Failure Knowledge Database / 100 Selected Cases

Explosion and fire caused by the breakaway of the cover platefrom the heat exchanger of the desulfurization equipment
Oct. 16th, 1992, Sodegaura-city, Chiba prefecture
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
An explosion and fire occurred at the Sodegaura-refinery of Fuji Se kiyu Corporation. Ten people died,and se ven people w ere i njured. Th e lock ri ng of th e co ver pl ate ( channel cover) of a BL C type h eat
exchanger was broken off, and the channel cover, the lock ring, and other parts were blown off and collided
with each other. Finally, an explosion and fire occurred that were caused by the leak of the hydrogen gas
from the heat exchanger.
1. Component
The BLC (Breech - L ock - Closure) type heat exchanger of t he desulfurization equipment (No.2 VGO
isomax equipment) for heavy oil
2. Event
For the purpose of rep lacing t he c atalyzer, the he at exchanger w as s topped o n O ctober 1. It res umed
operation on October 16. The hot-bolting process for the heat exchanger (E-2801A) began at about 15:30,
and it finished at about 15:42. After that, the hot-bolting process for the other heat exchanger (E-2801B)
began. At about 15:47, smoke was observed rising near the detection hole at the upper part of E-2801B, andthe process was stopped. At about 15:52, an explosion and fire o ccurred near E-2801B, the explosion andfire disaster occurred. Excluding the human damages, the cost of the physical damage was about 2.4 billionyen.
In particular, the channel cover, the lock ring, and other parts flew over 130 meters to hit and destroy anoil tank and pipes of Nippon Kougyou Corporation, located next to the Fuji Sekiyu Sodegaura-refinery.
3. Course
The damaged heat exchanger was a BLC type licensed by CHEVRON RESEARCH USA CO LTD andmanufactured by Chiyoda Kakoukensetsu Corporation.
Figure 1 shows the structure of the BLC type heat exchanger. Figure 2 shows the details of the airtightdesign. The pressure exerted on the channel cover of the heat exchanger is supported by the screw thread of
the channel barrel via the lock ring. The airtightness between the inside and outside of the heat exchanger iskept by gaskets through the torque of the bolt which fastens the lock-ring. The piping area and the bodyarea are separated by the tube sheet. The separation between the piping area that is at high pressure and thebody area that is at low pressure is kept airtight by gaskets through the torque of the channel cover set boltthat is applied through the special partition.
1

Failure Knowledge Database / 100 Selected Cases

Generally, hot bolting is required at maintenance of heat exchangers which are used under conditions ofhigh pressure and high temperature because the inside fluid tends to leak out with increases of pressure andtemperature when t he exchangers st art t o work. I n t he flange type heat exchanger, the bolts are big, andthere are many parts ne ar t he fla nge. T herefore, maintenance is d ifficult. On the other ha nd, i n the BLC
type heat exchanger, the bolts are small and all bolts are located in the front end of the heat exchanger. So,
maintenance is easier.

E-2801B was manufactured in February 1975. Maintenance was performed at sixtimes betw een 19 75 and 1 991. In t he l atest m aintenance operations (Jun 19 98 and Ju n 1 991), the heatexchanger w as moved to t he factory of Chiy oda-Protech Cor poration in K awasaki, an d th e maintenance
was perform ed there. T he a ccidents had a gre at i nfluence on so ciety, so an accid ent i nvestigationcommission w as estab lished. In the c ommission, t he ca uses of t he a ccident, methods t o p revent t herecurrence of the accidents were discussed, and a report was published.
4. Cause
The causes of the accident are as shown below.
1)

Although t he diameter of the g asket reta iner (dis k-shaped, SU S321) tha t w as set at t he b ack of thechannel cover to keep it air tight was being reduced by the repeated ratcheting, it was not corrlectlyreplaced.

2)

The thermal deformation of the inner parts of tu be area was absorbed by the destruction of the ti p of
the internal flange bolt set (10). However, the internal flange set bolts were n ot adequately replaced.
Therefore, t he loa d on th e channel c over set bolts (1 7) w as incre ased, and t he lock ri ng w as b ent.
Gradually, the diameter of the lock ring decreased.

3)

There were two main causes for t he increase of the diameter of t he channel barrel (1). O ne was thethermal defor mation caused by t he difference of temperature res ulting from t he rem oval o f theinsulation. The other was the variation of the inner pressure caused by the leak of hydrogen gas.

4)

Through the combination of the decrease of the l ock ring's diameter and the i ncrease of t he channelbarrel's diameter, the overlap of the screw threads was decreased. Finally, the lock ring was broken by
the plastic deformation of the top of the screw threads which were set in front of the channel barrel.

5)

The br eak off of th e lo ck r ing caused the lo ck ri ng (9 00kg) a nd the cha nnel c over (2000 kg) to fly
through the air. In addition, leaking hydrogen gas caused an explosion and fire to occur.
Figure 3 shows the faul t tree that is fo cused on the fracture m echanics and frac ture process. Figures 4and 5 s how the f ault trees focused on t he m aintenance. Fi gure 6 s hows t he event tree t hat describes t heseparation of the cover plate of the heat exchanger.
5. Countermeasure
The Ministry of International T rade and Industry a dvised counterm easure t o a ll of t he corporations
using the same heat exchanger, all petrochemical complex corporations including Fuji-sekiyu Corporation,and t he m akers of the dam aged he at e xchanger. A dditionally, th e H igh Pressure Gas Safety Instit ute ofJapan held a brief session about this accident.

2Failure Knowledge Database / 100 Selected Cases

6. Knowledge
The design of pressure pr oof parts is based on the concept of el astic design. However, the concept ofplastic design is required to design some parts. Either 1) fatigue and ratchet analysis to certify the lifetimesof these parts or 2) r egular part replacement are required. In these e xchangers, the gasket retainer and theinternal flange bolts are particularly relevant.
For regul ar re placement of parts, a basel ine a nd adequate m aintenance m anagement are requ ired. Inparticular, it is not clear who is responsible for the decision and confirmation of the parts replacement, theowner or the maker of the heat exchanger. The circumstances of owner and the convenience of maintenanceengineer often lead to accidents.
7. On the Side○ Gask et retainer
In general, the cover of a bottle is held to the bottle by the meshing of a screw cutting on the outside ofbottle's mouth an d o n t he insi de of th e bot tle's cove r. H owever, i n th e BLC ty pe h eat exchanger, thesituation is opposite. There is a screw c utting on the outside of the lock ring, which acts like the bottle'scover, and there is a screw cutting on the inside of the channel barrel's cup, which corresponds to the mouthof the bottle. In medicine bottles, there is a thin disk on the inside of the cover that is outside of the screw
cutting. This disk keeps the bottle airtight. The retainer for the BLC heat exchanger is located on the insideof t he s crew cutting. Although t his sit uation is o pposite from that of th e m edicine b ottle, t he r ole of t hegasket retainer is also to keep the heat exchanger airtight.
○ Ra tchet and ratcheting
The gear that is used t o ti ghten t he stri ngs of a tennis racke t is c alled a "ratc het". Deform ation thatoccurs to one way is cal led "Rat chet d eformation". O n co ndition t hat tensile str ess is ex erted to a rod ,tensile plastic strain in the direction of the tensile stress is accumulated by repetition of thermal stress andtensile plastic deformation. On the other hand, when compression stress is exert ed to a rod, compression
plastic strain is accumulated in the same direction. This is called "Thermal Ratcheting". Compression stressis performed to gasket retainer of which shape is disk. Then compression plastic strain is accumulated byrepetition of thermal stress. In the result, diameter of it decreases.
8. Primary Scenario
01. Misjudgment
02. Misjudgment of Situation
03. Organizational Problems
04. Poor Management
05. Usage
06. Maintenance/Repair
07. Parts Replacement
08. Incomplete Standard of Replacement
3Failure Knowledge Database / 100 Selected Cases

09. Gasket Retainer10. Bad Event
11. Mechanical Event
12. Pinching
13. Leak of Hydrogen Gas
14. Usage
15. Maintenance/Repair
16. Incomplete Standard for Replacement
17. Internal Flange Set Bolt
18. Failure
19. Deformation
20. Plastic Deformation
21. Lock Ring
22. Bad Event
23. Mechanical Event
24. Decrease of height of screw
25. Break Out of Lock Ring
26. Flight of Channel Cover
27. Secondary Damage
28. External Damage
29. Explosion and Fire Disaster4

Failure Knowledge Database / 100 Selected Cases

Fig. 1 Structure of BLC type heat exchanger.

5Failure Knowledge Database / 100 Selected Cases

Fig. 2

Detail of airtightness.

6