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)
