I'm putting together design-build requirements for a new single-story construction building. The flow test pressure is borderline in my opinion so I'll be calling for a fire pump.
When is a water storage tank required with a pump? I don't think the system needs one, but I just want to be sure. Posted anonymously for discussion. Discuss This | Submit Your Question | Subscribe
8 Comments
Dan Wilder
12/7/2020 08:43:42 am
You will need a tank when the water supply cannot meet the required GPM for the pressure and/or duration required per (typically) Appendix B of the Fire Code and/or the sprinkler demand plus any outside hose stream (whichever is greater). There are several other methods available depending on the AHJ adopted methods (ISO, NFPA, etc.)
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Franck
12/7/2020 09:26:46 am
If your flow test pressure is borderline, this is a good thing to plan for a dedicated fire pump.
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Casey Milhorn
12/7/2020 10:24:36 am
These gentlemen covered it pretty well. As an example of how this would work in your case, lets say your flow test was 50 psi static, 40 psi residual, at 400 gpm. Now lets say your building was classified as ordinary hazard 2 and 22' tall (so reduced area is out). Now lets say you estimated a demand of 50 psi @ 350 gpm. It's pretty evident from this that you will definitely need a booster pump (or let the sprinkler sub try and VE it out if the owner is up for it). So you could specify a 40 or 50 psi pump and that would take care of the pressure issue. The gpm in this pretend scenario is enough in the test without being close to that 20 psi minimum we all worry about. Also as Dan and Franck talked about, be very very careful about IFC fire flow demand, which will dwarf the sprinkler system demand typically. If private hydrants are coming off a new tap for this building, you can be stepping into a bear trap here. So other things to watch out for is if your flow test didn't actually perform to the gpm required, then I would recommend another flow test with two outlets or more open so that your demand point is within a real curve and not extended out into a projected area of the curve. Also if your flow test is flirting with 20 psi residual at or near your demand, then I would be nervous about using only a fire pump.
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Casey Milhorn
12/7/2020 10:44:53 am
Also, I failed to mention the 250 gpm hose demand in my pretend scenario and where it is added into the calculation based on public or private hydrants locations could definitely have an affect on the pump and tank sizing and requirements.
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Pete D.
12/7/2020 12:07:39 pm
The IFC appendix allows for a 75% reduction for fully sprinklered building with AHJ approval. Also, the tabulated duration typically only comes into play with a tank or pond. A lot of folks don't realize, if you have an automatic fill valve you can subtract the fill rate and reduce the tank size. [Required Fire Flow (0.25) - fill rate] * duration = tank volume.
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Franck
12/8/2020 03:03:34 am
Yes
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Dale Morris
12/8/2020 10:37:45 am
I worked on a project where we had to provide a tank due to it's Seismic classification. The building was a mixed use residential high rise, and was required to have two sources of water. The way the piping in the street was designed, there was no way to use a service that was separate from the only service in the street. So, a tank was installed in the parking garage to serve as the second source. The building has a roof top pool, and that was considered to be used in lieu of a tank. I don't remember what swayed them from not using the pool and installing the tank instead.
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Frank V
1/6/2021 12:15:34 pm
That's the situation we're facing on a very similar project here in Atlanta where we just learned of a heightened Seismic category. A couple of questions. 1) I assume since the tank is to serve as a second water source, that you cannot subtract the fill rate in order reduce the tank size, correct? 2) What flow rate and duration is ultimately required for the tank size - required flow or 150% of pump? Sixty minutes or what?
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