One reason is that some compressors have very small clearences between the impeller and the casing or diaphragm. If the impeller expands with a high temperature you could get an interference ("getting a rub". another reason is that the interstage seals in a multi wheel compressor often have a low temperature tolerence, often not much more than 120C.
I thought first about the head (power per volumetric or massic unit), What is the max. of head supported by compressor?
Because the temperature is about 120°C.So, Could the temperature causing thermal damage be the reason of TSHH and not the problem of the head which is directly related to the temperature?
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 vlid reason.
Usually, there is three TT location around the centrifugal compressor. One is
Located in the inlet line of compressor suction drum, the other is located in the compressor discharge line, and the last is located in the after-cooler of
the compressor discharge line.
If the compressor discharges TT got higher temperature, the compressor inlet
control valve will decrease the suction gas into the compressor.
