Fire Hydrant Flow Test and modifiers

[TravisMack]

I just ran into another situation today where someone providing a report of a fire hydrant flow test did not use the proper modifiers per NFPA 291.

When using the pumper connection when doing a flow test, you have to make an additional modification to your flow. If the pitot from the pumper connection on the hydrant is 7 psi or greater, you multiply the theoretical flow by 0.83 to get the actual flow.

Using a 2.5" outlet, the formula is

Q = 29.83 * Hyd Coeff * dia^2 * pitot^0.5

So, for a standard hydrant with a coeff of 0.9, a 2.5" outlet and a pitot of 40 psi, you get:

Q = 29.83*.9*2.5^2*40^0.5 or Q = 1061 gpm

If you use the pumper connection of a standard hydrant (4" for this example) and the pitot is 10, then the formula is:

Q = 29.83 * 0.9 * 4^2 * 10^0.5 * 0.83 or Q = 1127 gpm

From my experience, about 98% of those performing and reporting flow test data do not include this pumper modification. Now you know it is required, so be sure to include it in your reporting. For pitot readings less than 7 for the pumper connection, refer to NFPA 291 for the proper modifier.


Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[chicopee]

Normally the hose connections are used for such test, so what made those who did the test use the pumper connection? If the hydrant was a double head hydrant then the second hose connection should have been flowed had there been not enough in static pressure drop. Also inquire about the pitot tube used when the measurements were taken as it may have been calibrated for a 4" connection.

 

[TravisMack]

I have no idea why they did the pumper connection. I just know that I was provided with a report that showed it was flowed and pitot numbers. I often see a large local municipality that does all of the flow testing in their jurisdiction flow the 4" opening and not make the proper corrections. I wasn't trying to slam anyone. It is just every time I bring this up to some one doing the test, they always state: "I didn't know that was required." I was just trying to point it out to those on here.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[rmae]

Travis you know what the big problem regarding flow tests is? NFPA 13 does not have mandatory requirements regarding how or with what equipment a flow test is to be conducted. 23.2.1.2(2010)simply states 'The volume and pressure of a public water supply shall be determined from waterflow test data or other approved method'. The annex references NFPA 291 'Recommended Practice for Fire Flow Testing and Marking of Hydrants' as direction for further information but that's it. Unless a building code, bylaw, variance or AHJ (or engineer via specification)specifically requires testing to be completing in accordance with NFPA 291, then flow testing is a free for all for contractors. I don't know why the NFPA 13 technical committee does not make NFPA 291 mandatory.

As we all know the water supply is the number one variable governing sprinkler system design, yet most people seem to give little thought about it and most typical generic fire protection specs we see allow the contractor to do his own testing AFTER he gets the contract. In my fire protection specifications (for design-build projects)I always require that testing be done in accordance with NFPA 291 and that the test be witnessed by the engineer (me), or I conduct the test myself (much preferred). It's amazing how often I will show up to witness a test and the contractor has gauges and equipment that look like they were dug-up from a WWI trench and I ask him about the calibration dates and he looks at me and smiles, clueless as to what or why I am even asking the question. I bring up the spec. and the NFPA 291 reference and the contractor looks at me and says 'You're the first one who ever brougth this up'. So we have the city water department there, me, the contractor and the condundrum that I have to solve by letting the the contractor use my equipment (calibrated gauges, flow monsters etc.).

R M Arsenault Engineering Inc.

http://www.rmae.ca

 

[Sdpaddler50]

All good points. A few more, the coefficient to use will vary depending on the hydrant butt opening (.8 for square edge outlet, but .9 is a good rule of thumb). I have done flow tests out of a 2 1/2 in outlet, and have not had enough water to meet the hydraulic demand of the sprinklers and hose. I have then flowed the 4 in outlet at the same location, and have sometimes got better water - enough to meet the demand in at least a couple cases i remember. I am not 100% certain, but it seems flowing more water may cause additional pumps to possibly come on within the city grid. One last thing, we should be flowing the total water demand (sprinkler plus hose). You often cant do that with a 2 1/2 in, so its another reason why i ask we flow a 4 in. I have a 4 in flow tube which smooths out the water flow a bit, and allows me to use a .90 coefficient.

 

[TravisMack]

Yes. That is another point. I do outside design for contractors across the US, Carribean, some parts of Asia and just touching into Europe now. I always ask my customers to get a flow test where the flow is greater than the anticipated fire sprinkler + hose flow. I just don't want to have to extrapolate out to a point. In theory, it should work, but I would rather have the proven flow data measured.

Travis Mack
MFP Design, LLC

http://www.mfpdesign.com

 

[rmae]

Agreed!

As Hunter S. Thompson said 'If something is worth doing, its worth doing right'.



R M Arsenault Engineering Inc.

http://www.rmae.ca

 

[MGXFP]

There isn't much excuse to not have read NFPA 291. It is available for free to view online via NFPA's website and is brief enough to be read on a lunch break (at least the parts regarding flow testing).

Personally I consolidated the data (pitot pressures and flows, coefficients, rounding tolerances etc) into spreadsheets and have them laminated and with my flow test equipment