Can a Hydrant Flow Test Use Only One Hydrant?

11/29/2023

Can a hydrant flow test (for the purposes of a fire sprinkler design) be conducted using a single hydrant?

If so, how is this actually done? Is it recommended?

Thanks in advance.

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16 Comments

Dan Wilder

11/29/2023 07:38:56 am

NFPA 291

4.4 should be followed where 2 (or more) hydrants are utilized to define water supply as shown in Figure 4.4.4.

Can you perform it in this fashion, yes and the process would follow 4.5.

Should you, IMO, no as it does not accurately define a water supply, only the ability of that hydrant to flow the measured amount at the measured static/residual. It's the butt gauge on one 2½" outlet and the pitot/diverter/hose monster on the other 2½"/pumper outlet.

Quick search for a pic gave me this: https://bigislandgazette.com/fire-hydrant-flow-tests-scheduled-for-hilo-shipman-areas/

However, there are times when this is the only method available. It's typically worse than performing the test with 2 hydrants. If at all possible, try to find a nearby domestic/commercial backflow (test port #1 on the supply side or #2 if you need to close the valve to take the plug out) or other direct connection to the water supply to pull the static/residual.

Glenn Berger

11/29/2023 08:08:31 am

Dan gave an very good detailed answer. In Summary:

Can it be done? yes

Should you trust the results? NO

Anthony

11/29/2023 08:27:16 am

I agree with Glen and Dan here.

sean

12/31/2023 06:55:12 pm

the one hydrant method throws so much into that one point makes an issue that I really don't trust the numbers that come from it.

a two hydrant test can be messed up so easily that a one hydrant test is just that much more fraught with issues.

M. Newell

11/29/2023 08:19:14 am

AFSA just put out a posy about this on linkedin/facebook. It can be used to verify capacity of the hydrant but should not be used as a water supply evaluation. I would recommend following them on social media they have good informative posts about questions they get.

Alex

11/29/2023 01:22:53 pm

Agreed! I just read that. Highly recommended.

Chris

11/29/2023 08:30:17 am

If you are doing a flow test and by yourself, there are ways to do this and still flow 2 hydrants. What I have "heard" before was to use your phone to video the test hydrant while you flow the other. Then you can review the video after to determine the before, during, and after pressures.

***Note, in the summer heat you may need to make sure that your phone doesnt overheat!!!

Dan Wilder

11/29/2023 08:41:54 am

I've done this several times....just make sure that when you start the recording and set the phone down, you don't turn the recording off or the phone isn't angled to pick up the gauge or being pointed into the sun AND you check the recording prior to packing up....or plan on redoing the test (practice makes perfect I'm told...)

Jesse

11/29/2023 08:35:56 am

Dan is on the money here.

"Can" it be done? Yep. Should it be done? Probably not.

Casey Milhorn

11/29/2023 08:49:51 am

Yes, can be done and sometimes it's your only option. Will everyone accept it, no. Would I do it for an ESFR system, or other high demand system, no. Would I do it for sizing a fire pump, no. Would I use it for a residential or light hazard project, yes, but only if that was the only hydrant available. Also, there are situations where the city doesnt have reliable info on their system arrangement, closed valves, etc and the gauge or flow hydrant can't be verified if on the same system/zone. In this case I do a one hydrant and a two hydrant test and compare results. I really think the most important thing is to use commonsense in all situations, do your homework, and cover your tail. If you or your team doesn't understand the purpose and nuances in performing a flow test, stick to the book and ask someone if you run into a new situation or have concerns.

Jack G

11/29/2023 09:03:56 am

Agree that it is a bad idea.
What I found ( hydrant with 2 outlets at least ) is :
When you have no choice:
Gauge one side - treat it like it’s on a separate hydrant.
Collect the static pressure.
Open the other side ( in theory this should give you the pitot pressure for flow , but not the true residual pressure ) - pitot psi equal or near gauge psi, pressure there treat as residual pressure.
The resultant test will give you something to approximate a calculation, however the “ residual pressure “ will be lower than the actual residual pressure. This test could be used to get a permit but another flow test should be done.m
This is what I was shown when first breaking into the industry, 57 years ago, by an FM inspector in the Phillie area. ( FM had properties in the area, lower Delaware with nothing around.
I only ran into a one hydrant situation a couple times.
In those cases, after bringing in the underground supply, I would gauge the hydrant and flow the stub up ( placing a test header on it ) for actual flow at the base of riser. The Fire Marshals loved that test, for accuracy.
The one hydrant test is not recommended for a final hydraulic calculation.

Patrick Spillane link

11/29/2023 09:21:19 am

Just to clarify, NFPA 291 is a recommend practice, and not a standard, so there is a grey area.

For the purposes of design work, the Main Capacity Test (2+ Hydrants) should be performed.

The Hydrant Capacity Test (single hydrant) should be performed when evaluating and marking the hydrant for Fire Fighting purposes. (AA- Green, 1500GPM+, etc..)

Performing either test, you are always looking for 3 data points. Static Pressure, Water Flow in GPM and Residual Pressure.

When perform a single hydrant test, you will need a Gate valve (For steamer port, or 2.5" port), a Test Gauge Cap, and your water flow measuring device (ie Hose Monster).
Once you have all three data points, you can enter it at the free Fire Hydrant Flow Calculator on https://hosemonster.com/resources/hydrant-flow-test-calculator/
to determine with Main or Hydrant capacity at 20 psi.

Reach out if any questions. [email protected]

James Art, FPE

11/29/2023 10:33:33 am

Perhaps you can get the static somewhere else that is close ?

Pressure is pressure, so IF there is another connection,
like a Fire Sprinkler riser or water service* you may be able to get residual pressure from before *upstream" of other devices, like a backflow.
Some check valves are also equipped with plugs upstream of the clapper, so sometimes you can remove and use that.

Industrial: IF this is on a private fire loop, with hydrants, pressure on a riser, below the alarm check will do well.

*If this is a neighbor, you may need permission, and to be sure it is on the same line.
--------------------------------------------------------------------
Residential: For small 13D systems I sometimes ask for a domestic pressure, measured at a faucet or hose outlet (also sometimes at a neighbors) as a Preliminary, just to get an early idea of what pipe sizes may be needed.

Note for large houses, if you are sprinklering anyway it makes sense to go to a 4 sprinkler** calculation (or use 13R) to also protect the building.
I recommend that,
and several AHJs that I support have that in their Code Adoption.
Often the line is at 5000 sq. ft, or three stories.

**These are fire "sprinklers", not "heads"
A head is the bathroom on a ship!
Or an indication that a plumber may be in over his head!!

Fred Walker

11/29/2023 08:51:28 pm

Let add my concurrence to everyone above.

Mutiple hydrant (test and separate flow hydrant) should always be used for design and for network analysis.

Single hydrant test/flow may be used for system health testing and hydrant ITM efforts. But anytime significant variations between test occur, additional analysis is always prudent especially if you are providing an engineering report (signed, sealed, or stamped).

Chris

12/1/2023 02:26:49 pm

I would say a single hydrant should not be used for supply analysis if there are other options. Videoing a static/residual gauge is easy to do. There are some people that will recommend gauges that are digital with a "minimum pressure" recall feature to obtain the residual pressure on the gauge hydrant. But after evaluating that, against a regular gauge that is being observed, the "recall" gauge will read a lower pressure than what is observed on the regular gauge after the pressure settles once all the water is moving, and in my experience it can vary between 5% and 20% of a difference. So I used to tell guys it was fine for doing a pre bid flow test for estimation purposes but for the actual design it wasn't to be used.

Jim Amy

1/13/2026 03:01:04 pm

A single hydrant test is not a good measure. I have an old insurance manual with an appendix where this is discussed and they provide corrections for the residual pressure to account for turbulence in the hydrant (e.g. increase residual pressure by 7 psi for a flow rate of 625 gpm). That is beside the point though since the pressure in the hydrant is not the same as the pressure in the main. The pressure in the hydrant incorporates the pressure drop from the water flowing from the main to the hydrant and that value can vary based distance of the leg to the hydrant and elevation of the hydrant as compared to the main. While the elevation issue also shows up at the test hydrant when using a two hydrant flow test, we can at least find that elevation and account for it. We cannot add back in the pressure lost from the main to the hydrant. Bottom line is that is is a bad idea and should be avoided at all costs. Having completed over 1,000 fire hydrant flow tests, my team has never had to resort to a single hydrant flow test. There has always been another option.

Can a Hydrant Flow Test Use Only One Hydrant?

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