4 tanks with 80 m3 storage capacity for liquefied chlorine in a store housing 5 tanks. The tanks contained different amounts of chlorine with some containing only gaseous chlorine as a residue after discharging, and some being filled up to 20% of their capacity. Each chlorine tank was located in a concrete basin designed to retain the entire tank contents.
This tank with a capacity of 80 m3 was involved in the chlorine release on 23 August 2002.
Description
After being flooded with water exceeding the 'hundred year water level' by 1.3 m, the empty and less filled tanks were lifted by the buoyancy forces according to Archimede's law and displaced from their normal positions. The upwards movement of the tanks deformed and lifted the walkways situated above the tanks.
The safety fittings of the full tank got caught in the walkway and were completely torn off as the walkway kept moving upwards. As a consequence of the valves being torn off from the full tank, a massive leakage of chlorine occurred.
The plant had implemented 100-year anti-flooding measures. However, the flood exceeded the 100-year flood by 1.3 m, and the severity of the flood was not forecast by authorities and was hence not expected by the plant.
Lessons Learned
Lessons Learned on Equipment
After the accident, a guide-rail structure for vertical guiding of the tanks was installed. This will avoid unwanted horizontal motion of the tanks in case of displacement. This measure was accompanied by a segmentation of the walkways which will move together with the tanks in case of floating.
Lessons Learned on Organisational Aspects
Chlorine storage was reduced to 50% of the capacity before the accident and therefore to one storehouse only, thereby significantly reducing the accident risk.
Also, prior to each scheduled shutdown of the sodium hypochlorite plant, chlorine is removed from the manifolds and pipelines, and before each scheduled power outage or disruption of cooling water supplies, the pipeline connecting the tanks in the storehouse must be disassembled.