Explosion due to Ammonia vapour

A storage tank for aqueous ammonia solutions was up for maintenance (replacement of the bottom part). After mechanical completion of the replacement work, a trial had been undertaken to fill the tank up, but overpressure was registered and the flange connecting the feeding line to the tank leaked. The problems were reported to the maintenance department, the flange connection was repaired and the pressure relief line checked, the trial to fill the tank was not reported to the shift supervisor. The next day a safe work-permit was issued to the mechanics to disconnect the piping associated with this tank for further repair. The repair work proceeded and during the grinding of a disconnected pipe, a mechanic noticed a whistling sound and hid, together with the other mechanics, behind a concrete tankfarm wall. Soon afterwards the tank exploded. The top of the tank was blown over an adjacent building and the office buildings, and bumped into another office building (approximately 60 m away), which was empty. The explosion is believed to be caused by the ignition of ammonia vapour caused by the repair works. Also the pressure relief line failed to perform as expected.
Lessons
1. Improvement of procedures.
2. Improvement of communication.
3. Improvement of training of personnel.
4. Re-design of vapour relief lines.
Source:COMMUNITY DOCUMENTATION CENTRE ON INDUSTRIAL RISK, MAJOR ACCIDENT REPORTING SYSTEM LESSONS LEARNT FROM ACCIDENTS NOTIFIED, INSTITUTE FOR SYSTEMS ENGINEERING AND INFORMATICS, COMMISSION OF THE EUROPEAN COMMUNITIES JOINT RESEARCH CENTRE, 1991, ISBN 9282622894.