When 20 or fewer sprinklers are involved in a modification, codes and standards suggest new hydraulic calculations are not required.
What is the scientific or engineering basis for the "20-sprinkler" number?
Is there some study or analysis that found this to be a reasonable breakpoint?
I'm researching whether we should require full plans for less than 20 sprinklers. Thanks!
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4/2/2021 10:08:05 am
I do not know of any code or standard reference that suggests that calculations are not required for less than 20 sprinklers.
4/5/2021 11:58:05 am
You are right, however NFPA 13 Chapter 25 contains this statement: "Modification affecting 20 or fewer sprinklers shall not require testing in excess of system pressure" has been used as the basis for this statement in our rules:
4/2/2021 10:15:50 am
I don't think there is any scientific rule behind the 20. But maybe I'm wrong.
4/5/2021 12:02:33 pm
4/2/2021 10:46:04 am
I believe that you are confusing the allowance within the NFPA 72 standard which allows for a sprinkler system containing less than 20 sprinklers to forego Central Station monitoring.
4/2/2021 11:50:30 am
Franck is right on. I'm kind of split on putting a calc requirement based on QTY of sprinklers being added or remodeled. As Franck's examples show, there could be a case for the addition of one sprinkler requiring a calculation, or the addition of 1000 sprinklers not requiring a calc. For instance, if someone came in and built a 400 sqft paint application area in the middle of light or ordinary hazard building, you might only be adding 4 to 6 heads, but that demand could be more than the riser, main, or branch lines can handle, not to mention how do you know what size your new piping should be feeding the new heads? On the other hand, you could have an extra hazard multi-system building that was bought for office use, ACT ceilings were installed throughout with 1000 pendent sprinklers fed from 1" drop downs from the roof system. Common sense tells you that system is going to work extremely well and calculations are a waste of time. Some options would be just to trust the NICET (in most states) putting their signature on it and/or better training for the plan reviewer. I think NFPA stays away from this topic for these exact reasons and leaves it to the local/state AHJs to make their own rules on this.
4/5/2021 07:35:59 am
I think he may be referencing 188.8.131.52.1 in NFPA 13-2016 which deals with hydrostatic testing and acceptance requirements.
4/5/2021 12:05:03 pm
You see my question precisely. All I was hoping for was that someone would point me to a paper or analysis that justified 20 heads or fewer.
4/5/2021 11:47:14 am
This is a common practice in ALL jurisdictions. Why would an AHJ require expensive calcs for a 3 head relocate? Its plain silly.
4/5/2021 12:02:01 pm
Thanks. I understand that various jurisdictions differ. I was looking for a scientific, data-based analysis for 20 heads. Or any number of heads.
4/6/2021 03:32:01 pm
I've fought with my bosses about this for years (VA/NC/SC). Around here, 19-20 heads in a light hazard upfit didn't require a calc or submittal.
4/6/2021 06:08:00 pm
Yeah, I agree. There are times that adding flex on a non-flex job could require a calculation. I'm kind of split on this one too. I use a little common sense on this. You will gain some pressure due to elevation, typically the build out isn't in the remote part or floor of the building, and usually reduced area calculations aren't very common around here for shell systems and can be utilized for the buildout. Also be on the lookout from VIC, I think they said they are (or have) added other friction loss options for 1, 2, or 3 90 degree bends so you don't always have to use the worst case scenario. Remember that NFPA 13 already has tons of safety factors inherently in the original calc. Oversized area, elongated area, remote area is located at the worse case area, C-factor is based on future pipe conditions and not new and a lot of fire marshals throw some type of safety margin on top of that. Then you factor in that most light hazard fires are controlled or extinguished by just one or two heads. AND sometimes you pick up a little efficiency (back pressure) by restricting the non-driving heads with extra footage of pipe and fittings that you don't get with pure uprights. I just remember a lot of buildouts back in my early days coming out of 1/2" outlets. I worry more about that than I do flex. BUT all that being said, if I run into a build out on the top floor or I get a feeling of the system being marginal, I do a calc. You can't go wrong with doing a calculation when you suspect it's going to be tight.
4/9/2021 12:21:21 pm
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