Temperature reduction at gas delivery stations (GOS)

Setting the outgoing temperature at a gas receiving station takes place as follows:

• - The incoming gas is heated in a heat exchanger isobaric and adiabatic (at the same pressure and without heat exchange with the environment) to a certain temperature;
• - The pressure of the gas is reduced with a regulator, whereby the gas temperature also drops. The pressure of the gas is reduced with a regulator, whereby the gas temperature is also lowered.
• - The outgoing gas thus reaches the desired output pressure and temperature.

The process is shown schematically below:

In the above example, the inlet pressure and temperature are 40 barg and 10 °C respectively. Assuming G-gas, the enthalpy at that point is -18.4737 kJ/kg (with 0 °C and 0 bar as the reference situation with an enthalpy of zero).

At an exit temperature of 5°C and a pressure of 8 bar, the enthalpy of the gas is increased to 1.9549 kJ/kg. Because the expansion in the regulator is isenthalpic, the enthalpy between the heat exchanger and the regulator has also been 1.9549. It can then be calculated that at a pressure of 40 barg, the temperature at that point must have been 19.37 °C.

If we assume a flow rate of 1000 m3(n)/h, the mass flow through the street is 833.4 kg/h. The amount of heat to be delivered by the heat exchanger is then

833.4 x (1.9549 - -18.4737) = 17025 kJ/h.

The same can now be done for a situation in which the outgoing gas temperature is lowered to 3 °C. This is shown in the figure below.

It can be seen that the enthalpy of the outgoing gas has been reduced to -3.8576 kJ/kg, where a temperature of 16.70 °C belongs to a pressure of 40 barg (after the heat exchanger).

In this case, the heat supplied by the heat exchanger is therefore

833.4 x (-3.8576 - -18.4737) = 12181 kJ/h.

The difference between the set output temperatures of 5 and 2 °C therefore results in an energy saving of 17025 - 12181 kJ/h = 4844 kJ/h, or approx. 28.5%. In practice savings of 5% per degree Celsius are measured.

The MR gas quality prescribes a minimum gas temperature of 0°C. GTS therefore always delivers the gas warmer than 0°C.

The temperature to be delivered depends on the type of gas and the gas pressure. In general, high-calorific gas is very dry and can be supplied at 1°C for all supply pressures. Low-calorific gas is somewhat wetter and is delivered between 1°C and 7°C, depending on the delivery pressure. The proposed temperature for delivery is indicated in the letter you received from GTS about this subject.

The final temperature depends on the downstream network and the connected party and on further pressure reduction. For underground networks, the gas is usually warmed up by the soil. The average soil temperature is 7°C. The new delivery temperature is usually 3°C. This means that the gas is warmed up on the way. For above-ground networks, heating usually also takes place through the ambient air. Only on cold days can the gas cool down further. The temperature of the gas can drop further due to pressure reduction. A good rule of thumb is that a drop in pressure of 1 bar causes a drop in temperature of 0.5°C.

In principle, connected parties will not notice anything of the temperature drop. As a rule, the proposed reduction is small and can be implemented without effect.

If there is a large drop in temperature, it is advisable for a connected party's process engineer to discuss the possible effects with the GTS contact person. A gas turbine in particular can be sensitive to the temperature of the gas supplied. No effects are expected on other equipment.

Theoretically, this is possible if there is a significant pressure reduction in the connected party's process.

In general, no special effects are to be expected. In the case of regulators, it is necessary to check that breathing openings do not become blocked. A list of equipment and the extent to which this equipment is sensitive to icing is available.

Generally speaking, the failure of hydrocarbons must be prevented. Gas is currently very dry and in the short term there will be no adverse effects from a low temperature in the connected party's network. In the longer term, an accumulation of hydrocarbons may occur. For high calorific gas a minimum temperature of -3°C must be maintained. Below this limit, condensation of hydrocarbons can take place. For low-calorific gas, this temperature depends on the pressure. At a typical pressure of 8 bar the condensation temperature is 3°C under worst case conditions. If the gas is further reduced to, for example to, 0.1 bar, the temperature drops to approximately -0.5°C. Practical tests, carried out together with an RNB, indicate that no condensation occurs in their network.

Because the sensible heat of the gas is slightly lower, the gas consumption of the connected party will be tiny higher. Per degree of temperature reduction this will be 0,05 ‰. In practice this per mille is much lower due to the very rapid warming of the gas by the environment.

What temperature is permissible in the connected party's network in relation to the material specifications?