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Failure Analysis Case Histories
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Thermal Fatigue of Carbon Steel Pipe in Overhead Line
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ENVIRONMENT:
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Oil Refinery - Petroleum Products |
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EQUIPMENT:
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Fractionator Overhead in Fluid Catalytic Cracker |
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MATERIAL: |
42" Diameter Pipe - ASTM A-516, Grade 70 Carbon Steel |
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OPERATING TEMPERATURE:
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980F |
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SERVICE TIME:
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4 Years |
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FAILURE:
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Thermal Fatigue |
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This pipe failed due to cracking. Eight large, full penetration cracks, four
large cracks that had not yet made full penetration, and literally thousands
of small craze cracks which range in thickness from superficial to three
quarters (3/4) penetration were found, Figure 1. All the cracks were contained
in an area of about 8 to 10 inches wide and 15 feet long. |
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Typically, these pipes obtain a protective coating of coke during
operation. An on-side examination revealed that the area of cracking was devoid
of this coke build-up. Further, the cracks start immediately below a vent line
with a steam purge. Although the steam is not needed in the process, a small
flow was kept on to prevent clogging of the vent. |
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Analysis of the fractured surfaces revealed "beachmarks", Figure 2, typical of a fatigue failure. These are characteristic of interruptions in the propagation periods of fatigue fractures in relatively ductile metals.
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The steam entering the pipe was at approximately 300oF. The temperature differential between the steam and the process gases combined with a turbulent flow and/or variations in steam flow would cause the pipe to be either locally cooled by the steam or heated by the process gasses. This continuous heating and cooling is what induced the fatigue and eventually the cracks. |
