Establishing the Maximum Design Temperature
The maximum design temperature is based on the normal maximum operating temperature obtained from the process design. The recommended practice is to add a margin of at least 25°F above the maximum operating temperature, to assure safe operation in the event of minor temperature excursions. Exothermic reactions or other process characteristics that may cause higher temperature excursions will require greater margins, and should be discussed with the process designers before the maximum design temperature is established.
When the maximum operating temperature is below 650°F, it is usually desirable to increase the maximum design temperature to 650°F to provide greater flexibility for future changes in operating requirements. This is possible without significantly altering the design of the vessel (i.e., the minimum required thickness for major components), because the maximum allowable design stress for most of the materials of construction is the same for any design temperature up to 650°F.
However, the pressure ratings for ANSI B16.5 flanges does change at temperatures below 650°F, and increasing the temperature to 650°F may necessitate upgrading the flanges to the next higher class. If an increase in cost is unacceptable or a lower flange rating is desired for matching piping flanges, the maximum design temperature can be decreased to the rated temperature for the lower flange class.
Maximum design temperatures above 650°F significantly affect the design. Much lower stresses are permitted by the ASME Code for temperatures above 650°F for most materials of construction. Therefore, the required thickness for pressurecontaining components increases with increasing design temperature above 650°F.
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