Note about Laminar Corrections for Fittings

There is ambiguity in the literature about how to calculate fitting pressure loss for fittings. Here is a quick summary of different views:

No Laminar Correction Required

Crane (1988)Crane Co., Flow of Fluids Through Valves, Fittings, and Pipe, Technical Paper No. 410, Crane Co., Joliet, IL, 1988. suggests no correction is required. On pages 2-8 through 2-11 Crane discusses this. Worked examples 4-7, 4-8 and 4-9 use this assumption.

This assumption is the same as method 2 in the ATKF comparison section and Figure 1 (in the aforementioned topic), and is consistent with how AFT Fathom has always performed laminar calculations. This assumption is available by merely not applying the laminar methods discussed in this chapter.

Laminar Correction is Required

Publications such as Darby (2001)Darby, R., Chemical Engineering Fluid Mechanics, 2nd edition, Marcel Dekker, New York, NY, 2001. discuss the necessity for laminar corrections for fitting K-factors. On pages 209-214 “2-K” and “3-K” methods are discussed that purport to cover the entire range of Reynolds number.

Based on Equation 6 and Crane’s (1988) nomograph of page A-30 a laminar correction based on equivalent length methods can be applied. This gives similar pressure drop results as the 3-K method and is available through the Adjusted Turbulent K Factor method.

Low Reynolds Number Flow

In addition to the discussion above, there is a separate debate over whether flow at very low Reynolds numbers (less than 100) should be treated separately when determining if corrections are needed for fittings and losses. There are conflicting CFD studies and lab tests showing that corrections are needed, or are not needed. For additional details please see the below journal articles:

No corrections needed at low Re
- Liou, R. J., et.al, “Numerical Analysis of Steady Flow in a 90-deg Circular Section Bend”, J. Eng. Mech., v. 114 (1988), p. 1871.
- Berger, S. A., et.al, “Flow in Curved Pipes”, Ann. Rev. Fluid Mech., v. 15 (1983), p. 461.
Corrections needed at low Re
- Edwards, M. F., et.al, “Head Losses in Pipe Fittings at Low Reynolds Numbers”, Chem. Eng. Res. Des., v. 63, (Jan 1985), p. 43.
- Kittredge, C. P. and Rowley, D. S., “Resistance Coefficients for Laminar and Turbulent Flow Through One-Half-Inch Valves and Fittings”, Trans. ASME, v. 79, (Nov. 1957), p. 1759.