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CEESI Critical Flow Venturi/Sonic Nozzle Publications

The following are a selection of papers written by CEESI engineers regarding critical flow venturis (also known as "sonic nozzles). If you would like addition information on this type of meter, or on other flow measurement topics, search the Flow Measurement Technical Library. Please read the CEESI Disclaimer before downloading any CEESI publications.


The Premature Unchoking Phenomena of Critical Flow Venturis
Richard Caron, Charles Britton, and Tom Kegel,  2000, ASME Fluids Engineering Division Summer Meeting

Abstract:
The choking pressure ratio (CPR) for a Critical Flow Venturi (CFV) is defined as the ratio of the maximum permissible exit pressure to the inlet pressure that can exist across the venturi and still maintain sonic velocity at the throat. One dimensional isentropic flow theory states that when sonic velocity exists in the throat of a CFV, the throat static pressure is at a given percentage of the inlet stagnation pressure. The exact value is dependent upon the specific heat ratio of the gas. For air, the throat static pressure should be 0.528 of the inlet stagnation pressure. A conical diffuser is attached downstream of the throat of a CFV to assist in the pressure recovery of the device. Actual test data has shown that the static pressure downstream of the CFV can be substantially higher than the 0.528 pressure ratio between the inlet and throat. The seemingly obvious conclusion that can be drawn is that the minimum CPR for any CFV will be equal to 0.528 or higher. However, experimental data shows otherwise, yielding a phenomenon that we call “Premature Unchoking”. A more thorough investigation of the critical flow venturis that exhibit this oddity has been performed.

Experimental data is presented for a series of small critical flow venturis with throat diameters from 0.79mm (0.031”) to 3.18mm (0.125”) and with various diffuser geometries. Test data was obtained over an inlet pressure range from sub- atmospheric 6.0 psia (41 kPa) to 140 psia (965 kPa). The following test gases were used; Air, Argon, Helium and CO2. American National Standard ASME/ANSI MFC-7M-1987 entitled “Measurement of Gas Flow by Means of Critical Flow Venturi Nozzles” gives the design parameters for CFV’s, and predicts the choking pressure ratio for throat Reynolds numbers exceeding 200,000. For lower Reynolds numbers, the data presented in this paper will indicate where more stringent design parameters must be followed to insure accurate measurement.
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Measurement Uncertainty Considerations When Using an Array of Critical Flow Venturies
Tom Kegel, Charles Britton, and Richard Caron,  2000, 46th International Instrumentation Symposium

Abstract:
The critical flow venturi (CFV) is an excellent device for generating a known measured flowrate. The CFV exhibits a nominally linear relationship between inlet pressure and mass flowrate. A particular flowrate can be established by applying the proper value of inlet pressure. The range of available inlet pressure values will limit the range of flowrates that can be generated. For applications where the desired flowrate rangeability is greater than the available pressure range an array of CFVs can be used. There is concern that the use of multiple CFVs will result in greater uncertainty than that associated with the use of a single CFV. This paper describes a test program that has been implemented to identify whether this additional uncertainty is present.
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A Measurement Assurance Program (MAP) Using Critical Flow Venturis
Richard Caron, Charles Britton, and Tom Kegel,  1999, 4th International Symposium on Fluid Flow Measurement, Denver, Colorado

Abstract:
Ford Motor Company is involved in accurately measuring the air mass flow rate into their internal combustion engines for improved fuel economy and for meeting the required level of pollutant output. In this endeavor, Ford has constructed many different flow stands which measure air mass flow rates during various fabrication and manufacturing processes. To be assured that each flow stand is measuring the mass flow correctly, a Measurement Assurance Program (MAP) has been implemented.
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Repeatability and Reproducibility of Low Flowrate Standards
Tom Kegel,  1999, National Conference of Standards Laboratories (NCSL)

Abstract:
This paper describes initial results from a test program designed to evaluate the repeatability and reproducibility of low flowrate standards. Three standards of different design were tested: the critical flow venturi, the laminar flow element and the piston prover. The piston prover consists of a glass cylinder within which a lightweight piston travels. The gap between piston and cylinder is sealed with mercury. The critical flow venturi and laminar flow element were conventional in design.
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Compressible Flow Effects in Subsonic Venturis
Tom Kegel,  1999, 4th International Symposium on Fluid Flow Measurement

Abstract:
When a subsonic venturi is used to measure the flow of a compressible fluid there will be a density difference between the two pressure taps. This effect needs to be accounted for by a term called the gas expansion factor. A theoretical gas expansion factor is traditionally applied to measurements made with a venturi element. The degree to which the theoretical value is applicable to a particular venturi depends on the design as well as the flow conditions. In some cases a meter specific gas expansion factor must be determined.

This paper describes the process for determining gas expansion factor based on calibration data. A test plan has been developed and applied to two different devices. The effects of Mach number, Reynolds number and gas species are described. Calibration data from both devices are presented and discussed.
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Measurement Uncertainty Considerations When Using an Array of Critical Flow Venturies
Tom Kegel,  1999, Measurement Science Conference

Abstract:
The critical flow venturi (CFV) is an excellent device for generating a known measured flowrate. The CFV exhibits a nominally linear relationship between inlet pressure and mass flowrate. A particular flowrate can be established by applying the proper value if inlet pressure. The range of available inlet pressure values will limit the range of flowrates that can be generated. For applications where the desired flowrate rangeability is greater than the available pressure range an array of CFVs can be used. There is concern that the use of multiple CFVs will result in greater uncertainty than that associated with the use of a single CFV. This paper describes a test program that has been implemented to identify whether this additional uncertainty is present.
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Further Study of the Effects That Some Thermal Phenomena Have on the Accuracy of Critical Flow Venturi Based Flowrate Measurements
Tom Kegel and Richard Caron,  1997, ASME Fluids Engineering Division Summer Meeting

Abstract:
As air flows through a critical flow venturi (CFV), the Mach number increases steadily from the inlet to the exit. The dependence of static air temperature on Mach number results in significant thermal gradients through the venturi. The difference between inlet and exit temperature can be as great as 190K when the flow is fully expanded. Temperature gradients may result in distortion of the CFV geometry due to the material coefficient of expansion. When a venturi is used to measure flowrate, errors may be present if the geometric distortion is not accounted for. This paper reports on an ongoing study of the effects that some thermal phenomena have on the accuracy of CFV based flowrate measurements. A series of experiments has been performed where a venturi was instrumented with 31 thermocouples. the data in the present paper were taken over a broader range of temperature conditions than those presented previously by Kegel and Caron (1996).
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Recent Advancements In Critical Flow Venturi Nozzle Technology
Walt Seidl and Emrys H. Jones,  1997, ASME Fluids Engineering Division Summer Meeting

Abstract:
Critical flow venturi/nozzles are precise flow rate measurement devices that are used in a number if industrial and scientific applications. This paper provides an overview of their application and recent developments in the art of operating a venturi /nozzle. Data presented in the last few years and in this forum provide an excellent base for which to begin a revision of the current ASME standard. For one skilled in the art, current critical flow nozzle standards provide ample guidance on the design and installation of the device to use them for flow measurement, flow rate control, and as very reliable transfer standards. However, improvements in technology that make using nozzles easier are being made continuously and should be included in both American National and international standards. Developments in the technology base for the areas listed below should be included in a revised standard.
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Discharge Coefficient Changes in Critical Flow Venturi Nozzles After Severe Field Service
V. C. Ting, D.G. Ferguson, E. H. Jones Jr, and Steve Caldwell,  1997, ASME Fluids Engineering Division Summer Meeting

Abstract:
This paper presents calibration accuracy results of five sonic nozzles after three hundred hours of field service. In 1995, the sonic nozzles were removed from the system for inspection. Upon removing the nozzles, a thin layer of elemental sulfur, compressor oil, and glycol was found on the surface of the nozzles. The nozzles were then re-calibrated "as found" at the Colorado Engineering Experiment Station, Inc, (CEESI) to determine any drift in the discharge coefficient due to the surface contamination. K- Lab in Norway also found sulfur deposits on their nozzle system in 1989 and later installed zinc oxide scrubber to remove low level H2 in the gas stream.

Sonic nozzles were chosen as on-site transfer provers to verify and calibrate natural gas custody transfer turbine meters in Chevrons Venice flow metering station. The facility, located in Venice, Louisiana, USA, is owned and operated by Chevron U.S.A., Inc., since 1987 to meter dry processed natural gas downstream of a gas plant. The sakes capacity is 14.16 x 106std m3 (500 MMSCFD) at a nominal operating pressure of 6.9 MPa (1,000 psig). The system includes a bank of five sonic nozzles and master turbine meter, all connected in series with three 30.48-cm (12in) sales turbine meters.

The sales meter were tested monthly against the master meter for accuracy. The accuracy of the master meter was then verified with sonic nozzles every three months. If any turbine meters did not meet the accuracy requirement, on-site calibration was allowed using sonic nozzles to develop a new k-factor for the meter. The sonic constructed, calibrated, and operated under the guidelines of ANSI/ASME MFC-7M-1987 standard.

The "as found" discharge coefficient of all five contaminated nozzles were in the range of 0.1%to 0.3% lower than the original calibration data. However, the shift was still within the measurement uncertainty of the CEESI calibration facility. The results clearly indicate that a sonic nozzles is a robost flow proving device and that measurement accuracy was affected slightly when operating in a hostile environment.
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Some Effects of Thermal Phenomena on the Accuracy of Critical Flow Venturi Based Flowrate Measurements
Tom Kegel and Rich Caron,  1996, Forum on Wet Gas Measurement, Fluids Engineering Division

Abstract:
When the flow of air through a critical flow venturi (CFV) is fully expanded, the velocity increases steadily from the inlet to the exit. The dependence of air temperature on velocity results in significant thermal gradients through the venturi, the difference between inlet and exit temperature can be as great as 190 degrees Celsius. Temperature gradients may result in distortion of the CFV geometry due to the material coefficient of expansion. When a venturi is used to measure flowrate, errors may be present if the geometry distortion is not accounted for. This paper presents some result from an ongoing study of the effects that thermal phenomena have on the accuracy of CFV based flowrate measurements. A series of experiments has been performed where a venturi was instrumented with 27 thermocouples. Mass flowrate and transient as well as steady state temperature information were obtained. The investigation included several combinations of air and CFV body temperature values.
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Uncertainty Analysis of a Multiple Critical Flow Venturi Chamber
Tom Kegel,  1996, Measurement Science Conference

Abstract:
To meet the requirements set forth by the Clean Air Act and the CAFE Standards, Ford Motor Company has chosen an air mass flow sensor based powertrain control system for every automobile it manufactures. Flow test stands are used extensively in the engineering development and manufacturing of mass air flow sensors. Currently at Ford Motors Company, all air mass flow sensor flow test stands use critical flow venturies (CFVs) for flow measurement are used. The Ventures are sized in a binary fashion and mounted within a large plenum chamber. This paper presents a preliminary uncertainty analysis oaf single CFV and extends the analysis to a multiple venturi chamber. The analysis includes theoretical uncertainty and numerical simulation.
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Correlation of Hydrogen, Nitrogren, Argon, Helium and Air Flow in Critical Flow Nozzles
Gary Corpron,  1995, 3rd International Symposium on Fluid Flow Measurement

Abstract:
A toroidal throat critical flow venturi was calibrated against primary flow standards with air and hydrogen to determine whether any performance variations occurred with fluid property changes. These results were then compared to the results of Arnberg, et al.(a) The agreement between the air and hydrogen data was excellent, and although different calibration systems were used and there may have been small dimensional variations between venturis, there was also good agreement with the Arnberg results. This paper describes the experiments and presents the calibration results.
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Correlation of Hydrogen and Air Flow in Critical Flow Nozzles, Part 1: Primary Calibration Facility
Tom Kegel,  1992, 1992 Conference on Advanced Earth-to-Orbit Propulsion Technology

Abstract:
Operations of the Space Shuttle Main Engine (SSME) Test Bed requires accurate measurement of high flowrates of gaseous hydrogen, a critical flow venturi (CFV) is proposed to provide this measurement. Calibration of the CFV in a primary facility is costly, the cost can be reduced significantly if the meter is calibrated in air rather than gaseous hydrogen. he goal of this project is to determine correlating parameter that enable the nozzle to be calibrated in air and then used to measure the flow conditions expected is the SSME Test Bed. This paper consists of a primary calibration facility is described. This facility will be used to develop the correlating parameters.
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Oil Contamination in a Herschel Venturi
Taft Snowden, Stanley Schumann, and Gary Bramos,  1990, 2nd International Symposium on Fluid Flow Measurement

Abstract:
Performance measurement in gas compressors pose unique problems. Under normal operating conditions, refrigeration compressors circulate small amounts of lubricating oil with the refrigerant gas.

We designed an experiment to attempt to quantify the impact of small amounts of oil, below 5% by weight,on a Herschel subsonic venturi. A critical flow nozzle metered air at 2.06 MPa (300 psia ) through the test venturi at a known flow rate. An upstream metering pump injected oil into the flow stream. For an experimental run we established clean flow, recorded data for clean flow and while collecting data the oil injection began and the dirty flow recorded. At the conclusion of the test we stopped oil injection and allowed clean flow to again stabilize while collecting data.

A plot of the dirty to clean flow ratio as the ordinate with mass percentage of oil as the abscissa, Appendix 1, shows the relationship we found. We tried adapting Murdocks (1) method for two-phase flow determination for orifices but found our results to be many times greater than expected. We also tried to explain the differences using an area blockage technique, but results were no better.

We attribute this to oil cling on the venturi and pipe walls since the results were temperature dependent. As the flow stream temperature increased, and consequently the oil viscosity decreased, we observed a reduction in the impact of oil on the measurement.

At the conclusion of the experiment we discovered a significant shift in the discharge coefficient between a "dirty" venturi which had oil contamination earlier and the coefficient after a rigorous cleaning. The shift appeared even though the dirty venturi ran several days with clean air. We concluded oil contamination in a venturi has a long term impact on the measurement accuracy.
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Circular-Arc Venturi Meters at Critical Flow
B. T. Arnberg,  1974, ENERGY Pipelines and Systems

Abstract:
Abstract not available.
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Discharge Coefficient Correlations for Circular-Arc Venturi Flowmeters at Critical (Sonic) Flow
B. T. Arnberg, Charles Britton, and Walt Seidl,  1971, ASME Journal of Fluids Engineering

Abstract:
Data are presented which tend to verify the theoretically predicted discharge coefficients for circular-are venture flow meters operating in the critical flow regime (sonic) at throat. Reynolds numbers above . Extensive analysis of the data is presented using methods that ate presently in process of international standardization. The data tend to verify the theoretically predicted decrease of 0.25 percent in the discharge coefficient during transition from laminar to a turbulent boundary layer. The transition occurred at a throat Reynolds number of 2.2 106, but the transition point probably changes as a function of several influences. The mean line of the measured data fell between the theoretical values for laminar and turbulent boundary layers. The scatter in 55 data points was ±0.212 percent (95 percent confidence level) from the mean line equation for Reynolds numbers from 4.1 10 4 to 3.4 10 6, which included the effects of several variables. Data were obtained from 17 venturis with throat sizes from 0.15 to 1.37 in. with Beta ratios ranging from 0.014 to 0.25. Four different test gases and three primary flow measurement facilities were used. Additional data are presented extending down to throat Reynolds numbers of 1.3 104and throat diameters of 0.05 in which indicate special problems in these regions. The state of the art for measuring venturi throat diameters presented a major limitation to the correlation effort. Calibration is necessary in many cases depending on various parameters and requirements. Additional study is needed to determine the optimum geometrical parameters at low Reynolds numbers, the optimum approach configuration for Beta ratios above 0.1, the effect of surface roughness on the discharge coefficient, and the effect of various operational variables such as flow pulsation and approach velocity profile. Also, a continuous effort should be made to improve the critical flow functions for real gases as better gas property data become available, and to extend these calculations to broader ranges of pressures and temperatures, and to other gases.
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