Section 4.2: Water Supply and Fire Flow Calculations

Water Supply and Fire Flow Calculations

Water supply is the foundational element of active fire suppression. A plans examiner must verify that a building’s proposed water supply is adequate to meet both the building's automatic fire sprinkler demand and the fire-flow demand required for manual firefighting operations. The 2024 International Fire Code (IFC) Chapter 5 (Section 507) and Appendix B establish the criteria for determining required fire flow, the duration of that flow, and the allowable reductions for buildings equipped with automatic sprinkler systems.

Fire Flow and Residual Pressure Concepts (IFC 507.1 & Appendix B)

Fire flow is defined as the flow rate of a water supply, measured at a residual pressure of 20 pounds per square inch (psi), that is available for firefighting. The residual pressure of 20 psi is a critical threshold in municipal water systems for several reasons:

1. Cavitation Prevention: If fire engine pumps draw water from a hydrant at a pressure lower than 20 psi, the drop in pressure can cause cavitation inside the pump, damaging fire department equipment.
2. Main Protection: Drawing the pressure in municipal water mains below 20 psi runs the risk of collapsing the pipes due to external soil pressure or sucking in groundwater and contaminants through joints and cracks.

Therefore, all fire-flow calculations and hydrant flow tests must verify that the required gpm is available while maintaining at least 20 psi of residual pressure at the test hydrant.

Calculating the Fire-Flow Area (IFC Section B104)

Before determining the required fire flow from the code tables, the plans examiner must calculate the 'fire-flow calculation area.' Under IFC Section B104.1, the fire-flow calculation area is the total floor area of all stories of the building within the surrounding exterior walls.

However, IFC Section B104.2 provides an exception:

• Fire Walls: If a building is divided into separate portions by fire walls constructed without openings and in accordance with the International Building Code (IBC Section 706), the fire-flow calculation area is permitted to be the largest single area separated by these walls.
• Fire Barriers/Partitions: Portions of a building separated by fire barriers (IBC Section 707) or fire partitions (IBC Section 708) do not qualify for this reduction. Even if a stairwell or corridor is enclosed by a 2-hour fire barrier, the entire building area must be summed to determine the fire-flow calculation area.

Determining Fire-Flow Requirements (IFC Table B105.1(1) and Table B105.1(2))

IFC Appendix B uses two separate tables to establish the required water supply:

1. Table B105.1(1): Applies to one- and two-family dwellings, Group R-3, and Group R-4 buildings. For dwellings under 3,600 square feet, the required fire flow is 1,000 gpm for a duration of 1 hour. For larger dwellings, the flow increases up to 1,500 gpm.
2. Table B105.1(2): Applies to all other occupancies (commercial, industrial, institutional, etc.). This table cross-references the building's Type of Construction (Types I through V, protected or unprotected) and the Fire-Flow Calculation Area to determine the required flow in gpm and the duration in hours.

For example, a 60,000 square foot building of Type V-B construction (combustible, unprotected framing) has a higher fuel load and will collapse faster than a Type I-B building (noncombustible, protected). Consequently, the Type V-B building requires a much higher fire flow (typically 3,750 gpm for 3 hours) compared to a Type I-B building of the same size (which might require 2,000 gpm for 2 hours).

Sprinklered vs. Unsprinklered Flow Demands (IFC Table B105.2)

To encourage the installation of automatic sprinklers and offset the water demand of manual firefighting, IFC Section B105.2 and Table B105.2 permit significant reductions in fire-flow requirements for buildings equipped throughout with an approved automatic sprinkler system:

• Allowable Reduction: A reduction of 75 percent is permitted when the building is protected by an automatic sprinkler system installed in accordance with NFPA 13 (Section 903.3.1.1) or NFPA 13R (Section 903.3.1.2).
• Minimum Flow Limit: The resulting fire flow after the 75 percent reduction cannot be less than 1,000 gpm.
• Duration Reduction: The duration of the fire flow is also reduced. If the reduced fire-flow requirement is 3,000 gpm or less, the duration is reduced to 1 hour. If the flow exceeds 3,000 gpm, the duration is reduced to 2 hours.

Hydrant Flow Testing Mathematics (NFPA 291)

To verify if the municipal supply can satisfy these flows, a fire flow test is performed. The test requires a designated static/residual hydrant and one or more flow hydrants. The examiner evaluates three pressure readings:

• Static Pressure ($P_s$): The pressure measured when all hydrants are closed (no water is flowing).
• Residual Pressure ($P_r$): The pressure measured at the static hydrant while the flow hydrant is fully open.
• Pitot Pressure ($P_p$): The pressure measured using a pitot tube inserted into the flowing stream of the flow hydrant.

First, calculate the actual flow rate ($Q_f$) from the flow hydrant outlet using the pitot formula:

$Q_f = 29.84 \times c \times d^2 \times \sqrt{P_p}$

Where:

• $c$ = Discharge coefficient of the hydrant nozzle (typically 0.90 for smooth, rounded outlets).
• $d$ = Internal diameter of the hydrant nozzle in inches (typically 2.5 inches).

Once $Q_f$ is known, calculate the available flow at 20 psi residual ($Q_r$) using the relative discharge formula:

$Q_r = Q_f \times \left(\frac{P_s - 20}{P_s - P_r}\right)^{0.54}$

If $Q_r$ is greater than the required fire flow determined from the code tables, the water supply is deemed compliant. If the available flow is less, the building must be redesigned (e.g., changing construction type to reduce required fire flow) or a private water storage tank must be provided.

Worked Scenario

An examiner is reviewing a plan for a new two-story retail department store (Group M).

• Construction Type: Type III-B (Exterior walls are noncombustible, interior framing is combustible).
• Total Area: 45,000 square feet (22,500 sq ft per floor). The building is not divided by fire walls.
• Fire Protection: Equipped throughout with an automatic sprinkler system designed per NFPA 13.

The plans examiner performs the following review:

1. Unsprinklered Fire-Flow Determination: Using IFC Table B105.1(2) for Type III-B construction and an area of 45,000 sq ft, the baseline required fire flow is determined to be 3,250 gpm with a required duration of 3 hours.
2. Applying Sprinkler Reduction: Since the building has a full NFPA 13 sprinkler system, Table B105.2 permits a 75% reduction. $Reduced Flow = 3,250 \times (1 - 0.75) = 812.5 \text{ gpm}$
3. Applying Code Floor: The calculated reduced flow is 812.5 gpm. However, the code states the fire flow cannot be less than 1,000 gpm. Therefore, the required fire flow is adjusted upward to 1,000 gpm.
4. Determining Duration: Since the final fire flow is 1,000 gpm (which is less than or equal to 3,000 gpm), the required duration is reduced from 3 hours to 1 hour.
5. Flow Test Comparison: The plans examiner reviews the water utility's flow test report for the nearest hydrant: Static Pressure = 65 psi, Residual Pressure = 42 psi, and Flow = 1,450 gpm. Since 1,450 gpm is available at a residual pressure above 20 psi, the water supply is approved.

Summary Table of Fire-Flow Comparison