The sulphuric acid (H2SO4) production unit of a chemical plant was shut down after the high pressure alarm had activated. The sulphur burner outlet of the installation revealed a leak of vapour from the boiler tubes, which ensured heat exchange between the sulphur vapours and the steam. This leak had allowed water to enter into the acid production system and caused the acid concentration to decrease and the installation pressure to rise.

Boiler repair required pre-drying the unit by blowing air; this resulted in the release of 50 kg of sulphur dioxide (SO2) via the chimney. However, no impact was observed on measurements carried out by the local air quality surveillance system.

The unit was stopped for 1 month in order to carry out repairs. The production of sulphuric acid dropped from 200 to 95 t/d. Due to a lack of acid, the plant’s calcining furnace had to be stopped, resulting in a production loss calculated at 1,500 t of titanium oxide.

Corrosion by erosion of the welds in 28 out of the 975 boiler tubes was the cause of this incident. Dating from 1975, the boiler had been completely retubed in 1991, and the most recent periodic controls conducted in October 2006, in compliance with the Pressure Equipment Directive (PED), did not identify any specific defects in the welds.

Before restarting the unit, all of the welds on the boiler were checked, the tubes causing the leaks were plugged and a hydraulic test and helium test were carried out. Complete retubing of the boiler was planned for the future.