Explosion at a high-pressure furnace

The accident occurred during the manufacture of pharmaceutical intermediates using a high-pressure gas reactor. The reactor used a metal catalyst to react a liquid intermediate with hydrogen to produce a pharmaceutical raw material. The reactor's operating pressure (approximately 14 MPa) was controlled by adjusting the hydrogen supply quantity. The hydrogen was supplied by a battery of tubes on a trailer, at approximately 18 MPa.

The reactor was shut down due to a blockage in the sampling line caused by a powder metal catalyst. The reactor was then depressurised, and the sampling line was removed for cleaning and maintenance. The next day, a nitrogen gas cartridge was connected to the reactor's pneumatic test line to perform a leak test. However, the valve on the hydrogen supply line was not closed, allowing hydrogen to mix with the nitrogen gas. The mixed gas was then released into the sampling box, where it ignited and exploded due to static electricity.

The INITIATING CAUSE was a hydrogen supply valve accidentally left open when performing a leak test with nitrogen. When the testing gas was released via the high-pressure reaction into the sampling box containing air, hydrogen ignited.

The IGNITION SOURCE was static electricity discharge. The sampling box made of PVC, which is prone to static electricity. It was insufficiently grounded. The fact that the metal catalyst remained in the reactor during testing was probably an additional ignition factor.

The leak test was executed according to verbal instruction only. The fact that the supply valve was left open can be attributed to a human mistake, but it was triggered by the lack of written procedure for leak test and the absence of an independent check on a status of a component critical to safety. Deficiencies in the safety design contributed as well.

The process was working at high pressure (short below 15 Mpa). The proces cemperature is not provided. The event occurred the process suhut down and in the samling box, at a pressure probably not much higher than the ambient pressure.

The front door and the side panel of teh sampling box wasdamaged, for approximately 100,000 yen.

Timeline of Accident:
June 19: Reactor shutdown, sampling line removal, and maintenance
June 20:
- 9:00 AM: Nitrogen gas cartridge connected to reactor's pneumatic test line
- 11:44 AM: Explosion occurred during depressurisation of reactor
- 11:56 AM: Emergency services notified
- 12:00 PM: County fire department arrived at the scene
- 1:40 PM: Fire department left the scene

This event highlighted the importance of written and communicated procedures and the independent assessment of their execution.
As stated also by the KHK report, the event reminded:
1. The need for careful planning and execution of work involving flammable gases.
2. The importance of communication when executing work in a team.
3. The need for a regular check of the hazards and risks, in combination with safety inspections and safety training.

The production process was suspended until countermeasures were completed, two months later.

(1) Improvement of the design by adopting physical countermeasures: the sampling box was replaced with a metal one. Its grounding ensured and ignition source during sampling avoided.

(2) Management countermeasures (also avoiding the possibility of a human error): leak test instruction were written, a check on the valve status foreseen and a final formal communication that all preparatory steps had been executed.