## 1 Introduction  
One of the most important issues in industry is efficient and continuous provision of natural gas supply. Large amounts of natural gas are used in many industries, e.g., in metallurgy, power, chemical and petrochemical plants [1]. Natural gas is transported to these factories through transmission and distribution pipelines at different pressure ranges. Transmission pipelines can be classified by the gas overpressure range into the following categories [2]:  
* Very high pressure (1.6 MPa and more),
* High pressure (0.5-1.6 MPa),
* Medium pressure (0.01-0.5 MPa),
* Low pressure (lower than 0.01 MPa). The high gas pressure must be reduced before entering devices and machines (e.g., boilers, engines, metallurgical furnaces, etc.). The pressure reduction station is a commonly applied technology for reducing natural gas pressure both in gas transmission and distribution pipelines. The pressure reduction station maintains constant gas pressure at the outlet of the station, regardless of fluctuations in the flow rate and pressure at the inlet to the station [2]. Reduction of the natural gas pressure in a pressure reduction station is carried out via isenthalpic throttling without producing any energy [3]. The gas temperature drops while throttling (ca. in the range from 2 to 7 K/MPa) [2]. Due to the presence of steam in the gas, the temperature drop carries the risk of hydrate formation. In order to prevent this phenomenon, throttling is carried out in such a way that the gas temperature after throttling is higher than 0 \({}^{\circ}\)C (e.g., by pre-heating of the gas before the throttling). The assembly of a pressure reduction station is very simple. The main pressure reduction station elements are the gas filter, the gas heater, the gas pressure regulators (throttle-valves or expanders), the measuring devices (providing measurement of the gas temperature, pressure, flow rate and calorific value), the gas odorizing devices, and the safety fittings (quick-closing valves, etc.). Gas stations can be classified by application purpose into the following groups [2]:  
* Pressure reduction stations, used for reduction and stabilization of gas pressure,
* Pressure reduction and measuring stations, used for reduction and stabilization of gas pressure and measuring of gas properties,
* Measuring stations, used for measuring the gas flow rate or its calorific value,
* Distribution and measuring stations, used for separating the gas stream to at least two output streams and measuring its parameters. By the expansion range, pressure reduction stations can be classified into the following groups [2]:  
* First stage (high pressure reduction stations), applied in high pressure transmission pipelines, providing the reduction of the gas pressure to ca. 500 kPa,
* Second stage (medium pressure reduction stations) applied in distribution pipelines, providing the reduction of the gas pressure to ca. 10 kPa. The application of a pressure reduction station for reduction of the natural gas pressure is a simple method, however, it is affected by significant energy losses (as the gas pressure energy is being dissipated during the isenthalpic expansion process). An alternative to pressure reduction stations is a turbine or a volumetric expander. In such machines, the gas pressure reduction is achieved by polytropic expansion. The gas flow through the machine results in rotor movement. Thus, the gas pressure energy can be converted into mechanical energy useful for driving an electrical generator. However, when the gas is expanded in the expansion machine, the gas temperature drop is larger (ca. 15-20 K/MPa) [3] compared to isenthalpic throttling. This results in an increased quantity of energy needed for pre-heating of gas before expansion.