## LESSON 26. EMPRICAL EQUATIONS FOR FRICTION HEAD LOSS

### Hazen-Williams equation:

It was developed for water flow in larger pipes (D≥5 cm, approximately 2 in.) within a moderate range of water velocity (V≤3 m/s, approximately 10 ft/s). Hazen-Williams equation, originally developed for the British measurement system, has been written in the form

S= slope of the energy grade line, or the head loss per unit length of the pipe (S=hf/L)

Rh = the hydraulic radius, defined as the water cross sectional area (A) divided by wetted perimeter (P). For a circular pipe, with A=πD2/4 and P=πD, the hydraulic radius is

CHW= Hazen-Williams coefficient. The values of CHW for commonly used water-carrying conduits are given in Table 1.2.

The Hazen-Williams equation in SI units is written in the form of

Velocity in m/s and Rh is in meters

### Manning’s Equation

Manning equation has been used extensively open channel designs. It is also quite commonly used for pipe flows. The Manning equation may be expressed in the following form:

n= Manning’s coefficient of roughness. Typical values of n for water flow in common pipe materials is given in Table 1.3

In British units, the Manning equation is written as

Where V is units of ft/s.