According to the IEA, "Hydrogen is today enjoying unprecedented momentum". There is a growing interest, particularly in the pipeline market, about the possibility of mixing natural gas with hydrogen from few percentages up to 30%. Considering the millions of kilometers of existing gas pipelines globally, leveraging existing infrastructures would significantly save CAPEX for new-built hydrogen transport system as well as help reduce the CO2 footprint, while planning for long term pure H2 pipelines. However, transporting hydrogen has relevant impacts on centrifugal compressors, from mechanical and operability point of view.

Whenever dealing with lighter gases, to guarantee the same pressure ratio, a centrifugal compressor must be designed with high head impeller type, greater number of stages or higher impeller tip speed. While increasing the number of impellers may affect train lay-out (multi casings trains), increasing the rotating speed guarantees a reduced footprint. It allows to reduce costs for new units and a limited impact on existing compression station, in case of machine internals upgrade or full replacement.

Generally, the limits in increasing the rotating speed is given by mechanical stress of the impellers, rotordynamic criticalities and the loss of interference between impeller and shaft. The latest technology developed by Baker Hughes allows to extend significantly the maximum impeller tip speed, especially when considering different type of rotor design (High Pressure Ratio Compressor), where the impellers are kept on shaft by mechanical connections.

By using real case studies outcomes, it is possible to plot a map related to the compressor capabilities when the percentage of H2 gradually increases, up to the most challenging conditions (pure hydrogen, high pressure ratio and large flow). Depending on the specific process conditions and the evolution over time of the infrastructure planned by the pipeline operator, it is possible to optimize technically and economically the compression system.