Engineering task and calculation objective
The JTHO module calculates the outlet temperature for the adiabatic throttling of natural gas, i.e. the Joule-Thomson effect at pressure regulating stations, control valves and orifices. Given the gas composition (natural gas H, natural gas L or a freely defined mixture), the inlet pressure, the outlet pressure and the inlet temperature, the module determines the temperature downstream of the throttle for the isenthalpic change of state h = constant.
In practice, Joule-Thomson cooling of natural gas is a key design criterion for gas pressure regulating and metering stations: when gas is expanded from high transmission pressure to distribution pressure, natural gas typically cools by about 0.4 to 0.5 K per bar of pressure drop. Without preheating, hydrate formation, condensate dropout and icing of valves and fittings are a real risk. Anyone who wants to calculate the outlet temperature of a throttling process needs a real-gas model, because for an ideal gas the temperature change would be zero.
The module solves the problem using the real-gas equation of state of the specific gas mixture, providing the basis for sizing gas preheaters and for assessing whether minimum temperatures at downstream components are maintained.
Calculation workflow
- Define the gas composition: First the natural gas is defined: either as a typical natural gas H or natural gas L, or as an arbitrary composition of the individual components (methane, ethane, propane, nitrogen, CO2, etc.). The composition determines the real-gas behavior and thus the Joule-Thomson coefficient.
- Enter the state upstream of the throttle: The pressure and temperature upstream of the throttle are entered. From these values the module determines the specific enthalpy of the gas mixture at the inlet state.
- Specify the outlet pressure: The pressure downstream of the throttle is set as the second process parameter. The throttling process itself is assumed to be adiabatic and without work, so the enthalpy is conserved.
- Iterate the isenthalpic outlet temperature: The module searches along the isenthalpic line h(p1, T1) = h(p2, T2) for the outlet temperature T2 corresponding to the specified outlet pressure. The temperature drop results from the integral Joule-Thomson effect over the entire pressure reduction.
- Evaluate the result: The calculated outlet temperature is compared against the permissible operating limits, such as the hydrate formation temperature, the dew point or the lowest permissible component temperature, and the required preheating duty is derived from this comparison.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Before throttle | ⇒ | °C |
| After throttle | Drossel | °C |
| Before throttle | Drossel | Pa |
| After throttle | Drossel | Pa |
| Nitrogen | N2 | mol-% |
| Methane | CH4 | mol-% |
| Carbon dioxide | CO2 | mol-% |
| Ethane | C2H6 | mol-% |
| Propane | C3H8 | mol-% |
| n-Butane | C4H10 | mol-% |
| Type of natural gas | Erdgas | – |
| Hydrogen H2 | Konzentration | mol-% |
| Hydrogen | Zumischung | – |
| n-Pentane | C5H12 | mol-% |
| n-Hexane | C6H14 | mol-% |
Calculation options
Type of natural gas
Free Input of Concentrations · Natural gas L · Natural Gas H · Natural Gas H Russia according to DVGW Arbeitsblatt G290 · Natural Gas H North Sea according to DVGW Arbeitsblatt G290 · Natural Gas H Denmark according to DVGW Arbeitsblatt G290 · Natural Gas L Holland according to DVGW Arbeitsblatt G290 · Natural Gas L Germany according to DVGW Arbeitsblatt G290 · Biomethane H according to DVGW Arbeitsblatt G290 · Biomethane H + LPG according to DVGW Arbeitsblatt G290
Frequently asked questions
Why does natural gas cool down during throttling even though no heat is removed?
For real gases, enthalpy depends not only on temperature but also on pressure. During an isenthalpic pressure reduction, the gas molecules perform work against their intermolecular attractive forces; this energy is drawn from the internal energy and the temperature drops. For an ideal gas, the Joule-Thomson coefficient would be zero and the temperature would remain constant.
How large is the temperature drop for natural gas, typically?
As a rule of thumb, natural gas in the usual operating range of transmission and distribution networks cools by about 0.4 to 0.5 K per bar of pressure difference. The exact value depends on composition, pressure level and temperature, which is why the module calculates it from the real-gas behavior of the specific mixture rather than using a constant rule-of-thumb value.
Can the temperature also rise during throttling?
Yes. Above the inversion temperature, the Joule-Thomson coefficient changes sign and the gas heats up on expansion. For natural gas, however, the inversion temperature lies far above normal operating temperatures, so in gas pressure regulating stations cooling practically always occurs. Hydrogen and helium, by contrast, warm up during throttling even at ambient temperature.
Why is this calculation important for the design of gas pressure regulating and metering stations?
If the gas temperature falls below the water or hydrocarbon dew point, hydrate formation and condensate dropout can occur and block control equipment. In addition, minimum temperatures of the materials and of the downstream piping must be maintained. The calculated outlet temperature directly indicates whether, and by how much, the gas must be preheated before expansion.