Second 2.5" Outlet on Same or Different Hydrant?

9/13/2022

When conducting flow tests and the desired flow (gpm) for the test is not obtained via a single 2-1/2" fire hydrant outlet, should additional flow be achieved by opening an additional 2-1/2" outlet on that SAME fire hydrant, or should a single 2-1/2" outlet be opened on an ADDITIONAL fire hydrant?

So for multiple 2-1/2" outlets opening and flowing, should this be done:

(A) using one fire hydrant with two 2-1/2" outlets flowing, or
(B) with two fire hydrants with one 2-1/2" outlet flowing each?

I have heard people say that (A) is correct and (B) is incorrect, and vise versa. Thank you.

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11 Comments

Alex

9/13/2022 06:18:50 am

Hi,

(B). You want to use multiple hydrants.

NFPA 291 outline the testing procedures. Figure 4.3.4 outline suggested test layouts for multiple hydrants.

4.3.5 states the number of hydrants to be used depends on the strength of the system

4.3.6 states to obtain a satisfactory test, you need to achieve a drop in pressure of at least 25 percent, or to flow the total demand necessary for fire-fighting

4.3.7 states if the mains are small, a single hydrant can be flowed but..

4.3.8 states if the mains are strong, you need to flow multiple (seven or eight) hydrants

Thanks,
Alex

Dan Wilder

9/13/2022 07:59:56 am

This will be interesting...I am in the (A) school of thought.

The handbook commentary states

"In order to produce a pressure drop of 25 percent, or the total fire demand, multiple outlets on the flowing hydrant may need to be opened during the test. In some cases, this may require that several hydrants be flowed simultaneously to produce a sufficient pressure drop and/or flow." So, we open a hydrant to it's capacity before adding additional hydrants.

Our flow tests always start with the pumper outlet no matter what, then add in the 2½" ports on a single hydrant before adding in an additional hydrants. If we provide an compound test, we do follow the suggested test layouts per Figure 4.3.4. as best as possible with the known water supply routing.

We have an AHJ that will request us to provide minimum water required and provide a test meeting that minimum.....however, their own paperwork states that the test is invalid due to the 25% drop not being achieved. When we have requested a 25% drop, they won't do the test without a GPM (even if we say "X" GPM or 25%, whichever is greater). If we request a GPM that we think will produce a 25% drop based on previous tests, we get push back because of the manpower resources and water depletion required to get the 25%, and it takes weeks to complete....but they accept the invalid test for submittals so....

Eric R

9/13/2022 08:27:31 am

I don't see this question so much as an either/or but more of priority of selection. Opening the second 2.5" on the hydrant is an easy first step, and there certainly isn't anything wrong in doing so. From there, if you still need more water then a second hydrant should be opened up,

We generally default to two 1.75" hose monsters on our flowing hydrant which gives us a good range of throttling control and increased accuracy. I feel this is generally sufficient for a light or ordinary hazard system designs. Even for systems that only need 300-600 total gpm I think it's good practice to flow at least 1000gpm if the utility can handle it, as it provides a much more accurate curve to base a design on.

It gets a bit trickier when you get into the higher hazards however. In our area, some jurisdictions allow us to perform our own hydrant tests, while others require they be done by the utility. This can be problematic when the utility says they cant flow the demand we need due to lack of equipment, but also wont allow us to supply the necessary equipment/manpower to get the results ourselves.

For high demand systems like ESFR the flow test arrangement needs to be carefully planned based on knowledge of the underground layout, peak water demand times, and proper selection of hydrants to get a balanced flow per NFPA 291.

Jay

9/13/2022 10:01:17 am

The intent is to flow more outlets on a single hydrant. You flow more than one hydrant only if you can't achieve the pressure drop or flow you're seeking from the single hydrant.
The recommended 25% drop in pressure was changed to 10% in the 2022 edition of 291.
If you're flowing from the pumper outlet, there is a reduction factor (coefficient) that needs to be applied to the calculated flow (typically 0.83) due to the likelihood that cross sectional area of the large orifice is not completely filled with water (air pockets).

Jesse

9/13/2022 10:03:23 am

Multiple hydrants, and NFPA 291 has some great examples of where the additional hydrants can be in relation to the test hydrant.

Joe F

9/13/2022 10:20:38 am

I agree with Dan & Eric. (A) If you can achieve the desired 25% drop in pressure with a single hydrant flowing both ports that is completely acceptable. If not add additional hydrants as needed.
When we are talking about Hydrant Flow Testing, is the objective to obtain the data for the projects “Fire Flow Demand” or “Fire Sprinkler Demand” In Light and Ordinary Hazard Occupancies the demand is dramatically different. It is much easier to prove water availability for fire sprinkler demand vs the required fire flow for a project.
Another item to keep in mind is knowing the high elevation point of the water system. Our terrain is extremely hilly area, and the pressure difference between the high and low points of the water system can vary greatly. The residual pressure should not drop below 20 psi anywhere in the water system. Knowing the water systems high point is important and placing a residual pressure gauge on the nearest high point hydrant is a great safety precaution.

Dave

9/13/2022 11:05:18 am

I am with the majority (so far) above, the greatest majoriy of the the time we flow the (2) 2-1/2" on one hydrant, which in my experiernce is usually sufficient. If additional drop is needed, we flow two from an appropriately selected additional hydrant (if available). I agree with Eric as well, not so much an either/or.

Chris

9/13/2022 11:32:34 am

Where I live the typical water supply is very strong, sometimes as good as 90-100psi static pressure with drops of only 5-10psi while flowing 1100gpm+. So when working with LH and OH we generally only flow one outlet, but when working with ESFR or storage we flow multiple outlets.

I read 291 as multiple hydrants, except for 4.7.9 in the 2022 edition it says "...by the number of hydrants used and the number of outlets opened on each." Other than that I don't see anywhere else where it mentions multiple outlets on single hydrants.

My vote goes to option (B)

Food for thought, I conducted two flow tests at the same location one conducted like (A) where static was 62, residual was 56 and flow was 2,106gpm. Then I conducted one like (B) and got static of 66, residual was 50 and flow was 2,042gpm (Hyd#1 1,186gpm, hyd#2 856gpm). The tests were done over a year apart though, although development in the area between tests has been minimal.

Franck

9/13/2022 01:01:39 pm

You can do both
What is important is the total flow (that is what is leading the friction losses, and therefore the pressure drop).
Remember that with such a flow test you measure your system at the hydrant where you measure the pressure, not the flow.
This means that for a dead end line, there is no difference in the results if you open 2 outlets at one hydrant or 2 hydrants, as long as they are all located downstream the “pressure hydrant”. With a loop system, it becomes more problematic and you should chose carefully the location of the flowing hydrants compared to the pressure hydrant. See NFPA 291, they have nice sketches for the selection.
In that case, opening 2 outlets on the same hydrant (option A) will give you more accurate information.
After making a lot of hydrant testing in my long life of loss prevention engineer, I always use option A as it is easier (I am lazy), flow less water (if you test with a pitot tube and need to walk from one hydrant to the other… you will spoil a huge amount of water… This may be my environmental side), and is more accurate at the end.

Glenn Berger

9/13/2022 02:52:04 pm

As others have said, you need to follow NFPA 291 as the recommended practice for hydrant flow testing.

Questions to be answered prior to an appropriate answer:

1) Are you testing hydrants on a dead-end system?

2) Can you shut a valve such that you can simulate worst case scenario with a looped or gridded system out of service?

3) If testing multiple hydrants can you maintain line of sight and/or communication with the people at the other hydrants?

My personal preference is to simulate an one-way flow condition and then open as many outlets as required to achieve the proper pressure drop. A pressure drop of 25% is what is stated in NFPA 291, but not always easily achievable.

Before conducting the test, you need to understand the system configuration and preferably have historical data available to determine the adequacy of the test information recorded.

schulman

9/15/2022 06:12:55 am

constantly receiving flow tests NOT done to NFPA 291 ...makes my job difficult if not frustrating. Please do tests per NFPA 291 if you are a testing contractor !!!

Second 2.5" Outlet on Same or Different Hydrant?

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