Determining Required Fire Flow

Let’s take a further look at how fire flow is calculated. There are at least five recognized fire flow calculation formulas in use in the United States today. Some are intended for pre-incident assessment of a community’s flow rate; others are quick calculations for ICs who are faced with an emergency. The “community flow rate” calculations generally are meant for conflagration control rather than a single room and contents fire or building.

Fire Flow Computation Methods

One of the earliest methods for easily calculating fire flow on the fire ground was developed in the Midwest.

Iowa State University Method

The Iowa State University (ISU) method was created in the 1950s after a series of studies of fire in enclosed spaces. The ISU “Ideal Rate of Flow” formula:

Required fire flow (gpm) = V ÷ 100
Where V = the volume of the space is on fire.

Using the ISU formula, an IC can assess the conditions at a burning noncombustible mercantile occupancy that measures 50 feet by 75 feet and is one story (12 feet) tall. The IC can quickly determine that the volume of the structure is 45,000 cubic feet. Using this number and the ISU formula, the required fire flow for this structure would be 450 gallons per minute (45,000 ÷ 100 = 450).

National Fire Academy Method

The National Fire Academy (NFA) formula, taught in the Managing Company Tactical Operations (MCTO) classes, is similar to the ISU method, but employs different values and is nased on area, not volume. The NFA fire flow formula is:

Required fire flow (gpm) = (Length x Width) ÷ 3

Using the same example, an IC arrives at a burning mercantile occupancy that measures 50 feet by 75 feet and is one story (12 feet) tall. The IC quickly determines that the area of the structure is 3,750 square feet. Using this number and the NFA formula, the required fire flow for this structure would be 1,250 gallons per minute (3,750 ÷ 3 = 1,250).

Insurance Service Office Method

The Insurance Services Office (ISO) “Guide for the Determination of Required Fire Flow” has been a recommended practice for calculating fire flow for many years. It is more complicated than the ISU or NFA methods, but usually is employed when the fire department is establishing minimum flows for new construction, rather than for emergency operations. It also is an important consideration in a jurisdiction’s overall fire defense rating for insurance rates.

The ISO guide employs several steps in the calculations, but the fundamental fire flow formula is:

NFF = (C) (O) [1 + (X + P)]

Where:

NFF = Needed Fire Flow

(C) = Construction factor, including effective area

(O) = Occupancy factor

(X + P) = Exposures and communication (openings) factor

In order to solve this equation, the fire protection person must know how ISO establishes these “factors.”

First, another equation is needed to solve for C. That formula is

C = 18F√A

Where:
F = the coefficient related to the construction type

A = the effective building area

The construction type coefficient comes from the following table:

Using the same example, solve for C assuming a noncombustible building.

The building measures 50 feet by 75 feet giving us an area of 3,750 square feet. Inserting this value into the formula returns:

C = 18 (0.8) √3,750

C = 18 · (0.8) · 61.23

Thus, C = 881.712 (Round up to 882)

Round to nearest 250 gpm increment (1000 gpm)

The minimum value for C is always 500 gallons per minute.

Occupancy Factors

The next step in the ISO formula is determining the occupancy factor (O). This also is accomplished by referring to Table #1:

With this information, insert the combustibility class factor of our sample structure into the equation:

NFF = 882 · 1.00(X + P)

Exposure and Communication

The last portion of the ISO formula is

to determine the likelihood a fire might spread due to exposures or interior openings. To obtain this information, use the formula: 

Where
n = number of sides of subject building, and (X + P) has a maximum value of 1.75.

Values for both X and P come from others found in the ISO “Guide for the Determination of Required Fire Flow.”

To keep our example simple, we will presume this mercantile occupancy is located where exposures are not a consideration, and the values for (X + P) equal zero.

Our final fire flow formula using the ISO method:

NFF = (C) (O) [1 + (X+P)]

NFF = (882) · (1.00)  ·  [1 +0]

NFF = 882 gallons per minute

NOTE: ISO does not compute Needed Fire Flow for sprinklered buildings because it is presumed the calculated flow for the sprinklers and hose stream allowance is sufficient for the property. 

NFPA Standard 1142, Standard On Water Supplies For Suburban And Rural Fire Fighting Method

The NFPA 1142 fire flow calculation method is intended to be used in those areas where water systems are not readily available, and fire protection water must be retrieved from tanks, ponds, cisterns, and other sources.

The NFPA 1142 method uses the volumetric capacity of specific structure to compute the “minimum water supply.” The NFPA 1142 fire flow formula is:

Minimum water supply (MWS) = (Total structure volume) ÷ (Occupancy hazard classification number) * Construction classification number

Where:

• Total structure volume = the product of length · width · height
• Occupancy hazard classification number is obtained from a chart (see Occupancy Hazard Classification Table #2)
• Construction classification number is obtained from a chart (see Occupancy Hazard Classification Table #2)

The construction classification value comes from the following table:

Description and Classification Value:

Wood frame 1.5, Heavy Timber 0.75, Masonry (Ordinary) 1.0, Noncombustible 0.75, Fire-resistive 0.5.

Using our example of the 3,750 square feet, noncombustible mercantile occupancy with a 12-foot ceiling, our formula for minimum water supply would be:

MWS = 45,000 cu. ft. ÷ (4  · 0.75)

MWS = 45,000 ÷ 3

MWS = 15,000 gallons*

*Note that NFPA 1142 requires a minimum water supply of 2,000 gallons regardless of the formula results.

If the structure being evaluated has exposures within 50 feet, the resulting MWS must be increased by 50 percent, and the minimum supply increases.

International Fire Code
(IFC) Method

IFC Appendix B provides another method for computing the required fire flow for buildings.

The IFC method employs a table that lists the minimum flow based on the construction type and size of the structure or what is called the “fire flow calculation area.” The “fire flow calculation area” is the sum of all floor area between firewalls with no openings, or, in fire resistive construction, the sum of the area of the three largest successive floors.

In the example of the 3,750 square foot noncombustible mercantile occupancy, we find noncombustible construction (IIB) column on the table, read down to the corresponding area (3,750 square feet) and determine that the required flow is 1,500 gallons per minute for two hours. (See highlighted text).

The IFC also address fire flow in one- and two-family dwellings. Dwellings with fire areas less than 3,600 square feet may have a fire flow of 1,000 gpm. Dwellings with larger fire areas must use the flow information from the table.

IFC Appendix B also allows fire flow reductions between 50 percent and 75 percent if the structure is protected by automatic sprinklers and is a light hazard occupancy classification as determined by NFPA 13, Standard for the Installation of Sprinkler Systems.

Illinois Institute of Technology Method

The Illinois Institute of Technology (IIT) studied 134 fires in the Chicago area, and developed the following fire flow formula for nonresidential occupancies. The results of the survey were used with regression analysis to develop flow requirements based on building area:

Fire flow = -1.3 · 10-5 A2 + 42 · 10-2A

Where A = the area of the fire in square feet.

Using the mercantile example, and assuming that the building is fully involved, the fire flow figure would be

Fire flow = -1.3 · 10-5(3,7502) + 42 · 10-2 (3,750)
Fire flow = -1.3 · 10-5 (14,062,500) + 42 · 10-2 (3,750)
Fire flow + -1.3 · (1 ÷ 10,000) · (14,062,500) + 42 · (1 ÷ 100)(3,750)
Fire flow = -1.3 (0.00001) · (14,062,500) + 42 · (0.01)(3,750)

Fire flow = -182.825 + 42 · (37.5)

Fire flow = -182.825 + 1575

Fire flow = 1,392.175 gallons per minute

Mobile Water Supplies

Many jurisdictions must provide mobile water supplies in the form of tender shuttles or strike teams to sustain fire flow. While reliable in an emergency, mobile water supplies are not considered a code-compliant means of supplying fire flow.

Annex C of NFPA 1142 provides ICs a formula for calculating the amount of time it takes to sustain a specific flow using mobile apparatus.

The formula is:

Q = V/[A + (T1 + T2) + B] · 0.9

Where:

Q = maximum continuous flow capability

V = mobile water supply capacity

A = time for the mobile supplies to drive 200 feet, dump water into a drop tank, and return 200 feet to a starting point

T1 = time in minutes for the mobile water supply to travel from the fire to the water source

T2 = time in minutes for the same mobile water supply to travel from the water source to the fire

B = time for the mobile supplies to drive 200 feet, fill mobile water supply at the water source, and return 200 feet to a starting point

0.9 = amount of water supply considered unavailable due to spillage, under-filling, and incomplete unloading

The annex also address means to compute how safe travel times influence water delivery. It would be a good pre-incident planning activity to compute and drill on the amount of apparatus and time it would take to sustain a minimum fire flow for a target hazard.

Summary

You can see that fire protection is much more than “putting the wet stuff on the red stuff.”  Thoughtful analysis goes into the code officials’ determination of required fire flow.

There is a variety of methods to establish required fire flow for fire protection. All are based on mathematical formulas and range from simple to sophisticated. There is no single “correct” formula or method for fire flow calculations.

Based on the different methods, here are the fire flow calculation results for our hypothetical noncombustible mercantile occupancy that measures 50 feet by 75 feet and is one story (12 feet) tall.

Fire operations staff should meet with their fire protection personnel (fire marshal, inspectors, insurance underwriters, plant managers, building/facility engineers, etc.) to identify fire flow needs, and establish contingency plans for developing satisfactory flows during an emergency.