A boiler runs for thirty years. Its control system does not. At some point spare parts for a Simatic S5 or ET200 remote I/O are no longer reliably available, while the plant itself is nowhere near the end of its life. That is when a retrofit becomes due.
There is a standard for handling discontinuations in an orderly way. IEC 62402 describes obsolescence management as a continuous task, not a reaction. In practice the subject still comes up only once a spare part is already missing.
The spare part is not the problem
A successor can be found for every discontinued module. That is research work. It takes time, but it is predictable. What the new control system has to do is equally predictable. It is written in the old functional description.
A retrofit becomes incalculable somewhere else, namely in what happened between the last documentation update and today. A retrofitted interlock. A drive that was replaced and wired differently in the process. A measuring point that is still on the drawing and has been missing from the plant for years. Anyone planning from the existing documents is planning for a plant that no longer exists.
What the survey finds
So the starting point is not an order but a survey. The documents are checked against the plant, terminal by terminal, wherever the conversion touches. The result is not only a list of what has to be replaced. Above all it is a list of discrepancies. Nobody knows that list in full beforehand, not even the maintenance team that has looked after the same plant for twenty years.
That list is the real yield of the survey. It decides whether the planning holds or whether people improvise during the conversion.
The survey regularly turns up two labelling systems side by side as well. Why we usually continue the existing one in an operating plant rather than replace it is covered in IEC 81346 designations.
Will the new technology even fit?
A second question belongs before the order. Can the existing enclosure accommodate the new equipment? Modern modules are rarely larger, but they sit differently, need different clearances and some of them give off more heat. How much heat an enclosure can actually dissipate can be determined by calculation to IEC 60890, for example with our enclosure climate control calculator. Clarify that only during installation and the plant stops. Unplanned.
Conversion steps that fit into operations
Whether a retrofit succeeds without downtime depends on the plant and cannot be promised in general terms. What can be planned is how the work is cut. Conversion steps are kept small enough and described clearly enough for the customer’s own maintenance team to work through them in their own windows. That calls for documentation that is already correct before the conversion, not documentation that is brought up to date afterwards. Which documents belong to it and how they are structured is described in DIN EN 61082-1.
