function updateLabels() {
var system = document.getElementById('unitSystem').value;
var volLabel = document.getElementById('volLabel');
var unitFlow = document.getElementById('unitFlow');
var unitTotal = document.getElementById('unitTotal');
if (system === 'imperial') {
volLabel.innerText = "Vessel Volume (ft³)";
if (unitFlow) unitFlow.innerText = "SCFM";
if (unitTotal) unitTotal.innerText = "SCF";
} else {
volLabel.innerText = "Vessel Volume (m³)";
if (unitFlow) unitFlow.innerText = "Nm³/hr";
if (unitTotal) unitTotal.innerText = "Nm³";
}
// Clear results when switching units to avoid confusion
document.getElementById('resultDisplay').style.display = 'none';
}
function calculateNitrogenFlow() {
// Inputs
var volume = parseFloat(document.getElementById('vesselVolume').value);
var time = parseFloat(document.getElementById('purgeTime').value);
var initO2 = parseFloat(document.getElementById('initialO2').value);
var targetO2 = parseFloat(document.getElementById('targetO2').value);
var safety = parseFloat(document.getElementById('safetyFactor').value);
var unitSystem = document.getElementById('unitSystem').value;
var errorDiv = document.getElementById('errorDisplay');
var resultDiv = document.getElementById('resultDisplay');
// Validation
if (isNaN(volume) || volume <= 0) {
errorDiv.innerText = "Please enter a valid vessel volume.";
errorDiv.style.display = 'block';
resultDiv.style.display = 'none';
return;
}
if (isNaN(time) || time = initO2) {
errorDiv.innerText = "Target Oxygen must be lower than Initial Oxygen.";
errorDiv.style.display = 'block';
resultDiv.style.display = 'none';
return;
}
if (targetO2 <= 0) {
errorDiv.innerText = "Target Oxygen cannot be zero or negative.";
errorDiv.style.display = 'block';
resultDiv.style.display = 'none';
return;
}
errorDiv.style.display = 'none';
// Calculation: Dilution Purging (Perfect Mixing)
// Formula: N (exchanges) = ln(Init / Target)
var exchanges = Math.log(initO2 / targetO2);
// Apply Safety Factor
var totalExchanges = exchanges * safety;
// Total Gas Required
var totalGas = volume * totalExchanges;
// Flow Rate
var flowRate = totalGas / time; // This gives volume per minute
// Unit Adjustments
var flowDisplay = 0;
var totalDisplay = 0;
if (unitSystem === 'metric') {
// Input was m3, Time is minutes
// Flow is m3/min.
// Often metric flow is Nm3/hr, so let's convert m3/min to m3/hr
flowDisplay = flowRate * 60;
totalDisplay = totalGas;
document.getElementById('unitFlow').innerText = "Nm³/hr";
document.getElementById('unitTotal').innerText = "Nm³";
} else {
// Input was ft3, Time is minutes
// Flow is ft3/min (SCFM)
flowDisplay = flowRate;
totalDisplay = totalGas;
document.getElementById('unitFlow').innerText = "SCFM";
document.getElementById('unitTotal').innerText = "SCF";
}
// Display Results
document.getElementById('resFlowRate').innerText = flowDisplay.toFixed(2);
document.getElementById('resTotalN2').innerText = totalDisplay.toFixed(1);
document.getElementById('resExchanges').innerText = totalExchanges.toFixed(2);
resultDiv.style.display = 'block';
}
Understanding Nitrogen Flow Rates for Inerting
Calculating the correct nitrogen flow rate is critical for industrial safety, particularly when purging vessels, tanks, or piping systems to prevent combustion or oxidation. This calculator utilizes the dilution purging method to estimate the flow rate required to reduce oxygen concentration from atmospheric levels (typically 21%) to a safe target level.
The Dilution Purging Formula
For applications where nitrogen is mixed with the existing atmosphere (dilution), the calculation is based on the logarithmic decay of oxygen concentration. This is the standard method for determining the number of volume exchanges required.
N = ln( C_initial / C_final )
Where:
N is the number of volume exchanges required.
C_initial is the starting oxygen concentration (usually 21%).
C_final is the desired target oxygen concentration (e.g., 1%, 2%, or 5%).
Calculating Flow Rate (Q)
Once the number of volume exchanges is determined, the flow rate depends on how quickly the operation needs to be completed. The formula is:
Q = (V × N) / t
Q = Nitrogen Flow Rate (SCFM or Nm³/hr)
V = Volume of the vessel or tank
t = Time allocated for the purge
Difference Between Displacement and Dilution
Dilution Purging: Assumes the incoming nitrogen mixes perfectly with the gas inside the vessel. This is less efficient than displacement but is the most conservative and common assumption for safety calculations. This calculator uses the dilution model.
Displacement Purging: Relies on the nitrogen "pushing" the old gas out with minimal mixing, usually requiring fewer volume exchanges. However, achieving true plug flow is difficult in practice without specific vessel geometries.
Why Oxygen Concentration Matters
Different materials and processes require different Limiting Oxygen Concentrations (LOC) to prevent combustion:
Flammable Liquids: Often require O2 levels below 10-12%.
Dusts/Powders: May require O2 levels below 8-10%.
High-Sensitivity Chemicals: May require O2 levels below 1% or even ppm levels.
Common Units
In the United States, flow is typically measured in SCFM (Standard Cubic Feet per Minute). In metric regions, it is often measured in Nm³/hr (Normal Cubic Meters per Hour). Ensure you select the correct unit system in the calculator above to match your equipment specifications.