An uncertainty analysis of mean flow velocity measurements used to quantify emissions from stationary sources

Abstract

Point velocity measurements conducted by traversing a Pitot tube across the cross section of a flow conduit continue to be the standard practice for evaluating the accuracy of continuous flow-monitoring devices. Such velocity traverses were conducted in the exhaust duct of a reduced-scale analog of a stationary source, and mean flow velocity was computed using several common integration techniques. Sources of random and systematic measurement uncertainty were identified and applied in the uncertainty analysis. When applicable, the minimum requirements of the standard test methods were used to estimate measurement uncertainty due to random sources. Estimates of the systematic measurement uncertainty due to discretized measurements of the asymmetric flow field were determined by simulating point velocity traverse measurements in a flow distribution generated using computational fluid dynamics. For the evaluated flow system, estimates of relative expanded uncertainty for the mean flow velocity ranged from ±1.4% to ±9.3% and depended on the number of measurement locations and the method of integration.

Implications:

Accurate flow measurements in smokestacks are critical for quantifying the levels of greenhouse gas emissions from fossil-fuel-burning power plants, the largest emitters of carbon dioxide. A systematic uncertainty analysis is necessary to evaluate the accuracy of these measurements. This study demonstrates such an analysis and its application to identify specific measurement components and procedures needing focused attention to improve the accuracy of mean flow velocity measurements in smokestacks.

Acknowledgment

The authors gratefully acknowledge the technical and engineering support provided by Marco Fernandez, Laurean DeLauter, Doris Rinehart, and Anthony Chakalis and data acquisition support provided by Artur Chernovsky. They are grateful for calibration services provided by Iosif Shinder and the Fluid Metrology Group, and for technical guidance provided by Anthony Hamins, Jiann Yang, and Michael Moldover. Research support by the NIST Office of Special Programs—Greenhouse Gas and Climate Science Measurements, James Whetstone Program Manager—is gratefully acknowledged.

Additional information

Notes on contributors

Rodney Bryant

Rodney Bryant, Elizabeth Moore, Matthew Bundy, and Aaron Johnson are research scientists and engineers at the National Institute of Standards and Technology, in Gaithersburg, MD.

Olatunde Sanni

Olatunde Sanni was a student engineer in training at the National Institute of Standards and Technology during the execution of this study. He is currently an applications engineer with National Instruments in Austin, TX.