Fire Hydrant Flow Test and modifiers
Fire Hydrant Flow Test and modifiers
(OP)
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.
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
www.mfpdesign.com





RE: Fire Hydrant Flow Test and modifiers
RE: Fire Hydrant Flow Test and modifiers
Travis Mack
MFP Design, LLC
www.mfpdesign.com
RE: Fire Hydrant Flow Test and modifiers
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.
www.rmae.ca
RE: Fire Hydrant Flow Test and modifiers
RE: Fire Hydrant Flow Test and modifiers
Travis Mack
MFP Design, LLC
www.mfpdesign.com
RE: Fire Hydrant Flow Test and modifiers
As Hunter S. Thompson said 'If something is worth doing, its worth doing right'.
R M Arsenault Engineering Inc.
www.rmae.ca
RE: Fire Hydrant Flow Test and modifiers
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