A liquid ammonia NH3 tank failed catastrophically at the Azotas fertilizer plant in Lithuania in 1989. During maintenance on the boil-off system, operators accidentally created a backflow of liquid warm NH3 to the tank. Potentially after a “role-over”, NH3 evaporated quickly, creating pressure that could not be
relieved by the PSV. The tank was lifted, releasing its content of 7000 tons. The dike broke because of the impact, and the large ammonia cloud was ignited, generating toxic nitrogen gases, with 7 fatalities and 57 injuries.
Aspects:
➢A reliable handling of the boiloff gas is essential to control the pressure. The PSV was not designed for the scenario.
➢The backflow of warm NH3 to the tank can be made less likely by using non-return valves or interlocks.
➢Design atmospheric tanks in a way that they fail at the roof rather than at the bottom.
➢Temperature sensors with interlocks can help to avoid rollover.
Source: EPSC Learning Sheets