Creep and stress rupture are high-temperature mechanisms that depend on both the stress level and type of material. At high temperatures, metal components can slowly and continuously deform under load below the yield stress. Creep is defined as the time dependent deformation of stressed components under an applied load below the yield strength at the operating temperature of the material. Stress rupture is similar to creep except that the stresses are higher and the time to failure is shorter than those used for creep. Stress rupture failures are typically short-term failures while creep failures are typically long-term failures. Tubes that have been in-service for long periods of time can fail by stress rupture if the operating conditions are increased significantly during operation (e.g plugging of burner tips or readjustment of burners promoting flame impingement). Tubes are exposed to triaxial stress which are a combination of tube “hoop stress” due to the operating pressure, and longitudinal stresses from inadequate tube supports or inappropriate design/construction, which causes a localized high stress. Creep and stress rupture are described in API RP 571.
The metal temperature plays a major role in the type and severity of the deterioration of the heater tubes. The metal temperature of individual tubes or along the length of any specific radiant tube of a given heater can vary considerably. The principal causes of abnormal variation in metal temperature are internal fouling of the tubes which insulates the tube wall from the process and improper or poor firing conditions in the heater. Some potential signs of creep in tubes are:
a. Sagging. Excessive sagging is usually because of a decrease in the structural strength of the tube caused by overheating. It may also be caused by improper spacing of hangers, uneven metal temperatures, or failure of one or more tube supports or hangers.
b. Bowing. Excessive bowing is generally caused by uneven metal temperatures, which may be due to flame impingement or coke accumulation inside the tube. Heating on one side of the tube causes greater thermal expansion on the hotter side and bowing toward the heat source. Bowing may also be caused by binding of the tube in the tube sheets or improper suspension of the tube so that longitudinal expansion is restricted or by the use of improper tube lengths when individual tube replacements are made.
c. Bulging. Bulging is generally an indication of overheating. Continuing under the same temperature and stress conditions will eventually lead to creep and stress rupture. The amount of bulging varies with the specific metal and the type of damage, creep or overstress. If the bulge is attributed to overstress (short-term overheating), and the temperature and stress have been returned to normal, typically the life of the tube has not been reduced. Creep life will be reduced, if the bulge is the result of creep damage (long-term overheating). Bulging is considered more serious than sagging or bowing.