Page 154 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 121

air outlet temperature, the normal operating temperature of the tubes and associated
parts at the hot end is approximately 760°C (1400°F). This situation means the tubes
and tubesheet will be glowing red hot in normal operation. At this temperature, the
short-term strength of type 304 stainless steel is only approximately one-fourth of
that at room temperature. But at 760°C, short-term strength cannot be used for design
purposes because of creep.
Creep is the effect of slow extension of a metal under a tensile load at high tem-
peratures. Crack-like voids are created in the microstructure as grain boundaries
slowly move, and eventually the material breaks or ruptures at loads much less than
the tensile strength or yield strength. Creep strength is reported as a stress at a given
temperature to produce rupture in a certain period of time.
In general, once the temperature is greater than approximately 540°C (1000°F)
and a tensile load is present, the metal has a finite life and will eventually fail. The
creep life of the material is used up with time at high temperature. Similarly, the
fatigue life of a material is expressed as the number of cycles to failure at a given level
of alternating stress. Cracks grow in the metal with each cycle until the part eventu-
ally fails. Unfortunately both creep and fatigue create crack-like defects in the
microstructure, and the combined effect is greater than either one alone. Every hour
at high temperature and every cycle consumes some of the lifetime of the equipment.
Corrosion and erosion compound the problem.
Excursions or operation at higher temperatures have an exponential effect on both
creep rupture life and fatigue life. For an air preheater designed for 650°C (1200°F) pre-
heat, if it is operated at 700°C (1300°F), then the creep rupture life of the material will
be typically consumed at a rate more than ten times faster. This means the creep life-
time is reduced to only 10% of the creep lifetime at 650°C preheat. Figure 6.6 illustrates
this reduction in life versus temperature.
Failure eventually will occur by cracking. Cracking has been observed in tubes
near the tubesheet weld, expansion joints, and, more recently, in the support. A spe-
cific pattern of failure by type or area does not exist.
Timely inspection and repair of cracks by welding can extend the useful life of
the equipment, except for expansion joints. A typical unit can last approximately
10 years, but some units have lasted much longer and others have lasted a shorter
time. Units that operate with higher preheat temperatures than the design preheat
temperature usually have shorter lifetimes.
Newer units can be provided with an integral air-side bypass to control the air
preheat temperature. Cold air is bypassed around a portion of the tube bundle but