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Thermal Fatigue of Carbon Steel Pipe in Overhead Line

ENVIRONMENT:

Oil Refinery - Petroleum Products

EQUIPMENT:

Fractionator Overhead in Fluid Catalytic Cracker
MATERIAL: 42" Diameter Pipe - ASTM A-516, Grade 70 Carbon Steel
OPERATING TEMPERATURE: 980F

SERVICE TIME:

4 Years

FAILURE:

Thermal Fatigue
 

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.

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.

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.

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.

 
Figure 1. Cracks on the pipe ID (0.6X Original Magnification) Figure 2. Fracture surface showing "Beach Marks" (7X Original Magnification)

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