Another key to preventing high stress events in hot casing is to keep water outside the external lagging. Water and moisture coming in contact with hot casing thermally shocks the casing metal which can cause numerous cracks to form quickly. The unit shown in Figure 3 suffered from rain water exposure in addition to the inadequate insulation technique.
Superheaters are often lagged with lightweight corrugated sheet metal. At a minimum, the lagging should be installed so the over-lapping seams are orientated in a way that prevents driving rain from penetrating under the lagging. This may be difficult to achieve on the flat “roof” of the inlet duct where water can pool and seep through the seams. Switching to a heavier lagging with taller corrugations that provide a “standing seam” configuration can help protect against roof water.
To achieve maximum protection from water exposure, some plants have installed rain shields to cover the top and sides of the superheater. Even if there is only room enough to rain shield the inlet transition duct and front portion of the superheater, benefits can be well worth the expense.
If you operate or maintain a high-temperature superheater in your sulfuric acid plant, it may be beneficial to ensure the casing stiffeners are properly insulated, and the unit is adequately protected from environmental water.