Forward Flow: 2½" Necessary for Each 250 gpm?

1/22/2025

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?

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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.

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.

So if you measure 600 gpm and an outlet then you are measuring 600 gpm.

Knowing the residual pressure at the supply upstream of the outlet allows for the data to be entered as a flow curve where you can find other flow / pressure rates.

Randy Kimbro

1/22/2025 09:11:51 am

Daniel -

You are correct that we (AHJ) measure pressure via the pitot. And we utilize a chart to convert that to GPM. The charts take into account the relatively higher pressure as well as the outlet size and even the orifice shape to convert that pressure to a fairly accurate GPM.

One manufacturer has a statement on their chart stating, "THIS DEVICE IS FM APPROVED The pressure vs. flow rate data developed within this flow chart is based on the average K-factor measured during laboratory testing. This data has been determined to be within the acceptable limitations for accuracy."

Some example flow charts from a well known manufacturer can be found here:
https://hosemonster.com/resources/download/flow-charts/

Since the charts are pretty accurate, my AHJ question for the experts on this forum goes back to why does NFPA 13 require a 2-1/2” hose valve for every 250 gpm (950 L/min) of system demand when we can accurately measure more flow than that?

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?

I don't know, but it might simply try to address this issue in design, rather than leave it open-ended (performance-based) and have it fail in the field - putting everyone in a tougher position?

The requirement to achieve forward flow has been in the code for decades now, but the means to achieve it hasn't been prescriptive and therefore, sometimes if not often, few have complied and enforcement has been difficult.

I wonder if this isn't just a clear way to clean that up.

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.

The measurement for this data is taken from an "outcome" point. A discharge point.

Even with a flow meter in place at the pump, that data has to be proven by an actual flow to be considered an alternative location for flow data.

Personal opinion.

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.

The assumption that a backflow matches the system size isn't realistic when an underground vault is used. Often, a 6" backflow is installed in the vault, while the system inside the building is 2-1/2" or smaller. Basing the forward flow solely on the system demands of a 2-1/2" riser manifold may not be the correct approach in this situation.

In terms of flow dynamics, I’m not entirely sure at what point on the flow curve the check valves are considered fully open. From what I understand, the initial spike in flow is required to start cracking the valves open. The flow typically drops to its lowest point before gradually increasing. If my assumption is correct, this would mean that the check valves are fully open when the flow reaches the bottom of this curve.

If this is indeed accurate, a 2-1/2" C200 backflow would likely be fully open at around 150 gpm, although this figure seems a bit high. For a 6" C200, full opening would likely occur around 300 gpm. However, many 6" systems will have system demands of 600-1000 gpm, meaning that following NFPA guidelines would result in 3x-4x hose valves. In addition, as you pointed out, a hose valve can flow well beyond 250 gpm quite easily, meaning that the 3x-4x hose valves would likely be able to flow even more.

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?
Is there a way to safely estimate and prove to an AHJ that a single 2½" valve will provide more than 250 gpm?

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.

Based on David's response above, you don't. As you still would have proven the system as long as the extra 50 GPM over the 250 at one of the hose outlets.

But I'm not seeing code references to make me feel secure in that claim. Though Chris does give the note that NFPA has simplified this by stating that the system demand must be flown.

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.

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.

The intent is to be able to capture most of the situations for a common goal of the forward flow test. You may be able to argue that the 250GPM could be extended up to as much as 150% or 375GPM as generally approached from the NFPA 20 side, but all the way up to 600GPM would be very difficult as a solution to save the $200 in material and labor for the install.

Nothing says you have to use all the hose valves, just that they need to be provided and the forward flow test is to achieve the required GPM of the system. If you meet those two criteria, you're good to go.

1/22/2025 10:01:34 am

The author's question wasn't about design.

The question reads as during a performance test and are these results valid.

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.

While you can often eget more than 250-gpm from a 2.5" discharge, we need to remember that we always trade something for something else. In this case, we trade pressure for volume. Our water supply isn't infinite, and if we look at on a graphical curve we see that to get a higher volume; we have to sacrifice psi. So yeah, while you may be able to get more than 250-gpm through a 2.5" DC in some cases (and in some cases maybe not), you're sacrificing pressure to do so. So, to forward flow at system demand, we need a 2.5" discharge for every 250-gpm of demand

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.

2022 NFPA 13

16.14.5* Backflow Devices

16.14.5.1* Backflow Prevention Valves
A test connection shall be provided downstream of all backflow prevention valves for the performance of forward flow tests required by this standard and NFPA 25 at a minimum flow rate of the system demand including hose allowance where applicable.

16.14.5.1.1
A 2 1/2 in. (65 mm) hose valve shall be provided downstream of the backflow prevention valve for every 250 gpm (950 L/min) of flow rate required by the system demand including hose allowance where applicable.

16.14.5.1.2*
Existing hose connections downstream of the backflow prevention valve shall be allowed to be utilized.

16.14.5.1.3*
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.

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.

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.

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.

Forward Flow: 2½" Necessary for Each 250 gpm?

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