 Critical parts of the process would include the lighting of the burners, controlling the level of water in the drum, and controlling the steam pressure.What a SIS shall do ("the functional requirements") and how well it must perform ("the safety integrity requirements") may be determined from Hazard and operability studies (HAZOP), layers of protection analysis (LOPA), risk graphs, and so on.All techniques are mentioned in IEC 61511 and IEC 61508.