Hydrant · calculator
Fire Hydrant Flow Calculator (Pitot Pressure to GPM)
Estimate hydrant discharge in US GPM from pitot pressure, outlet diameter, and discharge coefficient.
Method record
What this calculator measures
Pitot pressure represents velocity pressure in the flowing stream. Outlet condition determines the discharge coefficient, so rounded, square, and projecting outlets can produce different flows at the same reading.
Q = 29.83 × Cᵈ × d² × √P Use it correctly
Calculation steps
- Confirm the effective outlet diameter.
- Measure pitot pressure in the center of the stream.
- Choose the coefficient that matches outlet condition.
Applicability
Use the model within its limits
Fire protection testing and available-fire-flow decisions require applicable standards, qualified personnel, and AHJ requirements.
What this result represents
- System
- Hydrant
- Formula
- Q = 29.83 × Cᵈ × d² × √P
- Review
- Verify safety-sensitive work with qualified guidance.
Worked Field Example
Worked Example: Municipal Fire Hydrant Pitot Test
Field Scenario: Measuring water flow from an open 2-1/2 inch hydrant butt during an NFPA 291 field flow test.
Given Input Parameters
Step-by-Step Mathematical Solution
- Compute orifice diameter squared: d² = 2.50² = 6.25 sq inches.
- Compute square root of pitot pressure: √36 = 6.0.
- Apply the NFPA 291 formula: Q = 29.84 × Cd × d² × √P = 29.84 × 0.90 × 6.25 × 6.0 = 29.84 × 33.75 ≈ 1,007 GPM.
Discharge flow rate is 1,007 GPM.
Professional Guidance: Verify that the pitot blade is held steadily in the center of the discharge stream, approximately half the nozzle diameter away from the orifice face.
Quick Answer & Key Principles
How to Calculate Fire Hydrant Flow from Pitot Gauge Pressure
Direct Answer: Water discharge from an open fire hydrant nozzle is calculated using the Freeman flow formula: Q = 29.84 × Cd × d² × √P.
- Q = Water discharge flow rate in US Gallons per Minute (GPM)
- Cd = Outlet nozzle discharge coefficient (0.90 for smooth rounded, 0.80 for square, 0.70 for projecting)
- d = Inside diameter of the flowing hydrant outlet nozzle in inches (typically 2.5" or 4.5")
- P = Velocity pitot gauge pressure reading measured at the nozzle centerline in PSI
Engineering FAQs
Frequently Asked Questions
How do I choose the correct hydrant discharge coefficient (Cd)?
NFPA 291 establishes three standard coefficients based on internal nozzle shape: Cd = 0.90 for smooth, well-rounded internal nozzle transitions; Cd = 0.80 for square, sharp-edged transitions that project slightly into the barrel; and Cd = 0.70 for rough, square edges that project substantially into the barrel.
What is the difference between static pressure, residual pressure, and pitot pressure?
Static pressure is the line pressure measured before any hydrant is opened. Residual pressure is the pressure measured at a non-flowing test hydrant while another hydrant is discharging. Pitot pressure is the dynamic velocity pressure measured in the center of the discharged water stream with a handheld pitot blade.
Why should pitot pressure be measured in the center of the stream?
Water velocity is highest and most uniform near the center of the orifice. The orifice discharge coefficient (29.84 × Cd) is calibrated specifically for pitot readings taken at the center of the stream orifice, held approximately half the nozzle diameter away from the orifice face.