On 20th March 1989, a cryogenic storage vessel containing approximately 7,000 tonnes of liquid ammonia catastrophically ruptured at its base at the “Azotas” fertiliser plant near Jonova, Lithuania. The rupture resulted in a large release of liquid ammonia, which subsequently evaporated, forming a gas cloud that ignited. The fire then spread to nearby fertiliser depots, causing the decomposition of thousands of tonnes of NPK fertiliser and the release of toxic nitrous oxides. The incident led to significant human casualties, environmental contamination, and widespread evacuation.
Date
Location
Industry
Substance
Cause
The official investigation in the USSR concluded that the tank failure was due to overpressure. This overpressure was attributed to the introduction of approximately 14 tonnes of warm ammonia (+10°C) into the bottom of the cryogenic reservoir (-32° to -34°C) due to an operating mistake in the ammonia plant. This warm ammonia may have formed unstable layers or lenses at the bottom of the tank, which did not immediately evaporate due to hydrostatic pressure. A subsequent sudden vaporisation of this warmer liquid is believed to have caused a rapid increase in pressure, exceeding the capacity of the safety valves and leading to the rupture of the reservoir at its base. Contributing factors included the fact that all refrigeration compressors were out of operation at the time. Furthermore, enquiries suggested that the resistance of the reservoir’s roofing was stronger than the attachment of the inside walls to the base, leading to the failure at the base. Modifications made during construction to reduce costs may have also compromised the integrity of the protective wall and possibly the reservoir’s foundations and anchoring devices.
Consequence
The rupture of the tank released approximately 7,000 tonnes of liquid ammonia, which spread over the ground. The ammonia vapour ignited, and the fire engulfed the surrounding area, including the control room, fertiliser factory, and conveyor belts. A burning conveyor belt fell onto a store of 15,000 tonnes of NPK fertiliser (11-11-11), initiating a self-sustaining decomposition that continued for three days, releasing large quantities of nitrous oxides. A toxic cloud composed of ammonia vapour and fertiliser decomposition products spread up to 35 km from the site, contaminating an area of 400 km². The cloud reached heights of 100 m at 5 km, 400 m at 10 km, and 800 m at 20 km from the site. The incident led to the evacuation of 32,000 people from Jonova and adjacent regions. Water curtains were deployed to reduce the impact of the cloud, and measures were taken to protect water sources like the NERIS river from pollution.
Injuries
57 employees were seriously injured, requiring hospitalisation lasting more than 24 hours.
Fatalities
7 operational personnel of the plant and construction companies working nearby were killed.
Lessons Learned
This accident highlighted several critical lessons regarding the design, construction, and operation of cryogenic storage facilities:
1. Cryogenic reservoirs can fail catastrophically, and unconfined ammonia clouds can ignite.
2. Ruptures at the base of a cryogenic tank can have more severe consequences than ruptures at the top. The structural integrity at both the base and the summit (frangible design) is crucial.
3. Introducing warmer substances into cryogenic storage, even in relatively small quantities, can lead to rapid pressure increases due to phenomena similar to roll-over. Filling cryogenic reservoirs from the top or using internal recirculation systems can help prevent roll-over.
4. Maintaining sufficiently low temperatures during storage and filling is essential.
5. Safety equipment such as bursting discs and valves must be correctly dimensioned and their evacuation conditions should be preventively examined. In this case, the protection valves were not sufficiently dimensioned.
6. Maintenance periods and unusual operations are high-risk situations requiring increased vigilance, prior risk assessment, and formal compensatory measures.
7. Retaining equipment like protective walls must be designed to withstand the dynamic pressure of released liquids and potential domino effects. The actual breadth of the protection wall was inferior to the design specifications due to cost-cutting measures.
8. Safety measures and emergency controls must be permanently accessible, even in degraded situations.
9. Redundancy in critical equipment reduces the probability of accidents but does not eliminate it. Implementing multiple independent layers of safety barriers is more effective.
10. Continuous recording of major operational variables with redundancy in the control room is vital.
11. Automatic safety systems, such as cut-off of ammonia feed at the base upon temperature rise, automatic gas diversion to a adequately sized flare stack (capacity of at least 20,000 Nm3/h), and remote start-up of recapture pumps for spills, are crucial.
12. Storage capacity should be limited (e.g., to 80% of the cylindrical volume).
13. Non-return valves should be installed to prevent the backflow of warmer substances.
Sources / References
- Aria Report No 717 ; Rupture of a Cryogenic Ammonia Tank
- Lithium Ammonia Accident, March 20th 1989 ; Paper for IChemE Symposium Series No. 124 ; Bengt Orvar Anderson
Image taken from: https://www.lrt.lt/mediateka/irasas/2000260243/jonavos-azoto-avarija-bei-jos-pasekmes at time 2:40
Analysis
The incident at the Jonova fertiliser plant was a complex event triggered by a combination of operational errors and potential design and construction deficiencies. The introduction of warmer ammonia into the cryogenic tank created an unstable condition that led to a rapid pressure increase. The fact that the plant’s refrigeration compressors were offline exacerbated the situation, preventing the removal of excess vapour. The safety valves, with a combined evacuation flow rate of 8,400 m3/h, were insufficient to handle the surge in pressure caused by the rapid vaporisation of the warm ammonia. The rupture occurred at the base of the tank, likely due to the greater resistance of the roof structure and potential weaknesses in the base attachments. The escaping liquid ammonia quickly evaporated, forming a flammable cloud that was ignited by an undetermined source, possibly the flare stack or sparks from the initial rupture. The subsequent fire spread to nearby fertiliser storage, leading to a prolonged and environmentally damaging decomposition process releasing toxic nitrous oxides. The inadequate construction of the protective wall around the tank also contributed to the severity of the consequences, as it was breached by the escaping liquid ammonia, allowing it to spread further. The large scale of the incident necessitated a significant emergency response and the evacuation of a large portion of the local population.