Thanks John. I knew there was a missing component in that experiment but couldn't (read wasn't interested enough) to put my finger on it. Thermal expansion--makes you go Hmmmmm?
On Thu, Mar 19, 2009 at 4:20 AM, John Drucker <[email protected]> wrote: > Thermal Expansion & Water Hammer > > One of the consequences of installing a backflow prevention assembly on any > water system is the system becoming a closed system. A closed system is a > system, which is not open to the atmosphere. A pressurized tank would be a > closed system, since there is no means for the pressure to escape. > A water user's system may be considered an open system if there is no > backflow protection. Water flows from the water supplier into the water > user's premises through the water meter. Should something cause the pressure > on the water user's property to increase, water can freely flow back out > into the water distribution system. > What can cause the water user's system to increase in pressure? > > A common phenomenon, which increases pressure, is thermal expansion. Thermal > expansion occurs when water in the pipes heats up. This may be due to > environmental conditions such as a hot room, or it may be caused by a piece > of water-using equipment that is generating heat. Regardless of what causes > the water to heat up, when it does it expands and requires more volume. This > is no problem in an open system, the water flows back through the meter into > the distribution system. However, when the system is a closed system, which > occurs when a double check valve assembly or a reduced pressure principle > assembly is installed; the water doesn't have anywhere to go. In these > conditions the pressure can increase dramatically. > > Another phenomenon occurring in hydraulic systems is a water hammer. A water > hammer occurs when the flow of water through a pipe is stopped quickly. This > may occur because of a solenoid valve, or even a quarter-turn ball valve > being closed too quickly. The flowing water is stopped when it hits the > closed valve, but a shock wave is sent back in the opposite direction. > Again, this wave would dissipate if the system was an open system. However, > in a system, which is closed, because of a backflow preventer, the shock > wave will hit the downstream check valve of the backflow preventer and the > wave will move back throughout the system. This may increase the pressure > throughout the system up to several times the normal water pressure. This is > enough to cause significant damage in equipment and water fixtures. In a > mathematical model, water flowing at 15 feet per second through pipe > generated an increase in pressure of over 900 pounds per square inch (psi), > when a downstream valve was closed instantaneously. > What can be done to prevent damage in a system subjected to either water > hammer or thermal expansion? > > Expansion Tank > Under the thermal expansion situation described above, the system could be > protected by the installation of an expansion tank. An expansion tank is > attached to the water system and may be pressurized with air on one side of > a diaphragm within the tank. When the water pressure increases due to > thermal expansion, the tank begins tfill with water pressurizing the air on > the other side of the diaphragm. Water is not compressible, so when the > water begins to expand due to heat, it needs to go somewhere. Air on the > other hand is compressible. So, the expanded water goes into one side of the > tank compressing the air on the other side. This prevents any part of the > system from giving way to the expanding water. > > Although an expansion tank may help solve a thermal expansion problem, water > hammer may be so powerful that an expansion tank won't take care of the > problem. A water hammer arrester may be installed to reduce the effects of > the water hammer. In some cases some re-engineering of the system may be > necessary. For example, at the industrial complex, the solenoid valve was > adjusted to close very slowly. So, instead of closing nearly > instantaneously, the closure was very slow and thus, the water hammer was > eliminated. > > Courtesy of the USC School of Engineering > Foundation for Cross-Connection Control and Hydraulic Research > http://www.wvdhhr.org/phs/water/Thermal_Expansion.pdf > > -----Original Message----- > From: [email protected] > [mailto:[email protected]] On Behalf Of Rodney Hamm > Sent: Wednesday, March 18, 2009 9:23 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) > > > _______________________________________________ > 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) > -- Ron Greenman at home.... _______________________________________________ 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)
