Typical Superheater inlet transition duct
Eliminating unnecessary downtime for critical process equipment is always at the forefront of any plant operator’s mind. While plant components such as converters, burners, pressure vessels, piping, and valves are obvious components to monitor and regularly inspect, other aspects of the plant can be easily overlooked. High temperature superheaters, boilers, and economizers and their external insulation is one key design feature that can easily go over-looked and unchecked. In recent years, we have seen problems arise with casing and stiffener cracks as well as stiffeners warping, all due to either improper insulation blanket installation or inadequate rain/water protection.
Over the past five years or so, Optimus has visited several sulfuric acid plants during their maintenance outages to investigate instances of casing cracks in the inlet transition ducts of high-temperature superheaters. These are superheaters downstream of the sulfur burner that receive flue gas exiting the first converter pass at about 1155°F. Some of the installations were more than 15 years old, but a couple were less than five years old and, as such, the casing cracks were unusually premature. Experienced plant maintenance personnel have provided useful insight as to what environmental factors can cause such failures, and our own 20+ years of experience with such equipment helped us identify some root causes.
The Image on the right depicts a typical superheater inlet transition duct which, due to the design conditions, feature 304H stainless steel casing and external stiffeners that are six to seven inches tall.