I have question regarding fire pump capacity.
In my situation the water department is requiring a city tap size to be 2 pipe sizes larger than the fire protection backflow preventer.
In my project, I was using a 4" backflow and 400 gpm fire pump so my underground/city tap would be required to be at least 6 inches.
The AHJ is requiring me to increase the area size in one of my remote areas. So, my sprinkler demand now becomes 582 gpm. To avoid having to use a 500gpm fire pump (with a minimum suction size of 6-inch) and increase the underground to 8-inch, could I still use the 400 gpm fire pump to supply the 582 gpm demand?
This comes out to about 145% of the pumps rated capacity.
I have never come close to the 150% mark before and just want to see what others think. If the underground size wasn't required to be 2 inches larger I wouldn't even think about it and go with the 500 gpm pump.
Thanks in advance.
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2/21/2023 08:12:09 am
There are a few things that need to be addressed.
2/21/2023 08:27:42 am
All good questions by Paul.
2/21/2023 08:28:54 am
Meant *combustible. Loose nut behind the keyboard apparently.
2/21/2023 08:51:39 am
Or is it in front of the keyboard?
2/21/2023 08:43:59 am
Riding a pump curve that far is never a good idea. There is no room for error and if the flow test ever changes then the owner is out of luck for future problems.
2/21/2023 09:00:39 am
I was thinking 140% was the max for designing out past the pump's rating. Not in front of NFPA 20 to verify, but I was pretty sure of this.
2/21/2023 09:18:10 am
I've always held onto the concept that many insurers don't recommend exceeding 120%, but that in worst-case situations not to exceed 140%.
2/21/2023 09:19:04 am
Then, additionally, ASK THE AHJ for Enhanced Content out of the NFPA 20 Handbook (highly recommend getting this):
2/21/2023 09:33:25 am
Without naming names, I think I know where this jurisdiction is, and this requirement has caused so many issues on installation.
2/21/2023 09:37:10 am
120% is about the highest I would go on the pump curve. Higher then 120% is opening yourself up for many other issues that have been discussed in the above responses.
2/21/2023 10:27:34 am
1) You comment says that you have a 400 gpm fire pump. This is not a standard size fire pump.
2/21/2023 12:32:15 pm
I will express my practical point of view, which is definitely not the one the AHJ may have.
2/21/2023 01:07:14 pm
Have you thought about using 2 backflow preventers in parallel? You could get two (2) 3'' to flow (600 gpm 300 through each), with no added pressure loss and keep your tap at 6''. I'm not sure how the money works out on that but it's an option worth considering I'd think.
2/21/2023 01:20:28 pm
Not the OP, but never thought of that. If this is the area I'm guessing it is, then this is an RPZ-required area, so that'd have to help. Nice!
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