our pressure is 50 Barg with same temprature as yours.
frankly i don't know about the concrete, all i know is that we had some sort of problem with the thermal isolation for the heater and the inside of it
we have the same system at our plant, what do you need to know exactly.
also the heater is on all the time what changes is its firing rate through controlling fuel gas flow.
the value isn't a fixed one, i think it depends on the design temprature for the supports for the downstream piping. the design temprature for the material will ofcourse be higher than 120C however the temprature limitation will be for the supports.
also what is stated by geordie87 could be a...