Techniques for Natural Gas Physical Properties Definition for Flow Rate and Volume Metering Systems

Abstract

The results of researching the techniques for natural gas properties determination for flow rate and volume metering systems are presented in the paper. The necessity of developing techniques for calculating the isentropic exponent of natural gas for pressure up to 25 MPa based on a simplified set of parameters of gas composition is shown. New techniques are proposed to define isentropic exponent and dynamic viscosity based on designed analytical dependence for the calculation the pseudo-critical density of natural gas, regression equation for isentropic exponent and improved equation of Dean and Steal for calculating gas dynamic viscosity for high pressure. Techniques adequacy verification is carried out relative to the arrays of calculated values of adiabatic index and viscosity obtained from high-precise equations of gas state and relative to the experimental data of viscosity and sound speed in natural gas. The techniques errors and the range of their application are defined using the verification results. 1. Definition of the problem to be solved Metering of natural gas flow rate and volume is carried out by metering systems based on microprocessor flow-computers. This enables the implementation of real-time calculation of gas flow rate and volume taking into account the parameters of its physical properties. In the most common metering systems, the parameters of natural gas physical properties are calculated in real time based on the measured values of gas pressure and temperature as well as on values of composition parameters which are previously defined and fed into computer. Accordingly, the flow rate and volume of gas are defined using the calculated values of physical properties and measured values of flow parameters. In such metering systems the calculating algorithms should be based on the precise techniques for calculating physical properties, particularly compressibility factor (density at operating conditions), isentropic exponent and viscosity of natural gas [1]. 2. Analysis of the recent publications and research works on the problem The state equations are the basis for calculating the gas mixture parameters, particularly of natural gas, and show the dependence between the compressibility factor and mixture state parameters. A large number of techniques are proposed to calculate the physical properties of hydrocarbon mixtures, particularly of natural gas. They are based on the principle of corresponding states, on the virial state equation and on special multi-constant state equations. The standards GOST 30319.1-3 96 [2-4], DSTU ISO 12213-2,3: 2009 [5, 6], ISO 20765-1: 2005 [7] are built on the basis of methods for defining physical properties of known world research centers. It should be mentioned that the standards [5, 6, 7] describe the methods for calculating the compressibility factor and do not consider compressibility coefficient, although it is used at equations implemented in the metering systems. The methods of standards [5] and [6] use complex iterative algorithm, the input data for the method