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Title: Comparison Of Five Natural Gas Equations Of State Used For Flow And Energy Measurment
Author: Aaron Johnson, Bill Johansen
Source: 2009 International Symposium on Fluid Flow Measurement
Year Published: 2009
Abstract: Measurements of the flow and the energy content of natural gas rely on equations of state to compute various thermodynamic properties including: 1) compressibility factor 2) critical flow factor 3) speed of sound and 4) isentropic exponent. We compare these computed properties using five equations of state (REFPROP 8, GERG-2004, AGA 8, AGA 10, REFPROP 7) for eight natural gas compositions. The molar compositions vary from 97 % methane to 80 % methane the latter has high levels of ethane, nitrogen, or carbon dioxide. These comparisons span the pressure and temperature ranges 0.1 MPa to 10 MPa and 270 K to 330 K. The five equations of state predict mutually consistent properties at low pressures. However, at higher pressures and lower temperatures inconsistencies among the speeds of sound are nearly 0.2 % and inconsistencies among critical flow factors are nearly 0.5 % for ethane-rich natural gas mixtures. For these mixtures the inconsistencies in the speed of sound are close to the AGA 9 limit of 0.2 %, and could influence ultrasonic flowmeter diagnostic capabilities that depend on accurate speed of sound predictions. In addition, the inconsistencies in the predicted values of the critical flow factor, if not resolved, could significantly reduce the accuracy of critical flow venturis when used in ethane-rich gases over select ranges of pressures and temperatures. The discontinuities discovered in the AGA 10 critical flow factors were as large as 0.075 %, and raise concerns about using this equation of state for critical flow venturi applications.

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