How Do I "Prove" Fire Flow in Design?

5/12/2022

Is there an established method to "prove" that we have enough fire flow for a project site?

I understand that the premise is to be sure we have the correct water supply available at the site. We do. I'm just interested in the technical 'design' portion to validate that concept.

Typically, I run a hydrant flow test at the site with nearby available hydrants. In this case, we're adding new hydrants but I understand the water supply nearby with recent testing.

Is a hydraulic graph (N^1.85) showing the available water supply at 20 psi sufficient to "prove" fire flow, or should I be conducting some type of calculation where I'm "flowing" hydrants?

We do sprinkler calculations all the time, but I'm just curious if I'm either overcomplicating things or if there's a process for fire flow that I'm not yet doing which I should be.

Trying to get things right. Thanks in advance.

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19 Comments

Alex

5/12/2022 06:14:09 am

HI,

I have never worked on a new 'development' before so I'm not to familiar here. I'm curious to see what others have to say as well.

With that said, how are you sure you have the correct water supply available? I would still conduct hydrant flow tests near your new site and calculate the loss for all new piping to the new hydrant locations. Once the new hydrants are installed, I would then conduct a new flow test to get an accurate graph.

Your city/town/jurisdiction should be consulted as well of course.

Looking forward to what others have to say.
Alex

5/12/2022 08:09:43 am

I have no problem conducting a flow test, but I guess my real question is if the flow test is not immediately adjacent to our site, then do I need to run a 'supply calc' to our site?

If so, am I just flowing every hydrant at my site at 20 psi and seeing what flow can be achieved?

I'm wondering what, if any, calculations need to be done to "prove" Fire Flow other than running a flow test chart all the way down to the 20 psi mark.

robert bennett

5/18/2022 01:27:04 pm

Correct answer, the proof is that the hydrant flows what is required. There are several ways of calculating the "Fire Flow" so make sure you know what method they are using before you start designing.

Glenn Berger

5/12/2022 08:07:27 am

A hydrant flow test is the only acceptable way to prove that there is sufficient flow and pressure. I have seen many projects where hydraulic modeling was not validated with a flow test.

5/12/2022 08:11:00 am

Right, and agreed. We have a flow test that is right at the location where we're going to tap the existing mains.

But do I then need to calc from that flow test to our site? If so, is it as simple as a supply-side calc flowing each hydrant at 20 psi?

Franck

5/12/2022 08:09:54 am

This is the only way to test it for real: make hydrant tests as close as possible to your future connection, with possible friction loss calculations if the test is far from your connecting point.

The only good advice is: flow as much water as possible during your test, this would help having a more accurate representation on your Q 1.85 graph when you will extrapolate up to 20 psi residual.

schulman

5/12/2022 08:17:33 am

I'm so excited to write about something that Alex does not know explicitly !!!

I design a lot of schools which are typically being built (in this area) on large parcels / properties without existing utilities (cheaper rural land) - we often use the Civil Engineers (sometimes working with the jurisdiction to bring water to a development site) who provides us with a WATER SUPPLY MODEL which can sometimes be calculated from a water source up to 10 or 20 miles away

these models are pretty good, should establish flow at 20 psi for the site and generally are close to the actual conditions we see upon construction ...

here's the kicker - I always, always require the contractor to do a Flow Test upon installation of the utility (sometimes years later) to PROVE (your word) that the installed water supply meets the designed conditions and if it does not, we pivot and readdress the issue. Get a copy of NFPA 291 if you dont have it ... cant believe how many flow tests I receive that dont comply ... Good Luck.

5/12/2022 04:32:55 pm

Too funny! I absolutely agree that all new hydrants must be tested upon installation.

Anthony

5/12/2022 08:28:12 am

Fire flow or available water supply is 'proven' empirically with a full flow test. Generally through either a hose monster or other contraption that relates velocity or pressure through a known orifice size into a "q" or volume per time.

Those flow tests then are used to interpolate the results using either graphical or computational methods to the 'demand' requirements.

So, yes, you can use the flow at 20 PSI and a static pressure to determine if you have enough "fire flow" or "water supply". HOWEVER, there are many factors that can change the water supply that gernally make it a good idea to flow the anticipated amount of water on or near the job site. Such as booster pumps that turn on at higher demand, local usage at different times, planed expansion or water service, etc.

Hydraulics specifically of water have been well studied and are well understood. I STRONGLY suggest if you haven't taken a college level fluids course that you spend some time on youtube and wikipedia researching it. Specifically the Bernoulli equation/principle. The information in any of the NFPA hand books and standards will tell you HOW things are done not WHY.

Matt

5/12/2022 08:33:23 am

Run a calculation from the flow test pressure hydrant to the private hydrants onsite similarly to what you would do with a sprinkler remote area. But instead of adding a k-factor at the hydrants use a hose stream.

I typically follow IFC Appendix C for the number of hydrants to be included in the calculation based on the amount of fire flow required (e.g. 2000 gpm = 2 hydrants at 1000 gpm).

5/12/2022 10:07:49 am

Exactly what I was thinking. Thanks for the input Matt.

Different Alex

5/12/2022 10:54:44 am

Hey Matt, do you run the calculation just for internal validation or do you actually include them in your submittals for plan check?

In my experience, the Civil engineer has typically been responsible for "Fire Flow Availability" and they always just post the results of the flow test taken upstream of the new work.

5/12/2022 03:07:50 pm

Yes, I always verify fire flow onsite as part of the water supply verification regardless if it needs to be submitted. This helps to avoid surprises at the 11th hour.

If the jurisdiction does require it to be submitted, then it is complete and ready to go.

Jesse

5/12/2022 08:51:19 am

N1.85 graph. Old school and I still use it today!

We have several AHJs that require this for new development. Essentially, we start with a hydrant flow test, and then model it through the new system in our design software. Adding a hydrant then in the system, we set the flow parameters for whatever our requirement is (and it varies for buildings that are fully-sprinklered and non-sprinklered), and run the calc.

Glenn and others are spot-on though: the ONLY way to prove it is to actually flow water from that hydrant. What we model doesn't always pan-out.

Anthony C. Brown

5/12/2022 09:05:20 am

Hello

As a matter of course I run my calculations back to the hydrant

. When I order a Hydrant Flow test I usually request a design flow of 150% more than what I anticipate as needed. This way if I think I may need 500 gpm and I order a 750 gpm and my design comes in as needing 570 gpm I'm good.

Now if I understand your question you want to know if when you do the calculations should you also include a flowing open hydrant then I would say no but if you want to stay "safe" add a 10% increase in gpm the same as we do with pressure.

5/12/2022 10:10:01 am

Anthony, I think you're correct - but that's specific for fire sprinkler systems.

In this case I'm talking on Fire Flow, which is the water available at the site for use at the hydrants at 20 psi.

Dan Wilder

5/12/2022 10:08:50 am

Working with several AHJ's that do not allow fire hydrant flow tests, many of our designs start with the water model provided by the AHJ. In these cases, they run out the fire flows for a 2 hour duration and only ask what GPM we need (Any more I just say 2000 for anything light/ordinary and 3000 for anything storage or EH related unless a pump sizing at 150% exceeds these amounts). Their water modeling software kicks out a Static + Min/Max expected pressure for that flow duration and we use the lower value + a safety factor to do calculations. When we perform pump commissioning is about the only time we see the actual performance of the water supply and have not had any issues yet (the numbers come out better in most cases).

This follows the approach that a higher GPM flow test is a more accurate test overall. Below is a good read as to why.

https://www.meyerfire.com/blog/advantage-of-flowing-more-water-in-a-flow-test

Salvatore Izzo link

5/18/2022 11:40:39 am

Available water supply for fire protection system design and for calculating fire flow capability are two different things as noted in the above discussions. And as stated, there is no substitute for actual fire flow testing in the area of a subject building to determine the main capacity at 20 psi and the flow capabilities of fire hydrants and their associated piping configurations and service levels at different points in the distribution system. Part of an ISO rating evaluation measures the capabilities of the water distribution system within a community at different locations based on the Needed Fire Flow of different types of buildings whether they are receiving fire sprinkler insurance rating credit or not. Knowing this information is critical in FD preplanning and water supply operations.

Calculating fire flow based on main capacity vs individual hydrants capacity are two different things. NFPA 291 methodology and language of classifying fire hydrants is both misleading and confusing.

Section 5.1 of NFPA 291 states that "Hydrants should be classified in accordance with their rated capacities as follows:" and then it lists the designations and associated capacities. Actually, the available capacity out of an individual hydrant does not have the ability to provide the actual capacity that may be available in the main (as demonstrated in two or more hydrant flow testing). Hydrants should be color-coded based on their single or collective ability to provide the rated capacity within the main, which is some flow at 20 psi. You may be able to get that rated flow out of using one or more hydrants, but not while using all of them simultaneously which would dictate using hydrants on another part of the distribution system or different service level. Just because a hydrant bonnet is colored coded light blue doesn't mean you are going to get all of that rated capacity out of a single hydrant. One would need to find another hydrant on that same portion of the distribution system piping for the additional capacity. I believe the new edition of NFPA 291 is now taking into account not only testing for main capacity, but for testing single hydrant capacity.

In my opinion, a hydrant classification color scheme involving two color code markings would optimize hydrant use; one color that represents the flow range based on the capacity of the individual hydrant, and one based on the main capacity serving a particular group of hydrants served by that main.

This would provide both effective preplan and fire ground operations information for effective firefighting….being able to make quick tactical decisions based on a visual determination to of available water sources to obtain the necessary flowrates needed for effective extinguishment based on the fire size upon arrival.

Bobby V

5/23/2022 11:54:56 am

Typically, the required fire flow is established by local building codes. For the IFC (International Fire Code), this fire flow is based on building construction type and square footage. Or, the local AHJ may allow reductions in required fire flow if the building contains a fire sprinkler system.

Since you are adding new fire hydrants, these are the fire hydrants the fire department will use, so the pipe size supplying the new fire hydrants must be adequately sized to handle the required fire flow at 20psi residual pressure.

A fire flow test must be conducted to establish your available pressure and flow. Then perform your calculations to ensure that the required fire flow at 20psi residual pressure can be achieved.

I presume the local AHJ would allow calculation printouts similar to fire sprinkler system calculations to prove your pipe size works.

How Do I "Prove" Fire Flow in Design?

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