L52299 Revised Analysis of Orifice Meter Expansion Factor Data

Abstract

Orifice meter expansion factor data collected at Southwest Research Institute (SwRI) between 2003 and 2005 have been reviewed to assess the effect of an assumption made during data reduction. In accordance with the North American orifice meter standard, AGA Report No. 3, Part 1, the data were originally analyzed using a constant value of the isentropic exponent, k = 1.3. By comparison, the expansion factor equation adopted by ISO employs the real isentropic exponent, K, which is a function of pressure, temperature, and gas composition. The SwRI orifice meter expansion factor data have been re-reduced, using values of the real isentropic exponent from archived test data in place of the ideal gas value of k = 1.3. The original expansion factor data were obtained in such a way that the expansion factor values themselves were unaffected by the value of the isentropic exponent. However, the use of k = 1.3 influenced the values of the acoustic ratio, the independent variable used with the AGA and ISO equations to compute values of the expansion factor in measurement applications. The use of the ideal isentropic exponent in the original SwRI data, instead of real values of the isentropic exponent, was found to have caused an average shift in the acoustic ratio.