Non-nulling protocols for fast, accurate, 3-D velocity measurements in stacks

ABSTRACT

The authors present protocols for making fast, accurate, 3D velocity measurements in the stacks of coal-fired power plants. The measurements are traceable to internationally-recognized standards; therefore, they provide a rigorous basis for measuring and/or regulating the emissions from stacks. The authors used novel, five-hole, hemispherical, differential-pressure probes optimized for non-nulling (no-probe rotation) measurements. The probes resist plugging from ash and water droplets. Integrating the differential pressures for only 5 seconds determined the axial velocity V_a with an expanded relative uncertainty U_r(V_a) ≤ 2% of the axial velocity at the probe’s location, the flow’s pitch (α) and yaw (β) angles with expanded uncertainties U(α) = U(β) = 1 °, and the static pressure p_s with U_r(p_s) = 0.1% of the static pressure. This accuracy was achieved 1) by calibrating each probe in a wind tunnel at 130, strategically-chosen values of (V_a, α, β) spanning the conditions found in the majority of stacks (|α| ≤ 20 °; |β| ≤ 40 °; 4.5 m/s ≤ V_a≤27 m/s), and 2) by using a long-forgotten definition of the pseudo-dynamic pressure that scales with the dynamic pressure. The resulting calibration functions span the probe-diameter Reynolds number range from 7,600 to 45,000.

Implications: The continuous emissions monitoring systems (CEMS) that measure the flue gas flow rate in coal-fired power plant smokestacks are calibrated (at least) annually by a velocity profiling method. The stack axial velocity profile is measured by traversing S-type pitot probes (or one of the other EPA-sanctioned pitot probes) across two orthogonal, diametric chords in the stack cross-section. The average area-weighted axial velocity calculated from the pitot traverse quantifies the accuracy of the CEMS flow monitor. Therefore, the flow measurement accuracy of coal-fired power plants greenhouse gas (GHG) emissions depends on the accuracy of pitot probe velocity measurements. Coal-fired power plants overwhelmingly calibrate CEMS flow monitors using S-type pitot probes. Almost always, stack testers measure the velocity without rotating or nulling the probe (i.e., the non-nulling method). These 1D non-nulling velocity measurements take significantly less time than the corresponding 2D nulling measurements (or 3D nulling measurements for other probe types). However, the accuracy of the 1D non-nulling velocity measurements made using S-type probes depends on the pitch and yaw angles of the flow. Measured axial velocities are accurate at pitch and yaw angles near zero, but the accuracy degrades at larger pitch and yaw angles.

The authors developed a 5-hole hemispherical pitot probe that accurately measures the velocity vector in coal-fired smokestacks without needing to rotate or null the probe. This non-nulling, 3D probe is designed with large diameter pressure ports to prevent water droplets (or particulates) from obstructing its pressure ports when applied in stack flow measurement applications. This manuscript presents a wind tunnel calibration procedure to determine the non-nulling calibration curves for 1) dynamic pressure; 2) pitch angle; 3) yaw angle; and 4) static pressure. These calibration curves are used to determine axial velocities from 6 m/s to 27 m/s, yaw angles between ±40°, and pitch angles between ±20°. The uncertainties at the 95% confidence limit for axial velocity, yaw angle, and pitch angle are 2% (or less), 1°, and 1°, respectively. Therefore, in contrast to existing EPA-sanctioned probes, the non-nulling hemispherical probe provides fast, low uncertainty velocity measurements independent of the pitch and yaw angles of the stack flow.

Acknowledgment

The authors thank Matthew Gentry of Airflow Science Corporation for many fruitful discussions. This research was partially funded by NIST’s Greenhouse Gas Measurements Program administered by Dr. James Whetstone. A Cooperative Research and Development Agreement (CRADA) between NIST and EPRI facilitated our access to a RATA at a coal-fired power plant.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The data that support the findings of this study are available from the corresponding author, ANJ, upon reasonable request email: [email protected] for data.

Supplementary material

Supplemental data for this paper can be accessed online at https://doi.org/10.1080/10962247.2023.2218827.

Notes

¹ In this manuscript the authors denote the standard uncertainty of a measurand x by u(x) and its relative standard uncertainty expressed as a percent of x by u_r(x) = 100 u(x)/x. Unless otherwise stated, all uncertainties are standard uncertainties with a unity coverage factor (k = 1) corresponding to a 68% confidence interval. Expanded uncertainties, which have a coverage factor of two (k = 2) and correspond to a 95% confidence interval, are denoted by U(x) = 2u(x) or U_r(x) = 2u_r(x), respectively.

² Certain commercial equipment, instruments, or materials are identified in this report to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.

Additional information

Notes on contributors

Iosif I. Shinder

losif I. Shinder is a physicist at the National Institute of Standards and Technology (NIST). His interest include thermodynamics, acoustics, fluid dynamics, fluid flow metrology, airspeed and standard development.

Aaron N. Johnson

Aaron N. Johnson is a mechanical engineer with expertise in fluid dynamics and flow measurement. His interests include measuring flue gas flows, measuring pipeline-scale natural gas flows, designing primary flow standards, analyzing the uncertainty of flow measurements, computational fluid dynamics, and modeling flow meters (e.g., critical flow venturi meters, turbine meters, ultrasonic meters).

B. James Filla

B. James Filla is a retired chemical engineer from the National Institute of Standards and Technology (NIST). During his tenure at NIST, Mr. Filla made significant contributions to areas such as wind speed, liquid flow, temperature, and thermal conductivity measurements. His work has had a major impact on the advancement of metrology, and he is being inducted into NIST Gallery of Distinguished Alumni in October 2023.

Vladimir B. Khromchenko

Vladimir B. Khromchenko is a physicist at National Institute of Standards and Technology working in air speed and radiometry (NIST).

Michael R. Moldover

Michael R. Moldover is a NIST Fellow and a Fellow of both the American Physical Society and the Acoustical Society of America. He received the Touloukian Award from the ASME, the Helmholtz-Rayleigh Interdisciplinary Silver Medal from the Acoustical Society of America and numerous awards from NIST and the US Department of Commerce.

Joey Boyd

Joey Boyd is a senior technician employed at the National Institute of Standards and Technology (NIST).

John D. Wright

John D. Wright is a Fellow of the American Society of Mechanical Engineers and has received numerous awards from the Department of Commerce for his contributions to flow metrology and leadership in the international flow community. Dr. Wright served as the Chairman of the Working Group for Fluid Flow (WGFF) from 2009 to 2018. The WGFF is a committee organized by the Bureau International des Poids et Mesures (BIPM) to coordinate calibration measurement capabilities and comparisons for national metrology institutes.

John Stoup

John Stoup is a mechanical engineer in the Dimensional Metrology Group in the Sensor Science Division of the Physical Measurement Laboratory (PML) at the National Institute of Standards and Technology (NIST).