The Effect of Using Real Gas Absolute Viscosity and Isentropic Exponent on Orifice Flow Measurement: Proposed Adoption of Refprop 8.0 as a Standard for the Natural Gas Industry
William R. Johansen
Abstract:
Constant values for isentropic exponent and absolute viscosity are commonly used for the calculation of gas flow rates through differential producing flowmeters. Errors in flow measurement due to the use of constants for thermodynamic and transport properties are bias errors. Bias errors in natural gas orifice flow measurement due to incorrect isentropic exponent values can exceed 0.15 %. When the adiabatic expansion factor is used to calculate natural gas flow rates the bias errors due to the use of a constant isentropic exponent can exceed 2 %. Bias errors in natural gas flow measurement due to incorrect absolute viscosity values can approach 0.2 % for natural gas. REFPROP 8.0 is recommended as a source of accurate thermodynamic and transport property values. REFPROP 8.0 can be used as a source of live values for use with flow computers or it can be used to select more appropriate constant values to minimize bias errors.
