Needed Fire Flow

Fireground size-up factors can have varying degrees of impact on a fire. Life safety is always the most important, but achieving all of our incident priorities is highly dependent on many factors: • Poor construction methods can accelerate a fire or make a building more prone to collapse...


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Fireground size-up factors can have varying degrees of impact on a fire. Life safety is always the most important, but achieving all of our incident priorities is highly dependent on many factors:

• Poor construction methods can accelerate a fire or make a building more prone to collapse

• The lack of clear distances between buildings can allow a fire to spread to adjacent exposures

• Height, occupancy, area, location, weather and lack of protective fire systems all can play a significant role in achieving or failing to gain fire control

• A shortage of personnel can cause a shift in strategies and tactics

• The location of the fire and the time it occurs can impact the situation

Yet, with all of the potential problems that accompany those size-up points, it has been my experience that a lack of or loss of water supply seems to always be followed by bad things happening. The water supply can be lost due to a burst length of hoseline or the engineer finding that the hydrant is out of service and the tank's water supply has been exhausted. In any case, fire burning unabated can spread rapidly, endangering occupants and firefighters. These situations can be compounded if ventilation activities have been initiated prior to the water supply being lost. The influx of fresh air can spread the fire to areas that seem to compound the problem the greatest.

Gauging Need for Water

A consideration at a fire scene for the initial incident commander is how much water will be needed for effective fire control. This information will impact the incident in terms of determining needed resources and the implementation of tactical operations.

Determining the amount of water needed to extinguish a fire in a specific building is best accomplished during the pre-planning stage. This can be achieved through a deliberate calculation of the occupancy considering conditions when establishing the needed fire flow. When pre-planned information is available to the initial incident commander upon arrival at an incident, strategic and tactical decisions can be made more readily and accurately.

Determining required fire flow during pre-planning activities requires the application of a "fire-flow formula" to conditions observed during an inspection of the premises. On many occasions, fire incidents are encountered where pre-planned fire-flow information is not available. Under these circumstances, experienced fire officers are able to determine the needed fire flow based on their experience and knowledge of similar situations they have encountered in the past.

There are occasions when a newly appointed or relatively inexperienced officer lacking the expertise of a seasoned officer must quickly judge the amount of water needed to effectively control a fire. The National Fire Academy (NFA) in Emmitsburg, MD, has developed a formula that allows for quick calculations. The formula was derived through a study of fire flows that were successful in controlling a large number of working fires along with interviews with numerous experienced fire officers from throughout the country regarding the fire flows they have found to be effective in various fire situations.

The NFA "quick-calculation" formula can be used as a tactical tool to provide a starting point for deciding the amount of water required at an incident scene. This will permit decisions to be made on the apparatus needed to deliver the water and the number of firefighters that will be needed to apply it. The information developed indicated that the relationship between the area which is involved in fire and the approximate amount of water required to effectively extinguish the fire can be established by dividing the square footage of the area of fire involvement by a factor of three. This quick-calculation formula is expressed as:

Fire flow = length times width divided by 3

This formula is most easily applied if the estimated square footage of the entire structure is used to determine an approximate fire flow for the total structure and is then reduced accordingly for various percentages of fire involvement.

The example shown below would illustrate how the formula can be applied to a single-family dwelling 60 feet long by 20 feet wide and one story high:

60 X 20 divided by 3 X 1 (floor) = 400 gallons per minute

100% involvement = 400 gpm

50% involvement = 200 gpm

25% involvement = 100 gpm

The formula indicates that if the dwelling were fully involved, it would require 400 gpm to effectively control the fire. If only half of the building were burning, 200 gpm would suffice and 100 gpm should be sufficient if one-fourth of the building were involved.

Multi-Storied Structures

In multi-storied buildings, if more than one floor in the building is involved in fire, the fire flow should be based on the area represented by the number of floors that are actually burning. For example, the fire flow for a two-story building of similar dimensions as the previous example would be:

60 X 20 divided by 3 X 2 (floors) = 800 gpm if fully involved

If other floors in a building are not yet involved, but are threatened by possible extension of fire, they should be considered as interior exposures and 25% of the required fire flow of the fire floor should be added for exposure protection for each exposed floor above the fire floor to a maximum of 5 interior exposures. (A fire on the first floor would threaten the second floor and 25% should be added. A second-floor fire would probably not threaten the first floor, so no interior exposure would need to be calculated.)

Exterior Exposures

Likewise, if exterior structures are being exposed to fire from the original fire building, 25% of the actual required fire flow for the building on fire should be added to provide protection for each exposure. The following example shows how to apply calculations for exposures to our previous one-story dwelling with two separate exterior exposed structures:

60 X 20 divided by 3 X 1 = 400 gpm

2 exposures: 400 gpm X (25% X 2) = 200 gpm

Total fire flow required = 600 gpm (for 100% involvement of the original fire area)

If the exposure actually becomes involved in fire (either additional floors of a multi-storied building or adjacent structures), the exposure(s) should then be treated as a separate fire area and calculated separately and then added to the required fire flow for the original fire area.

In using the quick-calculation method to determine required fire flows, it is important to remember that the answers provided by this formula are approximations of the water needed to control the fire. The formula is geared to an offensive attack and its accuracy diminishes with defensive operations. Don't forget that you are estimating both the area of the building and the amount of fire involvement within the building. Since firefighting is an inexact science to begin with, the use of the quick-calculation formula cannot be expected to determine the exact gpm that will be specifically required for full fire control. It has been found that as the amount of involvement reaches a stage where a defensive attack is necessary, that the needed fire flow will be found to be slightly greater than required.

Water Supply

In regard to water supply, firefighters operating in areas supplied by hydrants should have knowledge of the:

• Location of the hydrants

• Size of the water mains in the immediate and surrounding area

• Available water flow

Small water mains may not provide an ample water supply and hoselines will need to be stretched from distant hydrants that are fed by larger water mains. These hoselines can be stretched directly to the fire or used to supplement the water supplies of the units already operating on the scene.

In non-hydrant areas, firefighters must provide their own source of water. This can include front-line engine companies that have 2,000-gallon water tanks or larger. There will also be a need to have water tenders supplement them to ensure a continuous water supply. This can be accomplished by direct connection to the water tenders, or the use of dump tanks from which the engines can draft. If a water tender operation is to be used, a determination will have to be made of how many tenders will be needed.

Determining Fire Attack

Once the required fire flow has been determined, the capability of available resources will determine the strategy and tactics required to control the incident. If the fire-flow capability of available resources exceeds the required fire flow, an aggressive interior attack on the fire can usually be made. However, before this decision is implemented, the incident commander should consider:

• Do existing conditions allow sufficient safety for the firefighters on an interior attack?

• Is the fire area accessible?

• How many hoselines and firefighters are needed?

• Where is the best location to attack the fire from?

• What support activities are also required? (ventilation, forcible entry, etc.)

If the fire-flow requirements exceed the capability of available resources, a defensive mode of operation is usually required. In these situations, larger hose streams, more apparatus, more equipment and more personnel may have to be requested. The incident commander must recognize that situations will be encountered where nothing can be done with the available resources to save the involved building. In these circumstances, exposure protection becomes the primary objective.

Selection of Hoseline

Recognizing that a 1½-inch or 1¾-inch hoseline flows 125 to 175 gpm, and a 2½-inch hoseline flows approximately 250 gpm, we can estimate the number of hoselines and resources needed to control the fire. While the formula will provide the incident commander with a starting point to determine how much water may be needed for an effective fire attack in normal situations, common sense and good judgment are required to evaluate the effect of the water on the fire as it is being applied. Unforeseen factors may be impacting on the situation such as barriers that prevent the water from reaching the seat of the fire or building contents that cause unexpected fire behavior. If control is not achieved within a reasonable period, the amount of water may have to be increased. If immediate knockdown of the fire takes place, the amount of water being applied can be reduced to minimize water damage to the structure and contents.

Sprinkler Systems

If protective systems are contained within the structure, they should be noted in the pre-plan. Information on these systems is imperative for sound decisions to be made. (One major fire that destroyed a sprinklered building was due to a maintenance man shutting down the electricity, which shut down the water supply, and a malfunction prevented the backup water system from activating.)

A sprinkler system's supply must be maintained. If pumpers are hooked up to hydrants supplying the sprinkler system from a small water main, they may steal the water from the sprinkler system, a counterproductive move.

Summary

The calculations resulting from the needed-fire-flow formula is just another tool. An officer with little experience can find the tool helpful when facing a situation which he or she has not confronted before. How the tool is used can mean the difference between success and failure. Though no system will supplant experience, the utilization of a variety of methods will assist responding firefighters in implementation and deployment of resources at an incident.

JAMES P. SMITH, a Firehouse® contributing editor, recently retired as a deputy chief of the Philadelphia Fire Department. He is an adjunct instructor at the National Fire Academy and the author of the book Strategic and Tactical Considerations on the Fireground, second edition, published by Brady/Prentice Hall, and the accompanying Strategic and Tactical Considerations on the Fireground Study Guide, second edition, published by Trafford Press. He may be contacted at JPSmithPFD@aol.com.

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