PV = ZnRT | GPSA
Master PV=nRT, gas constant R values, combined gas law, Dalton's partial pressures, and when to use ideal vs. real gas equations
The ideal gas law (PV = nRT) assumes gas molecules have no volume and no intermolecular forces. The real gas law (PV = ZnRT) includes the compressibility factor Z to correct for non-ideal behavior at high pressures and low temperatures typical in midstream operations.
Standard volume is converted to actual volume using the ratio of actual to standard conditions: V_actual = V_standard × (P_std/P_actual) × (T_actual/T_std) × (Z_actual/Z_std). Standard conditions are typically 14.73 psia and 60°F per GPSA conventions.
The compressibility factor Z quantifies how much a real gas deviates from ideal gas behavior. For natural gas at pipeline pressures, Z typically ranges from 0.7 to 0.95. Ignoring Z in calculations can introduce errors of 5-30% in volume and density results.
Gas density is calculated as ρ = PM/(ZRT), where P is absolute pressure, M is molecular weight, Z is compressibility factor, R is the gas constant, and T is absolute temperature. For natural gas, molecular weight can be estimated from specific gravity as MW = SG × 28.97.