It is interesting because yesterday I sat through a seminar on "Air Mgmt and System Pressurization in Closed Loop Hydronic Systems". Some of the very same things we've talked about in regards to sprinkler systems and trapped air were talked about. One of the first slides "WATER IS INCOMPRESSIBLE", ok so we've established that. The expansion and management of air is a significant problem in hydronic systems. So much of what was dealt with has been discussed here in regards to sprinkler systems.
Water expands when heated, that's a given, as it does it has to go somewhere. The rate of expansion is also a given depending on the temperature. 2000 gallons of water at 40 degrees will expand to 2056 gallons at 180 degrees, it doesn't have to boil to expand. The percentage of volume increases as the temperature increases. Under the right pressure I can have 300 or 500 degree water, it will not boil. It will not boil or flash to steam until the pressure is released. This is basic thermodynamics and the information can be derived from the Steam Tables. Water does not have to boil to expand. In a closed loop hydronic system you often will have an expansion tank to accommodate the expansion of that water. As was discussed in the seminar, an area of piping that is in a high point of the hydronic system can act as the expansion tank if the system is not designed properly. As the water expands it will compress the air in the expansion tank, but the hydronic (water) side of the system pressure will remain constant. However if the expansion tank is not sized properly to allow for proper expansion you will then end up with increased pressure in the piping network. Since hydronic systems operate best with little to no air it's the desire to remove or at least control air volume with the system but also allowing for expansion of the water within the system. The expansion of water is control via the expansion tank and other relief devices. The system could not operate properly without taking into consideration the expansion of the water. So if you apply the theory of hydronic systems to sprinkler systems, (both closed systems) sections of trapped air in ends of the system or at high points would actually help cushion the system against water pressure fluctuations (acting like the expansion tank) resulting from increased water pressure due to thermal expansion of the water. But when there is not enough space for expansion of the water and eventually the trapped air sections become water logged as the air is forced into solution, the excess water volume becomes excess pressure and that pressure has to go somewhere. If there no pressure relief valves such as are required on gridded systems, and the pressure within the system exceeds the incoming water pressure, where do you think it's it going to go? One other point made is that as water is heated, it releases air, conversely as it cools it absorbs air. Water under pressure can retain more air, (the cup filled with saran wrap on top, no place for air to come out of solution), as the pressure is lowered, the water can not hold as much air in solution and the air escapes (cup half filled with saran wrap on top, water boils due to less pressure containment, air in solution is released into space already occupied by air, additional air = saran wrap breaks) this was very obvious in the analysis of sizing expansion tanks. The half filled cup was the undersized tank. There is s whole lot more to this whole thing then I care to get into at this time, but there were very obvious parallels in the two systems and the reactions of air and water. Basic analysis, air is not always bad and actually may stabilize system pressure variations due to thermal expansion of the water. Without some way to compensate for thermal expansion of water, sprinkler systems will continue to react as we've discussed. Craig L. Prahl, CET Fire Protection Specialist Mechanical Department CH2MHILL Lockwood Greene 1500 International Drive PO Box 491, Spartanburg, SC 29304-0491 Direct - 864.599.4102 Fax - 864.599.8439 [email protected] http://www.ch2m.com -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Chris Cahill Sent: Thursday, March 19, 2009 10:27 AM To: [email protected] Subject: RE: Excessive system pressure I find it VERY interesting that you find it VERY interesting there has been no findings. Hint of insecurity in your position? Why would anyone try your experiment when it is irrelevant to the constant volume container of a sprinkler system? No one "believes water compression" is responsible for anything. It's thermal expansion without compressible air. "Or could it possibly be that water and air act differently to temperature changes in Georgia than any other place in the world?" I guess that's as valid a theory as trapped air causes excess pressure. I have only driven through the great state of Georgia once so I'll defer to your expertise on matters in Georgia. You have made an extraordinary claim thus it is incumbent upon you to provide extraordinary evidence. Your claims go against the text book ('02-7.1.2.2, A.7.5.3.2 and the handbook) and the vast field experience of this forum. There's even the fancy math thingies with letters, subscripts AND symbols which us PE's like. I have the upmost confidence in my position so I'm not in any big hurry to propose and try a valid model experiment of the situation. If I had doubts it would be a higher priority. Given a choice I'll spend my time with friends and family. When those priorities allow (and I have my Guitar Hero fix for the day) you'll get feedback. And as far as your trees and grids. Sure sounds like what 7.1.2 describes. Imagine that the trees are OK and the grid is not and that's what the text book covers. Coincidence I'm sure. Granted I concede once text books had the center of the universe as Earth and the Earth was flat. I'd like to think we've come a long way and today's text couldn't be that wrong. But hey those that wrote those old texts probably thought the same thing, they couldn't be that wrong. Good news if you are right we'll probably name the principal after you. Bernoulli, Pascal, Hazen-Williams, Hamm! If you believed your theory why are you recommending the PRV? Shouldn't removing the air only be what you would advocate? If the PRV wasn't required when installed then it was and still is code compliant. Why would you need both seemingly counter solutions? Chris Cahill, P.E. Fire Protection Engineer Sentry Fire Protection, Inc. 763-658-4483 763-658-4921 fax Email: [email protected] Mail: P.O. Box 69 Waverly, MN 55390 Location: 4439 Hwy 12 SW Waverly, MN 55390 -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Rodney Hamm Sent: Wednesday, March 18, 2009 8:23 PM To: [email protected] Subject: Excessive system pressure I find it VERY interesting that it has been over a week since I suggested/challenged the forum participants to a simple experiment and there has been exactly ZERO responses to report any conclusions/findings. I am wondering if anyone actually tried the experiment referenced below....if so, I wonder why has there been no response through the forum? Could it be that some have spent the 4-5 minutes and have decided against posting a response because the experiment does not support their theory? Could it possibly be that the various forum participants who believe water compression is the root cause of high system pressures are to "old school" and "close minded" (to use a phrase that was used to describe me..heh heh heh..really funny if you knew me at all) to consider a simple experiment that might in any way challenge their way of thinking? Or could it possibly be that water and air act differently to temperature changes in Georgia than any other place in the world? I am having a little too much fun with this email. On a more serious note, I was at another plant yesterday which has the same exact problem. This is really why I am writing this email. Warehouse with 1 gridded system and 7 tree fed systems. The problem only exists for the gridded system; the remaining 7 systems had nominal pressure differences (0-10 psi difference) between the top & bottom gauges. On the gridded system, the public pressure upstream of the alarm check valve was fairly steady all afternoon at ~60 psi. At 1:50 PM, the system pressure downstream of the alarm check valve was 190 psi. I described the problem to the plant engineer and he wanted me to explain the problem to the plant manager. The three of us returned to the riser at 2:10 PM and the system pressure downstream of the alarm check valve was already up to 205 psi. We have not even entered our really hot days of summer yet! Riser nipples on the near main and the far main and the ITC for the system was attached to the bottom of the far main (in other words , the auxiliary or low point drain was converted into an ITC by installing the base of a sprinkler head at the end of the pipe and then someone placed an ITC sign on the valve). This system was installed prior to the NFPA requirement of a 175 psi PRV for all gridded systems. I have recommended the provision of a 175 psi PRV, installing air bleed off valves and removing a significant amount of the trapped air from the system. The ITC piping "SHOULD" be arranged to flow water through the highest point of the system whenever possible, but some people just don't give a damn! Rodney K. Hamm, P.E. President/Owner Falcon Fire Protection Office (478) 953-1677 Cell (478) 396-6988 Date: Wed, 11 Mar 2009 15:47:19 -0400 From: "Rodney Hamm" <[email protected]> Subject: RE: Test/Drain at the Riser To: <[email protected]> Message-ID: <[email protected]> Content-Type: text/plain; charset="us-ascii" I challenge you to try my suggested experiment! Even if you do not want to try it..try it with a preconceived notion that I am completely wrong. Take a completely 100% full glass of cold water (plastic would be best) pull some saran wrap over the top very tight and add a rubber band really tight to make sure no air leaks out. Make sure there are no air bubbles on top. Put the full glass of water in the microwave on HIGH for 2 minutes. No pressure builds up inside the glass because water is not compressible and does not expand. Pressure will eventually build if you let the water start to boil. THEN do the same exact thing with a glass half full of cold water. Using saran wrap and a tight rubber band, you will see the pressure increases dramatically and eventually will burst the saran wrap after the balloon effect! The results will be so obvious you will see my point. The plastic film on top allows you to see the dramatic difference in pressure increase (100% water vs. 50% air). My experiment is actually a closed system. I will agree that the volume is allowed to increase, but the only force which allows the plastic film to balloon is due to the increased pressure in the closed container. If the same percentage of water/air was in a sprinkler pipe, the increased pressure caused by the temperature increase would cause the system pressure to increase dramatically due to the fact that there is no change in volume (until a leak or system rupture occurs). Of course, the best test environment would be to use a sprinkler system in an oven with a calibrated pressure gauge, but most of our forum readers would never take this much time and effort. The experiment I suggested is VERY convincing if you are willing to spend 5 minutes of your time to try. Air is compressible and water is not compressible. Temperature increases or decreases do not affect the pressure of a section of pipe filled with 100% water until the water begins to boil. Temperature increases will cause the pressure to increase if the same section of pipe is filled with air or has a significant percentage of air along with some water. Temperature decreases will cause the pressure to decrease if the same section of pipe is filled with air or has a significant percentage of air along with some water. _ _______________________________________________ Sprinklerforum mailing list http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum For Technical Assistance, send an email to: [email protected] To Unsubscribe, send an email to:[email protected] (Put the word unsubscribe in the subject field)
