How to Best Determine the Available Fire Flow?

4/16/2019

I was needing a little help validating the methodology for fire flow requirements in my city. They do not allow you to verify fire flow per a fire hydrant flow test.

Our city always uses hydraulic calculations (a hydraulic model) to determine acceptable flow in an area. However, in their hydraulic model they always assume that the water usage that day is 19,000,000 million gallons which was established by taking the highest usage in the history of our community of all time, which occurred in 2012 when we had 30 plus days of 100 degree weather of 18M gallons and adding a 1 Million to that. Our average daily usage in our community is around 10 to 11 million gallons per day.

Anyway, the local engineers say that methodology is consistent with the International Fire code. What are your thoughts? I would love to understand what the IFC requires.

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WES

4/17/2019 06:22:43 am

If a water model is available and/or the water purveyor has input on seasonal and daily changes in the system, that would be the best information for available water to the site.

A flow test is only a single point-in-time data point in a water system that changes regularly, but without input from the purveyor it can sometimes be all we have to go on.

PETE

4/17/2019 12:15:18 pm

You are confusing the terminology. Fire flow is the required water supply for fire fighters based on the building size and construction type. The requirement is tabulated in the IFC. A flow test would be needed to confirm whether a particular building would be able to satisfy that requirement at its location on the water distribution network.
The model the city uses that gives the available water supply characteristics at a chosen point in the water distribution network probably uses software and it incorporates the use of either the Bernoulli equation, the Hazen-Williams equation, the Darcy Weisbach equation, or a set of linear algebra relations known as the Hardy-Cross method (also used in electrical grids).

Dean Melanson

4/18/2019 07:19:46 am

Your Water Department appears to be using calculations from their hydraulic modeling based upon the worst case scenario that the system has experienced. For the FD this is good. Should their modeling demonstrate an acceptable flow rate and pressure is available you know that it will be usable for the vast majority of times. A flow test, done in the evening in the off season only demonstrates a single point in time - and not under worst case conditions. There may be daily or seasonal conditions that sufficient water is not available as a result. I would imagine the developers would be the ones to complain if the available water is not acceptable as they may say these conditions will never happen.

How to Best Determine the Available Fire Flow?

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