Accident Reconstruction Links
Nitrogen Plant Explosion
A common method for the production of oxygen and nitrogen is the liquefaction of air. Air is compressed, expanded and cooled via the Joule-Thomson effect, and then re-compressed and expanded again and re-cooled and so forth until air is liquefied. Since nitrogen boils at a lower temperature than oxygen, the nitrogen can be distilled off the liquid air and then recompressed and re-liquefied.
A natural result of distilling off the nitrogen is that an oxygen-rich phase is
left in the bottom of the distillation column. |
Since the Joule-Thompson effect depends on the expanding air doing some work, a turbine type expander is sometimes used. It is important for any rotating machinery to have lubrication on the rotating shafts and a seal-gas system keeps the lubricating oil from contaminating the process stream.
A nitrogen plant that was servicing a large chemical complex tripped and went into automatic shutdown. When such a trip occurs, the chemical plant begins to draw nitrogen from liquid nitrogen reserves. The liquid nitrogen reserves are heated in vaporizers to produce higher-pressure nitrogen gas. Under normal conditions a check valve prevents the reserve nitrogen gas (created via the vaporizers) from flowing back to the liquified air distillation column. In this particular incident, a back-flow check valve was faulty, and the nitrogen gas from the vaporizers flowed back into the distillation column and forced the oxygen-rich liquid air in the bottom of the column into the turbine expander. Since the back-flow pressure was greater than the turbine’s seal gas pressure, oxygen-rich liquid and/or air flowed past the seal gas system and into the oil sump. This created an explosive mixture of combustible oil and oxygen-rich liquid air. Since the gas entering the sump was cold, the oil became cold and viscous. An operator turned on the oil sump heater in an attempt to warm the oil. Thirty minutes later the oil sump exploded and seriously injured two workers who were in the vicinity of the oil sump.
Chemaxx was hired to investigate the cause of the explosion. The ultimate physical cause of this explosion was traced back to the faulty check valve. A second physical issue was the seal gas pressure being lower than the back pressure of the distillation column. However, there were also technology transfer failures as the plant had recently been sold to new owners and operators who were less experienced than the previous owners.
Dr. Fox is an explosion expert, fire expert, and chemical expert with extensive experience in OSHA chemical regulations and chemical safety.
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