TP24-09 2023 CTI Pitot Tube Blockage Study Report

The Cooling Technology Institute (CTI) test codes have specified waterflow rate measurements by pitot tube traverse since the founding of the institute in the late 1950’s. In this process, a calibrated pitot tube is used to traverse a pipe to collect velocity pressure data at multiple points. The average pipe velocity is then used to calculate the water flow rate. For decades, anecdotal evidence indicated that the Simplex pitot tube traditionally used for water flow measurements had a high bias when used in locations with significantly disturbed velocity profiles. Since water pitots are frequently used in locations with disturbed flow profiles, CTI funded a study to assess the impact of disturbed flow profiles on flow measurement accuracy associated with both the Simplex and elliptical pitot tube designs.  The elliptical pitot tube was found to be much more accurate in disturbed flow than the Simplex design. Since the conclusion of the study in 2018, CTI has adopted the elliptical pitot for all pitot-based water flow measurements. In the process of measuring water velocity, the insertion of a pitot tube into a pipe changes the cross-sectional area in the traverse plane since the shaft of the pitot blocks a portion of the flow path. Whether and how to account for the pitot tube blockage introduced by the pitot tube in both pitot calibrations and water flow measurements have historically been open questions. The potential significance of pitot blockage has increased with the adoption of the elliptical pitot tube due to its larger cross-sectional area. The ellipsoidal pitot tube has now been fully adopted by CTI and is the default instrument to be used in the CTI STD 201 program and for all CTI field tests. In 2022, the Cooling Technology Institute funded a supplemental research project to investigate the effect of pitot blockage on flow measurements conducted with the elliptical pitot tube. The goal of this study was to inform the decision about the use of pitot blockage in pitot calibrations and water flow measurements.