Which Hydrant to Flow for Well-Gridded Supply?

3/24/2022

I'm quite familiar with conducting hydrant flow tests, NFPA 291, and have read articles such at the Sprinkler Age article from Feb. 2018. And I know the residual hydrant should be between the hydrant(s) to be flowed and the large source mains for the area.

But in terms of a specific project site (especially if the underground is well-gridded), would you chose the hydrant closest to your project to be the static/residual hydrant, or would you select the flowing hydrant to be closest to your project (and adjusting the static/residual for any elevation difference)?

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10 Comments

Jesse

3/24/2022 08:08:16 am

Sounds like you're pretty familiar with 291, which is good. I've kicked flow tests back to field crews because they did it wrong more times than I can count.

For me, I always specify the residual hydrant to be the one closest to the project. Our calcs effectively go from the pressure end head sprinkler through the AS piping, through the riser, and then from the base of the riser to the residual hydrant. Obviously we have to account for the friction loss realized in the underground so by having he residual hydrant closest, that's less pipe I have to account for.

I just had this conversation with a designer who put the water supply node about 6-ft from the base of the riser when in reality, its 1000-ft away through a network of UG pipe to the residual hydrant.

James Phifer

3/24/2022 08:10:07 am

From the Appendix of NFPA 13:A 24.2.2 (2016 ed)

The proper method of conducting this test is to use two
hydrants in the vicinity of the property. The static pressure
should be measured on the hydrant in front of or nearest to
the property and the water allowed to flow from the hydrant
next nearest the property, preferably the one farthest from the
source of supply if the main is fed only one way. The residual
pressure will be that indicated at the hydrant where water is
not flowing.

Brad K

3/30/2022 10:59:25 am

Great response from James P. here and correlates to Jesse's response.

Glenn Berger

3/24/2022 08:12:57 am

One thing that I have done in well-gridded systems is to shut off a valve and perform the test towards the shut-off valve.

3/24/2022 09:01:08 am

I always flow the hydrants closest to the building because that's basically simulating the flow of the fire sprinkler system in the direction of the building if there was ever a fire. If you think you have a abnormal site loop I would recommend conducting 2 or 3 flow tests to make sure your calculation works with the worst case scenario.

Alex

3/24/2022 02:22:18 pm

Hi,

I agree with NK. I would flow the hydrant closest to the site. Do you have a picture of the UG network? This could be used to tell where the building service is feed from. If you aren't familiar where the building service connects, I would recommend flowing 2 hydrants like NK mentioned.

Thank you!
Alex

Brian Gerdwagen FPE

3/24/2022 10:24:59 am

The residual hydrant is where your flow and pressure results are valid and where you stop your hydraulic calculations.

Casey Milhorn

3/24/2022 10:31:01 am

I love this topic because it leaves so much room for interpretation (due to actual placement of hydrants, elevations, etc.) and is something that so many AHJs, designers, engineers, etc. have questions on. James is correct, and you do want your static to be as close as possible to your connection point. I will add on a gridded city system, I believe you can place your test point (adjusted for elevation) at the system connection to the city main. Been awhile since I've read 291.
If on an unverified city gridded main, or a confirmed one way city main, the calculation should go back to the gauge hydrant. What is interesting is that many people will actually take it to the hydrant, through the gate valve, and whatever pipe is in the branch feeding the hydrant. By doing it that way you are adding friction loss that wouldn't occur in the normal flow path. IMO the test point should be at the tee of the gauge hydrant for a single direction (or dominant flow) city main and at the system connection point for a gridded or looped city system where pressures and flows are expected to be practically constant from hydrant to hydrant (relative to area and elevation).
An interesting thing about performing flow tests on a dominant or single direction city main is that if you reverse your hydrants, you are actually hurting yourself. You are robbing water before your gauge hydrant and you will see a drastic drop in residual pressure many times. If you design off of this flow test you definitely are overdesigning the system, or even adding fire pumps where otherwise would not be required.
I will say the most common issue I see is not accurately taking elevation into account and applying hose allowances in the wrong place in the calc. I would almost guess at 25% or more calculations that I see have one or both of these wrong.

Chris Logan link

3/24/2022 02:15:14 pm

I typically flow water past the project, and take residual from the hydrant closest to the project.

(A) (B) (C)

A-Flowing Hydrant
B-Static/Residual Hydrant
C-Project

Jack G

3/24/2022 02:44:24 pm

As a double check, since you need to flush your underground supply, I would gauge the hydrant closest to the property, put a test header on the F&S, after flushing. Roll out the hoses, pitot, and I don’t believe you can get base of riser flow and pressure more accurate than that. Opinion. Number of hoses depends on calculated demand.

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Which Hydrant to Flow for Well-Gridded Supply?

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