| Case Name |
Brittle Fracture of Hydrodesulfurization Reactor during Pressure Test |
| Pictograph |
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| Date |
April 1, 1980 |
| Place |
Tokuyama City, Yamaguchi Pref. |
| Location |
Tokuyama Refinary of Idemitsu Kosan Co., Ltd. |
| Machinery |
Reactor of desulfurizantion process(Fig. 2, 89t pressure wall of SB46M + 3.5t clad of SUS405) |
| Overview |
The reactor of the hydrodesulfurization plant for light oil, naghtha and kerosene failed during the hydrostatic test using N2 before the restart-up after scheduled shutdown inspection(Fig. 2, 9,000BPD).
The plant had worked for 21 years before the accident but the service time of the reactor was 16.5 years, and its operation conditions were 50kgf/cm^2 maximum (hydrogen partial pressure; 39kgf/cm^2 maximum) and 350degree C maximum. The reactor was fractured into 44 pieces(the heaviest one; 4,600kg) which gave severe damages to the equipment, pipings and structures around the reactor. But no casualty was counted because the accident occurred midnight. |
| Incident |
On March 1st, 1980, No.2 hydrodesulfurizaion plant was normally shut down and as to its reactor, applied were small modification work for increasing catalyst holding as well as normal scheduled inspections. After loading new catalyst, the hydrostatic test of the reactor was started using liquid N2 tank truck at 10:30 on April 1st, 1980. At 23:55 when the pressure of the reactor reached its regulated test one, the accident occurred. |
| Sequence |
The 44 pieces of the fractured reactor were collected and the map of fracture-pieces was made as shown in Fig. 4 where the arrows indicate the crack propagation directions. Based on the analysis results of Fig. 4 and the distribution of the catalyst blown out of the reactor, the starting point of fracture was presumed at (a) in Fig. 4.
Examination of fracture surface, microstructure analysis, electron probe micro analysis (EPMA), chemical analysis of (a) were carried out and it was concluded that the starting point was within the weld repaired part applied during fabrication of the reactor at shop.
The story of the repair work at shop was supposed as follows; In the nondestructive inspection of weld lines after completion of welding, found were defects in the circumferential weld line of the reactor. The defects were ground off and repair welding with D309 was applied. Finally, the inner surface of the repaired part was weld over laid with CR40Cb for matching the SUS405 clad (Fig. 5, 6). |
| Cause |
The main cause of the accident was that the maintenance team in charge of the reactor failed to anticipate (1)disbonding of base metal and repair weld metal and (2)hydrogen attack of base metal.
The progress of damage was presumed as follows;
Due to the repetition of plant startup and shutdown for many years, disbonding at the boundary of C-0.5Mo steel and D309(Dissimilar weld joint) occurred and worked as the starting point of crack(Fig. 5 I-IV). After the crack propagated and reached the inner surface of the reactor(Fig. 6 (A)), hydrogen attack of base metal occurred at the disbonded area and propagated toward the external surface of the reactor(Fig. 6 (B)). When the stress intensity factor of the crack tip reached the fracture toughness of the base metal under the hydrostatic test condition(wall temperature; 13degree C, pressure; 55kgf/cm^2), the accident occurred.
[Disbonding of Dissimilar weld joint]
There was no report of disbonding of C-0.5Mo steel-D309 dissimilar weld joint at hydrodesulfurization reactors for light oil in 1970's. But there were several informations about disbonding of ferrite-austenite dissimilar weld joint at heavy oil desulfurization reactors, although it was early 1980's that the detail reports on it were published. Therefore, it should have been anticipated the disbonding in this accident, although it should have been quite hard.
[Hydrogen attack of C-0.5Mo steel]
When we decide the operating conditions of equipment processing hydrogen at elevated temperature and pressure, it is the general practice to use API 941 for preventing hydrogen attack of steel. The API 941 being used in April, 1980 was API 941-1977 edition, and suggested that the operating conditions of the fractured reactor were in its safe region. Since 1977, however, there had been information and communications to API of plant experiences involving hydrogen attack of C-0.5Mo steel in the safe region of 1977 edition, although it was 1983 that API officially referred to the attack in safe region. |
| Countermeasures |
In situations in which hydrogen handling equipment of C-0.5Mo steel has been operated at elevated temperature and pressure, rigorous periodic inspections should be carried out, where the inspection techniques are required to be detectable defects within wall.
It is the general way to apply austenitic weld metal like 309 so as to avert PWHT when we carry out repair welding of equipments of C-0.5Mo steel. In case that the equipment repaired by this way is used under the conditions having hydrogen attack possibility, the practices of their periodic inspections should be discussed in detail and understood by all members concerned. A lot of inspection methods using ultrasonic techniques have been developed for detecting hydrogen attack of pressure vessels. |
| Knowledge Comment |
There are many standards for selecting construction materials of plants, which are based on plant experiences like API 941(Especially, standards for environmental embrittlement like stress corrosion cracking, hydrogen embrittlement, etc.).
When those standards are used for design, the owner of the plant as well as the people who were concerned in design, fabrication and construction should not neglect following up the information about the standards through out the plant life. |
| Scenario |
| Primary Scenario
|
Unknown Cause, Occurrence of Unknown Phenomenon, Disbonding of ferrite-austenite weld joint under high temperature and high hydrogen pressure, Insufficient Analysis or Research, Insufficient Prior Research, Overconfidence in API 941, Usage, Maintenance/Repair, Unsuitable Inspection Method, Failure, Fracture/Damage, Disbonding of Ferrite-austenite Weld Joint, Hydrogen Attack of C-.5Mo Steel, Failure, Large-Scale Damage, Fracture of Reactor, Partial Fall Down of Desulfurization Plant
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| Physical Damage |
Serious damage of desulfurizantion Plant |
| Multimedia Files |
Fig_2.Flow Sheet of Hydrodesulfuraization Plant.
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Fig_3.Structural Sketch of Hydrodesulfuraization Reactor.
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Fig_4.Map of Fractured pieces of Reactor.
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Fig_5.Schematic Representation of Fractured part weld repaired at shop.
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Fig_6.Schematic Representation of Crack Propagation.
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| Field |
Material Science
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| Author |
SHINOHARA, Takanori (Toyo Eng. Corp. OB)
KOBAYASHI, Hideo (Yokohama National University)
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