PSI - Issue 3

J.L. González et al. / Procedia Structural Integrity 3 (2017) 33–40 Author name / Structural Integrity Procedia 00 (2017) 000–000

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The data in table 1 shows that the displacements are result of combined vertical tilt and horizontal movements (referred here as lateral); the worst case in lateral displacement was drum 3, but the drum 2 has the greatest tilting, since it is where there is the most crack opening height. In terms of the directions of the displacement, the distribution is fairly random. 2. Analysis of the mechanical behavior of the cracked skirt junction The results of the hardness measurements indicate that the ASTM A 387 Gr. 11 CL2 steel plates of the skirt still met the tensile strength requirement, since the average measured hardness values of 156 HBN correspond to a tensile strength of 78 ksi, according to the table F.1 of the API579/ASME FFS-1 2007 standard [API570/ASME FFS-1 (2007)], which is greater than the minimum specified tensile strength of 75 ksi. Therefore there is no loss of strength of the skirt fabrication steel. Regarding the bulging, its maximum measured value was equivalent to a 0.6% Out-of-Roundness; this is less than the maximum allowed of 1.0 %, as required by the Subsection A, Part UG-80 of the ASME Section VIII Division 1 code [Code ASME (2001)], therefore it is acceptable, as for a Level 1 fitness for service assessment of Part 8 of the API579/ASME FFS-1 2007 standard. Based on the previous results, an analysis of the mechanical failure risks that could arise from the skirt junction cracking indicated that the most likely scenario of failure is the plastic collapse of some sections of the skirt caused by local high stress arising from the uneven vertical load distribution of the drum on the fractured edge of the skirt. The uneven vertical load distribution is schematically illustrated in the Figure 5 and is caused by the lateral and vertical displacements of the drum, which is free to move because of the fracture of the skirt junction, thus reducing the contact area of the mating surfaces of the cracked edges.

Fig. 5. Schematic representation of the formation of high stresses zone in the coker drum skirt due to the misalignment of the skirt junction.

Because of the very large diameter to thickness ratio of the skirt (ID/t = 334), even relatively small displacements will cause a dramatic reduction of the contact surface area, as shown in Table 2.

Table 2. Contact surface area of cracked edges of the skirt junction as a function of the lateral displacement.

Lateral displacement (mm)

Contact surface area of cracked edges (mm 2 )

0.0 2.5

683,010 639,528 465,601 356,897 291,675

12.3 19.1 22.9