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Flow-X─ Function Reference
Flow-X General functions - fxNX19_M
fxNX19_M
Description
The AGA NX-19 standard describes a method to calculate the super-compressibility for natural gases
and was developed in 1962.
The 1962 standard describes a standard method for calculating the super-compressibility factor that is
based on the actual pressure and temperature, the specific gravity and the mole fractions of the carbon
dioxide and nitrogen.
The 1962 standard also specifies 3 alternate methods, which are based on a full compositional analysis
(1st alternate method), the relationship between methane and specific gravity (2nd alternate method)
and the relationship between the heating value and the specific gravity
The function only performs the standard method as specified by the standard and none of the alternate
methods.
The function provides the option to perform the PTB G9 correction instead of the 1962 standard
method. This consists of the modified NX-19 method (NX-19-mod) per Herning & Wolowsky and the
additional ‘BR.KORR.3H’ correction for high-caloric gases (gross heating value >= 39.8 MJ/m3).
Note: the definition of the specific gravity as used in the standard is that of the
ratio of the density of the gas to that of air at base conditions, so the real specific
gravity or real relative density.
Compliance

AGA Par Research Project NX-19 - Manual for the Determination. of the
Supercompressibility Factors for Natural Gas, 1962

Berechnung von Realgasfaktoren und Kompressibilitätszahlen für Erdgas, Technische
Richtlinie G9 der Physikalisch - Technische Bundesanstalt für meßgeräte für Gas (PTB), TRG 9
8/82
Boundaries
The following bounds apply for the input values. Using the standard for conditions that lie outside this
range will yield to a higher uncertainty and is not recommended.
Input value AGA-NX-19
(1962) AGA-NX-19-mod AGA-NX-19-
mod.BR. KORR.3H EU
Pressure 0 .. 350 0 .. 137.9 0..80 bar(a)
Temperature -40..115.6 -40..115.6 0..30 °C
Relative density 0.554 .. 01.000 0.554..0.75 0.554..0.691 -
Gross heating value Not used 31.8..39.8 39.8..46.2 MJ/m3
Nitrogen 0.00 .. 0.15 0.00 .. 0.15 0.00..0.025 mol/mol
Carbon dioxide 0.00 .. 0.15 0.00 .. 0.15 0.00..0.07 mol/mol
Function inputs and outputs
Function inputs Remark EU Range Default
Name Optional tag name, tag description and
tag group
Pressure Observed pressure bar(a) 0..200 1.01325
Temperature Observed temperature °C -100..300 0
Specific gravity Ratio of density of gas and density of air
at the applicable reference conditions of
pressure and temperature, i.e. the real
specific gravity (real relative density).
If setting ‘PTB G9 correction’ is disabled
the reference conditions are 60°F and
14.73 psia. Else the specific gravity value - 0..2 0