Design Information Required
API-530 requires the user to specify certain design information: 1) design life, 2) whether the equivalent temperature shall apply, 3) temperature allowance, 4) corrosion fraction if special, and 5) whether elastic range thermal stresses shall apply. (See API-530 Paragraph 1.2.) Normal Company practice for each of these is listed below.
Design Life — (operating time in hours used as a basis for tube design). Normal Company practice is to use a 100,000-hour design life for furnace tubes. A 100,000-hour design life is also specified in HTR-MS-1350. If not specified by the user, API-530 defaults to a 100,000-hour design life. Hence, if this design life is to be used, it need not be specified. Other design lives, however, may be appropriate in special circumstances. API-530 includes provisions for using other design lives, and they must be specified to the heater designer if they are other than the usual 100,000-hour design life.
Equivalent Temperature — (calculated constant metal temperature which, in a specified period of time, produces the same creep damage as a linearly changing metal temperature). Normal Company practice is not to use this concept.
With API-530, it will not be used unless the user specifies that it should. The concept should be considered, however, for tubes designed to operate in the creep rupture range and which will operate over a wide (100°F or more) temperature range from start-of-run to end-of-run. Using the concept can produce a design temperature less than the maximum operating temperature. This may be hard to explain on a Safety Instruction Sheet. The concept is discussed in Paragraphs 2.8 and B.5 of API-530.
Temperature Allowance — (added to the calculated maximum tube metal temperature in order to allow for process or flue gas maldistribution, operating unknowns, and design inaccuracies). Normal Company practice is to use a 25°F temperature allowance for gas and naphtha fired heaters and 50°F for oil fired heaters and hydrogen reforming furnaces. The higher temperature allowance recognizes the greater possibility for flame impingement and heat flux non-uniformity in such furnaces. If not specified, API-530 defaults to a 25°F temperature allowance. Therefore, for oil fired heaters, a 50°F temperature allowance must be specified for API-530. A 25°F temperature allowance should also be used for the design of convection section tubes.
Corrosion Fraction — (for definition see API-530 Appendix B). Normal Company practice is to use the corrosion fraction, and API-530 defaults to using the corrosion fraction if not specified.
Thermal Stresses — (allowable stress for the elastic or creep- rupture range in psi). Normal Company practice is to apply the elastic range thermal stress limits. If not specified, API-530 defaults are used.