Example 3:
Laminar Flow resistance coefficients (K values) for pipe fittings


Background

The examples provide a comparison of AioFlo results with published data from well known and respected references that are generally accessible to engineers. This will allow prospective AioFlo users to validate its accuracy against a range of typical calculations. The worked examples can also be run by new users as part of their learning process. To learn more about AioFlo click on "Home" in the menu above.

Description

In Example 2 it was shown that resistance coefficients (K values) for pipe fittings are significantly higher in Laminar Flow than in Turbulent Flow. This example looks at resistance coefficients for elbows and globe valves tabulated in the well-known Perry's Chemical Engineers' Handbook.


Problem Reference

Kittredge and Rowley, Trans. Am. Soc. Mech. Eng., 79, 1759-1766 (1957). Quoted in Perry, RH and Green, DW. "Chemical Engineers' Handbook", 7th ed, McGraw-Hill, 1997, Table 6-5, Page 6-18.

Pipe Details

Pipe size : 0.5" NB
Roughness : Not applicable
Fittings : 90 ell, short radius
globe valve

To be Calculated

Compare the experimentally determined resistance coefficients for these fittings over a range of Reynolds Numbers in the Laminar Flow regime with the coefficients predicted by the Crane TP410M (1999) method and by AioFlo's implemetation of the Darby 3-K method.

Comparison of Results

Fitting Reynolds No Perry Crane AioFlo
90 ell, short radius
1000 0.9 0.54 1.31
500 1.0 0.54 2.11
100 7.5 0.54 8.51
50 16 0.54 16.5
globe valve
1000 11 9.18 10.26
500 12 9.18 11.76
100 20 9.18 23.75
50 30 9.18 38.74

Discussion

It is clear from the experimental results that the resistance coefficients (K values) increase as the Reynolds Number decreases in the Laminar Flow regime. Using fixed resistance coefficients determined for Turbulent Flow (as in the Crane TP410M procedure) introduces increasing errors as the Reynolds Number decreases. It must also be noted that there are differences between the experimental K values and those predicted by AioFlo's implementation of the 3-K method, but the agreement is markedly better for the fixed K values.

These differences between the experimental and AioFlo values highlight the fact that there will always be some discrepancies between design predictions using generic data and actual field results. The true K values for fittings and valves will vary from manufacturer to manufacturer, and in critical instances the manufacturer's data should be used in preference to generic data published in handbooks or used in software like AioFlo. It is for this reason that AioFlo has the provision to add in known K values. The K values vary from manufacturer to manufacturer even for seemingly "standard" items like bends and tees.