function calculateHydrantFlow() {
var diameter = parseFloat(document.getElementById('orificeDiameter').value);
var pitot = parseFloat(document.getElementById('pitotPressure').value);
var coeff = parseFloat(document.getElementById('coefficient').value);
var staticP = parseFloat(document.getElementById('staticPressure').value);
var residualP = parseFloat(document.getElementById('residualPressure').value);
var resultDiv = document.getElementById('flowResult');
var primaryDisplay = document.getElementById('primaryResult');
var secondaryDisplay = document.getElementById('secondaryResult');
if (isNaN(diameter) || isNaN(pitot) || isNaN(coeff)) {
alert("Please enter the required Orifice Diameter and Pitot Pressure.");
return;
}
// GPM = 29.84 * C * d^2 * sqrt(p)
var flowRate = 29.84 * coeff * Math.pow(diameter, 2) * Math.sqrt(pitot);
resultDiv.style.display = "block";
primaryDisplay.innerHTML = "Measured Flow: " + Math.round(flowRate).toLocaleString() + " GPM";
// Predict flow at 20 PSI residual (NFPA 291 Formula)
// Qr = Qf * ((Hr – 20) / (Hs – Hr))^0.54
if (!isNaN(staticP) && !isNaN(residualP) && (staticP – residualP) > 0) {
var pressureDrop = staticP – residualP;
var availableFlow = flowRate * Math.pow((staticP – 20) / pressureDrop, 0.54);
if (staticP <= 20) {
secondaryDisplay.innerHTML = "Static pressure must be above 20 PSI for residual calculation.";
} else {
secondaryDisplay.innerHTML = "Available Flow at 20 PSI: " + Math.round(availableFlow).toLocaleString() + " GPM";
}
} else {
secondaryDisplay.innerHTML = "";
}
}
Understanding Hydrant Flow Tests
A hydrant flow test is a critical procedure used by fire protection engineers, water departments, and fire marshals to determine the available water supply in a distribution system. These tests ensure that the fire sprinkler systems and firefighting operations have sufficient volume and pressure.
The NFPA 291 Calculation Formula
The standard formula for calculating the discharge from a hydrant orifice is:
Q = 29.84 × c × d² × √p
Q: Flow rate in Gallons Per Minute (GPM).
c: Coefficient of discharge (based on the shape of the hydrant outlet).
d: Internal diameter of the hydrant outlet in inches.
p: Pitot pressure reading in PSI.
Discharge Coefficients (c-factors)
The efficiency of the water flow depends on how the internal nozzle is shaped. Using the wrong coefficient can lead to an overestimation of available water:
0.90: Smooth, well-rounded transitions where the nozzle meets the barrel.
0.80: Sharp, square-edged transitions (the most common type).
0.70: Nozzle that protrudes into the hydrant barrel.
Predicting Flow at 20 PSI
While the test measures the flow at the current residual pressure, NFPA 291 requires calculating the Predicted Flow at 20 PSI. This is the industry-standard "safety" baseline, as most water systems require at least 20 PSI to prevent pipe collapse or backflow contamination. This is calculated using the Hazen-Williams based ratio of the pressure drops.
Example Calculation
Imagine you are testing a hydrant with a 2.5-inch outlet and a square-edged transition (0.80 coefficient). Your pitot gauge reads 16 PSI.
Square the diameter: 2.5 × 2.5 = 6.25
Find the square root of pressure: √16 = 4
Multiply: 29.84 × 0.80 × 6.25 × 4 = 596.8 GPM
Safety Note: Always open and close hydrants slowly to prevent water hammer, which can rupture water mains. Ensure proper dechlorination if discharging into storm drains.