Equivalent sand roughness

The equivalent sand roughness or equivalent sand roughness is a term used in fluid mechanics . It is used to compare the quality of different surfaces with regard to flow processes. ${\displaystyle k_{s}}$

Origin of the term

Since it is problematic to use different surfaces such as B. in wooden pipes, clean metal pipes and dirty drainpipes to compare with each other, an attempt was made around 1900 to determine a possibility for comparison.

For this purpose, pipes had to be manufactured that have exactly the same and precisely definable surface quality over a certain length . Since the inside of a pipe cannot easily be machined, it had to be coated in order to obtain the defined surface. The coating consisted of sifted sand , to which a certain roughness value can be assigned due to its grain size .

The flow properties of various coated pipes were determined through extensive tests.

Significance in technology

The equivalent sand roughness together with the pipe diameter D and the Reynolds number  Re determine the pipe friction coefficient : ${\displaystyle \lambda }$

1. by solving the universal flow formula according to Colebrook and White
2. using Miller's formula for practical use cases

or graphically

1. with the mood diagram
2. more convenient and more accurate with the two nomograms from the mock

wherein a laminar flow is ${\displaystyle \lambda =64/Re}$

In this way, the empirically determined coefficient of friction can be used to compare one pipe with another in terms of its flow quality. ${\displaystyle \lambda }$

The equivalent sand roughness of various pipe types is listed in catalogs. However, these values ​​are sometimes subject to considerable fluctuations, which is why a generous safety factor is necessary when designing a pipe system .