A University of California, Davis, physical chemistry teaching lab had a near miss when a bomb calorimeter exploded last month. Luckily, no one was injured as the lid of the calorimeter jacket hit the lab ceiling and pieces of the bomb head and a mercury thermometer blew into the room. Although a report of the incident (pdf) doesn’t outright say so, the cause of the explosion appears to be equipment failure due to lack of maintenance.
According to the report, the students in the lab were measuring the heat capacity of benzoic acid:
A nominal 1 gram sample of the material to be investigated is pressed into a pellet and placed in the sample cup of the bomb head. A nickel alloy fuse wire is attached to the two electrode leads and bent to contact the sample pellet. The head is assembled into the bomb vessel and sealed hand tight with the screw cap and o-ring. The vessel is purged and charged with oxygen. The bomb vessel is placed in a bucket of water (carefully measured) which is placed inside a jacket. On the underside of the top of the jacket a small mixer blade is installed that is used to stir the water in the bucket. Consistent mixing is important to achieve stable temperature. A thermometer is also built into the jacket top, extending down into the water in the bucket, to measure temperature change. The jacket top is installed on top of the bucket and the mixer engaged. Two electrode leads are plugged into the terminals on top of the bomb and then plugged into the ignition unit.
Once the temperature has equilibrated, the sample is ignited by pressing the switch on the remote ignition unit, located separately (typically 24”) from the calorimeter. Users should not stand over the calorimeter or approach the calorimeter for at least 20 seconds. The temperature is monitored over time until no further change is observed. The bomb is removed from the bucket and combustion gases are vented slowly (over a minute or more) through the exhaust valve into the fume hood.
The calorimeter in question exploded shortly after ignition. The investigation determined that the most likely cause of the explosion was that a valve seat failed and “allowed the combustion products to escape, causing the exhaust valve assembly to be ejected from the bomb head. The oxygen can act as a torch and is hot enough to burn through the bomb head.”
The calorimeter manufacturer, Parr, “recommends that all o-rings and valve seats be replaced annually or after 500 firings. Bomb vessels should be reconditioned after 5000 firings.” But there were no records of routine maintenance for the calorimeter, which was manufactured in 1985. Perhaps UC Davis’s experience can also be a “lesson learned” for other departments with equipment in a similar state.