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margin: 0 auto;
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.calculator-container {
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padding: 30px;
margin-bottom: 40px;
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.calculator-title {
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How to Calculate Actual Flow Rate
Calculating the actual flow rate of a liquid through a pipe is a fundamental task in fluid dynamics, engineering, and plumbing. The "actual" flow rate refers to the volume of fluid passing through a specific cross-sectional area per unit of time.
While theoretical flow rates can be estimated using pump curves, the actual flow rate is determined by the physical geometry of the piping system and the velocity at which the fluid is moving.
The Flow Rate Formula
The primary formula used to calculate the volumetric flow rate ($Q$) is based on the relationship between the pipe's internal Area ($A$) and the Fluid Velocity ($v$):
Q = A × v
Where:
- Q = Volumetric Flow Rate (e.g., m³/s, GPM)
- A = Cross-sectional Area of the pipe (e.g., m², ft²)
- v = Average velocity of the fluid (e.g., m/s, ft/s)
Step-by-Step Calculation Logic
To perform this calculation manually, follow these steps:
1. Determine the Internal Diameter
Measure the internal diameter (ID) of the pipe. Do not use the outer diameter (OD), as wall thickness varies by pipe schedule. For example, a 2-inch Schedule 40 pipe has a different internal diameter than a Schedule 80 pipe.
2. Calculate the Cross-Sectional Area
Since pipes are circular, use the circle area formula: A = π × r² or A = π × (d/2)².
Note: Ensure your units are consistent. If measuring velocity in meters per second, convert your diameter to meters first.
3. Multiply by Velocity
Multiply the calculated area by the fluid velocity. This gives you the flow rate in cubic units per second (e.g., cubic meters per second).
4. Convert Units
The raw result is often in cubic meters per second ($m^3/s$), which is a very large unit. You will typically convert this to:
- Liters per Minute (L/min): Multiply $m^3/s$ by 60,000.
- Cubic Meters per Hour ($m^3/h$): Multiply $m^3/s$ by 3,600.
- US Gallons per Minute (GPM): Multiply $m^3/s$ by approximately 15,850.
Factors Affecting Actual Flow
While the geometric formula above is precise, real-world conditions ("actual" flow) can be influenced by:
- Friction Loss: Roughness inside the pipe reduces velocity near the walls.
- Viscosity: Thicker fluids (like oil) flow differently than water.
- Turbulence: High velocity or obstructions (elbows, valves) create turbulence, affecting the average velocity profile.
For critical engineering applications, a flow meter is the only way to measure 100% actual flow, but the calculation above provides the standard operational flow rate based on known system parameters.
function calculateFlowRate() {
// 1. Get Input Values
var diameterInput = document.getElementById('pipeDiameter').value;
var velocityInput = document.getElementById('flowVelocity').value;
var dUnit = document.getElementById('diameterUnit').value;
var vUnit = document.getElementById('velocityUnit').value;
// 2. Validation
if (diameterInput === "" || velocityInput === "") {
alert("Please enter both Pipe Diameter and Fluid Velocity.");
return;
}
var diameter = parseFloat(diameterInput);
var velocity = parseFloat(velocityInput);
if (isNaN(diameter) || isNaN(velocity) || diameter <= 0 || velocity Result in Cubic Meters per Second
var flowM3PerSec = area * velocityInMPS;
// 6. Convert Results for Display
var resM3H = flowM3PerSec * 3600; // Cubic Meters per Hour
var resLPM = flowM3PerSec * 60000; // Liters per Minute
var resGPM = flowM3PerSec * 15850.32314; // US Gallons per Minute
var resCFM = flowM3PerSec * 2118.880003; // Cubic Feet per Minute
// 7. Update UI
document.getElementById('res-m3h').innerText = resM3H.toFixed(2) + " m³/h";
document.getElementById('res-lpm').innerText = resLPM.toFixed(2) + " L/min";
document.getElementById('res-gpm').innerText = resGPM.toFixed(2) + " GPM";
document.getElementById('res-cfm').innerText = resCFM.toFixed(2) + " ft³/min";
document.getElementById('results-area').style.display = 'block';
}