Since water is incompressible, its slight expansion with temperature leads to very large pressure increases. Water will expand about 0.3% in volume when heated from 40 to 80 deg F (density goes from 62.43 #/ft^3 to 62.22). That's not much but the cross sectional area of 2" sch 40 pipe will only increase about 0.025% with the same pressure swing (if I have my math right this morning).
A quick google search tells me that water will only compress 1 part in 5x10^7 with one atmosphere of pressure (14.7 psi). So contracting that water back to it's 40 deg volume would take BIG PRESSURE. Luckily, there is always some air dissolved in the water or we would have BIG problems. Why is air not a BIG problem? Air is compressible. The 8% temperature rise (8% Kelvin) will increase pressure 8%. (PV = nRT) Trevor Spain Fire Pro Corp 404 N Cedar Shelbyville, IL 62565 Rodney Hamm wrote: > I am sorry to be so blunt, but you need to think a little about what you > have stated. 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. Temperature decreases will cause the pressure to decrease if the > same section of pipe is filled with air. > > > > If you do not believe me, try a quick little experiment. 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! > > > > If you have significant trapped air pockets in a sprinkler system, the ONLY > time you do not need a PRV is if you are absolutely certain the sprinkler > piping does not experience significant temperature changes. High temps to > low temps will cause the pressure to decrease and can potentially allow > water to enter the system (i.e. false fire alarms) and low temps to high > temps will cause the pressure to increase and can potentially cause damage > to the sprinkler system piping (i.e. leaks, ruptures, deformation of > sprinkler plugs, etc.). > > > > You stated the following: "With a static system increasing in temperature > the air compresses and stabilizes the pressures." Actually, in this instance > increasing the temperature the air does not compress it actually expands and > instead of stabilizing the static pressure it drastically increases the > static pressure. > > > > Rodney K. Hamm, P.E. > President/Owner > Falcon Fire Protection > Office (478) 953-1677 > > Cell (478) 396-6988 > > > > Date: Tue, 10 Mar 2009 08:53:14 -0500 > > From: "Chris Cahill" <[email protected]> > > Subject: RE: Test/Drain at the Riser > > To: <[email protected]> > > Message-ID: <e86bfbe0cd6e4eae97886a3b39d3d...@chris2005> > > Content-Type: text/plain; charset="us-ascii" > > > > Rodney said "It is an indisputable fact that trapped air is a huge problem > > in gridded systems and THE reason why pressure relief valves are required on > > gridded systems is specifically because of the trapped air in these > > systems." > > > > Sorry it's the LACK of trapped air that is the problem as it relates to > > pressure. I suggest you read '02-7.1.2.2 and the handbook. If you have air > > you don't need a pressure relief on a system. Why? It's right in the text > > of the standard. Air is compressible. A spring if you will. For our > > purposes water is not compressible. With a static system increasing in > > temperature the air compresses and stabilizes the pressures. That's what > > the pressure tank on your well is doing, that is what an expansion tank on > > an anti-freeze is doing. Take the air out and there is nothing to compress. > > Minor temperature fluctuations then radically change pressure. I can show > > you systems that all the air has been deliberately removed. It takes a cup > > of water to change the pressure a couple psi. > > > > Ever opened an inspector test valve and waited 30-60 sec for the flow switch > > to change position. That's the trapped air expanding. Ever opened one and > > the flow switch moves immediately that's no air. For this purpose ignore > > the delay watch the lever/paddle position. > > > > A generalization gridded systems don't have air in them is also false. It's > > exactly related to the location of the ITV. If the ITV is at the end of the > > system on a grid air might be removed while filling or testing. If the ITV > > is at the riser (and there is no air relief remote) there is a similar > > amount of air in the system as a tree. OK all things being equal a gridded > > system has less volume compared to a tree system because of pipe size. But > > the volume of the system is similar in magnitude. > > > > I'm with Mark riser ITV with air relief remote and pressure relief someplace > > as the best of practice for all systems but I'll add not the minimally > > acceptable practice. > > > > Chris Cahill, P.E. > > Fire Protection Engineer > > Sentry Fire Protection, Inc. > > _______________________________________________ > 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) > ------------------------------------------------------------------------ > > > No virus found in this incoming message. > Checked by AVG - www.avg.com > Version: 8.0.237 / Virus Database: 270.11.9/1993 - Release Date: 03/10/09 > 07:19:00 > > _______________________________________________ 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)
