Water Conveyance Loss and Designing Conveyance System - Practices of Irrigation and On-farm Water Management - page 27

Determination of head loss (meaning loss of energy) in the pipe is necessary
because pump and motor (power) combination should be matched to flow and pres-
sure requirement. Oversizing makes for inefficiencies that waste energy and cost
money.

Components of Head Loss

Head loss in pipe may be divided into the following:

• Major head loss

• Minor head loss

Major head loss consists of loss due to friction in the pipe. Minor loss consists
of loss due to change in diameter, change of velocity in bends, joints, valves, and
similar items.

1.3.6.2 Factors Affecting Head Loss

Frictional head loss ( h L ) in the pipe can be functionally expressed as follows:

h L = f ( L , V , D , n , ρ , ν ) (1.19)

where

L = length of pipe

V = velocity of flow

D = diameter of pipe

n = roughness of the pipe surface (internal surface, over which flow occurs)

ρ = density of flowing fluid

ν = viscosity of the flowing fluid

The mode of action of the factors affecting head loss is as follows:

• head loss varies directly as the length of the pipe

• it varies almost as the square of the velocity

• it varies almost inversely as the diameter

• it depends on the surface roughness of the pipe wall

• it is independent of pressure

1.3.6.3 Different Head Loss Equations

Darcy-Weisbach Formula for Head Loss

Darcy-Weisbach formula for head loss in a pipe due to friction in turbulent flow can
be expressed as