). The pipe cannot expand. As soon as the middle valve is closed there is nothing pressing the water in.

The pressure energy is stored in the elastic deformation of all materials under load.

It’s an imaginary diamond pipe. It is not elastically deforming.

It is deforming. Diamond deforms under load. You can elastically deform a diamond with a gentle pinch between your fingertips.

[Ayahuasca]

Kawasicki said:

Ayahuasca said:

Kawasicki said:

Ayahuasca said:

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

OK, but where is the 15,000 PSI ‘stored’ ? The water cannot compress (much). The pipe cannot expand. As soon as the middle valve is closed there is nothing pressing the water in.

The pressure energy is stored in the elastic deformation of all materials under load.

It’s an imaginary diamond pipe. It is not elastically deforming.

It is deforming. Diamond deforms under load. You can elastically deform a diamond with a gentle pinch between your fingertips.

OK, for the purposes of the thought experiment, the pipe is made of a material that does not deform!

[Kawasicki]

Ayahuasca said:

Kawasicki said:

Ayahuasca said:

Kawasicki said:

Ayahuasca said:

mike_knott said:

Ayahuasca said:

You have a pipe made of a substance that will not stretch, say diamond for example, and pumped it to a pressure of say 15,000 PSI and then closed the middle valve to seal the end. As soon as the middle valve is sealed, what would the pressure in the sealed section be?

15,000 psi, unless I am misunderstanding (assuming the valves don't leak!)

It doesn't matter if the pipe stretches or not; the 15,000 psi in the pipe at the time of closing the valves would remain there.

Mike...

OK, but where is the 15,000 PSI ‘stored’ ? The water cannot compress (much). The pipe cannot expand. As soon as the middle valve is closed there is nothing pressing the water in.

The pressure energy is stored in the elastic deformation of all materials under load.

It’s an imaginary diamond pipe. It is not elastically deforming.

It is deforming. Diamond deforms under load. You can elastically deform a diamond with a gentle pinch between your fingertips.

OK, for the purposes of the thought experiment, the pipe is made of a material that does not deform!

Now we're talking!

So, can energy be stored as pressure in an incompressible fluid in an undeformable container.

My first thought is that no, it can't. Infinitely stiff objects don't deform, and therefore can't store and/or later release energy, etc.

I'm also assuming you haven't heated the system...or increased the height or added motion.

I'm just an engineer though... so maybe I'm missing something.

[mike_knott]

colin_p said:

Assuming the water was flowing and then suddenly wasn't, would it not get rather a bit hotter and thus the pressure would increase?

No, I think that in Ayahuasca's post the downstream valve is closed at the start (so the fluid isn't flowing) and the upstream valve is closed later, so there is no change in velocity.

[mike_knott]

Kawasicki said:

Now we're talking!

So, can energy be stored as pressure in an incompressible fluid in an undeformable container.

My first thought is that no, it can't. Infinitely stiff objects don't deform, and therefore can't store and/or later release energy, etc.

I'm also assuming you haven't heated the system...or increased the height or added motion.

I'm just an engineer though... so maybe I'm missing something.

I'm also just an engineer (so I could also be missing something!) but to me, incompressible only means that its density is not dependant on pressure. It doesn't mean it can't be under pressure.

In an ocean of incompressible fluid, could the bottom really be at the same pressure as the surface?

Mike...

[Kawasicki]

mike_knott said:

Kawasicki said:

Now we're talking!

So, can energy be stored as pressure in an incompressible fluid in an undeformable container.

My first thought is that no, it can't. Infinitely stiff objects don't deform, and therefore can't store and/or later release energy, etc.

I'm also assuming you haven't heated the system...or increased the height or added motion.

I'm just an engineer though... so maybe I'm missing something.

I'm also just an engineer (so I could also be missing something!) but to me, incompressible only means that its density is not dependant on pressure. It doesn't mean it can't be under pressure.

In an ocean of incompressible fluid, could the bottom really be at the same pressure as the surface?

Mike...

It can be under pressure. It just can’t store any energy. So at the bottom of a large column of incompressible fluid subject to gravity the pressure will, as you wrote, be higher. But because there is no deformation, there is no stored pressure energy.

If I had a 10km long infinitely stiff sealed pipe filled with an Incompressible fluid in space, then brought it to earth, and mounted it vertically... the pressure at the bottom of the pipe would be higher... and the pressure at the top of the pipe would be less. The overall average pressure would remain constant (obviously ignoring temp change between space and earth)... and the pressure energy stored in the pipe also constant.

I think.

[anonymous-user]

Todays earworm:

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

[Kawasicki]