Operations before opening containers such as the tower bottom

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There is a distillation tower containing a flammable liquid (with a flash point of around 300°C) and its vapor. If maintenance work on the tower is required, it is necessary first to remove the flammable liquid, then to wash the tower with water, and finally to displace the flammable gases inside the tower with nitrogen. I would like to ask what components need to be analyzed after replacing the combustible gas before the tower can be started up? What are the standards for these components to be considered qualified? Is there any evidence? Are the analysis requirements for starting up a tower the same as those for performing hot work on a tower? Please help resolve this confusion! I saw a set of operating procedures stating that nitrogen should be used to displace flammable gases before starting the tower, with the requirement that the explosive concentration be less than 5%. What does this mean? Right?