Is Flowing One Outlet in a Flow Test Acceptable?

5/30/2023

During a recent site visit, I conducted a 2-hydrant flow test with the assistance of a city engineer and the fire department.

The static pressure measured at one hydrant was 84 psi, and then I proceeded 650 feet downhill to the actual flow hydrant, where the pitot pressure was recorded as 70 psi.

A question arises due to the city's reluctance to test additional fire hydrants. My boss believes that testing more than one outlet is necessary to achieve a residual pressure drop. However, the city claims that their water distribution system is looped and, therefore, does not anticipate any pressure drop.

As a result, we are unable to calculate consistent numbers on a graph.

Additionally, we have come across information suggesting that there should be a 15 to 25% drop from static to residual pressure.

Is this a compliant approach? Should we be opening up more than one outlet on the flowing hydrant?

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13 Comments

danefre

5/30/2023 07:42:41 am

Yes it's acceptable, but additional outlets will provide a more accurate curve. Two points on the graph are good enough for most projects. Also, you didn't indicate what the residual pressure is.

Most of the time we only flow one outlet unless it's high piled storage where we try to exceed the system demand in gpm or sizing a fire pump.

See discussions on a similar topic:

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

Alex

5/30/2023 08:02:37 am

To ensure accuracy, you should achieve a 25 percent pressure drop between your static and residual pressure.

If this isn’t achievable, you want to flow your required sprinkler demand. This will ensure you are under your water curve without needing to extrapolate.

Paul Pinigis link

5/30/2023 08:08:09 am

Well, that depends. If your flow volume exceeded the demand flow of your sprinkler system (or the NFPA 1 flow demand) then your test is adequate. Sometimes it is impossible to get a reasonable drop in static pressure due to the configuration of the system (e.g. gravity tanks).

Dan Wilder

5/30/2023 08:12:10 am

Question, was the 70 an actual pitot pressure (flowing hydrant) or was that the residual PSI from the pressure hydrant (84 static/70 Residual) with an unknown flowing GPM (not provided)?

Nearly all of the flow tests we perform or are provided are from one hydrant and we never use the 2½" port anymore, always the pumper port. We do ensure that the flow is at least the estimated/actual water flow required for calculations.

One of our jurisdictions will not plot a curve on their results if the pressure drop is less than 25% and provides a warning (all we do is send in the request with the amount of water needed and the port requested to flow). Some of the water supplies in the developed metro area would take 4 or more hydrants to get a 25% drop (and I'm not even sure the water department is capable of flowing that many at one time with manpower and gear).

Glenn Berger

5/30/2023 08:12:33 am

Recommendation is to follow NFPA 291 for test procedure.

One flowing test outlet is acceptable. NFPA 291 has reversed itself and now only requires a 10% drop from static to residual, pressure which is met with the results stated.

However, having a pitot reading of 70 psi is excessive. NFPA 291 recommends that pitot readings should be more than 10 psi and less than 30 psi. I know that I cannot hold the pitot gauge in the flow stream at this pressure.

Anthony

5/30/2023 02:22:47 pm

I think Glenn has it right.

I'd also agree with others the more flow the better.

Jesse

5/30/2023 08:13:27 am

Yeah you can, but opening more than one outlet gives a better picture of the water supply characteristics.

NFPA 291 says you need to test to get the requisite pressure drop OR the required fire flow. Unless your protecting high piled storage, you probably got the required fire flow with the one outlet.

David Kendrick

5/30/2023 09:16:54 am

Looped mains are fine, knowing your supply arrangement is certainly important.
Finding that all control valves are operational and open is part of testing multiple locations.

"We always leave our control valves open."

Trust but verify.

Pete D.

5/31/2023 06:47:04 am

The 25% drop rule exists for a variety of reasons. 1) Extrapolated max flows at 20 psi residual pressure are theoretical data. Interpolated points between static and residual measurements are still considered experimental data. To minimize error in the extrapolated data, is the first reason. 2) Fully developed flow- in the large diameter city network, only flowing one 2.5" butt will give something like 500-650 gpm, which may result in laminar flow in a large pipe such as 16". Once a volume flow rate is achieved that creates turbulence, there will be an exponential friction increase, which will create a drop in residual pressure readings.Turbulent flow more accurately models the water supply.
Recently, I worked in a jurisdiction in which the water purveyor provided the minor losses associated with the meter and cross connection with their flow test letter. My flow requirement for the combine standpipe system business occupancy was <1000 gpm. I did not benefit from having a 3,500 gpm flow point, so I asked that it be omitted from the results. It was not a new development and I did not want a friction loss taken at 3,500 for the RPZ. So, there are exceptions to most rules. For new development, you will need flow test results to validate Fire Flow calculations, or you will end up needing a tank or reservoir.

5/31/2023 09:04:31 am

Pete D., Where is the 25% rule published?

Is it part of a local jurisdiction's rules and standards or is it found in some other publication?

Pete H

5/31/2023 09:37:11 am

Hey, I'm the wrong Pete, but I figured I'd take a stab at it.

NFPA 291 (2022) 4.4.6 claims a 10 percent drop between static and residual is recommended.

NFPA 291 (2013) 4.3.6 claims a 25 percent drop between static and residual is recommended.

So older versions of 291 called out for a 25 percent drop, but at some point that was reduced to a 10 percent drop.

5/31/2023 09:40:08 am

Thank you very much.
More than once we didn't have enough outlets to achieve a 25% pressure drop. We might have made those numbers if we drained the water tower. (humor)

Fred Walker

5/31/2023 01:28:55 pm

There is no right or wrong answer here; to large degree it "depends" on the purpose of the test. Are you getting design information to support an installed protection system which may have a 30 to 50 plus life expectancy, Assessing the health of the water distribution. Or determining available fire flow to support fire department's needs.
In the case of the fire suppression system design looking to a 20% - 25% drop between static and residual may well provide you the best overall long term health curve for the water supply and provided you a better level of confidence in the water supply supporting the system throughout its operational life.
The other uses indicated are subject to more frequent testing typically every 5 years or so; and normally are used as measures between the prior tests. During this testing, where poor system health is indicated then further testing is indicated or system maintenance / upgrade is identified.
So in the first case professional responsibility should be driving testing requirements not just what is the minimum testing to get through the AHJ review process. Just meeting the AHJ minimum for plan review approval does not automatically equate to having met our professional responsibility.

Is Flowing One Outlet in a Flow Test Acceptable?

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