C178 – NX-19 Gas Supercompressibility
Description
This calculates the supercompressibility factor at both line and base conditions of the input gas and uses this and the other input properties to calculate the volume correction factor and the line density of the gas.
References
Manual for the Determination of Supercompressibility Factors for Natural Gas- AGA PAR Research Project NX-19 (1962)
Kelton calculation reference C178
FLOCALC calculation reference F051
Options
Method
- Method 1: The specific gravity method
- Method 2:The analysis method
- Method 3: The methane-gravity method
- Method 4:The heating value method
Appropriate option should be chosen with regards to the input parameters the user has available to them.
Constants
Table 5
| Gas | Critical pressure (psi) | Critical temperature (°R) | Ideal specific gravity | Ideal gas total heating value (BTU/Sft3) |
|---|---|---|---|---|
| Methane | 673 | 344 | 0.554 | 994 |
| Ethane | 712 | 550 | 1.038 | 1742 |
| Propane | 617 | 666 | 1.523 | 2479 |
| iso-Butane | 529 | 734 | 2.006 | 3203 |
| n-Butane | 551 | 766 | 2.006 | 3213 |
| iso-Pentane | 483 | 830 | 2.49 | 3939 |
| n-Pentane | 485 | 846 | 2.49 | 3948 |
| n-Hexane | 440 | 911 | 2.973 | 4684 |
| n-Heptane | 400 | 972 | 3.46 | 5420 |
| n-Octane | 361 | 1025 | 3.943 | 6153 |
| Carbon Dioxide | 1072 | 510 | 1.519 | 0 |
| Nitrogen | 492 | 216 | 0.967 | 0 |
| Air | 547 | 239 | 1 | 0 |
| Oxygen | 731 | 278 | 1.105 | 0 |
| Helium | 33 | 10 | 0.138 | 0 |
| Hydrogen | 188 | 60 | 0.07 | 319 |
| Hydrogen Sulphide | 1306 | 678 | 1.176 | 630 |
Calculation
Supercompressibility and compressibility factors
The supercompressibility is calculated by:
| Where | ||
|---|---|---|
| Z | = | Line supercompressibility |
| Zb | = | Base supercompressibility |
The formulae for the compressibility factors are not explicitly given by NX-19 but are approximated using the above formulae. At pressures up to 5000 psi and temperatures up to 100°F gas compressibility factors should correlate to within 0.2% of AGA 8. The compressibility factors for line and base conditions are calculated by:
| Where | ||
|---|---|---|
| τ | = | Variable based on the adjusted temperature |
| π | = | Variable based on the adjusted pressure |
| B, D, n | = | Are variables based on τ and π |
Temperature and pressure adjusting factors
When using the specific gravity method the temperature and pressure adjusting factors are calculated by:
When using the analysis method the temperature and pressure adjusting factors are calculated by:
When using the methane-gravity method the temperature and pressure adjusting factors are calculated by:
When using the heating value method the temperature and pressure adjusting factors are calculated by:
| Where | ||
|---|---|---|
| G | = | Specific gravity of the gas |
| Xc | = | Mole fraction of carbon dioxide |
| Xn | = | Mole fraction of nitrogen |
| Xm | = | Mole fraction of methane |
| Tc | = | Critical temperature from Table 5 |
| pc | = | Critical pressure from Table 5 |
| H | = | Heating value (wet basis) |