difficult to answer, because many things are considered today to be "normal". And maybe after utilizing of blockchain will be not normal any more.
In general it works like this:
Data will be collected and analyzed by control system. all the logic is inside of it.
Normally there is a huge database which contains all the historical measured parameters, e.g. measured every 5 seconds. Some plants use neuronal networks to predicts some situations and partially control the operation.
Parallel to the normal control system there is a safety system which must prevent accidents by detecting dangerous situations and acting if necessary. It uses totally independent sensors and actors.