Overview: This article explains the concept of Required Fire Flow (RFF) and provides methods for its calculation, essential for firefighters, engineers, and safety planning. It details the National Fire Academy (NFA) formula and introduces the IOWA State University method.

Understanding Fire Flow: A Critical Metric

Fire flow defines the volume of water necessary to effectively control and extinguish a fire. This measurement, typically expressed in gallons per minute (GPM), quantifies the rate at which water must be delivered to the incident scene. Accurate determination of this value is fundamental to successful firefighting strategy and resource allocation.

A Guide to Calculating Required Fire Flow

Successful fire suppression relies on precise technical calculations. Determining the fire flow assesses the water volume needed to combat a blaze effectively. This section outlines proven methods for its computation.

The National Fire Academy (NFA) Method

To compute the Required Fire Flow (RFF) in GPM using the NFA method for a burning area, follow these steps:

  1. Multiply the length (L) and width (W) of the involved area, both measured in feet.
  2. Divide the resulting product by 3.
  3. Multiply this figure by the number of floors (F) that are actively burning.

The core formula is expressed as:

RFF = (L × W / 3) × F

Refining Precision with Exposure Factors

To enhance the precision of your estimate, you can account for the percentage of the building involved in fire and the number of exposed objects.

  • A building with 50% fire involvement would require half of the calculated RFF.
  • Each exposure typically adds an additional 25% to the total flow requirement.

Note: Interior exposures refer to additional floors within the same building that are threatened by fire spread. Exterior exposures encompass adjacent structures or buildings near the burning object.

Exploring the IOWA Fire Flow Formula

The IOWA State University (ISU) method provides an alternative calculation. The formula is: 

RFF = (L × W × H) / 100

In this equation:

  • RFF represents the Required Fire Flow in gallons per minute (GPM).
  • V signifies the total volume of the burning building.
  • L, W, and H correspond to the building's length, width, and height, respectively, used to derive the volume (V) as per a rectangular prism calculation.

Practical Application: A Fire Flow Test Example

Let's illustrate the use of the NFA formula with a practical scenario. Assume we are evaluating a single-story building.

  1. Determine the building's length: L = 20 feet.
  2. Establish its width: W = 20 feet.
  3. Apply the NFA formula: RFF = (20 × 20) / 3 ≈ 133 GPM.

It is crucial to remember that such calculations provide an estimated baseline. The actual required flow may vary due to conditions like fire type, wind speed, and combustible materials involved.

The Strategic Importance of Fire Flow in Suppression

Calculating fire flow is integral to optimizing firefighting operations. It enables crews to formulate a strategic attack plan based on the estimated fire size and available resources. Furthermore, accurate flow estimation helps prevent resource wastage and minimizes potential collateral damage.

Frequently Asked Questions on Fire Flow

What constitutes the maximum fire flow?

The maximum required fire flow for a single incident is generally considered to be 12,000 GPM (approximately 45,500 liters per minute). This term denotes the peak water volume a distribution system or source can deliver during firefighting activities.

What is the typical GPM requirement for a fire hydrant?

A standard fire hydrant's flow rate usually falls within a range of 500 to 1,500 gallons per minute (GPM). Specific requirements can differ based on local fire codes, assessed risk levels, and the capacity of municipal water infrastructure.

What is the procedure for calculating required fire flow?

To calculate the Required Fire Flow (RFF) for a structure: Measure the length and width of the burning area. Multiply the length by the width and divide the result by 3. Multiply this value by the number of floors affected by fire. The fundamental formula is: RFF = (length × width × number of floors) / 3.

What is the required fire flow for a 6 by 4 feet area?

For a single-level area measuring 6 feet by 4 feet, the required fire flow (RFF) is approximately 8 GPM (or about 30 liters per minute). This is derived using the standard formula with one floor involved: RFF = (6 × 4 × 1) / 3 = 8 GPM.