Gas and Liquid Pressure in a Closed System

Your post doesn't mention if the water/air mixture is stationary or moving.

If stationary in a closed vessel or pipe, there is a trend that both water pressure and air - vapor pressure go to an equilibrum, but that happens only over a time permitting the 2 media to reach the same temperature. Since the difference in thermal mass is high

short delta t's can cause different pressures in the resp. media.

Excellent point. I can see how the different heat transfer coefficients would allow for short term pressure differentials. Also, the dynamic aspect could cause local static pressure spikes.

Still, I believe we are talking about slow moving, low volume, uniform temperature conditions.

-A-

This scenario calls for a slightly different approach:

The nature of the "line", you call it is also important and can cause different pressures , since air can be compressed and water can't. Flow characteristics in certain places in the line like T's, elbows e.g. can create different pressures.

I refer to e.g. a water line in the house that has been serviced and had to be drained before doing that. To put that line back to use can cause a lot of noise and vibrations, before it is free of air. This can put a lot of stress on the pipes too, sometimes leading to failure in couplings or glue points.

balony

I'll approach this as a practical problem.

If the premise of the thread IS correct, a trompe would not operate. The pressure in the air reservoir would stop the water inflow, rendering the device inoperative.

However, a properly designed trompe does work, they are all over the place.

if somehow psi in a flow and some cyclic issue arises, intermittently some change in direction, holding a fluid longer than a gas and in a changing diameter of piping(s).. - maybe something, but below ~ 15 fpsec? same dia, etc with air? I would believe small glitches as to any steady state un-equilibrium existing, usually. ( so, Nah...), le'me see the gauges and learn of any unforeseen situation.

Using water flows and pressures of 1 psig to 130 and velocities of 1.1/2 fpsec and to "flushing not more than 4 fps" needed,

geothermal (near Earth surface systems) for loops of Earth-Coupling to a heat pump have ONLY and EVER shown identical air and water pressures flowing through the lines. - velocities under 3fps.

I have noted in any configuration , somewheres around 3 ft per second velocity, t-offs are affected by the inertia of the water flow to a point of 15% and 20% differences...? NO! maybe 1-3% relative to angular accelerations, like that of HVAC ducting T-off's have a speed number... relating to why air passes by a T , all under pressure (or creates negative pressure in a "draw").

noticeable over 6 or 7 ft per second and certainly at 15- 20 fps.. Too much variation in water and air movements together or isolated, was that that occurred in testing lines at a fiberglass tank OEM, that moved a few gal of fluid from 60 to 120 psig in 3/4" and 1/2" lines in 3 seconds T'd off of header line. operator simply corrected by equalizing T- constructs for equal inertial affects, to test 8 tanks at once on small , higher velocity lines.

Note:

how long will a 3" galv line last at 2200 GPM back and forth open reservoir for testing use, 100-hp pump, est- line-flow-speeds of over 90 ft per second , 8 cycles per minute of ~ some GPM-then (filled and drained from cycling tanks testing) pumped held released? over and over...

OVER 25 years... The lines are lined with bacterial scum , even protecting the 90's and T's (!)

The only way I can get there is for a pocket of air to be compressed within a pipe, when the flow runs into a much warmer environment, the air tries to expand and gains pressure, but only instantaneously and until the liquid is pushed back into equilibrium. It wouldn't last for long enough to measure.

I agree with Fredski, baloney.

Before I started my Engineering classes, I spoke with one of the mechanics for the pipeline company I was working on. We were talking about draining down a section of pipe for repairs, and once the repairs were complete, (because there was no vapor/air relief) I asked him what was to become of the air trapped in the pipe (about 100ft of 16in line). He told me that once the pipe was up to pressure the air would be compressed to a much smaller volume and in a very short distance/time it would be spread out into the pipe so as to not cause any fuel (water) hammer effect.

Now with all the engineering classes I have taken, I still cannot doubt what he said because it still makes sense. In your case, it would take a tremendous volume of air to not mix into the fluid as it passes through the pipe. Assuming that there is sufficient air for this to occur you would see tremendous surging of fluid and air. The pressure spikes he claims to witness could most probably be caused by the water hammer effect of the fluid surging.

Drew K

Your operator is confused.

Exactly. In your last paragraph, what you describe is also called vapor-propelled liquid, which is one of a few forms of "liquid hammer".

It's bo££ocks.

Agreed. The only possible explanation is in a dynamic flow situation where there is an exit stream (usually that will be slightly constricted), such that there will be large variation in flow depending on whether liquid or gas (air) is issuing from the exit port. Then there will be repeated events of water hammer.

This answer is correct, interestingly I read all the previous responses and no one appears to have answered the question until I got to this man!

Oliver Dunthorne ( Oil Hydraulic Engineer - retired)

several and myself addressed "NAH" operator is incorrect: Example: If the water pressure is 300 psig, the air trapped in the system could NOT be several hundred pounds higher. This has been NO explanation, in the REALITY for mechanical failure in the past.

This fellow goes so far as to say that gauges in the field have been used to support NONE of his theory...

UNLESS as many attributed to some DYNAMIC of an INSTANTANEOUS change in velocity or direction (def of acceleration) is applied to the MASS in that DYNAMIC situation.

Many in the thread have AGREED: "NAH", as to equilibrium arrives in a closed situation unaffected by CV (like a benchmarking) changes for a DYNAMIC WORKING CHANGING fluid vs pressure drops for an air flowing, but over all - al at the same pressure , instantly, if dynamics come to rest..

we have found because of varying inertial components of liquids and gasses in dynamic CLOSED SYSTEMS, that any fluid - accelerations, changes in speed or direction, (fittings, pipe dia , whatever)

such as an undersized header allowing liquid velocities high enough that there is even a NEGATIVE pressure going past say a 'T'

showed us has not necessarily been the case:

will always be at the same pressure

in a completely sealed , closed system.

high-enough velocity gasses dynamically appear as any slow moving fluid as well as probably all know anyway

Water hammer can certainly break things. I think that is the cause of your Operator's confusion. Or he might be thinking, perhaps incorrectly, of cavitation in a pump, which produces microscopic but extremely high pressures which can chip off pieces of even stainless pump impellers. See http://en.wikipedia.org/wiki/Cavitation

'Water hammer' is a phenomenon of force (mass and velocity). Here we are talking about pressure equilibrium. In a closed system gas and liquid cannot exist in different pressure but for a very short time and sooner would get to equilibrium.

Else, many liquid/gas apparatuses like these cannot exist.

Hi

I have been having trouble with this site on and off and need to test if this test comment will get through. Sorry it is off Topic but I am trying to find out what is wrong that it will not take my responces. So I am running a few tests.

Jim

I have been having trouble with this site on and off...

Same here. It's most annoying after spending time working on a good response having it disappear! I've started copying my responses into a doc file if I spent much time on them to be on the safe side.

Your taking too long to write your response. It is apparently caused by a time limit being imposed. Maybe to keep idle keyboards from tying up the system.

If this is a problem for you, as it for me, write your contribution in word, press "reply" at the posting you are replying to.Yes you will have a blank entry. Then highlight all text in word, push the left mouse key and hold it down. Drag this text to the previously entry space (an arrow and a small box will appear) and release the left mouse key. The text will be a negative of what you want. Simply find a blank part of the reply and push the left mouse key at it. You now have a complete entry that took longer than the allotted time.

An added advantage of this is the spell checker in word is much better than in CR4, Also when you move it from word, the word entry becomes blank. Touch the "Go Back" arrow (top arrow that is counter clock=wise) and the entry reappears. you can them save it to any place you want.

Good Luck, Old Salt

If you're talking about equilibrium (static pressure), it makes no sense. If the pressure is higher on one side of the interface, the surface will move to equalize the pressures. Liquid is not perfectly incompressible. Something has to change when a liquid is put under pressure, a minute reduction in volume.

The pressure readings will not be different over time, and the length of that time is usually very small. The premise being discussed is the basis for pressure tank design, usually placed in a coolant/ heat transfer or liquid supply line that may be subject to volume expansion due to thermal changes. A bladder separates the liquid from a pressurized volume of air, which is sealed. At any given time, the pressures will be equal when tested, (the air pressure reading on one side of the bladder and the liquid pressure reading in the line side, although changing, due to thermal expansion. Seems like a well documented proof.

So far, every answer proffered here will have been ignored by OP, due to this condition,

"I am expecting several answers to be standard CR4 responses, such as "I can't do the math, but I know this to be true/false." I will make an effort to overlook these comments."

You can't put more energy into a system than you have available. You can't get more pressure into a gas either. The volume just gets smaller.

Any air in a liquid piping system will cause any number of proplems from:

• Water hammer
• Cavitation of your pump
• Loss of pump prime
• Seal damage
• Pump overheat

Find the high spot in your piping and install an Air / Vacuum relief valve like the one I specified above. We have used these for decades in water and wastewater applications and are reliable and easy to maintain.

We've never established that there is a pump IN THIS HYPOTHETICAL question.

It's all pure speculation about a non-existent, undefined, closed system.

You can't get more out than you put in.

"a smart guy whom I respect, who is insisting something be true that is counter intuitive.

This fellow is telling me that air, trapped in a water line, will compress to much higher pressures than the liquid pressure in the pipe.

Example: If the water pressure is 300 psig, the air trapped in the system could be several hundred pounds higher. "

What Lyn ^...

what else is in "the example"

As Lyn put it simply and elegantly !

On the failures you mentioned: These will be due to other phenomenae due to dynamic actions, for example: Water hammers are due to sudden stoppage of the water flow in a pipe system (Sudden closure of a valve while there is a flow). A pressure wave will travel backward from the point of closure, upstream and might reflect back and forth until attenuated due to friction losses, unless its energy is absorbed by an exhaust device or an expansion tank. If the energy of the wave is high enough, then it might cause failures in other valves, devices or even the pipe work ...

In this context, if air is trapped in a pipe, in such a way that it gets compressed and decompressed due to the flow, thus creating intermittent cuts in the flow, then this can cause pressure waves to travel along the pipes in both directions, causing the same phenomenon as a water hammer. These waves can have peak pressures that can be higher than the normally expected. that's all for now.

If such a thing were true then many a submariner would never surface again.

I have spoken with my guy and, him being a rather intelligent fellow, quickly conceded that there must be some detail missing from his understanding of the system. Possibly a hidden check valve, or maybe the pressure spikes were occurring over a very short period of time, as in the case of water hammer.

We had a short discussion about the 2nd law of TD, and how there are some rules you just don't break.

Overall, I think it went well. Of course, there are still folks on the margins who continue to believe there is some mysterious "multiplication factor" that allows air to reach much higher pressures in a closed system with water. Unfoltunately these folks are beyond my sphere of influence.

As always, I will continue to do all I can for as long as I can. This and no more.

-A-

Sounds like a pretty good result to me.

Was Your mechanical failure of a blowout nature ?I have witnessed main line irrigation pipes to blow out at the end installed underground with no air release valve installed at the highest point. air in any water supply pipe/main is a big no no .

His theory was based on above ground systems where he has seen firewater pipe show different pressures with and without air in a wet pipe system. When we had underground failures, he concluded that there must be some difference between the air and water pressures. This theory was influenced by the exacting methods used to make sure all the air is purged from the underground firewater system after being serviced. This guy is NOT a firewater expert. Largely, these are theories based on second hand observations.

Please explane why any air in a water main is a big no no.

-A-