Air preheaters are subject to corrosion due to condensation during extended periods of downtime. Recuperative preheaters, both the tubular type and the plate type, are subject to severe corrosion when the element temperature is at or near the dew point. The severe corrosion is particularly prevalent at the cold air end. As much as possible of the recuperative-type preheaters should be inspected for corrosion. Usually, the conditions at the inlet and outlet ends will provide a good indication of what can be expected in the remainder of the preheater. It is not unusual to see extensive plugging of air preheaters when boilers are being fired with heavy oil.
Frequently, air preheater efficiency can be calculated to determine if its surface area is fouled or damaged. A heat balance can be performed around the air preheater to determine if it is leaking from the air-side into the flue gas-side. The quantity of leakage can be determined by measuring the oxygen on both sides of the air preheater. Fouling or other damage could be determined by measurement of the flue gas pressure differential across the air preheater.
Perforated tubes should be replaced or plugged. It is sometimes necessary to remove fairly good tubes or plates to get to the bad ones. Good judgment and consideration for future replacements are important factors in selecting the most economical method for repairing tubes and plates.
Regenerative preheaters require a more extensive inspection than do recuperative preheaters. Usually, rotating elements must be removed to clean the preheater. This affords an opportunity for close inspection of all parts. In most classes of regenerative preheaters, the incoming air enters at the same end that the flue gases leave, thus cooling that layer of rotor segments first. Corrosion will generally start at this point because of condensation and proceed toward the other end of the unit. Most preheaters have two sections, and if corrosion at the flue-gas exit ends is not too severe, the sections can be reversed; otherwise, new sections should be provided.
Rotor seals should be examined for corrosion. They can also be mechanically damaged by falling material, by high-pressure steam or water from soot blowers, or by being stepped on by maintenance personnel.
Soot blowers for regenerative preheaters are quite different from those used in other parts of the boiler. Manufacturers’ catalogs and drawings should be examined for points that require close inspection. Soot blowers should be inspected for deposits and leaky valves. Leaky valves and buildup of ash cause corrosion of nozzle tips, and subsequent malfunction of the blowers damages rotor seals and segments. Therefore, steam inlet valves should be inspected for tight shutoff, and drain valves should be inspected for correct operation.