Sensitivity of natural gas flow measurement to AGA8 or GERG2008 equation of state utilization

Abstract

The accurate measurement of natural gas flow at metering point is essential for custody transfer. The most used technologies for natural gas measurements are including turbine flow meters, ultrasonic flow meters and orifice flow meters. These flow meters are basically volume flow meter which measure natural gas volume at flow condition. Then the natural gas volume at flow condition should be converted to volume at base (standard) condition by appreciated procedure. Historically, the AGA8 equation of state (EOS) is widely used for such conversion. Recently, a new equation of state (GERG 2008) was presented by European Gas companies for natural gas thermodynamic properties calculation. The main objective of the current study is to present a comparison between AGA8 and GERG2008 EOS in computing conversion factor. Firstly, the Z-factor is calculated based on ISO20765-1 standard which is most recent version of AGA8 EOS. Then, ISO20765-2 standard has been employed to calculate Z-factor based on GERG2008 EOS. Comparing the calculated Z-factor and measured value of previous studies show that the accuracy of GERG2008 EOS is higher than AGA8 for all studied range of pressure and temperature and for various composition ranges. Results also show that GERG2008 equation of state predicts Z-factor higher than AGA8 standard in practical region of measurement. The results also reveal very important findings. Surprisingly, the relative difference is negative for the practical region of measurement. It means that by replacing AGA8 EOS by GERG2008 standard, the less flow is registered by the measurement system. In other words, simply by utilizing the GERG2008 EOS (instead of AGA8 EOS), the buyer benefits.