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'Absolute Pipe Roughness' (ε) is a measure of pipe wall irregularities of commercial pipes. Other than pipes, absolute roughness is also used for representing the irregularities of other equipment walls, for example, walls of heat exchanger shell. The absolute roughness has dimensions of length and is usually expressed in millimeter (mm) or feet (ft).

 

'Relative Roughness' or 'Roughness factor' of a pipe wall can be defined as the ratio of absolute roughness to the pipe nominal diameter. Relative roughness factor is often used for pressure drop calculations for pipes and other equipments. The relative roughness factor is an important parameter for determining friction factor based on Reynold's number for flow in a pipe.

 

Relative roughness = ε/D

 

Absolute Roughness is usually defined for a material and can be measured experimentally.

 

Following table gives typical roughness values in millimeters for commonly used piping materials.