City Pressure Req'd to Calc Higher than Demand?

8/28/2024

Where is the code section that states the numbers (calcs) on the supply side shall be greater than the numbers at the base of riser (system side)?

Does the code address this with a code section?

Thanks in advance.

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7 Comments

Peter Howard

8/28/2024 06:25:51 am

I don't know if it's code... but doesn't it have to work this way?

The supply side wouldn't have friction losses from fittings, valves, and piping that the system side inevitably would.

Anthony

8/28/2024 06:50:19 am

I think the following 2 code sections will suffice from NFPA 13-2016:

23.4.1.6 Hydraulic calculations shall extend to the effective point of the water supply where the characteristics of the water supply are known.

24.1.2 Capacity. Water supplies shall be capable of providing the required flow and pressure for the remote design area determined using the requirements and procedures as specified in Chapters 11 through 22 including hose stream allowance where applicable for the required duration.

11.1.5 Water Supplies.
11.1.5.1 The minimum water supply shall be available for the minimum duration specified in Chapter 11.

11.2.3.1.2 The minimum water supply shall be available for the minimum duration specified in Table 11.2.3.1.2.

I'd like to echo what Peter said; cuz ya know, that's how sprinkler systems work!?!?

Dan Wilder

8/28/2024 08:00:32 am

Unless the building is downhill from the flow test location and/or goes through a pump, having a required pressure greater than the available pressure equates to the system not working.

NFPA 13-22' Chapter 5 Water Supplies
5.1.2-Water supplies shall be capable of providing the required flow and ***pressure*** for the remote design area determined using the requirements and procedures as specified in Chapters 19 through 27 including hose stream allowance where applicable for the required duration.

***emphasis added

Glenn Berger

8/28/2024 08:23:29 am

I agree with Dan. I had to review the question posed today. The Sprinkler System Designer is REQUIRED to ensure that the water supply must be able to deliver sufficient flow and pressure to support the proposed sprinkler system.

Joe C

8/28/2024 08:24:54 am

I had one system "base of riser" that was about 80' lower in elevation than the flow test hyrant on the hill above adding about 35psi. An of course it's an obvious result if you have a pump in-between the "source" and the "base of riser'.

Jesse

8/28/2024 08:50:55 am

Dan's on the right track here.

In fact, even if the base of the riser were significantly downhill from the water supply - the water supply node on the design drawing - the supply side will always have higher numbers than the remote area (barring a pump). The lead in to the base of the riser is still on the supply side and isn't subject to the friction-loss generating system piping.

Jack G

8/28/2024 07:57:34 pm

Pretty sure you won’t find anything in the codes with that exact wording. NFPA 1142 shows how to evaluate the water supply..
Of course if the supply doesn’t meet the demand calcs, a fire pump/ pressure tank would need to be added.

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City Pressure Req'd to Calc Higher than Demand?

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