Simple Ways to Determine Your Fire Flow Requirements

Pros & cons of 2 common fire flow formulas

By Homer Robertson
Published Monday, February 1, 2010 | From the February 2010 Issue of FireRescue

Like most things in life, and especially in the fire service, the more you know, the more you need to know. Each new area in which we gain some useful knowledge becomes a stepping stone to another.

This month’s Quick Drill involves flow requirements and the formulas that can help you determine fire flow needs, which will in turn help you in several other areas.

Everyone’s Responsible
Having a good understanding of fire flow requirements is essential to everyone on the fireground—from the firefighter in the jump seat, who must know how much their handline is flowing and how many involved rooms it will put out, to the company officer making tactical decisions about where and when to attack the fire, to the chief officer defending their decision to dispatch four 1,500-gpm-rated engines to a building fire because it requires 6,000 gpm to control.

Ask a lot of firefighters how the whole extinguishment thing happens, and most will tell you the story about “putting wet stuff on the red stuff”—and in its own beautifully simple way, that’s true. It really is gpm vs. BTU, but there’s a lot more science to it than that. As a result, we must all have a basic understanding of the formulas that determine fire flow requirements.

2 Formulas
There are several fire flow formulas in use today, but for the sake of space, we’ll only touch on two: the National Fire Academy (NFA) formula and the Iowa Rate of Flow. Each has pros and cons.

The NFA formula is based on offensive interior operations where less than 50 percent of the building is involved. The formula is:

Needed fire flow = [(length x width) ÷ 3] x percent of involvement

Example: For a 30' x 50' building that’s 25 percent involved, the flow would be 30 x 50 = 1,500 square feet, divided by 3 = 500 x 0.25 = 125 gpm.

Based on that required fire flow, one handline pumped at the correct pressure should be able to produce the required gpm.

The Iowa Rate of Flow is based on work performed by Keith Royer and Bill Nelson while they were working for Iowa State University’s fire training program. While studying the effects of fog streams on interior fires, the pair demonstrated that all you need to do is put the right amount of water in the right place for the fire to go out—something that’s often overlooked by the modern fire service.

The Iowa formula is:
Required volume = (length x width x height) ÷ 100

Example: The same 30' x 50' building used in the first example is 10 feet high. 30 x 50 x 10 = 15,000, divided by 100 = 150 gpm.

The Iowa formula is based on science, while the NFA formula is based more on information gained from experienced fire officers from around the country. The Iowa formula is also based on a 30-second application rate, which is more applicable to today’s fireground, where application rates should be based on gallons per second, not gallons per minute.

Note: Because the NFA and Iowa formulas use different parameters, they’ll produce slightly different results for the same structure. Although I typically use the NFA formula for preplanning, I don’t prefer one over the other; they both get the job done.

Know Before You Go
When pre-planning target hazards, the fire flow formulas will add important information that can be reviewed prior to the incident. After calculating the total fire flow needed, break the flows down by 75 percent, 50 percent and 25 percent involvement. Remember: The Iowa formula doesn’t take into account percentage of involvement, it just gives you the maximum amount of needed flow. But you can easily calculate the different figures based on percentage involvement.

Fire flow formulas have a wide range of use, from the firefighter to the chief officer. Practice using the formulas to determine fire flows for structures in your response area. As you become more familiar with them, you’ll be able to determine which formula is best in a specific scenario.   

 

Drill 1:
What’s the Needed Fire Flow?

Discuss both the National Fire Academy (NFA) and the Iowa fire flow formulas. Ensure that each member understands and can apply both formulas.

Step 1:  On a whiteboard, draw different-size building layouts. Have members practice developing fire flows for the provided scenarios.

Step 2: Throughout the next several shifts, continue practicing on buildings in your response area. Ask crewmembers to quickly develop needed fire flows for various buildings by estimating approximate square footage. You should see improvement with practice.

Drill 2:
How Many Handlines & Master Streams?
Step 1:
  Using your department’s normal-length lines and pump pressures, use your apparatus flow meter or another measuring device to determine flows for each handline and master stream device.

Step 2:  Record these figures for reference later when determining fire flow based on the NFA or Iowa formula.

Step 3:  Using the whiteboard examples you used earlier, and based on needed fire flow requirements, determine what combination of handlines and master streams you would need to control each incident.

Comment Now: Post Your Thoughts & Comments on This Story

This building is about 50 x 50 x 10 on each floor. Applying the Iowa formula, we get: [50 x 50 x 10] x 2 (to account for the two stories) = 50,000 ö 100 = 500 gpm needed for the total fire flow. Photo Keith Muratori
This building is about 30 x 50 on each story and showing about 25 percent involvement. Using the NFA formula, the flow would be [30 x 50] x 2 (to account for the two stories) = 3,000 ö 3 = 1,000 x .25 = 250 gpm. Photo Keith Muratori



Simple Ways to Determine Your Fire Flow Requirements

Pros & cons of 2 common fire flow formulas House fire
This building is about 50 x 50 x 10 on each floor. Applying the Iowa formula, we get: [50 x 50 x 10] x 2 (to account for the two stories) = 50,000 ö 100 = 500 gpm needed for the total fire flow. Photo Keith Muratori

House fire
This building is about 30 x 50 on each story and showing about 25 percent involvement. Using the NFA formula, the flow would be [30 x 50] x 2 (to account for the two stories) = 3,000 ö 3 = 1,000 x .25 = 250 gpm. Photo Keith Muratori

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