At a chemical facility, a recently replaced graphite safety disc (nominal diam.: 250) calibrated at 3 bar broke on a 12-m³ reactor used to solubilize a Resol resin. This intermediate resinous agent, included in production activities for the hide tanning industry as well as in Bakelite manufacturing, was derived by: sulfonation of phenol with sulphuric acid, neutralisation with soda lye, and condensation with formaldehyde.
This solubilization step began on 3rd March and lasted 16 hours; the reactor, loaded at 2 pm (with 30% soda lye, sodium bisulphite and formaldehyde), started heating at 2:40 pm until reaching 125°C (3 bar of steam). After turning off the heat, the exothermic reaction drove the temperature up to 137°C. The technician tracked this value on a local thermometer and remained on-call to restore steam heating as needed. Samples were extracted every 2 hours (for a salt content) in order to monitor progress of the reaction. The measured index values matched predictions through 4:30 the next morning, i.e. 12 hours after initiating the solubilization process. Upon resuming his shift at 4:15 am, the technician recorded a temperature of 140°C and pressure lying between 2.4 and 2.6 bar. The salt content value at 4:30 had been constant for 2 hours. To maintain pressure at 2.5 bar, the reactor was heated with a small amount of steam around 5 am. The pressure oscillated from 2.4 to 2.6 bar and the temperature read 139°C when the disc broke at 5:40; 5,000 litres of reaction mix spilled outside falling onto 200 m² of the reserved warehouse zone. Fire-fighters equipped with self-breathing units first spread neutralising agents and absorbents, then a subcontractor collected the residue and cleaned the ground. The operator issued a press release.
An appraisal confirmed the hypothesis of a disc calibrated at a value too close to the reactor operating pressure (3 bar, tolerance: +/-10%; fatigue coefficient: 0.85; temperature: 140°C, thus favouring a lower bursting pressure). The installation was modified: high pressure alarm set with a threshold at 3 bar, exhaust valve calibrated at 3.5 bar, rupture disc calibrated at 4 bar for 140°C, rupture disc exhaust pipe rerouted to an airtight and little-used zone. The definition and design calculations of safety devices were revised. Operating protocols were updated (T and P indicated on the existing tracking sheet). The other safety measures applied onsite were catalogued and verified, as were the exhaust collection and associated discharge points.