Estimating Required Fire Flow:
The National Fire Academy Formula

Application of the appropriate flow rate is critical to fire control. However, how can we estimate the flow rate that is necessary?

There are a number of methods that can be used to estimate or calculate required flow rate for fire control. One method is to simply use your experience (which may work quite well if you have been to a large number of fires and paid attention to flow rate). However, if you do not have a large base of experience to draw on or need to apply flow rate estimation in a preplanning context, other methods are necessary. One of the most common methods used in the United States is the National Fire Academy (NFA) Fire Flow Formula.

Development of the NFA Formula

In the mid 1980s the development team for the National Fire Academy Field course Preparing for Incident Command developed this formula to provide a simple method for estimating the flow requirements for offensive, interior operations where a direct attack was used to control and extinguish the fire.

Interestingly enough the NFA Fire Flow Formula is not based on science (at least not physical science). The developers tapped into another valid source of information, knowledge of experienced fire officers.

The course developers designed a number of plot and floor plans showing different sizes of building with different configurations (e.g., rooms, doors, windows) with varied levels of involvement. These drawings were distributed to students attending the academy and they were asked how their fire department would control the fire (with the emphasis on the number, placement, and flow rate of hoselines).

There are three major parameters used for the scenarios based on these plot and floor plans.

• All scenarios were designed to involve offensive, interior firefighting operations and as such, fire involvement was limited to 50% or less of the total floor area of the building.
• Operations were to be conducted as they normally would, with initial operations started by the first arriving company and additional tactics implemented as resources arrive.
• Primary search and ventilation tactics would be performed concurrently with fire control operations.

The student’s responses were collected and analyzed. For each scenario, when the floor area of the involved area in square feet (ft²) was divided by the total flow rate in gallons per minute (gpm) for all hoselines used for attack, backup, and exposure protection; the average result was three. Turning this around, flow rate in gpm can be determined by dividing the area of involvement in ft² by three.

In that the exterior of the building can be determined more easily than the area of involvement, the formula was adapted to determine the flow rate based on building size and approximate percentage of involvement as illustrated below:

Note: This method does not translate easily into standard international (SI), simply converted the formula would be lpm = M²/0.07.

The course development team extended the application of this formula to include estimated flow required for exposure protection by adding 25% of the flow rate required for fire control (as determined by the basic formula) for each exposure. The full formula as used in preplan development is as follows:

The development team believed that this formula would also be applicable to defensive attack for levels of involvement above 50%. However, this was not validated using the same type of methodology as used to develop the base fire flow formula.

Limitations

It is important to remember the limitations of this fire flow estimation method:

• The NFA Fire Flow Formula is designed for offensive, interior operations involving direct attack.
• The formula becomes increasingly inaccurate if the level of involvement exceeds 50% or the resulting flow is greater than 1,000 gpm.
• This method is not designed for defensive, master stream operations (even though the developers believed that it would provide a reasonable estimate of required flow rate for defense.
• The formula is based on area, not volume. If the ceiling height exceeds 10’, the flow rate may be underestimated.
• The NFA Formula does not take into account the potential heat release rate of the fuel. Fuel with extremely high heat release rate may require a higher flow rate
• The developers of the NFA Formula made the assumption that the building was well ventilated (tactically). Increased ventilation can (if the fire is initially ventilation controlled) result in increased heat release rate.
• It may be tough to do the math at 0200 hours when faced with a rapidly developing fire! This method is best used in advance of the fire when developing preplans or working on tactical problems

Total Versus Tactical Rate of Flow

The most common application error is the belief that the formula determines the flow rate required for fire attack. This is incorrect! The formula determines the total flow rate required for attack, backup, and exposure protection lines. Use of this formula to determine the flow rate for the initial attack line (or lines) will greatly overestimate the required tactical rate of flow.

As discussed in It’s the GPM! and Choose your Weapon Part I, substantially exceeding the required tactical rate of flow has diminishing returns on speed of extinguishment and substantially increases the amount of water used. If excessive, water that is not used efficiently (i.e., turned to steam) increases fire control damage).

Using the NFA Base Fire Flow Formula (no exposures), roughly half of the flow rate is used for attack lines and the remainder is used for backup lines. The NFA formula provides an excellent method for estimating total flow rate requirements (which impacts on water supply and resource requirements). However, it must be adjusted (reduced by half) to determine the tactical rate of flow necessary for direct attack on the fire.

Other Approaches

As outlined in this post, the NFA Fire Flow Formula is intended for estimating the total flow rate required when making a direct attack and has a number of specific parameters that must be considered. Prior to introduction of the NFA formula, the Iowa Fire Flow Formula developed by Floyd W. (Bill) Nelson and Keith Royer. The Iowa Formula was developed quite differently, has substantially different assumptions, and will be the subject of my next post.

For more information on Fire Flow, visit my colleague Paul Grimwood’s website www.fire-flows.com. Paul has amassed a tremendous amount of information on this topic from around the world.

Ed Hartin, MS, EFO, MIFireE, CFO

Tags: Fire Control, fire flow, National Fire Academy, NFA fire flow formula