Discuss the effect of roughness parameter on friction factor. [5 Marks]

• In turbulent flow rough pipe leads to a larger friction factor than the smooth pipes for given Reynold’s number.

• If a rough pipe is smooth, the friction factor is reduced and ultimately a stage will come when further smoothing of the pipe does not reduce the friction factor for given Reynold’s number, the pipe is said to be hydraulically smooth.

• The height of a single unit of roughness is denoted by K and is called roughness parameter.

• The relative roughness is the ratio of the roughness parameter to the diameter of pipe, i.e. K/D the dependency of friction factor on the surface roughness is given through relative roughness.

• For larger values of Reynold’s number (NRe < 2100) the friction factor is independent of the surface roughness.<div>
  

• For low values of Reynold’s number (NRe > 2500) the friction factor is also function of surface roughness.

• At very large values of Reynold’s number, the friction factor is independent of NRe but is a function of surface roughness.