How to Calculate Gas Flow Rate from Pressure

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Calculate the flow rate of natural gas through a pipeline based on pressure differential, pipe diameter, and length.

function calculateGasFlow() { var p1 = parseFloat(document.getElementById('inletPressure').value); var p2 = parseFloat(document.getElementById('outletPressure').value); var d = parseFloat(document.getElementById('pipeDiameter').value); var lFeet = parseFloat(document.getElementById('pipeLength').value); var g = parseFloat(document.getElementById('gasGravity').value); var tempF = parseFloat(document.getElementById('gasTemp').value); if (isNaN(p1) || isNaN(p2) || isNaN(d) || isNaN(lFeet) || isNaN(g) || isNaN(tempF)) { alert("Please enter valid numbers in all fields."); return; } if (p1 <= p2) { alert("Inlet pressure must be higher than outlet pressure for flow to occur."); return; } // Convert to Absolute Units var p1a = p1 + 14.7; // psia var p2a = p2 + 14.7; // psia var tempR = tempF + 459.67; // Rankine var lMiles = lFeet / 5280; // Miles for Weymouth Equation // Weymouth Equation Simplified for SCFH // Q = 433.5 * (T_std / P_std) * sqrt( (P1^2 – P2^2) * d^5.333 / (G * T * L) ) // T_std = 520R, P_std = 14.7 psia var standardFactor = 433.5 * (520 / 14.7); var pressureDiff = Math.pow(p1a, 2) – Math.pow(p2a, 2); var diameterFactor = Math.pow(d, 5.3333); var denominator = g * tempR * lMiles; var result = standardFactor * Math.sqrt((pressureDiff * diameterFactor) / denominator); // Convert result from Daily (standard Weymouth is often per day) to Hourly if needed // The 433.5 coefficient is typically for SCFD, we adjust for SCFH (divide by 24) var resultHourly = result / 24; document.getElementById('flowValue').innerHTML = resultHourly.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}) + " SCFH"; document.getElementById('gasFlowResult').style.display = 'block'; }

How to Calculate Gas Flow Rate from Pressure

Understanding the relationship between pressure and gas flow is essential for mechanical engineers, HVAC technicians, and pipeline operators. In fluid dynamics, gas flow occurs when there is a pressure differential (a drop in pressure) between two points in a system. To calculate this flow rate, we typically use specialized formulas like the Weymouth Equation or the Panhandle Equation, which account for the compressible nature of gas.

The Science Behind Gas Flow Calculations

Unlike liquids, gases are highly compressible. This means that as pressure changes, the density and volume of the gas also change. When calculating gas flow from pressure, the following factors are critical:

• Inlet Pressure (P1): The absolute pressure at the start of the pipe.
• Outlet Pressure (P2): The absolute pressure at the end of the pipe.
• Internal Diameter (d): The size of the pipe significantly impacts friction and velocity.
• Pipe Length (L): Longer pipes result in more friction loss, reducing the flow rate.
• Specific Gravity (G): The density of the gas relative to air (Air = 1.0, Natural Gas is approx 0.6).
• Temperature (T): Higher temperatures increase gas volume and affect flow characteristics.

Common Formula: The Weymouth Equation

The Weymouth equation is one of the oldest and most widely used formulas for calculating the flow of natural gas through high-pressure pipelines. The simplified version used in our calculator accounts for the pressure drop across a specific length of pipe while considering the internal diameter to the power of 5.333.

Q = C × (T_std / P_std) × √[(P1² – P2²) × d^5.333 / (G × T × L)]

Practical Example

Imagine you have a natural gas pipeline with the following specifications:

• Inlet Pressure: 100 PSI
• Outlet Pressure: 90 PSI
• Pipe Diameter: 2 Inches
• Pipe Length: 500 Feet
• Gas Type: Natural Gas (Specific Gravity 0.6)

By entering these values into the calculator, you can determine the Standard Cubic Feet per Hour (SCFH). This helps in sizing regulators, selecting pipe diameters for new installations, or troubleshooting pressure drops in existing systems.

Why Pressure Differential Matters

If the pressure at both ends of the pipe is equal, the flow rate will be zero. The "driving force" of the gas is the difference between P1 and P2. However, if the pressure drop is too high, it may indicate an undersized pipe or a blockage, which could lead to inefficient appliance operation or safety hazards in industrial settings.

Factors Affecting Accuracy

While this calculator provides a highly accurate estimate based on standard equations, real-world variables such as pipe roughness (the material of the pipe), bends, valves, and fittings will create additional turbulence and pressure loss. For high-precision engineering projects, these "minor losses" must be calculated separately and added to the equivalent length of the pipe.