I'll just skip the other disagreements on this thread and speak to the trapped air issue:
Trapped air is NOT the cause of over pressurization. Gridded systems tend to have much less trapped air than tree systems because they have no (or limited) dead-end branch lines. From the day I professionally set foot into this industry working for FM I was told to include pressure relief valves on gridded systems because of the over-pressurization potential (I guess we don't disagree with this notion). Tree systems tend to not have that problem due to the trapped air. As we noted earlier: water in incompressible - air is compressible. When there is a lot of trapped air in a system, any temperature rise and resulting expansion of the water is easily absorbed by the compressible trapped air. When there is little or no trapped air, the expansion of water quickly overcomes the air and begins pressing directly on the interior walls of the piping system, resulting in a fast rise in system pressure. It is quite possible that I have had this notion of trapped air backwards all of these years, but I cannot understand how it would be 'indisputable' that tapped air can cause over-pressurization, given its compressibility. Conversely, how could a system with no air trapped within NOT quickly over-pressurize upon heating and expansion of the water? NFPA 13, as Ken noted, is moving towards requiring air bleed valves to remove as much trapped air as possible to reduce MIC and other corrosion potential. The resulting lack of trapped air is fueling the notion that ALL systems should have pressure relief valves to deal with the increased likelihood of over-pressurization resulting from temperature fluctuations. An ITC at the remote portion of the system CAN be a convenient way to reduce air within the system and isn't 'wrong' to locate it there. However, I feel the benefits of keeping it at the riser and reducing the amount of oxygenated water into the system outweigh the benefits of having the ITC at the end of the system. I.e. it isn't 'wrong' to have the ITC at the riser either; and NFPA-13 has modified its position lately to suggest it is more right to locate the ITC at the riser. Mark -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Rodney Hamm Sent: Monday, March 09, 2009 5:25 PM To: [email protected] Subject: RE: Test/Drain at the Riser Mark also wrote the following: "The problems associated with air in the system are valid; that's why a means should be provided to remove the air. But excessive pressure from temperature rises is not one of those problems. In fact, it is precisely because of trapped air that most tree systems don't have excessive pressure problems. The grids tend to have these issues because air can't be as readily trapped. That's why gridded systems should have a pressure relief. Of course, if you facilitate the removal of most air from a tree system, then a pressure relief is warranted on that system as well." In a more direct response to Mark, I really do not know where to start. 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. The industry had a significant problem with leaks and accidental system activation due to air pockets and temperature changes prior to the requirement of pressure relief valves on these systems; the PRV simply prevents the system pressure from exceeding 175 psi. The systems were experiencing system pressures well above 175 psi (sometimes >300 psi) specifically due to the trapped air and temperature changes caused by weather fluctuations. ) _______________________________________________ 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)
