Failure to Manually Purge Hazardous Gases

Key Instruction Points

  • Don't rely on procedures - insist on engineering controls.

  • Use engineering controls to protect against unforeseen hazards.

An experienced physical scientist (a Ph.D. of 15 to 20 years) at  an industrial research lab used hydrogen and phosphine gas, along with other materials, in a metal organic chemical vapor deposition system. This was a slightly modified commercial system that operated at atmospheric pressure. The reactor was contained in a secondary enclosure with exhaust ventilation and toxic and flammable gas detection equipment linked to automatic gas shutoff valves. The equipment operating procedure involved a manual inert gas purge prior to flow of hazardous gases.

When considering a modification of the system to make the purge process automatic, the manual procedure was thought to be acceptable since the only one who operated the system was the physicist who bought and built the equipment and his coworker, who was also experienced and trained.

The addition of the automated purge feature was considered an unnecessary hardship. Three months after startup, an over pressurization of the reactor occurred, cracking the glassware and leaking the gas into the secondary containment. The gas monitor detected the leak and caused automatic shutdown of gas flow. No gas escaped secondary containment. The physicist indicated that he had forgotten the purge step of the process.  

Corrective Action

The physicist modified the equipment to include an automatic inert gas purge. This involved very little time and expense. This incident is consistent with other mishaps in which highly intelligent, experienced, and well trained personnel miss a critical step in the process. The need for engineering controls for high hazard processes was emphasized in employee training and hazard reviews.