Required Pressure & Flow Through A Hydrant?

5/6/2020

I haven't had any experience doing site calculations before, but I'm curious how it works from a practical standpoint. Fire Flow is required by the International Fire Code here locally, and there's guidance (albeit not formally adopted) in Appendix B of the IFC for a total demand. Additionally, there's hydrant spacing requirements for any particular building, and guidance on how far the hydrants can be from a building.

In order to determine how the hydrants are fed (dead-end vs. looped and size of pipe), are there specific flow and pressure amounts that each hydrant has to be calculated at?

Is it similar to a standpipe calculations where each hydrant has to have a specific flow?

I'm not performing the design work myself, but I'm just curious how that is typically done and pipe size determined.

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11 Comments

Hector R

5/6/2020 11:54:48 am

Yes, it's done in a similar way as standpipes, hydrants will require 1500 GPM at 20 PSI.

Nick

5/6/2020 11:58:01 am

Where does the 1,500 GPM per hydrant come from? Am I missing that somewhere?

Mike

5/6/2020 12:27:24 pm

Piping size is Chap. 7 of NFPA 24.

Jesse

5/6/2020 12:55:03 pm

Good question. Its not dissimilar to any other volume calc you would do for automatic sprinkler design. I guess the process would depend on which software suite you're using. For a program like AutoSPRINK its pretty simple, but it starts with the fire flow requirement in your local code.

In my area of Austin, Texas several FDs have amended the code to require 1500-gpm at 20-psi minimum for hydrants on a private fire main.

What design software are you running this through?

Franck

5/7/2020 10:49:33 am

1500 gpm per hydrant?
Are you sure?
I never see an hydrant able to flow, on its own, 1500 gpm.
I can understand that the manual firefighting demand is 1500 gpm, with several hydrants. For example, NFPA requirements for manual firefighting (hoses + hydrants) normally range between 100 and 500 gpm in NFPA 13.
In Europe, requirements from Fire Departments are normally 250 gpm at 15 psi by hydrant. The number of operating hydrants at the same time depends on the occupancy hazard and can range from 1 to 6 (250 gpm to 1500 gpm), regardless of possible sprinkler installation.

Jesse

5/7/2020 12:23:29 pm

Absolutely. I spent 19 years as a career firefighter before fire protection engineering. I could count on one hand the number of hydrants that didn't flow at least 1500-gpm. Right now I'm looking at flow test data that flows 86 static, 85 static @ 1007 gpm. You don't think that's not flowing way more than 1500-gpm as the flow curve progresses?

Another project, the city adjusted flow is 56 static, 51 resid @ 1186. Follow that flow curve to 20-psi and your moving 3445-gpm.

This isn't through a single 2-1/2 port mind you. But since we're not in the 70s anymore, the FD isn't flowing a 2-1/2 supply line.

I'd be happy to post a flow curve showing this.

Bill

5/7/2020 05:40:09 pm

Keep in mind NFPA 13 is for the hose stream allowance, in addition to the sprinkler demand. It has no bearing on the fire flow requirements for a hydrant. The IFC or NFPA 1 give the minimum flow requirements.

Jesse

5/7/2020 12:24:47 pm

That was meant to say 86 static, 85 residual @ 1007 gpm

Shaikh

5/8/2020 04:56:44 am

Even i have concern on pressure requirement for hydrant.

Some says minimum preasure for industrial application hydrant to be 100 psi and for residential application to be minimum 20psi.

Need a solid ungerstading on this concept as per nfpa becuase i need to do calculation and size the rated pressure requirement for pump.

Please guide

Jesse

5/8/2020 07:38:27 am

Shaikh - I'd look at that specification a little deeper. 20-psi is always our benchmark minimum residual so we don't collapse a main or pull a draught from the suction of a pump. I would speculate that the 100-psi minimum is along the lines of "must maintain a minimum static pressure of 100-psi. So 100-psi static with our rated flow somewhere at a lower pressure, and system not to have a residual pressure lower than 20-psi.

Muhammad Jawad

12/14/2020 12:48:10 pm

JESSE: How to find out the Gpm of fire system? let's say we have 6 hydrants in one system & 15 in another.

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