How the Pressure Drop Calculator Works
This pressure drop calculator estimates pressure loss in a pipe using the Darcy-Weisbach equation. It calculates flow velocity, Reynolds number, Darcy friction factor, friction pressure loss, elevation pressure change, and optional minor losses from fittings or valves.
Darcy-Weisbach Pressure Drop Formula
The pressure drop due to pipe wall friction is:
where:
- f = Darcy friction factor
- L = pipe length (m)
- D = pipe internal diameter (m)
- ρ = fluid density (kg/m³)
- v = average flow velocity (m/s)
Velocity and Reynolds Number
Velocity is calculated from flow rate and pipe area:
Reynolds number is:
Friction Factor
For laminar flow:
For turbulent flow, this calculator uses the Colebrook-White equation:
Transitional flow is uncertain, so the calculator gives an estimate and displays a warning when Reynolds number is between approximately 2300 and 4000.
Elevation and Minor Losses
Elevation pressure change is:
Minor losses from fittings, valves, bends, entrances, and exits can be approximated as:
The total required pressure difference is:
Typical Pipe Roughness Values
| Pipe Material | Typical Roughness ε (m) |
|---|---|
| Smooth plastic / PVC | ≈ 1.5×10⁻⁶ |
| Drawn tubing | ≈ 1.5×10⁻⁶ |
| Commercial steel | ≈ 4.5×10⁻⁵ |
| Galvanized iron | ≈ 1.5×10⁻⁴ |
| Cast iron | ≈ 2.6×10⁻⁴ |
| Concrete | ≈ 3×10⁻⁴ to 3×10⁻³ |
How to Use the Calculator
- Enter the internal pipe diameter.
- Enter the volumetric flow rate.
- Enter fluid density and dynamic viscosity.
- Enter the pipe length used for friction loss.
- Enter pipe roughness.
- Enter elevation change. Use a positive value if the outlet is higher than the inlet.
- Optionally enter a minor loss coefficient K for fittings, valves, entrances, exits, and bends.
Example Calculation
Suppose water flows through a pipe with:
- D = 0.05 m
- Q = 0.002 m³/s
- ρ = 1000 kg/m³
- μ = 0.001 Pa·s
- L = 25 m
- ε = 0.00015 m
- Δz = 2 m
- K = 1.5
The calculator first finds velocity and Reynolds number, then calculates the Darcy friction factor and pressure loss terms.
Important Assumptions
- The pipe is circular and flowing full.
- The flow is steady and incompressible.
- Fluid properties are constant.
- The Darcy friction factor is used.
- Minor losses are approximated using a single total K value.
- For gases, compressibility may become important at high velocity or large pressure drops.
Frequently Asked Questions
What is pressure drop in a pipe?
Pressure drop is the reduction or required change in pressure as fluid flows through a pipe. It is caused by wall friction, fittings, valves, bends, and elevation differences.
What is the Darcy-Weisbach equation?
The Darcy-Weisbach equation relates pressure loss to friction factor, pipe length, pipe diameter, fluid density, and velocity.
What is the difference between Darcy and Fanning friction factor?
The Darcy friction factor is four times the Fanning friction factor. This calculator uses the Darcy friction factor.
What is a minor loss coefficient?
A minor loss coefficient K represents losses from fittings, valves, bends, entrances, exits, and other local disturbances.
Can this calculator be used for water?
Yes. For water at room temperature, density is often approximated as 1000 kg/m³ and dynamic viscosity as 0.001 Pa·s.