Failure Knowledge Database / 100 Selected Cases

Explosion of air bottle by mixing oil
July 31st, 1995, Okegawa-city, Saitama Prefecture
KOBAYASHI, Hideo (Tokyo Institute of Technology)(Summary)
The accumulator in the Okegawa factory of Mitsubishi Materials Corporation exploded at 8:27 am on
July 31st, 1995. Okegawa is in Saitama Prefecture, Japan. Eighteen people were injured. Two of the injured
people were neighborhoods of the factory, and one of neighbors died after t hirteen days. Two air bottles,
which were parts of the accumulator, burst. In addition to those of the plants of factory, the windows, roofs,and walls of houses that were outside of the factory but were less than 1500m from the accumulator, werebroken. Parts of the broken accumulator flew 1200m.
Old oil t hat h ad been mixed t o the a ir bottles ignited, ca using the explosion. It w as rec ognized ag ain
belatedly that t he mixture of o il a nd h igh-pressure a ir is da ngerous. Th e m anagement of o il q uality a ndcleaning need to the air bottle which is recognized to have no risk about the explosion.
1. Component
The accumulator (high-pressure gas equipment) is a pa rt of a 1650t extrusion press that makes jointlesscopper tubes. It consists of an air compressor, two air bottles, the accumulator and so on. The accumulator
is a vertical hydraulic type, an d th e pi ston compresses an d ex pands air. Th e t wo ai r bo ttles are id entical.
One is cal led A-ty pe air bott le a nd t he other is B-ty pe. The a ir bo ttle is m ultilayer cy linder ve ssel. The
design pressure and the volume of the vessel are 210kg/cm2 and 1.8m3, respectively.
2. Event
At 8:27 am on July 31st, 1995, in the Okegawa factory of Mitsubishi Materials Corporation in Saitama
Prefecture, a worker opened a valve t hat w as i n the upper part of t he ac cumulator i n order to run t heextrusion press equipment. Suddenly, the two air bottles exploded. The outline of the accumulator is shownin Fig.1.
Fig.2 shows the layout of the factory and the situation of this accident. The two air bottles broke into
small parts th at flew in to the neighboring areas a nd broke the buildings, roofs a nd walls of t he buildings
there.

The A-type air b ottle broke into t hirteen parts, a nd t he m aximum flight le ngth of th e par ts was

630m. The B-type broke into eleven parts, and the maximum flight length of the parts was 1200m.
The damage to the accumulator and other nearby parts of the extrusion press was not so bad. However,
the wall, the roof and the center pillar of the building in which t he press equipment was housed received
greater damage. Furthermore, the windows, roofs and walls of neighboring houses that were not inside thefactory b ut w ere less t han 1500m fro m the air b ottles, w ere serio usly dam aged. Eig hteen people w ere
injured, two of whom were the neighborhood of the factory. Thirteen days later, one of them died.

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Failure Knowledge Database / 100 Selected Cases

Because there are so m any plants that are similar the one in the accident, the Ministry of Interna tional
Trade and Industry est ablished a committee for investigating the c ourse of the acci dent and devel oping
measures to prevent its recurrence.
3. Course
The air bottles were installed as parts of the accumulator that was added to the extrusion press in 1966.
It was thirty years before the accident. The latest regular self-inspection was performed on Dec 11th, 1994,
and th e l atest safety ins pection was performed on J uly 6th, 1 995. However, t he insides of th e air b ottles
were not appropriately cleaned since the time that they were built.
The air part and the oil part were not completely isolated from each other because the accumulator usesa piston as a com partment. The oil le aks a long the side of the p iston i nto t he air p art.

Through theinvestigation of the accident, it was d etermined that oil had collected in the hand valve, the inside of t heconnection tube and the lower part of the air bottle. Also, the operating oil of accumulator was leaking out
because of the accident, so the piston was at its lowest position. Oil had accumulated on top of piston to aheight of 42m m (about 14L). Although it is not clear whether the oil had accumulated before or after theaccident, it is certai n t hat there was oil i n the air bo ttles and in the upper part of the piston before theaccident.
Although t he am ount of o il t hat ha d a ccumulated in the air b ottles w as verifie d usin g l iquid levelindicator, the quality and amount were not verified. It discharges to the drain gutter.
From the results of gas chromatography of the oil left in the accident plants, lubricating oil for the a ir
compressor was found only near the compressor and the connecting tubes. The rest of the oil was the press
oil for t he accumulator. T herefore, t he oil ca using the e xplosion is c oncluded to be th e oil for t heaccumulator. Through repe ated c ompression and e xpansion of air over t he t hirty years of operati on, th equality of oil deteriorated, and hydrocarbons with low boiling temperature accumulated. Consequently, theignition temperature of the old oil was 60 degrees C lower than that of new oil, and it could easily explode.
Until July 29th, the plant was operated. The start and stop process in which the open and close processof hand valve was accompanied often occurred. At 15 p.m. on J uly 29th, when the plant was stopped, thepressure of air bottle was 195kg/cm2.
The leak of p ress oil w as v erified by the accident's in vestigation. It w as verified by the results of th eleak test of the solenoid valve that is set in the pipes. Therefore, the press oil leaks from the solenoid valve
when the plant is stopped for long time or when the pressure inside the accumulators drops to atmospherepressure. From the analysis of the accident, the length time that had passed from the time that the plant wasstopped on July 29th to the time when it was re-started on July 31st was estimated to be about 41 hours andthe pressure inside the accumulator was estimated to be 22.5kg/cm2. When the hand valve was opened, thehigh press ure air in t he a ir bot tles c ombined with the oil i n t he b ottles to form an air -oil mixture t hat
splashed into the accumulator where the pressure was low, and t he inside of ac cumulator was m isty withthe air-oil mixture.
In the hand valve and the connecting pipes of the air bottles, the phenomenon of plastic deformation,

2Failure Knowledge Database / 100 Selected Cases

which caused the expansion of the pipes, was confirmed. It was hypothesized that the flame had propagatedalong the air-oil mixture in pipes before causing the explosion of the air bottles.
Considering the f act t hat the two air b ottles had e xploded at t he s ame tim e, an d from knowl edge offracture mechanics, the ignition was hypothesized to have occurred in the inside of the accumulator and theconnection pipes. The possible causes of the ignition are shown below, but which actually occurred could
not be determined.
(1) Rise of temperature by adiabatic compression(2) Elec trostatic discharge(3) Friction heat by solid
In summary, first ignition occurred in the accumulator. Next, the flame propagated through the inside of
the pipes and reached the two air bottles at the same time. Suddenly, an explosion occurred in both bottles
by the air-oil mixture and the old oil.
Moreover, a new impact mark made by the explosion was confirmed on the sheet of safety valve.
Fig. 3 s hows the fault tree describing the fracture mechanics and process. The event tree of a ir bottles'
explosion is shown in Fig. 4.
4. Cause(1) Deterioration of press oil
Operation was continued without confirming the pressure difference between the accumulator and theair bottles when the plant began to operate.
When the hand valve is opened under the condition of low pressure in the accumulator, the temperaturein the accumulator increases by adiabat ic compression. Through the repetition of this process, the oil thatwas mixed with air in the accumulator and the air bottles was deteriorated (carbonized). This carbonizationof the oil resulted in the formation of hydrocarbons with a low boiling point, causing the risk of ignition
and burning to increase. Checking of the quality of the oil and the drain and cleaning of the equipment wereinadequate. It is thought that regular cleaning would have prevented the accident.
(2) Difference of pressure
The main cause of this accident was the formation of the burnable mixture of air and fuel that occurred
when t he h and va lve w as opened u nder the c ondition of a lar ge difference of pressure bet ween t heaccumulator and the air bottles.
5. Countermeasure
The countermeasures proposed to prevent future accidents are shown below.
(1) Use of nitrogen instead of air. This measure is costly.
(2) Keep the air and oil isolated. It is difficult to keep them perfectly isolated.
(3) Change system to check condition of inside of equipment(4) Check the quality of the drain and oil, and clean the equipment regularly(5) Change the mechanism to prevent impact of airflow when the hand valve is opened, or fix the process
to open the valve
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Failure Knowledge Database / 100 Selected Cases

The countermeasures that were finally adopted are (3) and (4). They are daily maintenance. Prior to theaccident, this factory ha d not pai d at tention t o t he leak of press ure oil in the ac cumulator and ha d notcleaned the inside of the air bottles for thirty years.
6. Knowledge○The quality of oil changes over time
The quality of even non-burnable oil used for long time will change and become burnable. Removal ofthe old oil and regular cleaning of the equipment is required.
○Risk of mixtures of high-pressure air and oil
The risk of fire when old oil is mixed with high-pressure air must be recognized.

7. Primary Scenario
01. Organizational Problems
02. Poor Management
03. Poor Process Management
04. Disregard of Procedure
05. Ignorance of Documentation
06. Ignorance of Process Documentation
07. Usage
08. Maintenance/Repair
09. Management of Oil's Quality
10. Cleaning
11. Accumulator
12. Bad Event
13. Thermo-Fluid Event
14. Fluid Reaction
15. Difference of Pressure
16. Burnable mixture air
17. Bad Event
18. Chemical Phenomenon
19. Ignition
20. Burn
21. Failure
22. Large-Scale Damage
23. Burst
24. Air Bottle
25. Secondary Damage
26. External Damage
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Failure Knowledge Database / 100 Selected Cases

27. Equipments of Factory
28. Buildings Outside of Factory
29. Bodily Harm
30. Death
31. Bodily Harm
32. Injury

5Failure Knowledge Database / 100 Selected Cases

Fig. 1 Outline of Accumulator.

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Failure Knowledge Database / 100 Selected Cases

Fig. 2

Layout of Factory and Details of Damage.

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Failure Knowledge Database / 100 Selected Cases

Burst of Air BottleorStarting point(time dependent)

Rapid Fracture(time independent)
orBrittle Fracture

Ductile Fractureor

Plastic damage by excessive pressure

Fracture by external forceor
Increase of pressure with time

Excessive increase of pressure

Burn (explosion)andSource of injunction
and / or
Adiabatic
Compression of Gas

Static electricalchargeChange of Oil's quality

Difference of Pressure

Mixture of oil

Fig. 3 Fault tree focused on fracture mechanics and process.

8Frictional heatFailure Knowledge Database / 100 Selected Cases

Pressure oil foraccumulator

Change ofquality

Managemantof quality
and cleaning

Differenceof pressure

Injection

Flame
Deformation
propagationof pipes

Explosion ofaccumulator

ResultBurst

Crack
HealtyHealtyHealtyHealtyHealtyHealtyHealtyFig. 4

Event tree of breakaway of heat exchanger’s cover plate.

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