Abstract
After 39 years of operation, the orifice plate in a black liquor incinerator reactor has seen over 250,000 hours of exposure to operating temperatures in the range of 480-595°C (900-1100°F), and has cracked in numerous locations. This article describes the results of a metallurgical evaluation of plate samples removed from the unit. Although a 0.5Cr-0.5Mo high-temperature alloy was used for the orifice plate, this evaluation showed that the orifice plate had undergone irreversible elevated temperature degradation of its mechanical properties, in addition to the cracking observed. High temperature degradation mechanisms found in the plate include: creep, thermal fatigue, surface cracking, grain growth, temper embrittlement, and low toughness at temperatures below 85°C (185°F). A fracture mechanics evaluation of the plate subjected to the operating stresses and the effects of the existing cracks showed that static loads alone would not induce brittle crack growth and the plate would not collapse. However, below 85°C (185°F) impact loads could be high enough to trigger brittle fracture and a collapse of the orifice plate. Impact loads could originate from process material build-up falling from walls of the reactor chamber or by a power loss where the fluidized bed suddenly rests on top of the plate.
