A violent explosion occurred at 07.50 a.m. in a chemical company manufacturing stable isotopes for the diagnosis of disease and for research using a continuous cryogenic distillation technology.
The distillation units separate stable NO isotopes (containing 15N and 14N) from high-purity liquid nitric oxide in specially designed columns (91 m long, sealed inside a 40-cm-diameter insulating vacuum jacket) in below ground well casings.
at 7:30 am, the on-call system operator received an automatic pager alert indicating an alarm condition in a cryogenic nitric oxide (NO) distillation unit. Arriving at the facility at 7:50 am, he observed reddish-brown gas venting from the distillation unit vacuum pump exhaustwhich indicated a breach in the column piping within the vacuum jacket. He notified his supervisor, who called the 911 dispatcher. By 8:15 am, employees secured the leak by closing the vacuum pump suction valve.
At 10:15 amwith no warninga violent explosion destroyed the distillation column, the blast containment structure, and nearby buildings. Windows were blown out in a 42 m radius, projections of concrete and metal were propelled up to 300 m causing damage to 3 residential houses. One employee was hurt by glass shards. A large steel panel from the blast containment structure struck and dislodged a 25 t gaseous carbon monoxide (CO) storage vessel, pushing it about 3.5 m off its foundation. A second steel panel severely damaged adjacent equipment. A ruptured fill line vented CO gas, which then ignited and burned for about 1 hour, until the vessel was empty. As a precaution, the fire department requested that the police evacuate a 1.6 km radius to protect the community from metal shards or other debris in the event that the CO vessel exploded. The evacuation order was lifted after 24 hours.
The CSB investigated the case and identified the following failure scenario from the analysis of available physical evidence (most remains were inaccessible because of damage and debris from the cave-in at the upper section of the well casing) :
- corrosion, fatigue cracking, or other degradation produced a leak in the distillation column piping, releasing NO into the vacuum jacket. The loss of vacuum in the jacket seriously degraded its insulating capacity, thus increasing the heat load on the column.
- Closing the vacuum jacket isolation valve to stop the NO leak into the environment pressurized the vessel as the liquid nitric oxide began to boil.
- Nitric oxide continued entering the vacuum jacket through the leak in the pipe and detonated, crushing the column piping and bursting the jacket.
The experts underlined that the company did not complete a full process hazard analysis to evaluate safety and design protection systems (e.g. blast shielding and overpressure protection calculations). The company had experienced 3 other distillation unit failures (2 involving the detonation of liquid NO), but the accidents had not been thoroughly investigated to implement corrective actions to prevent recurrence.
Recommendations were also made on land use management and emergency planning.