Phase Behavior and ThermodynamicsGas Properties
Real Gas Density Formula
Real gas density estimates gas density at pressure and temperature using gas molecular weight or gravity and the real-gas z-factor.
How engineers use this formula
Use it for gas PVT checks, pressure-gradient estimates, and gas-property calculations where nonideal gas behavior matters.
Assumptions
- The gas z-factor is representative at the stated pressure and temperature.
- Pressure and temperature are absolute where required by the equation.
- Gas composition or gas gravity is known.
Limitations
- Does not model compositional phase behavior or liquid dropout.
- Accuracy depends on the z-factor and gas-property inputs.
Common mistakes
- Using gauge pressure instead of absolute pressure.
- Using Fahrenheit where Rankine is required.
- Applying a dry-gas calculation to wet gas without checking assumptions.
Default example
Using the default inputs, rho_g equals 10.32018 lb/ft³.
Ppsia
3000
M_glb/lbmol
18.82855
zdimensionless
0.85
Rpsia ft³/(lbmol °R)
10.732
T°R
600
Inputs
P
psiaPressure
M_g
lb/lbmolGas Molecular Weight
z
dimensionlessGas Compressibility Factor
R
psia ft³/(lbmol °R)Gas Constant
T
°RTemperature
Outputs
rho_g
lb/ft³
Real Gas Density
P
psia
Pressure
M_g
lb/lbmol
Gas Molecular Weight
Source and review
reviewedPenn State PNG 301. Natural Gas Properties, Equation 3.71.
Source