ABSTRACT

This paper explores the various degradation mechanisms experienced by a refinery’s RFCC catalyst cooler aeration piping system. A detailed analysis of the most recent failure and a past failure was conducted to determine the various metallurgical and mechanical degradation mechanism(s) that led to these failures. The analysis which covered both the old UNS S30409, TP304H stainless steel, and newly upgraded UNS S34709, TP347H stainless steel catalyst cooler aeration piping system, with the same design, showed the dominant degradation mechanisms to be low cycle thermal fatigue and low cycle fatigue- creep for the TP304H and TP347H stainless steel aeration system’s header and sub header. The TP304H aeration system failed after three years in-service and the TP347H aeration system after six months in-service. The proposed solutions to prevent recurrence include a design change of the aeration piping system, stricter quality control during fabrication, installation of liquid knock-out drums to prevent water ingress resulting in thermo-mechanical strains and a return to the TP304H metallurgy for the aeration system pending a detailed investigation into the TP347H failure.

The paper also explores the technical reason(s) for the metallurgy upgrade from TP304H to TP347H and how this change affected the aeration piping’s service life.

INTRODUCTION

A refinery’s Residue Fluid Catalytic Cracking Unit (RFCC) experienced repeated failures of its Regenerator Catalyst Coolers’ aeration piping system. The RFCC Catalyst Coolers are external vertical shell-and-tube type heat exchangers. In the present unit, two Catalyst Coolers, 1 and 2, are located on either side of the Regenerator. The purpose of the catalyst coolers is to reduce the temperature of the regenerated catalyst by generating of steam from the circulating boiler feed water on the tube side of the exchanger. During operation, regenerated catalyst flows over the entire cross sectional area of the tube bundle. The aeration system provides uniform air distribution on the shell side of the catalyst cooler exchangers thereby fluidizing the catalyst to allow for better catalyst flow and a uniform heat transfer coefficient. A catalyst cooler failure results in a forced refinery stoppage/shut-down for repairs. The primary location failure is at the weld joints of the aeration piping system due to various stresses and strains acting on the aeration system. The catalyst cooler failures follow a general sequence of events, as described below:

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