Failure Knowledge Database / 100 Selected Cases

Brittle Fracture of Hydrodesulfurization Reactor
during Pressure Test
April 1st, 1980: Tokuyama-city, Yamaguchi Prefecture
SHINOHARA, Takanobu (Toyo Eng. Corp. OB)
KOBAYASHI, Hideo (Tokyo Institute of Tchnology)(Summary)
The reactor of the hydrodesulfurization plant for light oil, naghtha and kerosene failed during thehydrostatic test using N2 before the restart-up after scheduled shutdown inspection(Fig. 1, 9,000BPD).
The plant had worked for 21 years before the accident but the service time of the reactor was 16.5years, and its operation conditions were 〜50kgf/cm2(hydrogen partial pressure; 〜39kgf/cm2) and 〜
350℃. The reactor was fractured into 44 pieces(the heaviest one; 4,600kg) which gave severe damages tothe equipment, pipings and structures around the reactor. But no casualty was counted because the accident
occurred midnight.

1. Component
Reactor of desulfurizantion process(Fig. 2, 89t pressure wall of SB46M + 3.5t clad of SUS405)

2. Event
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 scheduledinspections. After loading new catalyst, the hydrostatic test of the reactor was started using liquid N2 tanktruck at 10:30 on April 1st, 1980. At 23:55 when the pressure of the reactor reached its regulated test one,the accident occurred.

3. Course
The 44 pieces of the fractured reactor were collected and the map of fracture-pieces was made as shownin Fig. 3 where the arrows indicate the crack propagation directions.

Based on the analysis results of Fig.

3 and the distribution of the catalyst blown out of the reactor, the starting point of fracture was presumed at(a) in Fig. 3.
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 weldlines after completion of welding, found were defects in the circumferential weld line of the reactor. Thedefects were ground off and repair welding with D309 was applied. Finally, the inner surface of therepaired part was weld over laid with CR40Cb for matching the SUS405 clad (Fig. 4, 5).
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Failure Knowledge Database / 100 Selected Cases

4. Cause
The main cause of the accident was that the maintenance team in charge of the reactor failed toanticipate (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 ofC-0.5Mo steel and D309(Dissimilar weld joint) occurred and worked as the starting point of crack(Fig. 4 I〜IV). After the crack propagated and reached the inner surface of the reactor(Fig. 5 (A)), hydrogen attackof base metal occurred at the disbonded area and propagated toward the external surface of the reactor(Fig.
5 (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; 13℃, pressure; 55kgf/cm2), the accident occurred.
[Disbonding of Dissimilar weld joint]
There was no report of disbonding of C-0.5Mo steel-D309 dissimilar weld joint athydrodesulfurization reactors for light oil in 1970's. But there were several informations about disbondingof 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 thisaccident, 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 temperatureand 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 thefractured reactor were in its safe region. Since 1977, however, there had been information andcommunications to API of plant experiences involving hydrogen attack of C-0.5Mo steel in the safe regionof 1977 edition, although it was 1983 that API officially referred to the attack in safe region.

5. Countermeasure
In situations in which hydrogen handling equipment of C-0.5Mo steel has been operated at elevatedtemperature 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.

6. Knowledge
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.).

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Failure Knowledge Database / 100 Selected Cases

When those standards are used for design, the owner of the plant as well as the people who wereconcerned in design, fabrication and construction should not neglect following up the information about thestandards through out the plant life.

7. Primary Scenario
01. Unknown Cause
02. Occurrence of Unknown Phenomenon
03. Disbonding of ferrite-austenite weld joint under high temperature and high hydrogen pressure
04. Insufficient Analysis or Research
05. Insufficient Prior Research
06. Overconfidence in API 941
07. Usage
08. Maintenance/Repair
09. Unsuitable Inspection Method
10. Failure
11. Fracture/Damage
12. Disbonding of Ferrite-austenite Weld Joint
13. Hydrogen Attack of C-0.5Mo Steel
14. Failure
15. Large-Scale Damage
16. Fracture of Reactor
17. Partial Fall Down of Desulfurization Plant

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Failure Knowledge Database / 100 Selected Cases

ReactorFeed Heater

Low Pressure Separator

Fractionator
High Pressure Separator

Desulfurized Naphtha
Desulfurized Kerosene
Desulfurized Light Oil

NaphthaKerosene
Light OilFig. 1

Flow Sheet of Hydrodesulfuraization Plant.

4Failure Knowledge Database / 100 Selected Cases

Bubble Cap Tray

1" Ceramic Ball
1/4", 3/8" Ceramic Ball

1/16" CDS-D5 New Catalyst

1/16" HR-304B New Catalyst

1/8" Crashed Catalyst
1/4" Ceramic Ball
1" Ceramic Ball

Catalyst Unloading Nozzle
3Total Volume : 31.0M
3Catalyst Bed Volume : 23.7M

Fig. 2

Structural Sketch of Hydrodesulfuraization Reactor.

5Failure Knowledge Database / 100 Selected Cases

Circumferencial Weld(e)Longitudinal Weld(d)
(a)
(c)
(b)

Fig. 3

Map of Fractured pieces of Reactor.

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Failure Knowledge Database / 100 Selected Cases

Upper Part of Reactor

Inside of Reactor

CircumncrefeielWlad

Outside of Reactor

(a)

(b)
Fig. 4

Schematic Representation of Fractured part weld repaired at shop.

71cm

Failure Knowledge Database / 100 Selected Cases

Weld Metal made by Ferritic Type CR 40Cb Filler Metal

Weld Metal made byAustenitic Type D309 Filler Metal(A)

Base Metal(C 1/2Mo)(B)

Fig. 5

Schematic Representation of Crack Propagation.

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