A desuperheater can be considered as part of instrumentation, since as you have mentioned it controls the superheat of the material passing through it - the final control element.
Dear Shobi
A desuperheater can be considered a item of instrumentation but this depends on which system you use. Lets get practical: there are control valves that are specifically designed to desuperheat steam (like Samson type 240 or pressure and desuperheater type 281), this are clearly part of instrumentation. There are receivers or spray nozzles system used for this too and, even its use to control the temperature, I would not classify this as an Instrument.
If one identifies it with an instrument tag then it has become an instrument. If it is identified with an equipment number then it is not an instrument.
If one is wondering which identity to apply then it would seem that it would come from a definition of how one defines each.
Six of one and half a dozen of another?
In any event any inline device should meet the piping specs. And any appertuance should meet the appropiate specs. Usually an isolating valve at the first opportunity.
The end result, the same information has to be available to come up with the appropiate solution.
