|
(This is part one of a two-part series.) Retired Phoenix Fire Chief Alan Brunacini is famous for distilling firefighting into “putting the wet stuff on the red stuff.” However, every experienced firefighter knows if there is not enough “wet stuff” the “red stuff” is going to win every time. Huge building fires that provide dramatic video for the evening news often are the result of an inadequate suppression water supply in the right place at the right time. |
Fire flow calculations should include a building’s construction type, size, use and exposures. Photos by Rob Neale |
The water supply required for manual or master streams is called “fire flow,” shorthand for the water volume and pressure needed to extinguish a building fire and, perhaps, keep it from spreading to nearby properties. It is a foundational truth of fire physics that fire flow’s heat-absorbing capacity must exceed a fire’s heat-generating energy or the fire fight will be lost.
When a fire occurs, it may be too late to think about the amount of water needed for fire control. The fire department’s objective may be as simple as finding any kind of water supply much less worrying about its capacity. Good pre-incident planning is important to identify fire flow resources.
Both the International Building Code® and International Fire Code®, the foundations for the North and South Carolina construction safety codes, require fire protection water supplies before or at the time of new construction. The building or fire code official, often in consultation with the local fire chief, should identify the water supply requirements for a project.
The codes refer to requirements for an “approved” water supply, and leave it to the code official to determine the requirements. Water supplies must “consist of reservoirs, pressure tanks, elevated tanks, water mains or other fixed systems [emphasis added] capable of providing the required fire flow. Where public water supply is inadequate or not available, an approved water source meeting the fire flow requirements shall be provided.” This is according to the International Fire Code (2015), Section 507.
The term “fixed systems” is emphasized in the previous paragraph because despite the long-standing tradition and use of mobile water supplies — water tenders or tankers — the building and fire codes do not recognize them as an approved fire protection water source.
Fire protection water supplies must be fixed facilities based on the fire protection demand. This small tank at a California winery is intended to provide enough water for the fire attack.
Elements Affecting Fire Flow
How is fire flow established? There is no single “correct” method for establishing fire flow: the Incident Commander’s (IC’s) overall objective is to provide enough water effectively at the right place to suppress a fire’s heat energy. The capacity of the water system and the fire department’s ability to deliver flow may be the limiting factors.
Several mathematical formulas exist to determine water supply requirements. Some of these formulas are intended to be applied during building construction, and others are intended to be quick references for an IC confronted with an emergency. Some of the formulas are mathematically complex while others can be worked out in a few seconds in one’s own mind.
When a fire protection engineer, inspector, building official, IC, insurance representative, or contractor discusses fire flow, six significant features must be considered:
- Building construction type
- Occupancy type
- Building size
- Exposures
- Percentage of involvement
- Automatic fire protection
Taken together, these elements are evaluated to establish how much water is needed to control and suppress a fire.
Building Construction Type
Do the building materials used in construction contribute to the fuel load? Wood, plastics and other combustible materials provide fuel. Steel, concrete and gypsum wallboard do not increase the fuel load of a structure.
The International Building Code® classifies buildings into five major construction categories: fire resistive, non-combustible, ordinary, heavy timber and combustible. Each category is further refined by whether the structural elements are protected by fire resistant materials such as gypsum wallboard, concrete or spray-on fire resistive finishes.
For determining fire flow, it is generally assumed that buildings which are partially of wholly combustible materials will require more water for fire suppression than buildings that are predominantly non-combustible or fire resistive.
Occupancy Type
A structure’s use, or occupancy, is a major factor in evaluating water supply requirements. A mercantile occupancy likely has a heavier fuel load than an office use; a rack storage warehouse generally has more potential fuel than a dwelling.
The various fire flow formulas used in the US handle occupancy type and contents differently. Several have specific, detailed adjustments based on the contents’ assumed combustibility and fire load — the amount of combustible materials per square foot. The simpler calculations either ignore the contents factor or average the fire across all occupancy types.
Secondary or alternative water supplies, like this abandoned quarry, may provide substantial reserves and should be included in pre-incident planning.
Building Size
Generally, the larger the building, the more water will be needed to control a fire in it. A 100,000-square-foot building, for example, will require a higher flow rate than a 28,000- square-foot building, because the larger building can contain more fuel. Likewise, a tall or multiple-story building may include more fuel, so the structure’s volume may be a consideration.
Exposures
An important part of fire strategy is to prevent fire from spreading from the burning building to adjacent structures or exposures. Master streams or large hand-lines often are used to protect exposures so an adequate water supply must be calculated to include them. Usually, the closer the exposures are located to one another, the greater water demand is for exposure protection. The openings — windows and doors — and combustibility of adjacent structures also should be taken into account.
Percentage of Involvement
Part of an effective size-up includes determining how much of a building is burning. If the fire has gained headway when the fire suppression forces arrive, it will take a greater amount of water to control and, eventually, to suppress the fire.
The formulas intended for the IC during an emergency may or may not take the “percentage of involvement” into account. The IC may have to make adjustments based on experience and observation.
Automatic Fire Protection
If installed and operating properly, automatic sprinklers will confine a fire and keep water demands low. Effective sprinkler protection also minimizes the water needs for hand-lines.
The Insurance Services Office (ISO) “Guide for the Determination of Required Fire Flow” that will be explained in the next issue, makes it clear that if a building is protected by an approved sprinkler system designed in accordance with NFPA 13 “Standard for the Installation of Sprinkler Systems,” no additional fire flow calculations are required for hand-held streams.
The reason ISO does not require additional fire flow is found within NFPA 13: it requires an adequate water supply for the sprinkler system plus what it calls “hose stream allowance.” The hose stream allowance is the amount of water needed to suppress a fire once the fire department arrives. The volume and pressure requirements are computed in the sprinkler system design.
Some jurisdictions reduce fire flow calculations if the building is protected by sprinklers and a complete smoke detection system that reports to the fire department. Their justification is that if the fire suppression forces get early notification and respond, the fire will be small upon their arrival and less water will be needed. While this may be appropriate for some buildings, smoke detection systems may not be appropriate in all occupancies where dust and high air flows are routine. For example, a smoke detection system installed in a wood working plant would be susceptible to regular and annoying false alarms.
As a final note, in addition to the required fire flow, planners and incident commanders should consider access to secondary or alternative water supplies. In some cases, existing water service may be adequate to sustain an initial fire attack, but if a fire cannot be controlled more water may be needed. The fire code notes this deficiency by stating “Where public water supply is inadequate or not available, an approved water source meeting the fire flow requirements shall be provided.”
These supplies should be identified and included on pre-incident plans in case of failure of the primary supply or conflagration.
Summary
An adequate fire protection water supply – fire flow – is essential to successful fire outcomes. Building and fire codes give code officials the authority to establish fire flow requirements for the buildings in their jurisdiction.
The fire flow should be based on sound engineering principles and not solely reliant on what may exist already at a project site or can be transported by mobile apparatus.
Part two of this article “Determining Required Fire Flow” will explain the variety of methods that can be used to help the code official establish an “approved” fire flow.
By Rob Neale
11/07/2017 – 
