Failure Knowledge Database / 100 Selected Cases

Burst by Hydrogen attack ofResidue Hydrodesulfurization Unit Piping
March 31st, 1982, Kashima city, Ibaraki Prefecture
HASHIMOTO, Tetsunosuke (Chiyoda co., OB)
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
The pro cess f luid began to le ak from the d ownstream p iping of t he safety valve of t he car bon ste el
by-pass pi ping in the residu e hydrodesulfurization unit, du ring unit operation on March 31 st, 19 82. As aresult, the piping exploded, and a fire oc curred. This process unit had been operating for 12 y ears prior t o
the explosion. This by-pass piping was located downstream of the safety valve, and it was connected to themain stai nless stee l piping from the reac tor e xit t o the hi gh-pressure separa tor. T he main sta inless steel
piping was operat ed a t a t emperature of 335 ℃, a t otal pressur e of 140 kgf/cm2, and a hy drogen p artialpressure 120kgf/cm2. However, the process fluid did not flow to the piping of the safety valve downstream
during normal operation, and therefore the internal temperature designed for the by-pass piping was low. A
survey of the damaged parts showed that the piping had been damaged by an intense hydrogen attack. The
strength of the part of the piping suffering from the hydrogen attack was decreased, and it was thought that
this decrease in strength caused the piping to explode. Figure 1, Figure 2, and Figure 3 show the appearanceof the crash site. Five operators died in this accident, and three people were seriously injured, and the unit
suffered enormous damage.
1. Component
Residue hydrodesulfurization unit
2. Event
The process fluid, including high temperature high pressure hydrogen, flow s to the main piping fro m
the reactor exit to the separator.

Stainless steels (20B, SUS321) were used for this main piping.

Branchpiping 1 ,300mm in len gth of sam e material (6 B, SU S321) had been i nstalled horizontally i n the m ainpiping. Th e damaged carb on st eel by-pass pi ping (6B , STPT38) of t he s afety valve downstream w asconnected with the stainless steel branch piping (6B, SUS321) by a flanged connection (Refer to Figure 4).
During regular operation, the fluid does not flow through the pipe. Therefore, the system had been designed
based on the assumption that the temperature of the carbon steel by-pass piping would not rise.
The burst ha ppened in the direction of the tube ax is from a distanc e of 36 mm from the flange welding
line to a distance of about 1.2m.
As a result of studies including microscopic examination, it was concluded that the hydrogen attack had
occurred on the inner side of the piping, and that the decarburization had occurred in the flange weld of thecarbon steel piping resulting in m icro intercrystalline cracks in a part of th e piping about 5.5 m in le ngth.
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Failure Knowledge Database / 100 Selected Cases

Micro cracks extended throughout the whole area of the tube section in the vicinity of the part of the pipingthat had burst as show n in Figure 5, and it is thought that the first l eakage had occurred through a microcrack. (Refer to Figure 6). Judging from the Nelson chart (API 941) that shows the boundary of safety fromthe hydrogen attack , the damaged part was presumed to have been kept at a temperature of 230℃ or more
for a long time. However, although the interaction between the carbon steel branch piping and the stainlesssteel piping of the high temperature driving condition (335℃ and hydrogen partial pressure 120kgf/cm2)
was examined, the temperature rise mechanism of this carbon steel piping was not determined conclusively.
The mechanism of the heat-pipe phenomenon was presumed as a strong inference of the possibility. That is,when the steam was added to promote the hydrodesulfurization reaction, it condensed in the carbon steel
piping of t he line in the part where the fluid did not flow during operation. The condensed water flowed
backward to the main piping, causing the inflow of t he high temperature fluid to the damaged part, whichraised the temperature of the carbon steel piping.
3. Course
After having detected the pressure abnormality, eight people who were in the operation room gathered
to the leakage site f or the purpose of t he site confirmation. The explosion occurred just 4-5 minutes after
the leakage was discovered, resulting in extensive damage, five deaths, and three serious injuries.
4. Cause
It was thought that the line stopping carbon steel branch piping was designed to be maintained at a lowtemperature. However, the piping act ually reached the high tem perature of 230℃ or more because of theheat-pipe phenomenon caused by the condensation of the steam added to the reactive system.

The carbon

steel pipi ng r eceived t he hydroge n a ttack c ausing t he mechanical s trength of piping t o decre ase, a nd theleakage of the process fluid happened after twelve years of operation.
5. Immediate Action
Currently, plans for improvements in construction and operation are being examined for the preventionof a reoccurr ence of t he s ituation. Moreover, the manual an d the sy stem for regular checks are being
reviewed. It i s confirm ed whether t here is a part t o have ca used t he rise in he at ab normally with a rea lfacility e ven i f it is a part judged for the p ossibility of ca using the hy drogen a ttack b ecause it doesn't
become a high temperature when equipment is produced for similar equipment not to exist on the site.
6. Countermeasure
In order to prevent man-made disasters resulting from explosions, the work standard must be reviewed
and the ed ucation a nd training f or making a ppropriate em ergency responses should be enhanced,
particularly fo r th e s ituation whe n a leakage is discovered in e quipment t hat treats high temperature andhigh pressure hydrogen.

2Failure Knowledge Database / 100 Selected Cases

7. Knowledge
In t he d esign of e quipment for trea ting h igh tem perature a nd h igh pressure hydrogen f luid, of c ourse
materials and structure des ign are se lected co nsidering the p ossibility of hy drogen atta ck. When a of fset
pipes and other peripheral equipment are designed, it is important to examine carefully the possibility of arise in temperature when operating.
It may be dif ficult to assess all of t he possi bilities of a tem perature rise d ue to som e heat -pipe
phenomenon occurring in offset piping.
However, some measures such as securing enough room are necessary if there is uncertainty.
For example, the m easurement of the temperature of t he tube wal l after it begins operation is als o aneffective measure.
8. Sequel
Investigations were conducted to determine if any similar, abnormal temperature rise had occurred in asimilar part o f sim ilar hy drodesulfurization u nits used in eac h refin ery in Japa n. Moreover, a n em phatic
check whe ther worried about t he hydrogen at tack a bout a part concerne d was exec uted. M oreover, thepiping design was reviewed processing it. On the other hand, the revision of the Nelson Chart is one by one
executed in API (American Petroleum Institute), and the improvement of the reliability is attempted.
At the same time, concerns regarding the effectiveness of the method of nondestructive inspection fromthe o utside of th e p iping g ave rise t o t he an xiety ab out t he possibility of hy drogen a ttack of p iping andother of damage before and after this accident. The nondestructive inspection technology from the o utsideof pi ping w as exam ined aggressively under the c ooperation of all rela ted pr ocessing c ompanies andinspection companies in Japan, and it was executed on the site.
In th e i nvestigation of this acc ident, 2 30℃ in t emperature is pres umed fro m the hydr ogen partialpressure by using the Nelson Chart of the temperature to which the carbon steel part of the damaged part ispresumed. The Nelson Chart sets the marginal condition based on the test research of the hydrogen attack
and the data of the accident case etc. on t he site, and the accuracy is reflected in the design and operation
assuming that there is reliability enough under the present situation. The accuracy will be expected for thedata of t he case etc. to be added and to be improved further in the future. The validity of t he judgment of
having r eached 230℃ or more is incontrovertible t hough t he Nelson C hart i s used fo r th e te mperaturepresumption of the carbon steel part by this case, and hold time at the high temperature has not been fixed.
For th e connection d estination of bl ow up si de of t he safety v alve, an o pinion s uggests that the l ow
temperature part such as the suction side etc. of the pump usually done from the high pressure part of thehigh temperature is preferable.
9. Information Source(1) Institution for Safety of H igh-Pressure Gas Eng ineering: Kas hima refinery of Kashima oil Ltd.:
Residue Hydrodesulfurization Unit Explosion and the Fire Accident Investigation Report (1982).

3Failure Knowledge Database / 100 Selected Cases

10. Primary Scenario
01. Unknown Cause
02. Occurrence of Unknown Phenomenon
03. Heat Pipe phenomenon of branch piping of the line stopping
04. Planning and Design
05. Poor Planning
06. The rise in heat of the by-pass piping is not forecast.
07. Application of improper structure to by-pass piping
08. Failure
09. Fracture/Damage
10. Hydrogen attack of by-pass piping and leakage of process fluid
11. Failure
12. Large-Scale Damage
13. Burst of by-pass piping
14. A large amount of gush of process fluid including hydrogen
15. Misjudgment
16. Misjudgment of Situation
17. Correspondence to leakage event
18. Secondary Damage
19. External Damage
20. Fire
21. Bodily Harm
22. Death
23. Accidental death
24. Loss to Organization
25. Economic Loss
26. Damage to Plant Unit

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Failure Knowledge Database / 100 Selected CasesFig. 1 Accident site.

5Failure Knowledge Database / 100 Selected Cases

Fig. 2

Burst part of the safety valve downstream piping.

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Failure Knowledge Database / 100 Selected Cases

Fig. 3Passage of a fire and explosion.

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High pressure separator

Failure Knowledge Database / 100 Selected Cases

ReactorBurst position
Safety valve

Fig. 4

Hydrodesulfurization precess flow seat and burst position.

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Failure Knowledge Database / 100 Selected Cases

Fig. 5Distribution of decarburization part of tube section.

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Failure Knowledge Database / 100 Selected CasesFig. 6

Distribution of direction of tube axis of decarburized layer and micro crack.

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