I agree on the whole with Duane, but for a couple of clarifications to his statements that I'll hit on later. With over 40 years in the industry and 20 years in special hazards here's my take: it is true, using different detection technologies and cross zoning them gives better assurance against false or unwanted activations. In most cases it is also true that this kind of setup will delay the activation of the valve. My question would be - is this critical, given that the water is not going to deploy until a head fuses?
Modern technology in detection and control systems provides a range of choices, I'll attempt to hit some highlights without getting long winded. In a setting where critical operations or artifacts are at risk we should as an industry do what we can to ensure suppression is employed only when needed, so double up on automatic detection via cross zone and even multiple technologies. Or, as Duane suggests use double interlock. On the other hand, if protecting a generator enclosure an accidental dowsing of water is probably not going to be catastrophic; here single interlock on a single detector activation rather than cross zone works just fine. My practice has been to use heat detection in this setting and preferably flame detection as well. Sometimes these are cross zoned, sometimes not. Speaking to Steve's comment about incipient fires and their rate of growth, Class B fuels do not take long to woosh into fully involved; flame detection will give the quickest reaction for these fuels. Some will point out that flame detectors have caused them misery and pain due to false activations. The industry and technologies have come a long way from the first UV detectors (which caused me misery and pain back when) to the point that when properly applied they are very reliable. I would also mention that for automatic smoke or heat detection used for suppression one should always reduce listed detector spacing by at least half, sometimes more depending on conditions. Bottom line is that application of detection is application specific. Air sampling smoke detection is a great solution and provides lots of flexibility in design and implementation - I love it. Spot smoke detection works well but has challenges in high airflow or dirty environments. Heat detection is reliable but generally slower to respond unless the fire has moved beyond the incipient stage. As an aside, FMG's take on pre-action is to use heat detection and NOT cross zone; whether single or double interlock. As this is a technical forum I want to point out a couple misstatements in Duane's info. Nothing personal Duane, just sharing info. Firstly, ionization smoke detectors are no longer available from any of the system manufactures (hazmat disposal issues). Second, alarm verification is not allowed in releasing applications per NFPA 72. Cheers, Pat Thompson NICET IV Special Hazards ATS Alaska Direct: 907-375-4176 -----Original Message----- From: Sprinklerforum [mailto:[email protected]] On Behalf Of Duane Johnson Sent: Friday, November 13, 2015 10:38 AM To: [email protected] Subject: RE: Cross zoned preaction detection Cross zoned works, but personally I like double interlocked over cross zoned single interlock when you are discussing mitigation of damage. Cross zoned will not help 'detect' a wider range of fires, it helps 'reduce' the amount of false activations. The more fire signatures you require (ie. Heat, smoke, IR, etc) prior to activation, the less false activations you will have. For example, a welders torch will activate both a heat detector and sprinkler (seen it), but not likely a photoelectric smoke detector and a sprinkler. So you add smoke detection as a way to mitigate non-fire heat sources from tripping the system. There is a good range of smoke detector types (ie photo, ionization, air sampling, etc) and fire alarm program adjustments (ie. Alarm verification, dirty, etc.) that can help reduce false activations. One type of detection that seems good, at least on paper, is video detection. But I can't confirm field success. Duane Johnson, PE Design Manager Strickland Fire Protection 5113 Berwyn Road College Park, MD 20740 301-474-1136 Office 301-455-0010 Cell -----Original Message----- From: Sprinklerforum [mailto:[email protected]] On Behalf Of Steve Leyton Sent: Friday, November 13, 2015 12:37 PM To: [email protected] Subject: Cross zoned preaction detection Does anyone have any strongly held opinions about cross-zoning of detectors for single-interlock preaction systems? I haven't done one, and have usually fallen back on double-interlock when there is a heightened concern about accidental activation (earthquakes and all that). My understanding is that when two different types of detectors are cross-zoned, it can help to detect a wider range of fires that may grow out of the incipient stages at different speed and/or with different release characteristics. And cross-zoning with the same type of detector can give a heightened level of fail safety, so that seems more comparable to double-interlock. I'm interested in hearing any suggestions, opinions of the value of this design (or not) and good or best practices. Steve Leyton _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org _______________________________________________ Sprinklerforum mailing list [email protected] http://lists.firesprinkler.org/listinfo.cgi/sprinklerforum-firesprinkler.org
