Fire protection · calculator
Available Fire Flow Calculator (NFPA 291 Residual Projection)
Project measured hydrant test flow to a selected residual pressure using static and residual readings.
Method record
What this calculator measures
A hydrant flow test establishes one measured point on the water-supply curve. The projection estimates flow at another residual pressure, commonly 20 PSI.
Projected flow = test flow × [(static − target) ÷ (static − residual)]⁰·⁵⁴ Use it correctly
Calculation steps
- Record static pressure before flowing.
- Measure residual pressure and test flow simultaneously.
- Project only within a defensible range and document test conditions.
Applicability
Use the model within its limits
Follow current NFPA 291, utility, department, and AHJ test procedures. A weak pressure drop can make extrapolation unreliable.
What this result represents
- System
- Fire protection
- Formula
- Projected flow = test flow × [(static − target) ÷ (static − residual)]⁰·⁵⁴
- Review
- Verify safety-sensitive work with qualified guidance.
Worked Field Example
Worked Example: Projecting Available Fire Flow at 20 PSI
Field Scenario: Evaluating municipal water main capacity from an NFPA 291 dual-hydrant flow test.
Given Input Parameters
Step-by-Step Mathematical Solution
- Pressure drop during test: P_S - P_F = 78 - 56 = 22 PSI.
- Allowable drop to 20 PSI: P_S - P_R = 78 - 20 = 58 PSI.
- Ratio: 58 / 22 ≈ 2.636.
- Apply NFPA 291 0.54 power equation: Q_R = 1,150 × (2.636)^0.54 = 1,150 × 1.684 ≈ 1,937 GPM.
1,937 GPM available at 20 PSI residual pressure (NFPA Class AA Hydrant).
Professional Guidance: Paint bonnet light blue per NFPA 291 to designate capacity exceeding 1,500 GPM.
Engineering FAQs
Frequently Asked Questions
Why does the NFPA 291 formula use an exponent of 0.54?
The 0.54 exponent is derived directly from the Hazen-Williams friction equation, where pressure loss scales as flow to the 1.852 power (since 1 / 1.852 = 0.54). It models the non-linear relationship between pressure drop and discharge rate in municipal water mains.
What are the NFPA 291 hydrant color-coding classifications?
Hydrant bonnets and caps are painted according to available flow at 20 PSI: Class AA = Light Blue (1,500 GPM or greater); Class A = Green (1,000–1,499 GPM); Class B = Orange (500–999 GPM); Class C = Red (less than 500 GPM).
How much pressure drop is required during a valid hydrant flow test?
NFPA 291 recommends creating a residual pressure drop of at least 25% (or a minimum of 10–15 PSI) during the flow test to ensure the projection calculation is mathematically robust.