Most fired heater tubes are made of steel. The use of a steel at high temperature levels will result in creep or permanent deformation even at stress levels well below the yield strength of the material. The tube will eventually fail by creep-rupture, even when a corrosion or oxidation mechanism is not active. For steels at lower temperature levels, creep effects are negligible, indicating that under such circumstances the tube will last indefinitely unless corrosion or oxidation cause failure.
There are, therefore, two different design considerations for heater tubes:
1. At lower temperatures, in the elastic range, tubes are designed to prevent failure by bursting during a maximum pressure condition near the end of the design life, when the corrosion allowance has been used up.
2. At higher temperatures, in the creep-rupture range, tubes are designed to prevent failure by creep-rupture during the design life. Temperatures are discussed below.
API-530, “Recommended Practice for Calculation of Heater Tube Thickness in Petroleum Refineries” discusses these two design philosophies. Paragraph 2.3 shows the tube thickness calculations for elastic design (lower temperature) and Paragraph 2.4 shows the calculations for the rupture design (higher temperature).
Both methods need to be considered when specifying tubewall thickness in the transition temperature area.
API-530 also provides allowable stress charts for different materials, calculation of maximum tube skin temperatures and sample calculations.
Choosing Between the Low- and High-Temperature Calculations. The temperature which separates the elastic and creep-rupture ranges of a furnace tube is not a single value but is a range of temperatures which depends upon the alloy. For example, for carbon steel tubes the lower end of this transition temperature range is about 800°F; for Type 347 stainless steel the lower end of this transition temperature range is about 1100°F. In the temperature region where the elastic and creep rupture allowable stresses cross, both design equations must be used to determine which is the controlling design parameter.
The 1988 version of API-530 is located after this section of this manual. It is the user’s responsibility to obtain the latest version of API-530.
Steam Reforming Tubes. The rupture allowable stresses and the design equations in API-530 are appropriate for designing steam reforming tubes. The equation form using the inside diameter should be used since the inside diameter is an important process variable in steam reforming tubes.
Maximum Metal Temperature. The allowable stress curves in API-530 show a maximum metal temperature for each alloy. The maximum metal temperature is the upper limit of the reliability of the rupture strength data. There are other considerations which may require lower temperature limits such as oxidation, graphitization, carburization, hydrogen attack, or other effects. For these limits, refer to Section 700 of this manual, or contact a materials specialist.
Oxidation Limits and Maximum Temperatures. Oxidation limits and recommended minimum corrosion allowances are listed in Figure 400-2. The metal temperatures shown are the minimum recommended values for continuous service. Higher temperatures can be tolerated for limited periods of time, if consistent with strength requirements of the metal. Oxidation is generally not a factor for very short times such as are experienced during steam-air decoking. Consult a materials specialist to determine the time-temperature limits for any prolonged operation above the temperature limits given in Figure 400-2.
If the tube material is carbon steel and has welds, the weld areas have stricter temperature limits than those shown in Figure 400-2. The temperature of carbonsteel welds should not exceed 800°F. Carbon-½Mo welds should not exceed 850°F. These limits are set to avoid graphitization at the weld’s heat affected zone. Graphitization limits do not apply to Cr-Mo or austenitic stainless steel tubes.
Arbitrary Minimum Thickness. The stress thickness, ts, (Equations 2 and 4 in API-530) should not be less than the minimum thickness of Schedule 40 average wall pipe for ferritic steel (CS through 9 CR alloys) and not less than the minimum thickness of Schedule 10S average wall pipe for austenitic steels (SS, HP, HK, etc.). These lower limits are based on mechanical strength for tube handling considerations. If these limits apply, then design temperature or pressure can be increased rather than the corrosion allowance, to take advantage of the full thickness.
Operating Pressure. The rupture design pressure used in the design equation is usually the pressure at the mid-length of the tube, or
where D Pmax is the maximum pressure drop across the tube. This average pressure is used because the highest tube metal temperature occurs in the outlet half of the tube, where the pressure is lower.