| Case Name |
Leakage and fire of hydrogen during exchange of a dehydrogenation catalyst at an alkylbenzene manufacturing plant |
| Pictograph |
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| Date |
April 14, 1994 |
| Place |
Kawasaki, Kanagawa, Japan |
| Location |
Chemical factory |
| Overview |
A catalyst in the dehydrogenation reactor, which usually was operated under hydrogen atmosphere, was changed with separating the reactor and its peripheral part from the slightly pressured part by closing a 20-inch remotely controlled valve. The hydrogen pressure of the peripheral part was set at 20 KPaG, and the reactor was opened to the atmosphere. Considering some hydrogen leakage, suction from piping was done with a vacuum device and, in addition, nitrogen sealing was performed. In piping restoration work after changing the catalyst, flames spouted from the clearance of the flange and two workers got burnt. The cause of the fire was mal management of the method of catalyst changing. |
| Incident |
A catalyst exchange was carried out at a reactor where a dehydrogenation reaction of normal paraffin was done at a linear alkylbenzene manufacturing plant (LAB plant). For piping restoration of the reactor after the exchange, a top flange with an elbow was mounted, and nitrogen purging of the reactor and piping started. Leaked hydrogen gas ignited in the piping and flames spouted out through the flange during temporary tightening work. The fire ignited the clothing of plumbers who were working near the flange. Two plumbers got burnt. Refer to Fig2. |
| Processing |
Manufacture |
| Individual Process |
Maintenance |
| Process Flow |
Fig2.Unit process flow sheet
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Fig3.Process flow sheet
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| Substance |
Hydrogen, Fig4 |
| Type of Accident |
Leakage, fire |
| Sequence |
At 05:30 on April 12th, 1994, the LAB plant was shut down for a catalyst exchange.
At 13:10, the remotely controlled air-driven valves at the inlet and the outlet of the dehydrogenation reactor were closed. The top and bottom piping of the reactor was kept at 20 KPaG with hydrogen gas.
On April 13th, the reactor was left in a sealed condition at 40 KPaG with nitrogen gas.
At 09:00 on April 14th, the reactor was depressurized, the top cover (the inlet side) was removed, and the blind flange (a partition board installed at the end of the piping) was set at the end of the inlet piping, and evacuation started with an ejector.
At 11:00, catalyst extraction started.
At 14:00, catalyst extraction finished.
At 16:00, new catalyst filling finished.
At 16:10, the blind flange was detached.
At 16:30, the top cover of the reactor was mounted, and substitution of air in the reactor with nitrogen started.
At 16:40, Flames spouted out through the flange, and two plumbers got burnt during tightening flange bolts. |
| Cause |
Piping around the reactor were kept at 20 KPaG with hydrogen gas, which was the main gas on-stream, to protect air contamination from atmosphere. Separation of the hydrogen-pressurized section and the reactor at atmospheric pressure was executed by closing only one valve. A steam ejector was running to remove leaked hydrogen. (Refer to Fig. 2) In the last stage of work to mount the upper piping to the reactor top flange after exchanging the catalyst, the blind flange was detached, and it was intended to connect the upper piping. At the piping where the blind flange was set, removal with the steam ejector had already started for the hydrogen leakage through the valve. Therefore, atmospheric air came into the piping from the flange during temporary tightening work. The air and the hydrogen that leaked from the remote control valve generated a combustible gas mixture in the piping. The gas mixture seems to have ignited by a static electric spark, which might have been generated from solid material adhering to the internal wall of the piping and gas leaked.
According to the plan, injecting nitrogen gas would purge the leaked hydrogen. In fact, the nitrogen was drawn prior to the leaked hydrogen due to the drawing point was inadequate as shown in Fig. 2, so it was not useful for the nitrogen seal. |
| Response |
The injured persons were taken to hospital by ambulance. The hydrogen flames automatically went out. |
| Countermeasures |
The catalyst exchange method was returned to the previous method with nitrogen purging of the whole reaction vessel and piping. |
| Knowledge Comment |
1. They tried to make a positive change to increase production and decrease a workload, but failed to notice negative elements of the change.
2. This is also an example of a change that could cause trouble. It is necessary to thoroughly simulate the negative elements of the change.
3. Although the valve stops a flow, it is necessary to consider leakage. A valve might leak often due to fouling and solid material between seals and so on. |
| Background |
Apparently there were two causes: one was a leaking valve, and the other was poor positioning of nitrogen injection. However, the principle of the work plan should be on the assumption that a valve might leak. In treating hydrogen gas, which easily leaks, the idea of using only one gate valve with a big 20-inch diameter to shut off hydrogen will naturally cause leakage. Such small countermeasures as drawing with an ejector or sealing with a small quantity of nitrogen should not be taken, because the amount of leaked hydrogen could not be estimated.
The catalyst in this reactor is exchanged at about 55-day intervals. Therefore, to decrease the catalyst exchange operation period, a new dividing method with only one remote air-driven valve between a slightly hydrogen pressured section and a released-to-atmosphere section was proposed as a convenient method, and was approved by the factory manager. Although a senior manager did not direct the change, the true cause might have been the change in the work method without a careful study for increasing the production quantity.
Especially, lack of a basic quantitative study was a problem such as an estimation of the leakage quantity, injection volume required for evacuation etc.
In a word, the rough and insufficient study of safety executed for the production increase in a perfunctory manner and at a low technical level seems to be the true causes. |
| Incidental Discussion |
The hydrogen flame spouted instantaneously, and no fire-extinguishing equipment was used. There was no damage to the plant. The clothing of the workers caught fire and they got burnt. It was not recognized as a fire because there was no special damage, and it was automatically extinguished. A fire was recognized from the clothing of the workers who got burnt. Fortunately, they were only slightly injured and their lungs did not get burnt because they did not swallow the hydrogen flame. |
| Reason for Adding to DB |
Example of accident caused due to inadequate management of change. In particular, it is an example showing what must be considered on a valve leak |
| Scenario |
| Primary Scenario
|
Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Insufficient Analysis or Research, Insufficient Practice, Lack of Imagination, Organizational Problems, Poor Management, Slackness of Management, Planning and Design, Poor Planning, Poor Planning of Repair, Bad Event, Mechanical Event, Valve Seat Leakage, Secondary Damage, External Damage, Fire, Bodily Harm, Injury, 2 person injured
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| Sources |
Kawasaki City, Fire fighting station, Prevention division, Peace section. N petrochemilcal Co., Ltd.. K operation center. Alkylbenzene manufacturing plant fire accident. Material of the Kawasaki City Complex safety countermeasure committee
High Pressure Gas Safety Inst. of Japan, High-pressure gas protection overview , pp.260-261(1996)
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| Number of Injuries |
2 |
| Financial Cost |
¥1,000. ( Material of the Kawasaki City Complex safety countermeasure committee) |
| Multimedia Files |
Fig4.Chemical formula
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| Field |
Chemicals and Plants
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| Author |
KOBAYASHI, Mitsuo (Office K)
TAMURA, Masamitsu (Center for Risk Management and Safety Sciences, Yokohama National University)
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