@Julien Marrec you are on the right track with getOptionalDoubleArgumentValue, because your argument is not required as indicated by the false in OpenStudio::Ruleset::OSArgument::makeDoubleArgument('fan_pressure_rise', false). And you are correct that all you need to do is retrieve the double from the optional double if it is not empty?. To do that simply use optional's get method like this selected_fan.setPressureRise(fan_pressure_rise.get).

Take a look at the Measure Writing Guide. In the run section there is a deeper explanation of the optionals.