In reviewing the top articles from 2024, I re-read #11 regarding forward flow. As an AHJ, this is something we have been focusing on for the last four years and have uncovered multiple water supply issues.
My question is related to NFPA 13 and the 2-1/2” hose valve that is required for every 250 gpm (950 L/min) of system demand. From the fire suppression side, we usually generalize that a 2-1/2” hose valve can only flow 250 gpm. However, in our forward flow testing and research, we have found that a 2-1/2” hose valve off a main riser can actually flow almost 600 gpm. So my question to the forum technical design experts is, if we are getting an adequate gpm flow for system demand from the pitot reading, do we really need to flow a hose valve for every 250gpm of system demand? In other words, if system demand is 450 gpm and we are getting that from a single 2-1/2” hose valve, is that accurate and acceptable? Sent in anonymously for discussion. Click Title to View | Submit Your Question | Subscribe
16 Comments
Daniel LeFave
1/22/2025 08:15:51 am
Some one correct me if I am wrong, but I believe when you measure flow through a pitot you are recording pressure and using a chart/formula to give you an idea of what the flow would actually be. So for your example, one hose valve being opened has a very high pressure due to a large flow rate trying to push through a small opening (no different than placing your finger over a garden hose - there's a pressure increase but less water is flowing), but you aren't actually flowing 600 gpm. The added hose valves will ensure that the proper amount of gallons are forward flowing the backflow.
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David Kendrick
1/22/2025 08:27:47 am
The pitot measure at a known outlet with known physical properties is the actual quantity of flow.
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Randy Kimbro
1/22/2025 09:11:51 am
Daniel -
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Wes
1/22/2025 09:17:38 am
Is this a case where determining exactly how many hose outlets would require a calculation, whereas an overly conservative prescriptive requirement makes it black and white to design and review?
David Kendrick
1/22/2025 08:18:51 am
If proof you seek is the quantity / pressure records from the system it seems from your statement you have achieved your goal.
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Chris
1/22/2025 08:26:35 am
NFPA has simplified this issue by stating that the system demand must be flown. However, the primary purpose of forward flow is not just to meet system demand, but to exercise the backflow's check valve springs in order to prevent them from seizing. The industry has oversimplified this by comparing the system demand to the riser size, assuming that because the backflow is roughly sized for the system, flowing the system demand should be sufficient to open the check valves.
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D
1/22/2025 08:52:41 am
Addressing the 250 gpm limit per valve specifically, this is something that has come up several times recently for our office. One great example is an ESFR system where we were able to get away with no fire pump. The backflow was 8", the system demand was roughly 1500 gpm, necessitating six 2½" hose valves to get the full forward flow per the latest requirements of NFPA 13 (even if we had a fire pump, this test header would be typical anyway). But what if you had that same 8" backflow to feed multiple OH2 systems, where the greatest demand was only 550 gpm including hose allowance? First of all, do we really think we need to add a 3rd hose valve to get the final 50 gpm? Secondly, is 550 gpm adequate to fully exercise the backflow?
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Pete H
1/22/2025 09:07:30 am
I'm not sure about the answers to this question, but I'm hoping someone who is responds to it.
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Ivan Humberson
1/22/2025 08:57:47 am
The provision of a 2-1/2" hose valve for every 250 gpm of demand has been a nominal requirement in several NFPA standards for many, many years. That being said, it is not to say that a single 2-1/2" hose valve can only flow 250 gpm. Flow rate is going to be dependent on the size of the supply pipe and the available flow and pressure of the water supply. It is not uncommon to be able to flow several hundred gallons per minute from a single 2-1/2" hose valve, given adequate pressures and flows supplying it. The minimal standard to supply one 2-1/2" hose valve for each 250 gpm of demand is to ensure you can meet the demand flows, even with minimal supply pressures.
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Dan Wilder
1/22/2025 09:13:40 am
While your building may have the available water supply to achieve the flow rates greater than the stated 250 GPM per 2½" hose valve, there are locations that would not be able to make this work with a single valve.
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David Kendrick
1/22/2025 10:01:34 am
The author's question wasn't about design.
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Jesse
1/22/2025 10:02:22 am
Hi all. Back from vacation so Ithought I'd chime in, I want to reiterate what Dan said (seems I do that a lot). Seems I do that a lot which may mean Dan is freaking brilliant or I'm just lazy. Or maybe both.
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Shane D
1/22/2025 10:24:57 am
See below, other means shall be permitted as long as the system doesn't require modification to perform the test, and it is sized to meet the system demand.
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MIGUEL ANGEL D'ADARIO
1/22/2025 12:59:14 pm
The main challenge is that a very high flow rate through a smaller orifice generates a high velocity stream, which causes the pressure reading on the Pitot gauge to be highly variable and unstable.
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Glenn Berger
1/23/2025 09:01:30 am
If you try to flow more than roughly 250 gpm through a 2-1/2 inch outlet the pressure drop may more than desired.
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Randy Kimbro
2/3/2025 02:18:03 pm
Very interesting discussion. Thank you all for your expertise and input. This will help us as an AHJ but also help as we teach this concept to new fire inspectors.
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