Inside a molecular sieve plant, an ammonia tank was placed back into service after welding a new bottom; afterwards, the evening shift carried out filling tests without making note in its end-of-shift report. During the next shift, a new series of repairs were conducted on the tank, which was assumed to be empty. When grinding of a pipe, sparks caused a deflagration of the confined ammonia vapours along with the ejection of the tank bottom a distance of 60 m against one of the site’s administrative buildings. A hissing sound prior to the explosion alerted employees, and as a result none were hurt. The tank had contained 50 litres of 25% ammonia.