Is there an FM Global requirement for a minimum safety factor in a hydraulic calculation?
We have a project that is following FM requirements, and the calculation "safety cushion" is coming in at 2 PSI.
We have looked through the FM Data Sheets, but cannot find anything to give a minimum criteria.
Thanks in advance.
Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
4/26/2023 08:07:29 am
Doesn't FM Global require a 1.4?
4/26/2023 08:19:44 am
Edit: I mean isn't it a 1.4 design factor. Not the same thing I was thinking. I typically use 10 psi across the board in my intent drawings. That typically keeps everyone at FMG happy.
4/26/2023 08:41:43 am
FM Data Sheet 3-0 - Hydraulics of Fire Protection, Section 18.104.22.168.3.1 - Density/Demand Area Format will show when the 1.2 vs 1.4 shape factor is utilized.
4/26/2023 08:17:27 am
Typically, the requirement for safety factor is from the AHJ and they derive it from IBC. In the FM criteria projects I've designed, I had to meet the AHJ requirement for +10-psi.
4/26/2023 08:18:31 am
We have always used a safety factor of 10 psi as minimally acceptable for our industrial clients.
4/26/2023 08:21:36 am
There is no specific safety factor listed in FM Global's public Data Sheets. You can ask the FM engineer assigned to the project or use 10-psi or 10% of the demand pressure whichever is less. Personally, I would not accept a 2-psi cushion.
4/26/2023 08:54:59 am
I strive for a 10 psi safety, minimum. Even with a 10 psi safety, I take into consideration my velocity. Once you start getting velocities above 20 (25/30), you are going to lose pressure quickly through additional fittings that may not be within your calc.
4/26/2023 09:00:41 am
This is a good thing to point out. I have a note in my 0 sheet that limits the velocity to not exceed 30 ft/s for Hazen-Williams equations.
4/26/2023 08:57:41 am
NFPA nor FM require a safety margin. My opinion, though it might be unpopular, is that if additional safety margin was vital to these systems doing their job, they would be required. NFPA and FM build in safety margins already, in the density, in the design area, in the C factor. They are X% of the time overdesigned, which is a good thing when it comes to a life safety system. Cost to the end user is a factor in promoting our industry and is an important factor that we must take into consideration. More than once I've had to have the conversation of needing a fire pump on a light hazard project because the water supply was mediocre and due to an attic, or elevation, or some other reason, we couldn't quite get the "extra safety margin" that an AHJ wanted, even though it would work with a few psi "extra" safety margin. NOW, I will say under certain circumstances I do err on the side of caution, especially when it comes down to a 13D or 13R system. I am in favor of some safety margin. Number 1 because a 2 or 4 head design is cutting things a little close (not a lot of room for error), and number 2, because of the bucket test. Also if I suspect the water supply is inconsistent, or I'm not familiar with it, etc... Again, just my opinion and the "extra" safety margin does have it's place, but needs to be looked at on a case by case basis.
4/26/2023 09:00:32 am
I assume this project has a municipal supply as opposed to a water storage or break tank. I also assume this is not a manual standpipe system, but rather a standard sprinkler system calculation. That said I have done a lot of FM projects and 10psi
4/26/2023 09:02:28 am
Just about all the projects I have done included a water supply summary from the FM Global Rep for the site/area or we just followed the local AHJ water supply reduction requirements or jobsite specifications.
4/26/2023 04:01:05 pm
FM also sets the discharge coefficient for a smooth outlet at 0.8.
4/26/2023 09:46:39 am
This will probably come across as a rant. Apologies to all.
4/26/2023 10:47:58 am
I use 10% of supplied pressure. The many AHJ that I deal with all accept this number. The only exception is if the local water authority supplies me with a "Historical average low pressure" number than I will use that regardless of the current pressure reading.
4/26/2023 11:05:44 am
The answer to the question is specific to the local FM rep or the local AHJ, and maybe the FPE.
4/26/2023 11:11:28 am
The published pressures, quantities, spacings are all the minimum requirement printed with the safety factor already included.
4/28/2023 02:29:56 pm
I agree with Casey. Examining the loss data, most fires open, and are controlled by, one to three fire sprinklers. In storage scenarios, 4 sprinklers typically open. For a light hazard system with remote area decreases, there are 9 sprinklers calculated on average. This gives a safety factor of 3 to 9. For the storage scenario, this also gives a safety factor of 3 if we consider an ESFR system. So a safety factor of 3 is typically built in. Additional safety factors don’t seem to be needed.
4/28/2023 05:27:42 pm
I don't think 2 PSI is wise regardless of any published or required minimum. It's so easy to go over that by only adding a few offsets in a bulk main or end of line. There are always unforeseen circumstances.
Leave a Reply.
Subscribe and learn something new each day:
Top May 2023 Contributors
Get 100 Days of Free Sample Questions right to you!
PE PREP SERIES