API 521 typically recommends 0.01 to 0.1 ft/s to prevent air ingress.
Required Flow Rate (SCFH):–
Required Flow Rate (Nm³/h):–
Husa Correlation Estimate (N2):–
The Husa estimate is based on the empirical formula Q = 0.003528 × D^3.46.
Always verify calculated rates against local safety standards and API 521 guidelines.
Understanding Flare Header Purge Rates
In industrial process safety, maintaining an adequate purge rate in flare headers is critical to prevent the ingress of air. Air infiltration into a flare system can create combustible mixtures which, if ignited, may lead to catastrophic flashbacks or explosions within the piping network. This calculator assists process engineers in estimating the required purge gas flow rates based on header geometry and industry-standard velocities.
Why Purge Calculations Matter
Flare stacks are open to the atmosphere at the top. Without a continuous flow of purge gas (typically Nitrogen or Fuel Gas), the buoyancy differences between the hot waste gas and the ambient air, or cooling of the gases inside the stack, can create a vacuum or internal circulation that draws air down into the stack.
The purge rate is defined as the continuous flow of gas required to maintain a positive pressure or a specific exit velocity that prevents this air ingress. The most common standards for these calculations come from API Standard 521.
Methodologies Used
This tool provides two calculation methods to ensure a comprehensive safety assessment:
Velocity Method (API 521): This is the primary output. It calculates the volumetric flow required to maintain a specific velocity (typically 0.04 ft/s for Nitrogen) across the cross-sectional area of the header. This is a geometric calculation:
Q = Velocity × Area
Husa Correlation: Developed by H.W. Husa in 1964, this empirical formula estimates the purge rate specifically required to keep oxygen concentrations below safe limits (typically < 6%) deep within the stack. The formula is:
Q (SCFH) = K × D^3.46 Where D is the diameter in inches and K is a constant derived from experimental data (approx 0.003528 for Nitrogen).
Purge Gas Selection
Nitrogen is the preferred purge gas because it is inert and does not support combustion. It increases the safety margin significantly. Fuel Gas (Methane/Natural Gas) is also used; however, because it is lighter than air (buoyant), it helps create a buoyancy seal, but it adds combustible load to the system and has economic implications.
Key Considerations
When designing purge systems, engineers must also consider the molecular seal or velocity seal installed at the flare tip. These mechanical devices can significantly reduce the required purge rates compared to an open pipe. The values generated by this calculator assume an open pipe configuration or a conservative baseline for systems with fluidic seals.
function calculatePurgeRate() {
// 1. Get Input Values
var diameterInput = document.getElementById("headerDiameter").value;
var velocityInput = document.getElementById("purgeVelocity").value;
var gasType = document.getElementById("gasType").value;
// 2. Validate Inputs
if (!diameterInput || diameterInput <= 0) {
alert("Please enter a valid positive header diameter.");
return;
}
if (!velocityInput || velocityInput Radius is D/2, converted to feet is D/2/12 = D/24
// Alternatively: Area = Pi * (D_ft/2)^2. D_ft = D_inches/12.
var D_feet = D_inches / 12;
var radius_feet = D_feet / 2;
var area_sqft = Math.PI * Math.pow(radius_feet, 2);
// 5. Calculate Volumetric Flow Rate (Velocity Method)
// Q (cfs) = V (ft/s) * A (sq ft)
// Convert to SCFH: cfs * 3600
var Q_cfs = V_fps * area_sqft;
var Q_scfh = Q_cfs * 3600;
// 6. Convert to Metric (Nm3/h)
// 1 SCFH approx 0.0283168 m3/h (Normal cubic meter depends on Temp/Pressure std, usually close factor)
// Standard industrial conversion: 1 SCF = 0.0268 Nm3 (depending on norm) or 0.0283 Sm3.
// We will use 1 SCFH = 0.02832 Nm3/h for approximation.
var Q_nm3h = Q_scfh * 0.0283168;
// 7. Calculate Husa Correlation (Empirical Check for Nitrogen)
// Formula: Q = 0.003528 * D^3.46
var husa_factor = 0.003528;
var husa_exponent = 3.46;
var Q_husa = husa_factor * Math.pow(D_inches, husa_exponent);
// 8. Display Results
document.getElementById("resSCFH").innerHTML = Q_scfh.toLocaleString("en-US", {maximumFractionDigits: 1}) + " SCFH";
document.getElementById("resNm3h").innerHTML = Q_nm3h.toLocaleString("en-US", {maximumFractionDigits: 2}) + " Nm³/h";
// Show Husa result
document.getElementById("resHusa").innerHTML = Q_husa.toLocaleString("en-US", {maximumFractionDigits: 1}) + " SCFH";
// Make result box visible
document.getElementById("resultBox").style.display = "block";
}