ABSTRACT

Samples of carbon steel tubes were removed from the radiant section of a fired heater after 40 years operation at 950oF to 1000oF (510oC to 538oC) tube metal temperature. The tubes exhibited significant microstructural degradation in the form of advanced pearlite decomposition and graphitization (graphite nodules). However, no evidence of linking of graphite nodules was observed. This paper highlights the results of an API 579 Fitness for Service analysis using the MPC Omega creep methodology to help show that the tubes have significant remaining creep strength and are suitable for continued service, contrary to the results of an API 530 analysis1,2.

INTRODUCTION

The Hydrodesulfurizer (HDS) rerun column feed heater described in this paper was originally installed in the 1950?s and is configured with vertical carbon steel tubes of diameter NPS 6 in the radiant section (Figure 1). In June 2003, the heater was opened for inspection during a planned turnaround.

The four outlet tubes (#11, #22, #33, #44) were found to be severely bowed and oxidized and were upgraded to 1.25Cr material. The remaining carbon steel tubes showed minor scaling due to oxidation, and no significant loss in thickness was measured. Tube strap measurements showed 1 to 2% diametrical growth in the remainder of the furnace tubes.

Although no significant signs of visible damage were evident on the remaining tubes, the refinery was concerned with continued operation of the heater considering the long term history of operation heater at elevated temperature, well beyond the original 100,000-hr design life. An important consideration is that carbon steel is susceptible to a form of damage known as graphitization in which the microstructure of the steel forms graphite nodules (described in more detail below) after extended operation of carbon steel above 800oF (427oC). This form of microstructural degradation also reduces creep life.

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