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Gas and Liquid Pressure in a Closed System

08/09/2013 11:58 AM

A question for Engineers.

I have an Operator, 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. This has been an explanation, in the mind of this Operator, for mechanical failure in the past.

This fellow goes so far as to say that gauges in the field have been used to support his theory.

Is there any known theory out there that could be used to explain this phenomenon?

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.

This is an excellent example of someone believing something they can't explain. This is the type of thinking I have come to expect from Operator types. This is the type of thinking I expect an actual Engineer to avoid.

Please provide any legitimate and verifiable explanations you might have.

I look forward to reading you responses,

-A-

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#1

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 12:16 PM

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.

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#2
In reply to #1

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 12:23 PM

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-

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#3
In reply to #2

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 12:44 PM

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.

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#4

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 1:06 PM

balony

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#5

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 1:26 PM

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.

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#6

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 3:03 PM

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 (!)

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#7

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 3:41 PM

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.

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#8

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 4:06 PM

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

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#9

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 5:03 PM

Your operator is confused.

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#10

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 5:54 PM

Some folks and maybe particularly with a goodly amount of field experience often have reasons, somehow rooted in the real world where seems eventually a lot of s#@%&* (stuff) happens, for thinking the way they think. This one as stated, however, is kind of hard to reconcile in the face of Pascal's et al principles say at the interface of confined air and water fluids (see some explanation even from rocket scientists at http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/Pascals_principle.html ).
That being said, air (trapped and/or moving/expanding) in confined piping systems can indeed be a problem in many respects, and at least even per some authorities in the generation of quite high pressure surges (in otherwise normally/seemingly rather low water pressure piping systems) that can do damage to particularly weaker pipes. See e.g. the site at http://www.femoranshs.com/pressure-surges-article/ where there is effort to explain some of these mechanisms (and also it appears providing an illustration of a broken, it appears restrained, pvc pipe connection to prove their point). It has increasingly also been revealed in recent years that air (perhaps near inevitable?) in pipelines is a factor in "rapid crack propagation" again at low application pressures, that one would not normally think would cause such damage. This is again most prevalent in weaker piping materials with glued, fused or welded joints, with really not much to arrest same. See the 63 slides with field photographs at http://www.plasticspipe.com/Plastic-Pipe-for-Water-Distribution-0313-final.html that allegedly illustrate this point (the book at http://books.google.com/books?id=baWyaC3w3hcC&pg=PA379&dq=crack+destroying+several+miles+pvc+pipe&hl=en&sa=X&ei=v2MFUtbgN-qY2QWtyoHYDA&ved=0CEgQ6AEwAA#v=onepage&q=crack%20destroying%20several%20miles%20pvc%20pipe&f=false even talks about an instance where "several miles" of pvc pipe was destroyed!.
Upon a line break or other abrupt loss of pressure, I would also hazard to guess trapped slugs of air under high pressure could also well propel slugs of water at abnormally quite high velocity and momentum, and toward what ends or constrictions (as I have seen "water rockets" work)? All have a good weekend!

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#11
In reply to #10

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 6:05 PM

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

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#12

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 6:13 PM

It's bo££ocks.

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#19
In reply to #12

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 9:22 AM

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.

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#13

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 9:55 PM

Your operator is incorrect.

Gas or liquid in a closed system will always be at the same pressure.

Not even experienced operators can overcome the laws of Physics.

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#18
In reply to #13

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 8:42 AM

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)

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#28
In reply to #18

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 9:56 PM

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..

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#30
In reply to #28

Re: Gas and Liquid Pressure in a Closed System

08/12/2013 10:58 AM

OPERATOR = topic

several, were already specifically noting some confusion of the 'operator'= topic

myself= on topic

inserted words FROM the TOPIC per my conjectures = on topic

"DYNAMIC CONDITION" could attribute to some fluxuation in gauges read at points on a piping path...

= all on topic

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#31
In reply to #13

Re: Gas and Liquid Pressure in a Closed System

08/12/2013 11:06 AM

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

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#14

Re: Gas and Liquid Pressure in a Closed System

08/09/2013 11:59 PM

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

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#15

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 1:00 AM

'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.

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#16

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 4:53 AM

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

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#26
In reply to #16

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 11:41 AM

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.

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#36
In reply to #16

Re: Gas and Liquid Pressure in a Closed System

08/13/2013 6:38 PM

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

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#17

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 7:59 AM

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.

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#20

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 9:29 AM

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.

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#21

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 9:39 AM
This will vent the air from the system and suppress any surges.





Surge-Suppression Air Valves are Combination Air Valves equipped with Regulated-Exhaust Devices (slow-closing devices) to limit secondary surges during column separation. The Val-Matic® Surge-Suppression Air Valve provides full airflow into the pipeline during vacuum conditions to prevent a vapor pocket (vacuum) from forming.

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  • Provides full vacuum protection for the pipeline
  • Provides controlled venting to suppress surges in pipeline
  • Minimizes water blow-by during Air Valve closure
  • Allows the use of smaller valve size by utilizing a maximum sizing differential pressure of 5 psig
  • Releases entrained air while pipeline is operating to maintain pumping efficiency
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  • Ability to adjust air exhaust for greater surge suppression
  • Provides full vacuum flow port for maximum pipeline protection

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#22

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 9:59 AM

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.

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#23

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 10:51 AM

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.

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#24
In reply to #23

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 11:03 AM

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.

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#32
In reply to #24

Re: Gas and Liquid Pressure in a Closed System

08/12/2013 2:41 PM

"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"

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#25

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 11:34 AM

If your piping system is at 300 PSI then any air in the piping is at 300 PSI.

Like Lyn said, "You can't get more out than you put in."

You can compress air, you cannot compress water. Any air in the piping will cause surges and cause your instrumentation to give false readings i.e.:

  • Incorrect flow rate.
  • Varied pressure readings.
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#27

Re: Gas and Liquid Pressure in a Closed System

08/10/2013 1:41 PM

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.

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#29

Re: Gas and Liquid Pressure in a Closed System

08/11/2013 8:05 PM

Are you sure that your operator isn't simply getting confused with the fact that water is "incompressible" and air is compressible. The result of this is that when a vessel fails pneumatically, the contents (air) wants to expand and propels parts of the vessel around the general area - typically causing much more damage to the surroundings than hydrostatic failures?

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#33

Re: Gas and Liquid Pressure in a Closed System

08/13/2013 5:05 PM

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

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#34

Re: Gas and Liquid Pressure in a Closed System

08/13/2013 5:15 PM

While I personally kind of like and am certainly entertained by rather straightforward answers myself, I did notice the OP said they respected this operator/info source. While short, glib answers can no doubt be accurate and easier, if the OP simply tells the operator his/her point is "baloney", "bo££ocks", or he is simply "confused", could his future relationship be harmed (depending unfortunately on some human nature and thin skin?) Maybe now however with some firm third-party principles and references from this thread, the operator can at least be let down a little easier.

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#35
In reply to #34

Re: Gas and Liquid Pressure in a Closed System

08/13/2013 5:20 PM

I prefer a firm correction to an operator who proposes weird science than to let him potentially corrupt individuals who may listen to him, and being my operator, cast doubts on my own credentials by association.

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#37

Re: Gas and Liquid Pressure in a Closed System

08/14/2013 8:58 AM

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-

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#38
In reply to #37

Re: Gas and Liquid Pressure in a Closed System

08/14/2013 9:38 AM

A simple test jig could be built of a small loop of piping, an O shape. You have 2 pressure guages, one in the lower half and one in the upper half. The upper portion will have an air fitting for pressurization. Fill the pipe to almost full with water. Then seal the system with only the air fitting access. apply air pressure to the fitting and observe the 2 guages. they should correspond. Mathematics aside, its a simple proof, probably one from early high school physics classes. Given this, one can assume that the operator in his theory is applying dynamic conditions to a static situation, because only a dynamic action can result in an imbalance, and only for a very short time. For the air to achieve a higher pressure, the water would have to occupy more space, but since water does not compress, it also would not expand in a static system. if an imbalance were to be introduced by dynamic influences, then the pressures would equalize quickly. This can also be shown by injecting water into the loop and observing the guages. My analogy to submariners was a little vague but if you imagine that the water could force a higher pressure into the air, then very soon the sub would not be able to overcome that pressure and vent its tanks to surface.

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#39
In reply to #37

Re: Gas and Liquid Pressure in a Closed System

08/14/2013 6:48 PM

Sounds like a pretty good result to me.

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#40

Re: Gas and Liquid Pressure in a Closed System

08/15/2013 5:00 AM

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 .

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#41
In reply to #40

Re: Gas and Liquid Pressure in a Closed System

08/15/2013 9:23 AM

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-

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#42

Re: Gas and Liquid Pressure in a Closed System

08/15/2013 9:40 AM

-A-

You might try a gauge like the one below. it is a cheap way to see if you are getting spikes in pressure.

I know the pressure scale on the gauge shown is 100 PSI.

However they are available in many pressure ranges.

The second needle is a MAX indicator showing your operators what the maximum pressure in the pipe reached.

It will record the pressure of a spike or hammer and if you are getting high spikes, you may need to add some type of surge suppression to your piping.

There are also digital gauges that do the same type of recording and may be able to trend the spikes, i.e.

  • Time of day the spike occurred? (sometimes this is the most informative)
  • Pressure of spike?
  • Multiple spikes?

You may also want to invest in a chart recorder like the one below?

This will give you a look at a 24 hour period so you can trend the pressures!

I have a couple of these, however I'm not sure of the pressure rating on them?

A smart operator should be willing to discuss your concerns, possibly he got some bad information years back and it has stuck with him. Better to educate your operators with good info rather than rumor.

Good luck

BA

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