FPEeXTRA Issue 104

Fire Flow in Fire Protection Engineering

By: Ahmed Ibrahim, PE, PMSFPE, Fire and Life Safety Consultant

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Introduction

Fire flow refers to the amount of water required for firefighting operations, delivered at a rate sufficient to control a significant fire within a structure while maintaining a residual pressure of 20 psi for a specified duration. This water supply supports fire suppression, helps prevent the fire's spread, and minimizes damage to the structure and surrounding areas. Typically, fire flow is supplied by fire hydrants connected to the municipal water network. However, if the public water system flow is insufficient to meet the required demand, private hydrants, fire pump and water storage tanks may be needed to fulfill fire flow requirements.

History of Fire Flow

Early fire protection efforts relied on basic methods such as bucket brigades, where individuals formed a line to pass buckets of water from a source to the fire, one person at a time. Although useful for small fires, these techniques were often insufficient against larger, more intense fires that required substantial water flow to control effectively. Such events underscored the need for standardized fire protection systems and dependable water supplies to support effective firefighting.

As cities expanded and building density increased, modern water distribution systems were developed to meet urban needs, including those of firefighting. The introduction of fire hydrants connected to municipal water networks was a significant advancement, offering firefighters a readily available, pressurized water source. This improvement transformed firefighting by reducing response times and allowing for a sustained water supply, which is crucial in mitigating larger fires.

To establish adequate fire flow, various calculation methods were introduced, such as the Iowa State Formula, the National Fire Academy Formula, and the Insurance Services Office (ISO) Formula. These methods account for key factors, including building area, construction materials, and other structural details, to ensure that the water volume and pressure provided are sufficient to control and contain fires effectively.

Today, in various regions across the U.S. and other countries, different fire codes are adopted to regulate fire safety measures. In the U.S., for example, either the International Fire Code (IFC), published by the International Code Council, or NFPA 1, published by the National Fire Protection Association, is typically adopted. These codes, collectively referred to as fire codes, are widely used not only in different U.S. states but also in other countries around the world. Both codes utilize similar methodologies for calculating fire flow, with minor differences, to ensure a reliable water supply that protects structures and minimizes the risk of extensive fire damage.

Calculating Fire Flow

The IFC addresses fire flow requirements in Appendix B, which are advisory unless formally adopted by state, national, or local jurisdictions. In contrast, NFPA 1 specifies fire flow requirements in Section 18.4, making compliance mandatory unless exclusions or changes are specified in the adopted code version by these authorities. Regardless of whether these requirements are advisory or mandatory, fire protection engineers and Authorities Having Jurisdiction (AHJ) should carefully assess fire flow to ensure it meets firefighting needs. Insufficient fire flow can lead to uncontrolled fire growth, potentially spreading to adjacent structures and causing significant losses.

Both the IFC and NFPA 1 use identical tables for calculating fire flow, following methodologies developed by the Insurance Services Office (ISO). These tables take into account factors such as building construction type and total floor area, measured within the perimeter of exterior walls, to determine the required fire flow and duration. By incorporating ISO's guidelines, these codes provide a standardized approach to fire flow calculations.

Building construction type, defined by the materials used and the fire resistance ratings of structural elements, plays a critical role in determining fire flow requirements. Both the IFC and NFPA 1 classify buildings by fire resistance, ranging from Type I (most fire-resistant) to Type V (least fire-resistant):

• Type I and II: Constructed with noncombustible materials like concrete and steel.
• Type III: Noncombustible exterior walls with combustible interior components.
• Type IV: Heavy timber construction with large wooden structural members.
• Type V: Primarily wood-framed construction.

The calculation also includes the duration for which the fire flow must be sustained. For example, according to IFC Table B105.1(2), a Type II noncombustible building with a floor area of 10,000 square feet would require a fire flow of 1,500 gallons per minute (gpm), sustained for two hours.

Impact of Sprinkler Systems on Fire Flow Calculations

Automatic sprinkler systems have proven highly effective in controlling and extinguishing fires, significantly influencing fire life safety design, including fire flow requirements. When sprinklers are installed, codes permit substantial reductions in the required fire flow; however, minimum thresholds must still be maintained. This reduction is justified because sprinklers reduce the risk of fire spreading, significantly lowering the potential for fire growth and minimizing the chance of it spreading to nearby buildings.

For instance, the IFC allows up to a 75% reduction in fire flow for buildings equipped with an approved automatic sprinkler system. However, the code mandates minimum fire flow levels based on building type, with specific minimum values set for one- or two-family dwellings and other types. In a non-residential scenario where the original fire flow requirement is 1,500 gpm, a 75% reduction would bring it down to 375 gpm. Despite this reduction, the minimum fire flow for such buildings cannot be lower than 1,000 gpm. Therefore, in this case, the fire flow would be adjusted to 1,000 gpm to meet code requirements.

Reduction of Fire Flow Requirements Through Fire Walls

The required fire flow under both the International Fire Code (IFC) and NFPA 1 can be reduced by dividing a building into smaller sections using fire walls. A fire wall is a fire-resistance-rated wall designed to stop fire from spreading between different parts of a building. These walls extend continuously from the foundation up to, or even through, the roof, creating a solid, protective barrier. They are generally built to remain standing even if fire causes surrounding areas to collapse, which means a failure would likely occur only on one side of the wall, leaving the other side protected.

By using fire walls to divide a building into multiple fire areas, each section can be considered independently when calculating the required fire flow, making it easier for larger buildings to comply with fire safety requirements. This approach allows fire protection for larger structures by treating each section as a separate area, which reduces the total fire flow needed.

Fire walls can include protected openings, such as fire-rated doors or windows, as long as they meet specific code standards. However, the codes vary in requirements for fire flow reduction: for a fire wall to qualify for reducing fire flow requirements, the IFC mandates that it must have no openings at all. In contrast, NFPA 1 permits fire walls with openings to qualify for reduced fire flow, provided those openings meet designated safety standards.

Fire Flow Requirements for Rural Buildings

Fire codes allow the Authority Having Jurisdiction (AHJ) to reduce fire flow requirements for buildings constructed in rural regions. The risk of large fire growth in rural areas is generally lower, especially for sprinklered buildings where outside hose allowances are included in the sprinkler calculations. In fully sprinklered buildings located in isolated rural areas, the calculated fire flow may sometimes be overlooked, as the hose allowance could provide sufficient water for firefighting operations.

However, full fire flow should still be considered in areas where buildings are close together, as additional fire flow is crucial to prevent fire from spreading to adjacent structures. Therefore, the decision to reduce or omit fire flow in sprinklered buildings in rural areas should be evaluated on a case-by-case basis. This decision should involve coordination between the fire protection engineer and the AHJ. By considering specific circumstances, safety measures, available water resources, local codes, and relevant past experiences, an appropriate determination can be made.

Common Oversights in Implementing Fire Flow

1. Water Storage and Fire Pump Capacity

When a fire pump and tank are part of a private fire protection system due to flow and pressure limitations in the main water network, and private fire hydrants within this system are designated for firefighter operations, the fire pump selection must account for the required fire flow. The rated flow of the fire pump does not need to match or exceed the required fire flow directly. Instead, the pump’s performance curve should be evaluated to ensure it can deliver the necessary fire flow while maintaining a residual pressure of at least 20 psi (138 kPa). This minimum pressure, recommended by water authorities, is essential for the safe operation of fire engines and helps prevent negative pressure in the water main, which could otherwise compromise the water supply system. Maintaining this level of pressure is crucial for effective firefighting operations and the stability of the water infrastructure.

A fire pump with a rated capacity of 1,000 gpm may still meet a fire flow requirement of 1,500 gpm if it can maintain this higher flow rate while keeping the residual pressure above 20 psi. This underscores the importance of reviewing the pump’s performance curve and its maximum flow capabilities rather than relying solely on its rated capacity. Additionally, the fire water tank must be able to store the required fire flow for the duration specified by fire codes, ensuring that, along with the pump’s performance, there is sufficient water available to meet firefighting needs effectively.

2. When Fire Flow Is Required

Fire flow is typically supplied by public hydrants. However, when a group of buildings has private hydrants within its fire protection system, these hydrants must be included in the fire flow requirements. In such cases, the private fire protection system, which may include private hydrants, fire pumps, and water storage tanks, is responsible for providing the required fire flow. Conversely, if hydrants are only located on public streets, fire flow requirements do not apply to any private fire protection system components, such as fire pumps or water tanks, as they are not necessary for firefighting operations in this setup.

3. Adding Fire Flow to Sprinkler and Standpipe Systems

Fire codes do not require that the fire flow be added to the sprinkler and/or standpipe systems in all cases. Both NFPA 1 and the IFC require using the higher flow value between the required fire flow and the sprinkler or standpipe demand, not the sum of both. This approach ensures that the most demanding requirement is met without unnecessarily oversizing the water supply system.

4. Flow Test Verification

When fire flow is supplied by public hydrants, conducting a hydrant flow test is essential to confirm that the water supply can deliver the required flow at a minimum pressure of 20 psi. Fire flow calculations must also be submitted to the Authority Having Jurisdiction (AHJ), taking into account the building's area and construction type as specified in the relevant fire codes. The AHJ plays a crucial role in enforcing fire flow standards and may adopt specific code appendices or additional local requirements that impact how fire flow is calculated and applied. Fire protection engineers should collaborate closely with the AHJ to ensure full compliance with all applicable regulations, including fire flow requirements.

5. Coordination Among Stakeholders

Effective fire flow determination requires collaboration among fire protection engineers, civil engineers, architects, local fire departments, and other relevant Authorities Having Jurisdiction (AHJ). This coordination should occur in the early stages of the project to ensure that the fire flow is accurately calculated and integrated with other design systems. In some cases, especially with large, non-sprinklered buildings, the required fire flow may be significantly high. Early discussions can help explore options to reduce fire flow requirements, such as using fire walls. Engaging all stakeholders, including building owners, early in the process is essential to address potential issues and streamline the approval process.

Areas of Improvement in Fire Codes

Fire flow calculations are mostly based on observed data and standard guidelines rather than detailed mathematical or physical models. This reliance on empirical methods means they may not fully account for important factors that can affect how much water is needed to control a fire. Some of these factors include:

• Building Surroundings: If a building is in an open area with no nearby structures, the risk of fire spreading is lower. Standard fire flow requirements might not account for this reduced risk, especially if the building has a sprinkler system. While it is up to the local fire authority to decide on any reductions, clearer guidelines in the codes could help adjust fire flow based on the actual fire risk.
• Hazardous Materials: The amount and type of materials stored in a building affect fire risk, but standard fire flow calculations do not always consider this. Buildings with high-risk contents, like flammable materials, might need more water than general calculations suggest, while buildings with low-risk contents might need less.
• Ambiguity in Code Provisions: Fire flow provisions in the codes are structured independently of sprinkler and standpipe system designs. The codes lack clear guidance on whether the outside hose allowance or standpipes can fully satisfy fire flow needs in certain cases.
• Lack of Clear Statements: Neither the IFC nor NFPA 1 clearly states that fire flow requirements are intended only for fire hydrants, which can lead to misunderstandings. Some may assume that fire flow also applies as an extra safety measure for other water-based systems, like sprinklers and standpipes, when this is not the primary intent.

Conclusion

While fire flow calculation methods may not be rooted in exact science, they offer a standardized approach adaptable to various situations and building types. Properly understanding and applying fire flow requirements is essential to effective fire protection engineering, helping ensure an adequate water supply is available for firefighting efforts. Achieving compliance with fire flow standards requires thorough coordination among all stakeholders, including fire protection engineers, architects, Authorities Having Jurisdiction (AHJs), building owners, and municipal water engineers. Through collaborative planning and adherence to these requirements, fire protection systems can be tailored to meet unique project needs, enhancing safety for occupants and reducing the risk of fire damage to structures and surrounding areas.