Inside a vinegar plant, a 150-m³ tank containing 38.5 m³ of an alcohol vinegar mixture exploded around 9:40 am. The 3,000-m² site had been in activity start-up mode, but engineering works were still underway. The tank bottom became detached from the shell. It’s upper part, measuring 13 m high, 4 m in diameter and weighing some 3.5 tonnes, was projected upwards, by missile effect. It perforated the building roof and landed 80 m away on a rail line. Two subcontractors were injured, one sustaining serious burns. Both required hospitalization. Fire-fighters extinguished a fire outbreak in the company offices, most likely due to a compromised electrical installation. Nearby rail traffic
was suspended. The vinegar spilled on the floor and was collected in the site’s retention basin. The building adjacent to the tank was severely damaged and at risk of collapse. The neighboring tanks, not fastened to the ground, were deformed either by the explosion blast or subsequent to striking one another. A 42-m² opening was visible in the siding near the tank. The projection through the roof left a 20-m² hole.
ORIGIN AND CAUSES OF THIS ACCIDENT
Hazardous substances present prior to the completion of works
The plant was being restarted following the relocation of its installations. More specifically, all of its pipe connections had not yet been finalized. The facility operator had initiated the transfer of activities prior to the complete execution of the various on-site works. Ongoing production runs were stored in the tanks while awaiting the hookup step. The operator had validated the policy whereby all vessels containing liquids with an alcohol content of less than 11% were to undergo assembly work without requiring preliminary drainage.
The tank involved had been installed on its base without being fastened. It had been filled 5 months before. The 38.5 m³ contents came from various mixes prepared at the former production site. This volume was composed of water, vinegar, alcohol and acid. The degree of alcohol in this liquid had been estimated at 9%. The planned operation consisted of welding 2 pipe brackets onto the shell. No prevention plan or hot work permit had been issued prior to initiating these works. The 1st weld was set 2.10 m above ground (i.e. 1 m below the liquid level), while the 2nd one was positioned at an elevation of 5.70 m (2.60 m above the liquid surface). The explosion occurred when readying for the 2nd weld. A pressure release noise could be heard by welders just a few seconds before the blast.
Unfamiliarity with the ignition risk
An expert appraisal was conducted by a specialist body in order to identify the causes of this accident. The first point appearing in the operator’s post-accident assessment was that the mix contained in the tank had an effective degree of alcohol near 20%. At this concentration, the flash point of the mix is 36°C. The study indicated that the act of fastening the first bracket by welding, performed below the liquid level, caused local heating. The liquid temperature, initially estimated at 30°C in light of meteorological conditions, most likely rose to a temperature near that of its flash point. The heat created an explosive atmosphere within part of the tank’s expansion space. The 2nd weld, which proceeded during the gaseous phase, provided enough energy to ignite the gaseous mix. According to the study, only 10% to 20% of the expansion space volume needed to be at the lower flammability limit concentration in order to trigger the observed effects.
Another possibility is that the current used to perform the weld (TIG) triggered a phenomenon of electroerosion eating away the tank’s stainless steel material. This reaction could have produced hydrogen since the liquid mix did contain acetic acid. More specifically, the reaction between steel and diluted acids caused a hydrogen release. As for the ethanol concentration already present in the expansion space, it is possible that a slight addition of hydrogen (with a lower flammability limit at 4%) was enough to render the entire mix flammable.
Moreover, the installation configuration featured a number of conditions needed to generate the observed effects, i.e. :
- a pressure-resistant and non-fragile tank with respect to the shell/dome connection;
- an elongated and vertical tank shape;
- a pressure peak encountered during the blast due to gas ignition until rupture of the shell/dome connection;
- high-speed ejection of the liquid through the tank bottom, thus increasing the propulsion output. Given the amount of energy available, the shock wave was nonetheless significantly dissipated due to the proportion of energy spent tearing the tank, roof and siding and projecting the tank, thus limiting the consequences of this explosion.
ACTIONS TAKEN
Subsequent to this accident, the plant operator took a number of corrective actions in order to satisfy regulatory compliance, with priority assigned to defining the explosion risk zones. The operator also adopted the following measures:
- preparation of a single document along with workstation data sheets for the purpose of informing employees of the risks present. Part of this document was devoted to the set of measures relative to explosion protection;
- definition of the ATEX (explosive atmosphere) zoning, giving rise to the implementation of all necessary prevention measures. Accordingly, the operator displayed regulatory posters, including pictograms indicating the presence of flammable liquids in certain tanks;
- development of a mandatory prevention plan along with a hot work permit system to better supervise all maintenance work or installation modifications.
SOURCE: European Union Network for the Implementation and Enforcement of Environmental Law (IMPEL)
https://www.impel.eu/wp-content/uploads/2018/01/Brochure_IMPEL2017_EN.pdf