K-factor Method

The excess loss in a fitting is normally expressed in a dimensionless “K” factor. The excess head loss (dH) is less than the total by the amount of frictional loss that would be experienced by straight pipe of the same physical length. The loss coefficient Kf , depends on the Reynolds number of the flow. The values of Kf at low Re can be significantly greater than those at high Re. Additionally, valves and fittings do not scale exactly. e.g., the loss coefficient for a 1/4 in. valve is not the same as that for a 4 in. valve.

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.