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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Things are busy around here - despite the PE Prep "offseason" beginning, I've been working on improvements and construction of a handful of promising tools.
One basic but very much needed update is an improvement to the Obstruction Calculator. Now, you can enter either the horizontal distance of a sprinkler or the vertical distance of the sprinkler, and get minimum and maximum feedback based on each.
During design, many of us know the depth of the sprinkler and depth of the obstruction prior to determining where (horizontally) the sprinkler is going to be located away from an obstruction. Now the tool helps support that effort.
If you're a Toolkit user, you have immediate access to these updates and can download the latest updates on the dashboard here.
As always, thank you to those who have sent ideas and feedback! Stay tuned for next week on a new database launch for Toolkit users.
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Based on some feedback and good ideas I've been experimenting with graphing fire pump & flow test curves with usable data outputs. Below is the first iteration for drawing a fire pump curve alongside a water supply curve.
Determining ideal fire pump configurations for sprinkler and standpipe systems can be
an important part of optimizing fire suppression design
Here's the help I could really use from you - what else would be included in your ideal pump curve?
Would you prefer this be on a logarithmic x-axis?
Want 175 & 300 psi limit lines shown?
Would you want to see at what height in a building the 175 psi threshold would occur - on this graph?
System demand and hose?
I'm open to any and all ideas - in the end I think it'd be great if this tool was the quickest & best method for summarizing and analyzing fire pump output. Share your ideas in the comments here, thanks in advance!
When conducting or reviewing hydraulic calculations, I very often face scenarios where the initial (very first) hydraulic demand exceeds the potential for the water supply.
At that point I lose all hope and add a fire pump to the job.
Just kidding, of course - there's at least a half dozen hydraulic elements I analyze and refine to better match the capabilities of the water supply to the design of the sprinkler system.
Refining Hydraulic Calculations with K-Factors
One of the more fine-tooth aspects I look at is the k-factor used on the sprinklers.
The k-factor for a fire sprinkler is the discharge coefficient, or in normal human terms just relates to the amount of water that is permitted through the sprinkler.
The k-factor is dependent upon the orifice diameter of the sprinkler - a low k-factor (such as K2.8) restricts the flow of water, while a larger k-factor (such as K22.4, K25.2, or K28.0) permit much more water to flow through. K-factors were originally created to be multiples of the discharge of a K5.6 sprinkler. A K2.8 sprinkler, for example, is 50% discharge of a K5.6 sprinkler, while a K11.2 sprinkler is 200% of the discharge of a K5.6. NFPA 13-2016 Table 18.104.22.168 shows this well.
Use In Design
We find K5.6 sprinklers in light hazard all the time. Residential sprinklers often have k-factors less than 5.6. ESFR and CMSA require minimum K11.2 (NFPA 13-2016 22.214.171.124). ESFR are tied directly to the hazard it protects.
Back to refining the hydraulics in a system - increasing the k-factor of a sprinkler allows more water to flow through a sprinkler with less pressure loss. This becomes very important when trying to reduce pressure loss in a system.
Light Hazard Example
A light hazard system (0.10 gpm/sqft) with widely spaced sprinklers (at 225 sqft each) would require a minimum flow through each sprinkler of 22.5 gpm (0.10 gpm/sqft x 225 sqft = 22.5 gpm).
In order to flow 22.5 gpm, a sprinkler with a k-factor of 5.6 now requires 16.1 psi to do so (Q=k√p, or rearranged, p=(Q/k)^2). This is 9.1 psi higher than 7 psi, or the minimum that NFPA 13 requires.
In order to flow 22.5 gpm, a sprinkler with k-factor of 8.0 only requires 7.9 psi to do so, or less than 1 psi more than the minimum NFPA 13 requires.
In this scenario, flowing the same amount of water (22.5 gpm) results in a 8.2 psi difference in the pressure required at the most remote sprinkler. Can 8.2 psi be important? Absolutely!
Similarly, consider Ordinary Hazard Group 1 (0.15 gpm/sqft) and Ordinary Hazard Group 2 (0.20 gpm/sqft) systems.
For Ordinary Hazard Group 1 and sprinklers spaced at 130 sqft, a K8.0 sprinkler requires 5.1 psi less than a K5.6 sprinkler (7.0 psi vs 12.1 psi).
This same methodology applies to extended coverage sprinkler requirements, specific densities for traditional storage design, and more.
The K-Factor Selector
Want to quickly compare fire sprinkler k-factors across different design densities and sprinkler spacing? Easy. Here's the calculator I've created that quickly compares pressure requirements and flow rates across different sprinkler k-factors.
Want all these tools in a downloadable, printable & PDF-saving capability? Great! The MeyerFire Toolkit will include this tool as well. You can download and try it out now through September for free.
Other than the Toolkit, users of the comprehensive Fire Sprinkler Database can sort & search among k-factors as one of the parameters when comparing sprinklers.
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Joseph Meyer, PE, is a Fire Protection Engineer in St. Louis, Missouri. See bio on About page.