Gas Compressibility Factor

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Gas Compressibility Factor

Gas Compressibility Factor ( Gas Deviation Factor)

The gas compressibility factor or gas deviation factor is a measure of how close a real gas is to an ideal gas. The compressibility factor Z is a dimensionless number close to 1.00. It is independent of the quantity of gas. It depends on the gravity, temperature, and pressure of the gas.

For example, a sample of natural gas may have a Z value of 0.8595 at 1000 psia and 70°F. Charts are available that show the variation of Z with temperature and pressure

A related term called the supercompressibility factor F_pv is defined as follows:

Equation 1
F_pv = 1 / Z^1/2

Equation 2
Z = 1 / F_pv²

Several methods are available to calculate the value of Z at a temperature T and pressure P. One approach requires knowledge of the critical temperature and pressure of the gas mixture. The reduced temperature and pressure are calculated from the critical temperatures and pressures as follows:

Equation 3
Reduced temperature = T / T_c

Equation 4
Reduced pressure = P / P_c

where temperatures and pressures are in absolute units.

The value of the compressibility factor Z is calculated using one of the following methods:

Standing and Katz method
Hall-Yarborough method
Dranchuk, Purvis, and Robinson method
AGA method
CNGA method

Reference:

Piping Calculations Manual,
E. Shashi Menon
SYSTEK Technologies, Inc