Acting as the buffer tank between a continuous process and a discontinuous process in a chemical plant, a vertical 200m³ GFRP (Glass Fibre Reinforced Plastic) tank cracked rapidly, then collapsed to the ground causing a break in its retention system: 100 t of a mixture of 70% of sulphuric acid at 60% and 30% of lactam (acid organic polymer) spilled on the ground. The shell of the tank ruptured under the action of a vacuum created by the sudden drainage (the GFRP equipment did not withstand the vacuum). The polluted soils were neutralised with calcium carbonate. The gravel excavated over a depth of 20 cm was stored on the site in swaths equipped with waterproof liners. The effluent spread over the asphalted surfaces was collected and directed towards a buffer tank in order to be treated; no discharge into the GAVE river was observed. Piezometric monitoring of the pH was installed; no impact was indicated. SO2 and H2S measurements taken on the property’s boundary in the vicinity of the tank were negative. All sections of the production workshop were shut down, and the operating, safety and maintenance teams were mobilised. This shutdown was continued for 9 days, which was the time needed to find an alternative solution.

According to the operator, the accident was caused by the combination of 3 factors:

• the presence of a fixed point against the tank’s wall (solid walkway of the retention wall) which constitutes an area of significant mechanical stresses. The liner was cracked at this level, causing acid to enter into the resin constituting the tank’s shell;
• the use of isophythalic resin, whose acid resistance is limited, which resulted in decohesion of the stratification in presence of the infiltrated acid;
• the vertical shape of the tank produced significant pressure in its lower part, pressure which finally caused rupture of the resin weakened by the acid attack (the buffer function of the tank requires frequent drainage and filling cycles, the number of cycles permissible with mechanical fatigue reduces rapidly when stresses increase; due to lower heights of liquids, horizontal tanks are less sensitive to these phenomena);

However, use of these resins does not present any danger so long as the integrity of the waterproof and anti-corrosion internal liner is preserved. The accident analysis retained a tank design error as the main cause given its usage in the process. Through domino effects when the tank fell, this could have caused the pipes to be wrenched off or the storage tank or a production unit to be split open. For feedback purposes and based on similar accidents that have occurred on other sites (ARIA 6838 and 32680), the operator modified the design principles of the tanks intended for this type of use:

• to avoid constructing tanks in GFRP with a thermoplastic liner acting as the anti-corrosion barrier (the difference in the mechanical behaviour of the 2 materials promotes the cracking of welds at singular points)
• to avoid using tanks that are high in relation to the diameter (the tank affected was 12 m high)
• to use a high chemically-resistant resin (vinylester type) for the GFRP structural layer
• to use a non-pigmented external finishing layer to facilitate early detection of degradation of the structure
• to limit the tank’s attachment points on walls, or distribute the loads.