This always gets me going. I love when AHJ's put in an arbitrary maximum velocity without any logical backup, other than they read that it's better to keep them low. (Usually in some plumbing magazine where noise is an issue.) While we all usually do a demand calc, with a nice safety margin, like 15Psi for a 140Psi supply, so let's say that we are limited by the AHJ to 30 FPS. Our calc's all work out and we're off. But what actually happens in a fire? Does the fire say "I only want this much water, and no more!"? Of course not everyone knows it takes everything the sprinkler system will give. If the fire is small, all the affected heads overflow because they have a higher residual pressure, than was calc'ed. If the fire is the exact size of the design area, the heads still over flow the design because of the "safety factor" that extra 15Psi. and if the fire overwhelms the system design, well again we overflow the system but not necessarily individual heads. So which scenario created higher velocities than were designed. ALL OF THEM!!!
I always love to give the AHJ a supply calc. (After they return my approved submittal) and show them what really happens if those same heads and area are now flowing, and if I have time I like to do one for some other area as well. If you've got the Psi use it. If there is no limit why not use what works? Thom McMahon, SET Firetech, Inc. 2560 Copper Ridge Dr P.O. Box 882136 Steamboat Springs, CO 80488 Tel: 970-879-7952 Fax: 970-879-7926 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Chris Cahill Sent: Monday, June 02, 2008 8:47 AM To: [email protected] Subject: High velocities A question for where there is healthy pressures. Around here 60 is considered high. I know places with 28 static. Got a OHII system that needs to be changed to EXII. That's not negotiable. It's a 40 yo pipe schedule system. That appears to be in good shape. The BL need to be replaced for 100 sq.ft. but the main even the last 12' of 2.5", 25' of 3" and 36' of 3.5" can fly without being replaced. The openings are where I need them. There is a FP with about 140 psi at the 900gpm demand. The "problem" is the velocities in one segment of pipe are just about 40 (last piece of 3") and about 30 in the 3.5". I quote problem because I know 13 has no limit, this is not FM. There is still better than 15 psi safety. So the question is where you have pressure do you typically burn it up with small pipe size at the cost of velocities getting high or do you leave a lot of safety? Again not FM and OK per 13. Any thoughts on why this is a bad idea? It's easy to say just pull the offending pipe. In reality though you all know the competitive market and spending owners money. 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 _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum To Unsubscribe, send an email to:[EMAIL PROTECTED] (Put the word unsubscribe in the subject field) _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/mailman/listinfo/sprinklerforum To Unsubscribe, send an email to:[EMAIL PROTECTED] (Put the word unsubscribe in the subject field)
