function calculatePressureDrop() {
var Q_m3h = parseFloat(document.getElementById("flowRate").value);
var D_mm = parseFloat(document.getElementById("pipeDiameter").value);
var L_m = parseFloat(document.getElementById("pipeLength").value);
var rho = parseFloat(document.getElementById("fluidDensity").value);
var mu_cp = parseFloat(document.getElementById("dynamicViscosity").value);
var epsilon_mm = parseFloat(document.getElementById("pipeRoughness").value);
if (isNaN(Q_m3h) || isNaN(D_mm) || isNaN(L_m) || isNaN(rho) || isNaN(mu_cp)) {
alert("Please enter valid numeric values for all fields.");
return;
}
// Convert to SI units
var Q = Q_m3h / 3600; // m3/s
var D = D_mm / 1000; // m
var mu = mu_cp / 1000; // Pa.s
var epsilon = epsilon_mm / 1000; // m
// 1. Calculate Velocity
var area = Math.PI * Math.pow(D, 2) / 4;
var v = Q / area;
// 2. Reynolds Number
var Re = (rho * v * D) / mu;
// 3. Friction Factor (f)
var f = 0;
var flowType = "";
if (Re = 2300 && Re <= 4000) {
// Transition zone: simple linear interpolation or use turbulent approx
var f_lam = 64 / 2300;
var re_turb = 4000;
var f_turb = 0.25 / Math.pow(Math.log10((epsilon / (3.7 * D)) + (5.74 / Math.pow(re_turb, 0.9))), 2);
f = f_lam + (f_turb – f_lam) * (Re – 2300) / (4000 – 2300);
flowType = "Transitional";
} else {
// Turbulent: Swamee-Jain equation
f = 0.25 / Math.pow(Math.log10((epsilon / (3.7 * D)) + (5.74 / Math.pow(Re, 0.9))), 2);
flowType = "Turbulent";
}
// 4. Pressure Drop (Darcy-Weisbach)
// deltaP = f * (L/D) * (rho * v^2 / 2)
var deltaP_Pa = f * (L_m / D) * (rho * Math.pow(v, 2) / 2);
var deltaP_bar = deltaP_Pa / 100000;
// Display Results
document.getElementById("resultsArea").style.display = "block";
document.getElementById("resPdBar").innerText = deltaP_bar.toFixed(4);
document.getElementById("resPdPa").innerText = Math.round(deltaP_Pa).toLocaleString();
document.getElementById("resVelocity").innerText = v.toFixed(2);
document.getElementById("resReynolds").innerText = Math.round(Re).toLocaleString();
document.getElementById("resFlowType").innerText = flowType;
document.getElementById("resFriction").innerText = f.toFixed(5);
}
Understanding Pressure Drop and Flow Rate
In fluid dynamics, calculating the pressure drop in a piping system is critical for selecting the correct pump size, ensuring system efficiency, and maintaining safety. The Pressure Drop Flow Rate Calculator utilizes the Darcy-Weisbach equation, the gold standard for calculating friction losses in pipes.
Key Parameters Explained
Flow Rate: The volume of fluid passing through the pipe per unit of time (m³/h).
Internal Pipe Diameter: The actual inside diameter of the pipe. Note that nominal pipe sizes (NPS) differ from actual internal diameters.
Fluid Density: The mass per unit volume. For water, this is typically 1,000 kg/m³.
Dynamic Viscosity: A measure of a fluid's resistance to flow. Water at 20°C has a viscosity of approximately 1.0 cP (mPa·s).
Pipe Roughness: The average height of surface irregularities inside the pipe. For example, stainless steel is typically 0.015mm, while rusted iron can exceed 1.0mm.
The Science: Darcy-Weisbach Equation
The total pressure loss due to friction is calculated as:
ΔP = f · (L / D) · (ρ · v² / 2)
Where:
ΔP: Pressure Drop (Pa)
f: Darcy Friction Factor
L: Pipe Length (m)
D: Diameter (m)
ρ: Density (kg/m³)
v: Velocity (m/s)
Laminar vs. Turbulent Flow
The calculator automatically determines the flow regime using the Reynolds Number (Re):
Laminar (Re < 2300): Fluid moves in smooth layers. Friction is purely a function of Re.
Transitional (2300 – 4000): A mix of both regimes; flow is unstable.
Turbulent (Re > 4000): Fluid moves chaotically. Friction is determined by both Re and the relative roughness of the pipe.
Practical Example
Imagine you are pumping water through a 50mm diameter steel pipe (roughness 0.045mm) over a distance of 100 meters at a rate of 10 m³/h.
1. The velocity will be roughly 1.41 m/s.
2. The Reynolds number will be high enough to indicate Turbulent flow.
3. The resulting pressure drop will be approximately 0.19 bar.