Simplified GERG Virial Equation for Field Use (include associated paper 23568 )

Abstract

Summary. A GERG (Groupe European de Recherches Gazires) equation of state (EOS) is presented to calculate the compressibility factor of natural gases. The equation, which does not require detailed gas analysis, can predict the compressibility factor when three of the four following gas properties are known: the gross calorific value, the relative density, and the mole fractions of N2 and CO2 The new equation, known as the SGERG-88 virial equation, is based on ideas presented in an earlier study. The new equation, however, applies to wider ranges of temperature and pressure and can be applied to gases containing hydrogen if the mole fraction of the hydrogen. The equation was tested on natural gases purchased, transported, and marketed by the European gas companies. Excellent agreement between computed and experimental compressibility factors was found. This difference is, in most cases (94% of the data points), less than 0. 1 % in the temperature range of 265 to 335 K and for pressures up to 12 MPa. Introduction In the natural gas industry, large-volume flows are measured at transmission-line pressures and temperatures. In custody-transfer operations, however, transmission-line conditions must be converted to reference conditions. This requires accurate knowledge of the compressibility factor at both flow and reference conditions. A modified version of American Gas Assn. (AGA) NX-19 is frequently used to calculate the compressibility factor. The modified AGA NX-19 is able to predict the compressibility factor of natural gases with a calorific value not larger than that of methane with an accuracy of about 0. 2 %. In western Europe in the last decade, however, many gases have been introduced with compositions differing considerably from these low-calorific-value natural gases. For these types of gas, modified AGA NX-19 is unable to predict the compressibility factors accurately. Differences of more than 1 % appear between calculated and measured compressibility factors. In addition to modified AGA NX-19, other methods are available for calculation of the compressibility factor of natural gases-e.g., the AGA NX-19-BR. KOR (especially derived for high-calorific-value gases), the Benedict-Webb-Rubin EOS, and cubic EOS, such as Soave-Redlich-Kwong or Peng-Robinson EOS. None of them, however, can predict the compressibility factors very accurately for a wide range of natural gases. Recently, the European gas companies that make up GERG (Groupe European de Recherches Gazirres) supported the Van der Waals Laboratory in developing a virial EOS, known as the MGERG-88 equation, for the prediction of compressibility factor of natural gases from a detailed gas analysis. The accuracy of the MGERG-88 equation, which was tested for 84 natural gases (more than 4,000 data points), is approximately 0.1 % in the temperature range of 265 to 335 K and at pressures up to 12 MPaIn many situations, however, the composition of a natural gas is not available. Consequently the SGERG-88 equation was devised. Its input parameters are comparable to those for the modified AGA NX-19 method: the gross calorific value, Hs, the relative density, y, and the mole fractions of N2 and C02 (and hydrogen, if present).