Example of improper technique of insulating the external casing.
Failures in the superheater inlet duct can often be attributed to improper external insulation techniques. To properly insulate stiffened, hot casing, such as shown in Figure 1, even layering of insulation blanket is critical. If the field insulation contractor is inexperienced and not following well established technical specifications, they may be tempted to pack insulation into the areas between stiffeners and leave the tops of the stiffeners uncovered, or insufficiently covered.
Three-year-old Superheater inlet with inadequate insulation technique.
This condition allows the stiffeners to act as heat-wicking fin with a potentially enormous thermal gradient through the height of the stiffener. Where the stiffener is attached to the casing plate and well insulated, the metal temperature will be near the hot flue gas temperature and see the most thermal expansion. At the outer tip the stiffener will be much cooler with much less thermal expansion. The effects of which include wavy and distorted stiffeners, sometimes generating stresses high enough to break the stiffener-to-casing welds. The image on the right is a recent picture of an inlet duct where the tips of the stiffeners were not adequately insulated. Notice the waviness and distortion featured in some of the stiffeners.
Example of a good casing external insulation technique.
The advisable method is to contour the rolls of insulation blanket continuously over both the casing plate and the stiffeners, thereby creating an even insulation factor across the entire vessel. The image on the right is a depiction of good insulating technique. This results in a low temperature differential across the height of the stiffeners, thereby keeping stresses relatively low.