Cracking of SA213 Gr22 BFW Preheater Heat Exchanger tubes ends

摘要

This paper reports the premature failure of heat exchanger tubes after less than two years of service in Ammonia Plant. The investigation includes visual inspections, hardness measurements, and microscopic examinations. Material SA-213 T22 is a 2 1/4Cr-1Mo ferrite alloy seamless steel pipe used for heater Heat-Exchanger Tubes when process conditions require both high temperature corrosion resistant material and low sensitivity to creep phenomena. Tube bundle heat exchanger, employed to cool the synthesis gas coming from Ammonia converter, exchanges with boiler feed water (on the shell side) producing medium temperature/pressure steam. The high temperature Synthesis Gas rich of H2, NH3 and N2, requires metallurgy able to prevent brittle failure due to atomic hydrogen permeation into metal matrix. The main defense to this phenomena is employing correct materials (chemical composition) and less than 225 HB hardness. The failure observed has been fully described and examined in view of the unusual location where this phenomenon occurred. The failed Preheater tube shows intergranular cracks and micro crack starting from the tube end. The failure mechanism is brittle intergranular crack ascribed to a High Temperature Hydrogen Attack (HTHA) also known as Hydrogen Assisted Cracking (HAC).