Page 153 - Wastewater Solids Incineration Systems

120 Wastewater Solids Incineration Systems

4.1.5 Expansion Joints
The most common location of heat exchanger failure in earlier incinerator service was
at expansion joints. The bellows in such joints must not only face repeated cycling
loads, but are thin to permit flexing and are highly stressed. The most prevalent
failure mode has been stress corrosion cracking from chlorides. As with tubes, it is
virtually impossible to prevent the formation of corrosive condensate throughout the
entire operating cycle. Consequently, the only practical solution with chloride-rich
feeds is to use construction materials of higher quality. Because bellows materials
have been upgraded, failures are now uncommon. Design improvements minimize
leakage in the event of bellows cracking so that the effect on operation is minimized.

4.1.6 Materials Recommendations
As discussed in the preceding section, upgrading materials from conventional stain-
less steels of the type 300 series may be necessary in special situations, most often to
provide improved resistance to corrosion cracking from chloride stress. Unfortu-
nately, it is difficult to formulate strict guidelines. In general, experience indicates
that the frequency of problems with stainless steels increases considerably when
chloride levels in the flue gas reach approximately 100 ppm and becomes progres-
sively worse with increasing amounts of chloride.
For chloride levels greater than 100 ppm, intermediate alloys such as alloy 20
800H, and type 825 have been used with success. For higher chloride levels, greater
than 1000 ppm, alloy 625 is required. Hot tube sheets follow the same pattern and
should be compatible with welding the tubes to the sheets. The lower tube sheet is
typically composed of carbon steel. It is sufficiently cool and protected from flue gas
exposure by refractory and insulation so that failures are rare.
Expansion joints are now almost exclusively furnished with alloy 625 bellows,
which effectively resist corrosion cracking from chloride stress because of their high
nickel content. Although this alloy is costly, the overall increase is limited by the
small amount used in the thin bellows. This extra cost is easily justified when com-
pared with the high total cost of a typical heat exchanger and the additional life that
can be obtained by using this alloy.

4.1.7 Long-Term Operation
Unfortunately, nothing lasts forever. The typical operating environment for an FGTT
air preheater is severe: extreme high temperatures, a corrosive and erosive environ-
ment, and cyclic operation. For a typical air preheater designed for a 650°C (1200°F)