Determining fire flow can be a tricky subject. This week I'm breaking down one common method of determining fire flow requirements and hopefully exposing some myths about the process.
Not an Exact Science
First, determining the exact amount of water required to manually suppress a fire is dependent upon so many variables. The amount of water used could depend on the building size, hazard, outdoor conditions, speed of fire growth, fire department response time, whether the building is protected by sprinklers, and on and on.
The methods used to calculate fire flow are different methods at estimating the amount of water required to manually suppress a fire. It is not an exact science.
What is Fire Flow?
I'll start by what fire flow is not. Fire Flow is not the volume of water required for the fire sprinkler system. I couldn't count the number of projects where Fire Flow has been assumed to be sprinkler-related.
Fire Flow is formally defined as the "flow rate of a water supply, measured at 20 psi (138 kPa), that is available for fire fighting." (IFC 200-2018 Appendix B Section B102)
Fire flow is used to determine the quality of a water supply to an area. It's used as an aid to determine pipe size and arrangements to delivery water to a specific area.
Fire Flow is important for emergency response at it is the total capacity of the system that the fire department has available for use in response to a fire.
How Is Required Fire Flow Determined?
In short - it depends.
There are many methods for determining fire flow. The most common cited in US circles include the Insurance Services Office (ISO) Method, Iowa State Method, and the Illinois Institute of Technology (IIT) Method. At least a dozen other methods exist (for more on these, the Fire Protection Research Foundation provides great analysis in Evaluation of Fire Flow Methodologies research paper).
The International Fire Code (IFC) offers Appendix material that provides guidance for determining the required fire flow, which is based on the ISO Method. It is not a mandated code requirement unless a jurisdiction adopts the Appendix.
Many jurisdictions I've worked with do not have an ordinance that adopts the appendix, but when asked they are typically open to using the IFC Appendix B method of determining fire flow. The International Fire Code, which is widely adopted in the US, only requires that an approved water supply "capable of supplying the required fire flow" be provided to buildings.
This process will be explored in more detail here.
1. Determine Baseline Fire Flow
The first step in this overall determination of water supply to a site is to determine the required fire flow.
Using the IFC Method, Appendix B has a reference table that stipulates a minimum fire flow and flow duration based upon building size and construction type (2000-2012 Table B105.1, 2015-2018 Table B105.1(2)).
2. Reductions & Increases
Once a baseline value for flow and duration is taken from the table, it can be reduced based on the presence of sprinkler system.
Section B105 details the adjustments that are available for buildings with a sprinkler system. A reduction of up to 75% can be permitted for buildings with a fire sprinkler system.
It's important to note that up through the 2012 edition of the International Fire Code, a reduction of fire flow had to be approved, meaning the AHJ must agree on the reduction. This may not make a difference if a jurisdiction hasn't adopted the appendix and the entire calculation has to be approved anyways, but in the case where Appendix B is adopted and you're under IFC 2000 through 2012, you'll need AHJ buy-in to use the reduction.
The 2015 and 2018 edition of IFC removed the approval necessity for sprinkler flow reductions.
As part of this process the Fire Chief is also authorized to decrease the required fire flow, based on building isolation or impracticality. Alternatively, the Fire Chief is also authorized to increase based on unusual susceptibility for the facility. These stipulations come with Section B103 of Appendix B (all editions).
Fire Flow is used to quantify the available water supply for manual firefighting operation.
3. Verify Provided Fire Flow
The best way to verify fire flow for a location is to conduct a flow test at the site itself. This of course can be difficult to impossible for new-construction projects on virgin sites.
For developed areas or building expansions, this may not be difficult to accomplish.
I have a current project we're working on that is a major building expansion. Fire flow needed to be assessed based on the new expanded building and whether a single 8-inch feed would still meet the minimum requirements. A flow test on the site itself confirmed that we are just short of required fire flow which prompted a healthy discussion with the AHJ.
4. Calculate from Flow Test to Site (if necessary)
Sometimes a flow test can't be conducted on the site itself.
When this is the case, a hydraulic calculation can be run between the water supply source (nearby flow test, a water tower, reservoir, or pump) and the project site to estimate what the available fire flow will be. This calculation incorporates the pressure loss of the pipe network as water is constricted between a source and a project site. The best way to confirm actual fire flow (in my opinion) is to verify with a flow test once any extension is installed.
Easy Tools for Fire Flow & Water Supply Analysis
There's a new tool in the arsenal around here that directly addresses fire flow requirements.
It's the Fire Flow Calculator that's now a part of the Toolkit. If you're already a Toolkit subscriber, download it today.
The Fire Flow Calculator uses the IFC method based on your project parameters to quickly grab the baseline fire flow and duration, and make adjustments for sprinkler protection. Now you have extremely quick access to determine required fire flow, and the documentation to support your process.
This is a tool I'm happy to debut and have used with great client feedback.
On a side note, Toolkit subscribers also now have access to last week's Design Checklist with user-provided feedback. The download update includes both tools. Give them a download and let me know what you think!
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Joseph Meyer, PE, is a Fire Protection Engineer in St. Louis, Missouri. See bio on About page.