In Italy the gas transmission network, more than 23.000 km long, has 21 compressor stations with a total power of 500 MW. Some of the stations are used only for transmission purposes while others can also be used alternately or at the same time to compress gas into storage fields (Fig.1).
Italy has one of Western Europe’s biggest gas reserves in storage fields: in fact the working gas used at the moment is about 12 Gm3 and the gas in place is about 24 Gm3. The total compression capacity is about 90 Mm3/d with discharge pressures of up to 150 bar.
All the stations are remote controlled with no personnel on site. For this, very reliable station control systems are used to control plant, engines and all the station’s auxiliary equipment.
A compressor unit consists of a centrifugal compressor driven by a gas turbine. The heavy duty gas turbines have an output of between 3.7 and 10.4 MW, while the jet derivative turbines have an output of 20.5 MW. The gas compressors have two separate stages (sections) that can be put in series or in parallel according to the required compression ratio and flow.
The special configuration of the stations gas piping is designed to match operating requirements.
Examples of series/parallel configurations of the compressors and of the gas piping are given with an explanation of the pros and cons.