AGA 8

Compressibility Factors Of Natural Gas And Other Related Hydrocarbon Gases

This standard calculates the compressibility and density of a gas along with other properties from its composition in accordance with the options selected


Composition Limits


The table below lists the composition limits for both the Gross (Normal Range) and Detailed (Extended Range) methods.


Component Extended Range Normal Range
Min.% Max.% Min.% Max.%
Methane 0 100 45 100
Ethane 0 100 0 10
Propane 0 12 0 4
iButane 0 6 0 1
nButane 0 6 0 1
iPentane 0 4 0 0.3
nPentane 0 4 0 0.3
neo-Pentane 0 4 0 0.3
nHexane 0 100 0 0.2
nHeptane 0 100 0 0.2
nOctane 0 100 0 0.2
nNonane 0 0.2 0 0.2
nDecane 0 100 0 0.2
Hydrogen 0 100 0 10
Nitrogen 0 100 0 50
Carbon Dioxide 0 100 0 30
Hydrogen Sulphide 0 100 0 0.02
Water 0 100 0 0.05
Helium 0 3 0 0.2
Oxygen 0 21 0 0
Carbon Monoxide 0 3 0 3
Argon 0 1 0 0

Upstream Compression Factor


The standard density is determined from gas compression factor at standard conditions for temperature and pressure. The compression factor is calculated using the detail characterisation method defined in American Gas Association Report No.8 of the year selected in the options.

Upstream Density


The upstream density and gas compression factor are related as follows:
Upstream Density

Symbol Description Units
ρ1 Density at the upstream pressure point P1 kg/m3
m Gas molar mass kg/kmol
p1 Upstream pressure bar a
z1 Compression factor at upstream conditions -
t1 Upstream temperature K

Constant Description Value Units
R Universal Gas Constant 8.314459848484848 J/mol/K

Since the AGA-8 does not cater for the neo-pentane component, this component is added to the iPentane or the nPentane dependant on the users selection from the configuration settings prior to performing the AGA-8 calculation.
Similarly the Benzene and Toluene components are also not catered for. The Benzene component can be added to the nPentane component and the Toluene component can be added to either the nHeptane or nHexane components dependant on the users selection from the configuration settings.
Adding the Toluene component to nHeptane is consistent with ISO 12213-2:1997 whereas adding the component to nHexane is consistent with ISO 12213-2:2006.

Standard Compression Factor - 1994


The standard density is determined from gas compression factor at standard conditions for temperature and pressure. The compression factor is calculated using the detail characterisation method defined in American Gas Association Report No.8 1994.

Standard Density - 1994


The standard density and gas compression factor are related as follows:
Standard Density

Symbol Description Units
ρb Density at standard conditions kg/m3
m Gas molar mass kg/kmol
pb Standard pressure bar a
z1 Compression factor at standard conditions -
tb Standard temperature K

Constant Description Value Units
R Universal Gas Constant 8.314459848484848 J/mol/K

Speed of Sound and other thermodynamic properties - 2017


The speed of sound, isentropic exponent and other properties are determined by differentiation of the fundamental Helmholtz energy equation as defined in American Gas Association Report No.8 - 2017

References