A Practical Method for Measuring Pressure Loss Characteristics in Tube Fittings

Abstract

Pressure loss varies significantly among fittings, yet such data are rarely provided by manufacturers. This gap largely stems from the lack of a measurement method that balances accuracy and practicality. Conventional testing requires each fitting to be connected to straight tubes of compatible material and connection type, with multiple upstream and downstream pressure taps. Given the wide range of materials, geometries, and connection types used in modern fittings, fabricating dedicated instrumented test sections for each fitting is impractical. This paper presents a streamlined and widely applicable method that enables consistent and accurate testing of various fittings using the same instrumented straight tubes. The method is demonstrated using three elbow fittings (glue-joint CPVC, solder-joint copper, push-to-connect brass) and a PEX tube bend, all with ¾-inch (19 mm) nominal tube size. A comprehensive uncertainty analysis is conducted. Results show that measurement uncertainty in pressure loss is strongly influenced by the extent to which the flow is fully developed at the pressure measurement locations. Screening data based on the straight-tube friction factors effectively identifies and removes data associated with developing flow, thereby reducing the uncertainty. The measured pressure loss coefficients agree with predictions from existing methods based on bend radius. By simplifying the test setup and streamlining the data analysis, the method lowers the practical barrier for manufacturers to generate pressure-loss data, improving the accuracy of design and evaluation of fluid piping systems.