Page 152 - Wastewater Solids Incineration Systems

Heat Recovery and Reuse 119

Although problems and failures have been greatly minimized because of proper
design, heat exchangers still experience several material and mechanical failures.
Some of these are related to upset conditions, such as overheating incidents, which
can cause rapid oxidation, distortion, and cracking of the hottest parts, such as tubes,
tube sheets, and expansion joints.

4.1.3 Tubes
Austenitic Cr-Ni stainless steel such as type 304 typically possesses adequate resis-
tance to corrosion by hot flue gases, even those with high sulfur content. Alternating
oxidizing and reducing conditions may greatly accelerate corrosion by destroying the
protective oxide films that rapidly form on stainless steel in excess-air environments.
Reducing conditions also create the risk of afterburning of incompletely combusted
substances inside the bundle, which can lead to tube failure. This problem is best
addressed by better controls and tighter operating practices. Alloy 625 and alloy 20
provide added protection against chlorides and are often specified. Although more
costly, the added life is considered worthwhile and should be evaluated and used if
no standby capacity is available.
A common mode of tube failure is stress corrosion cracking (SCC) caused by
chlorides in the flue gas. Many stainless steels are susceptible to SCC; maximum sus-
ceptibility occurs when nickel content is approximately 8%. Using an alloy with no
nickel or one with a high nickel content (greater than 30%) results in significant
improvement to resistance.
Stress corrosion cracking occurs in austenitic stainless steels only when tensile
stress, chlorides in solution, and elevated temperature are present. During cool down
(shutdown), water condensation occurs and promotes chloride concentration.
During startup, moisture from the flue gas will condense on cooler surfaces. As the
equipment heats up, the liquid boils away and concentrates dissolved salts until the
liquid is saturated. Because it is impractical to prevent condensation formation, espe-
cially in cyclic operations, the solution typically lies in upgrading materials.
4.1.4 Tubesheets

The main concern with tubesheets is cracking of the highly stressed upper tubesheet,
particularly in the area of the tube attachment weld. Cracking may result from
thermal fatigue caused by repeated cycling with high thermal stresses or from high
temperature creep, or both. As with tubes, upgrading materials to alloy 625 or alloy
20 is justified in many instances.