* STP defined as 0°C (273.15K) and 1.01325 Bar Absolute.
* "Actual" refers to flow at operating pressure/temperature.
* "Normal" (Nm³) refers to flow corrected to standard conditions.
function calculateGasFlow() {
// 1. Get input values
var diameterMM = parseFloat(document.getElementById("pipeDiameter").value);
var velocity = parseFloat(document.getElementById("gasVelocity").value);
var pressureGauge = parseFloat(document.getElementById("gaugePressure").value);
var tempC = parseFloat(document.getElementById("gasTemp").value);
// 2. Validate inputs
if (isNaN(diameterMM) || isNaN(velocity) || isNaN(pressureGauge) || isNaN(tempC)) {
alert("Please enter valid numbers for all fields.");
return;
}
if (diameterMM <= 0 || velocity < 0) {
alert("Diameter and Velocity must be positive numbers.");
return;
}
// 3. Constants
var standardPressure = 1.01325; // Bar absolute (1 atm)
var standardTempK = 273.15; // 0 degrees Celsius in Kelvin
// 4. Calculate Area
// Convert mm to meters: diameter / 1000
// Radius = diameter / 2
var diameterM = diameterMM / 1000;
var radiusM = diameterM / 2;
var areaM2 = Math.PI * Math.pow(radiusM, 2);
// 5. Calculate Actual Flow Rate (Q = v * A)
// Result in cubic meters per second (m^3/s)
var flowM3s = velocity * areaM2;
// Convert to cubic meters per hour (m^3/h)
var flowActualM3h = flowM3s * 3600;
// 6. Calculate Absolute Conditions
var pressureAbs = pressureGauge + 1.01325; // Bar absolute
var tempK = tempC + 273.15; // Kelvin
// 7. Calculate Standard Flow Rate (Ideal Gas Law Correction)
// Q_std = Q_act * (P_act / P_std) * (T_std / T_act)
var flowStandardNm3h = flowActualM3h * (pressureAbs / standardPressure) * (standardTempK / tempK);
// 8. Display Results
document.getElementById("resArea").innerHTML = areaM2.toFixed(6) + " m²";
document.getElementById("resActualFlow").innerHTML = flowActualM3h.toFixed(2) + " m³/hr";
document.getElementById("resStandardFlow").innerHTML = flowStandardNm3h.toFixed(2) + " Nm³/hr";
// Show result div
document.getElementById("resultsArea").style.display = "block";
}
How to Calculate Gas Flow Rate Formula
Calculating the flow rate of gas through a pipeline is a critical task in engineering, HVAC, and industrial process control. Unlike liquids, gases are compressible, which means their volume changes significantly with pressure and temperature. Therefore, understanding the difference between "Actual" flow and "Standard" flow is essential for accurate measurements.
The Basic Continuity Equation
The most fundamental formula for calculating the volumetric flow rate of any fluid (gas or liquid) moving through a pipe is derived from the continuity equation:
Q = v × A
Where:
Q = Volumetric Flow Rate (e.g., m³/s or ft³/min)
v = Average Flow Velocity (e.g., m/s or ft/s)
A = Cross-Sectional Area of the pipe (e.g., m² or ft²)
Step-by-Step Calculation Guide
1. Calculate Cross-Sectional Area
First, you must determine the internal area of the pipe. If you have the internal diameter (d):
A = π × (d / 2)²
Note: Ensure your units are consistent. If diameter is in millimeters, convert it to meters before calculating area in square meters.
2. Calculate Actual Flow Rate
Multiply the velocity of the gas by the area. This gives you the Actual Flow Rate at the current operating conditions (pressure and temperature).
Correcting for Pressure and Temperature (Ideal Gas Law)
Because gas compresses under pressure and expands with heat, the "Actual" volume (m³/hr) does not tell you the mass of gas moving through the pipe. To compare flow rates accurately, engineers convert the actual flow to Standard Flow (Standard Cubic Feet per Minute – SCFM, or Normal Cubic Meters per Hour – Nm³/hr).
The formula to convert Actual Flow ($Q_{act}$) to Standard Flow ($Q_{std}$) is:
Pstd = Standard Pressure (typically 1 atm or 1.01325 bar)
Tact = Absolute Operating Temperature (Kelvin = °C + 273.15)
Tstd = Standard Temperature (typically 273.15 K or 0°C)
Example Calculation
Let's say you have a pipe with an internal diameter of 50mm carrying gas at a velocity of 10 m/s. The gauge pressure is 5 bar and the temperature is 20°C.
Area: 50mm = 0.05m. Radius = 0.025m. Area = π × 0.025² ≈ 0.00196 m².
Actual Flow: 10 m/s × 0.00196 m² = 0.0196 m³/s. Multiply by 3600 to get 70.56 m³/hr.
Correction Factor: Pressure Ratio = (5 + 1.013) / 1.013 ≈ 5.93
Temperature Ratio = 273.15 / (20 + 273.15) ≈ 0.93
Standard Flow: 70.56 × 5.93 × 0.93 ≈ 389 Nm³/hr.
This result shows that while the physical volume moving through the pipe is only ~70 m³/hr, the amount of gas (mass equivalent at standard conditions) is nearly 390 Nm³/hr due to the compression at 5 bar.