Is a 4"-6" Discharge OK for Sprinklers & Hydrants?

5/17/2024

Can a 4" pump with 4"-6" discharge fitting be used on a private 10" combination line (fire suppression and hydrants)?

NFPA 24 Section 13.1 states that no line less than 6" shall service a hydrant. But does the fact that NFPA 20 Section 4.16 allows the 4"-6" discharge fitting as part of the pump assembly overrule this?

Thanks in advance.

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4 Comments

Dan Wilder

5/17/2024 07:58:34 am

Two separate standards...cross compliance that is not specifically called out in either standard is not a requirement. Also, based on the question, the 10" feeds the 6" laterals to the hydrants via the combined service piping, the pump sizing would not play into that pipe size requirement, only volume. As a '4" pump' is only available in 300 or 400 GPM volumes, that would be my concern more than the sizing...6" discharge requirements of 750 & 1000 GPM volumes but would already comply with NFPA 24 only because it's a requirement from NFPA 20 (Table 4.28), they are not inclusive or a prerequisite between them.

Even if the pump directly fed the hydrant (like 5' away from each other....) NFPA 20 pipe sizing scope starts at the "10 diameters upstream of the pump suction flange"(20-24'-4.16.3.3) with everything prior to that being sized to comply with 4.16.3 and then stops at the Discharge Pipe & Fittings, specifically at the system side of the discharge valve, butterfly or OS&Y. (14-22'-4.17.1)
If the discharge fitting of the pump was 4", that would comply with the NFPA 20 requirements. Once out of the scope of NFPA 20 (the previously mentioned discharge valve), the piping must be a minimum of 6" to the hydrant per NFPA 24. (24-22'-13.1), again, assuming the hydrant is directly attached to the pump and not fed from an above ground or underground service main.

The illustration for A.3.3.13 shows this as the "end of private fire service main".

Brett

5/17/2024 08:31:50 am

The requirement for a hydrant to be feed with a minimum 6" pipe is to ensure a minimal volume of water availability at the hydrant. When designing any system, but especially one which includes a pump, you really need to thoroughly evaluate the purpose of the system. This will help you determine how much water is going to be needed. The pump can only affect pressure, not volume. However, it must be sized to not reduce the required volume necessary for the system.

Jesse

5/17/2024 09:16:30 am

I typically refrain from making an affirmative statement about something either way without calcs, but I'm gonna say this probably isn't a good idea.

First of all, we dealing with two different standards here.24 doesn't come into play until after the discharge flange and valve on the DC side.

I steer my design team away from referring to pumps by their discharge size, and prefer their rated capacity. In this case, you're looking at a pump of no more than 400-gpm. Its likely that your fire flow requirement at the hydrants far exceeds 400-gpm

Jack G

5/17/2024 09:39:39 am

Residential neighbor hood, H2, commercial?
All that will come into play when figuring the fire flow.
If a 400 gpm pump the 150 % is 600 gpm.
Once the cart in 20 is satisfied, the 6 inch minimum comes into play. I probably would make the lead into the pump 6 inch as some manufacturers recommend to ensure the 150 %.
The 10 inch may be a “ trifle” overkill. But may be sized this way for volume available to the fire department.
Given all of the data, I probably would not install the above scenarioo as was presented.,

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Is a 4"-6" Discharge OK for Sprinklers & Hydrants?

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