The operating temperature is the temperature which prevails inside equipment and piping during any intended operation. A process engineer determines the operating temperature.
In many cases temperatures can be different from those required for the intended operation can occur, e.g. during starting up, shutting down, drying, regeneration, steam-air decoking, heating up to maximum ambient temperature, including solar radiation, fouling of heat exchangers, failure of cooling medium and heat exchanger by-passing, etc.
The maximum operating temperature is the highest temperature which provides sufficient flexibility for the control of the intended operation.
In many cases this flexibility is not required and in those cases the maximum operating temperature is equal to the operating temperature. The maximum operating temperature is normally used as the basis for materials selection with respect to long term corrosion and/or material degradation. Therefore, any subsequent proposed change in MOT shall be scrutinised to confirm the continued suitability of the material. The maximum operating temperature is determined by the process engineer in consultation with the process control engineer.
The design temperature is the highest temperature at which equipment may be subjected to the upper and/or lower design pressure. The design temperature is typically 10 °C above the operating temperature and shall not be lower than the maximum operating temperature. The design temperature is initially selected by the process engineer and finally determined in close consultation with the mechanical design engineer.