Creep Rupture and Thermal Shock Failures of Nickel-based Alloy Reformer Tubes: A Comparative Analysis

Abstract

This paper examines the failures of radiant tubes within a fertilizer plant's steam reforming unit between the creep rupture and thermal shock, encompassing discussions on the characteristics of the failures, microstructural analysis of the failures, the mechanisms of degradation at high temperatures, and forthcoming preventive actions. The primary reformer is where the initial stage of the steam reforming process occurs, involving the downward passage of a preheated hydrocarbon and steam mixture through radiant tubes containing catalysts. The resulting reformed gas, enriched with carbon monoxide, is further processed into ammonia- rich synthesis gas. These radiant tubes, crafted through centrifugal casting and primarily composed of 35% nickel and 25% chromium, operate at temperatures of 9100C. The discovery of a leaking radiant tube necessitated an unscheduled shutdown, with the failure manifesting as an elliptical window-shaped rupture situated 590 mm from the tube's upper end. The affected tube was subsequently removed, and the outlet weldolet was sealed. During an online refractory repair operation, a catalyst tube exhibited a circumferential crack at a location 770 mm from the tube's upper end. At the same time, another catalyst tube was also observed with a longitudinal crack located at 5130 mm from the tube’s upper end. To address these issues, both tubes were isolated from service through crimping at the top inlet hairpin and bottom outlet pigtail. Detailed metallurgical and failure analyses unveiled that one of the failures was initiated by intergranular cracking at the outer diameter, associated with thermal shock-induced cracking, followed by fatigue-driven crack propagation due to thermal stress and cyclic loading. Meanwhile, the other failed tube identified creep as the failure mechanism due to a combination of time, temperature and stress under such extreme operating conditions. The paper further explores the similarities and differences between these two radiant tube failure incidents and associated failure morphology and mechanisms.