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Frontiers of Mechanical Engineering

ISSN 2095-0233

ISSN 2095-0241(Online)

CN 11-5984/TH

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Front. Mech. Eng.    2018, Vol. 13 Issue (4) : 554-562    https://doi.org/10.1007/s11465-018-0513-y
RESEARCH ARTICLE
Optimal slot dimension for skirt support structure of coke drums
Edward WANG, Zihui XIA()
Department of Mechanical Engineering, University of Alberta, Edmonton T6G1H9, Canada
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Abstract

The skirt-to-shell junction weld on coke drums is susceptible to fatigue failure due to severe thermal cyclic stresses. One method to decrease junction stress is to add slots near the top of the skirt, thereby reducing the local stiffness close to the weld. The most common skirt slot design is thin relative to its circumferential spacing. A new slot design, which is significantly wider, is proposed. In this study, thermal-mechanical elastoplastic 3-D finite element models of coke drums are created to analyze the effect of different skirt designs on the stress/strain field near the shell-to-skirt junction weld, as well as any other critical stress locations in the overall skirt design. The results confirm that the inclusion of the conventional slot design effectively reduces stress in the junction weld. However, it has also been found that the critical stress location migrates from the shell-to-skirt junction weld to the slot ends. A method is used to estimate the fatigue life near the critical areas of each skirt slot design. It is found that wider skirt slots provide a significant improvement on fatigue life in the weld and slot area.
Keywords coke drum      stress analysis      cyclic stress      fatigue life      skirt slots     
Corresponding Author(s): Zihui XIA   
Just Accepted Date: 22 January 2018   Online First Date: 15 March 2018    Issue Date: 31 July 2018
 Cite this article:   
Edward WANG,Zihui XIA. Optimal slot dimension for skirt support structure of coke drums[J]. Front. Mech. Eng., 2018, 13(4): 554-562.
 URL:  
https://academic.hep.com.cn/fme/EN/10.1007/s11465-018-0513-y
https://academic.hep.com.cn/fme/EN/Y2018/V13/I4/554
Fig.1  Circumferential sandwiched plate skirt support structure
Fig.2  Integral skirt attachment design
Fig.3  Coke drum vessel and skirt dimensions (unit: m) and detailed dimensions of junction weld (unit: mm)
Fig.4  Important dimensions of considered skirt slot designs
Parameter Original slot value/mm Proposed slot value/mm
d 76.2 92.1
L 304.8 203.2
w 3.175 50.8
rk 9.525 25.4
s 101.6 101.6
Tab.1  Dimensions for considered skirt slot designs
Temperature, T/°C SA387-12-2 TP240-410S
Ea)/GPa Syb)/MPa Etc)/MPa CTEd)/(10−6°C−1) E/GPa Sy/MPa Et/MPa CTE/(10−6°C−1)
20 202.4 435 10714 12.3 178.0 272 13333 11.0
100 192.9 393 10333 12.8 175.8 270 9705 11.2
250 185.0 362 10000 13.6 161.1 220 11111 11.6
480 170.7 330 8441 14.7 161.5 188 6878 12.1
Tab.2  Material properties of SA387-12-2 and TP240-410S [5]
Fig.5  Cyclic symmetry cut boundaries (model domain)
Process stage Time/s h/(W·m−2·oC−1) Tb /°C P/kPa
Steam testing 7200 113.4 142 300
Vapor heating 7200 54.9 316 300
Oil filling 36000 141.0 482 300+ Psb)
Water quenchinga) 7200 345.0 93 300+ Ps
Unheading 5400 63.7 38 120
Tab.3  Prescribed boundary conditions for each process stage [7]
Fig.6  Temperature history over a complete operation cycle at inner skirt junction. ST: Steam testing; VH: Vapor heating; OF: Oil filling; WQ: Water quenching; UH: Unheading
Fig.7  Skirt deformation during the water quenching phase showing “vasing” effect (scaled by factor of 20)
Fig.8  Examined slot design effects on equivalent stress near top of skirt during quenching stage
Fig.9  Comparison of maximum plastic strain magnitude between examined slot designs
Design Equivalent stress/MPa Equivalent plastic strain/%
Maximum Range Maximum Range
NS 452.5 258.5 0.92 0.69
OS 369.5 256.3 0.17 0.14
PS 373.9 258.8 0.16 0.10
Tab.4  Junction inner surface stress and strain result summary
Design Equivalent stress/MPa Equivalent plastic strain/%
Maximum Range Maximum Range
OS 469.1 442.0 1.30 1.15
PS 403.3 390.5 0.66 0.63
Tab.5  Top keyhole stress and strain result summary
Design Deeff/% Salt/MPa N
NS 0.69 636.8 1343
OS 0.28 256.7 29332
PS 0.24 221.9 51138
Tab.6  Estimated fatigue life of junction weld
Design Deeff/% Salt/MPa N
OS 1.27 1174.0 282
PS 0.79 730.9 911
Tab.7  Estimated fatigue life of slot area
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