Repair work for liquefied carbon dioxide at a low-temperature storage tank (CE) in a steel mill was carried out. At the time, the main block valve below the safety valves was closed, while liquid CO2 remained in the tank. In addition, the heat source from the tank heater was still active. Therefore, in the tank, the temperature rose and pressure increased, and the low-temperature storage tank exploded. A vapor explosion of carbon dioxide occurred due to the rapid pressure drop.
Incident
On a low-temperature storage tank for liquefied carbon dioxide, the main block valve below the safety valve(s) was closed, while liquid carbon dioxide was being stored for repair work. In addition, the power supply to a refrigerator was stopped. However, the power supply to a heater remained on. Heat from the heater and heat absorbed from ambient air caused the temperature and pressure of liquefied carbon dioxide in the tank to rise. The tank’s internal pressure exceeded the MAWP of the tank. A crack was generated at the tank wall. Because of the crack, the internal pressure of the tank was equalized with the atmospheric pressure suddenly, and liquefied carbon dioxide in the storage tank generated a vapor explosion due to the rapid pressure drop. (i.e. Cold BLEVE)
Sequence
- Although the lowering level of liquefied carbon dioxide in a low-temperature storage tank was big, the flow rate at the receiver became low. A person responsible judged that there was a leakage at a relief valve or other valves, and started repairs. (This is speculated from the report.)
- Main valves of a safety valve, a rupture disk, and a pressure relief valve were closed at the time, the refrigerator’s power supply is cut, and the power supply to the heater was on.
- The tank suddenly ruptured, and was reduced to seven pieces of debris.
Cause
Maintenance work for a storage tank of liquefied carbon dioxide was underway.
- Closing the valve below a safety valve, a rupture disk, a pressure relief valve, and a safety valve, and other safety valves had no effect.
- Liquid was present.
- The power supply to the heater was not cut off. Pressure increased due to the heat generated by the heater and heat absorbed from ambient air.
Countermeasures
A repair manual should be written in detail and concretely. It should be written so that even an outside person can easily understand.
Comment
- The inside pressure of a closed vessel in which liquefied gas, etc. was charged becomes the equilibrium pressure for the temperature at that time. It must be understood sufficiently that the inner pressure of equipment in which there is a heat absorption effect from outside, such as a low-temperature storage tank, increases to the equilibrium pressure for the outside air temperature.
- Errors happen frequently during repair work. There are many factors, for instance: 1) it is not regular work, 2) many subcontractors’ workers who do repair work might not be educated well on safety, 3) there might be insufficient safety management because work is done in a hurry, etc.
Background
- The main reason was lack of knowledge. The pressure became the equilibrium pressure for the temperature because liquid co-existed with gas in a closed vessel. At a low-temperature storage tank, heat absorbed from the atmosphere raises the temperature and pressure increases even when a heater is turned off. The refrigerator stopped, main safety valves, etc. were closed, so a rupture was the natural outcome.
- Repair work was carried out with safety valves not functioning while liquid was inside.
Incidental Discussion
- When a liquid, which has a temperature higher than the boiling point under pressurized conditions, is subjected to a rapid lowering of pressure, a vapor explosion might occur following rapid evaporation from not only the surface but also inside. This might increase the damage, and must be sufficiently understood.
- Insufficient education of a subcontractor’s workers can be considered. The reasons seem to be the following: education would not be useful in the future because the work is temporary, business secrets might leak, safety management is part of the know-how of the enterprise, etc. However, there would be larger problems if an accident occurs.
Scenario
Ignorance, Insufficient Knowledge, Insufficient Study and Experience, Poor Value Perception, Poor Safety Awareness, Inadequate Risk Recognition, Organizational Problems, Inflexible Management Structure, Insufficient Education/Training, Malicious Act, Rule Violation, Safety Rule Violation, Usage, Operation/Use, Wrong Stopping Method, Failure, Large-Scale Damage, Rapture, Bodily Harm, Death, 3 person died, Bodily Harm, Injury, 38 person injured, Loss to Organization, Economic Loss
Sources
1) High Pressure Gas Safety Inst. of Japan. Rupture of liquefied carbon dioxide in a low-temperature storage tank. Accident investigation report (outline) (1969)
2) High Pressure Gas Safety Inst. of Japan. Examples of accidents. Collection of examples of high-pressure gas accidents. pp.236-237 (1982)
3) Tetsuzo Kitagawa. Vapor explosion in liquid dioxycarbon tank. Analysis of explosion hazard. pp.321-322 (1980)
Number of Deaths: 3
Number of Injuries: 38
Date: March 1, 1969
Place: Fukushima, Japan
Location: Steel mill
Physical Damage
A storage tank was destroyed, and pieces of debris of the shell were scattered, the longest blowing distance was about 60 meters. A slate-roofed factory within a 50 m radius was completely destroyed, only its pillars remaining. Roofing tiles were scattered within a 100 m radius, damaging doors and windows. Windows of houses located within a 500 m radius were damaged.