Internal
1. Waterlines. Tubes. Due to oxygen and organic materials during boiling process.
2. Vicinity of boiler water dicharge resulting in oxygen release.
3. Top of lower tube sheet because of scale formation.
4. On and arround of stay bolts as a result of stresses imposed and the subsequent expansion and contraction (stress corrosion).
5. In water legs especially at the bottom - pitting scale.
External
1. Top of tube sheet, bottom tube sheet and tube ends. Acid formed.
2. Arround manhole, handhole and washout openings. Due to leakage and thermal expansion.
3. At the bottom shell, water legs and furnade sheet as a result of soot attack.
4. Arround all openings including valve, steam connection, feedwater connection, blowdown, water column and leakage.
For reasons of efficiency, more and more condensing boilers are being selected for replacement and new construction.
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