The Equivalent length (L/D) Method

The equivalent length adds some hypothetical length of pipe to the actual length of the fitting.
However, the drawback is that the equivalent length for a given fitting is not constant, but depends on Reynolds number, pipe roughness, pipe size, and geometry.
 

Kf = f(Leq/D)

Leq = Kf .(D/f)

Every equivalent length has a specific friction factor. The method assumes that

• Sizes of fittings of a given type can be scaled by the corresponding pipe diameter.
• The influence of Reynolds number on the friction loss on the fitting is the same as the pipe loss.

However, neither of the above assumption is accurate.

Furthermore, the nature of the laminar or turbulent flow field within a valve or a fitting is generally quite different from that in a straight pipe. Therefore, there is an uncertainty when determining the effect of
Reynolds number on the loss coefficients. This method does not properly account for the lack of exact scaling for valves and fittings.