GPSA / API 617
Enter mole percentages (must sum to 100%)
Understand Z-factor calculation methods, equation of state, and reduced properties
It calculates Z-factor, k-value (Cp/Cv), gas density, molecular weight, specific gravity, Cp/Cv on mass and molar basis, pseudo-critical properties (Tpc, Ppc), and reduced properties (Tr, Pr) at compressor operating conditions.
You need to provide gas composition (mole fractions of up to 13 components), pressure, and temperature at compressor conditions. Preset compositions are available for natural gas, rich gas, lean gas, and sour gas.
The Z-factor (compressibility factor) accounts for real gas deviation from ideal behavior. A 5% error in Z can produce a 5โ10% error in head and power calculations, leading to undersized or oversized compressor selection.
It uses the Dranchuk-Abou-Kassem (DAK) correlation, a numerical fit to the Standing-Katz Z-factor chart for natural-gas-like mixtures. DAK is valid for reduced temperature Tr > 1.0 and reduced pressure up to Pr โ 30. For hydrogen-rich gas, near-critical conditions, or custody-transfer accuracy, use AGA-8 detailed characterization or a process simulator.
For mixtures containing HโS or COโ, the Wichert-Aziz correction is automatically applied to the pseudo-critical temperature and pressure before computing Z. This correction accounts for the polar interactions of acid gases that Kay's mole-weighted mixing rule alone cannot capture, and is most important above ~5% acid-gas content.