API STD 530-2004 pdf download.Calculation of Heater-Tube Thickness in Petroleum Refineries.
3 Terms and Definitions For the purposes of this document, the following terms and definitions apply. 3.1 actual inside diameter D i Inside diameter of a new tube. NOTE The actual inside diameter is used to calculate the tube skin temperature in Annex B and the thermal stress in Annex C. 3.2 component fitting Fittings connected to the fired heater tubes. EXAMPLES Return bends, elbows, reducers. NOTE 1 There is a distinction between standard component fittings and specially designed component fittings; see 5.9. NOTE 2 Typical material specifications for standard component fittings are ASTM A234/A234M  , ASTM A403/A403M  , and ASTM B366  . 3.3 corrosion allowance δ CA Additional material thickness added to allow for material loss during the design life of the component. 3.4 design life t DL Operating time used as a basis for tube design. NOTE The design life is not necessarily the same as the retirement or replacement life. 3.5 design metal temperature T d Tube-metal or skin temperature used for design. NOTE This is determined by calculating the maximum tube metal temperature (T max in Annex B) or the equivalent tube metal temperature (T eq in 3.8) and adding an appropriate temperature allowance (see 3.16). A procedure for calculating the maximum tube metal temperature from the heat-flux is included in Annex B. When the equivalent tube metal temperature is used, the maximum operating temperature can be greater than the design metal temperature.
When the equivalent tube metal temperature is used to determine the design metal temperature, this design metal temperature is only applicable to the rupture design. It is necessary to develop a separate design metal temperature applicable to the elastic design. The design metal temperature applicable to the elastic design is the maximum calculated tube metal temperature among all operating cases plus the appropriate temperature allowance. 3.6 elastic allowable stress time-independent allowable stress σ el Allowable stress for the elastic range. See 6.2. 3.7 elastic design pressure p el Maximum pressure that the heater coil can sustain for short periods of time. NOTE This pressure is usually related to relief-valve settings, pump shut-in pressures, etc. 3.8 equivalent tube metal temperature T eq Calculated constant metal temperature that in a specified period of time produces the same creep damage as does a changing metal temperature. NOTE The equivalent tube metal temperature concept is described in more detail in 5.8. It provides a procedure to calculate the equivalent tube metal temperature based on a linear change of tube metal temperature from start-of-run to end-of-run. 3.9 inside diameter Inside diameter of a tube with the corrosion allowance removed; used in the design calculations. NOTE The inside diameter of an as-cast tube is the inside diameter of the tube with the porosity and corrosion allowances removed. 3.10 minimum thickness δ min Minimum required thickness of a new tube, taking into account all appropriate allowances. NOTE See 5.4, Equation (5). 3.11 outside diameter D o Outside diameter of a new tube. 3.12 rupture allowable stress time-dependent allowable stress σ r Allowable stress for the creep-rupture range. See 5.4.
3.13 rupture design pressure p r Maximum operating pressure that the coil section can sustain during normal operation. 3.14 rupture exponent n Parameter used for design in the creep-rupture range. NOTE See Figures E.2 through E.65 and Tables E.1 through E.22 (and Figures F.2 through F.65 and Tables F.1 through F.22). 3.15 stress thickness δ σ Thickness, excluding all thickness allowances, calculated from an equation that uses an allowable stress. 3.16 temperature allowance T A Part of the design metal temperature that is included for process- or flue-gas mal-distribution, operating unknowns, and design inaccuracies. NOTE The temperature allowance is added to the calculated maximum tube metal temperature or to the equivalent tube metal temperature to obtain the design metal temperature (see 3.5).