The simultaneous activity and synergistic interplay of the hydrogen-enhanced localized plasticity (HELP) and hydrogen-enhanced decohesion (HEDE) mechanisms of hydrogen embrittlement (HE) was fully confirmed for the first time in low carbon grade 20 - St.20 (or 20G, equivalent to UNS G10200, AISI 1020) steel specimens (boiler tubes) in situ enriched and embrittled by hydrogen during boiler exploitation in a thermal power plant (TPP), and not only through lab experiment, simulation or modelling. The proposed HELP+HEDE model of HE in low carbon steels is based on the correlation of mechanical properties to SEM microscopy fractography analysis of fracture surfaces in the presence of simultaneous action in a cooperative manner of the HELP and HEDE micro-mechanisms of HE depending on the local concentration of hydrogen in the investigated steel. Also, the effect of hydrogen on the material mechanical properties and ductile to brittle fracture transition resulting from the concurrent and synergistic (HELP+HEDE) or competing action of HELP and HEDE mechanisms of HE is discussed. The effects and correlations between microstructure, fractographic observations (light microscopy and scanning electron microscope - SEM) and macro-mechanical testing data (tensile testing, impact strength on instrumented Charpy machine and hardness) are also considered. This paper also gives an overview of the application of a model for structural integrity analysis of boiler evaporator tubes in TPP made of low carbon steel exposed during operation to a local corrosion process and multiple hydrogen assisted degradation processes. The proposed model enables enhanced predictive maintenance of industrial component systems.