Remaining Life Predictions for Reformer Catalyst Tubes

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By John Brear at ERA Technology Ltd., John Williamson at ERA Technology Ltd., and Bobby Wright, P.E. at Stress Engineering Services. This article appears in the May/June 1997 issue of Inspectioneering Journal

Reformer catalyst tubes are commonly manufactured from high strength, creep and corrosion resistant alloys. They are relatively thick walled and are usually produced by centrifugal casting. Their lives are limited by creep, driven by a combination of internal pressure and through-wall thermal stresses generated by operational transients. The typical composition for alloys for reforming service is 25/20 Cr/Ni with the generic reference HK40.

Tube design is ordinarily based on pressure stresses, conservative outside wall temperatures and lower bound materials rupture lives. While this is sufficient for the achievement of design life, it does not provide a basis for remaining life assessment. Life prediction by an inverse design process using actual materials properties and service conditions results in highly optimistic estimates of future operational capability which are not borne out by service experience This is due to the effects of thermal stresses resulting from normal cyclic operation and from other transients, such as process upsets. Realistic life assessment must therefore take account of the full range of operational behavior - transient and process upsets as well as steady state.

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