What Is Friction Loss?
Friction loss (or head loss) is the energy lost by fluid flowing through a pipe due to viscous effects and wall friction. This energy is converted to heat and represents a permanent pressure reduction in the system. It is the primary consideration in pipe sizing and pump selection for fluid distribution systems.
The Darcy-Weisbach equation provides a universal method for calculating friction loss that works for all fluids and flow regimes. The equation shows that friction loss is proportional to pipe length, the square of velocity, and inversely proportional to pipe diameter. Doubling pipe diameter reduces friction loss by a factor of 32 for the same flow rate.
Darcy-Weisbach Equation
Where h_L is head loss in meters, f is the Darcy friction factor, L is pipe length, D is diameter, v is flow velocity, and g is gravitational acceleration.
Typical Friction Losses
| Pipe Size | Flow (L/s) | Velocity (m/s) | Loss per 100m |
|---|---|---|---|
| 25mm | 0.5 | 1.02 | 3.2 m |
| 50mm | 2.0 | 1.02 | 0.8 m |
| 100mm | 8.0 | 1.02 | 0.2 m |
| 200mm | 32.0 | 1.02 | 0.05 m |
Frequently Asked Questions
How does pipe diameter affect friction loss?
Friction loss is extremely sensitive to pipe diameter. For a given flow rate, head loss is proportional to D^-5 (fifth power of diameter). This means a pipe twice the diameter has 32 times less friction loss, which is why upsizing pipes often pays for itself in reduced pumping energy costs.
What is a reasonable flow velocity in pipes?
For water supply, 1-2 m/s is typical. Higher velocities increase friction loss quadratically and can cause erosion and water hammer. Lower velocities may allow sediment to settle. Industrial systems may use higher velocities if pumping costs are acceptable.