A spill of nitric acid occurred from a nitric acid absorber
tower when a flange failed. An operator noticed a small leak from the flange
bolts and proceeded to climb the tower towards the top isolating valve. At this
time the flange bolts failed and sprayed acid into the absorber tower bund. The
operator climbed to the next level and shut down the operation of the plant.
Once the acid level had dropped to below the level of the flange, gaseous
oxides of nitrogen were released. The operator then used water spray to reduce
the impact of gas and acid mist. Industrial neighbours were notified of the
release and the acid in the bund was neutralised with lime and sand, and later
used in another onsite manufacturing process.
Key learning points
The investigation revealed that the bolts in the flange were mild steel bolts,
which were not appropriate for use in nitric acid applications. These bolts
were found to be corroded, which eventually led to the leak. As a follow-up,
the company reviewed all the bolts used in the process. Five other flanges were
identified as having mild steel bolts rather than stainless steel bolts. It was
found that these bolts had not been replaced since plant commissioning. All
inappropriate bolts were immediately replaced with stainless steel bolts
suitable for acid applications. To prevent a recurrence the company implemented
quality control procedures and inspection test plans for new projects and
maintenance work. Verify that components, such as flanges and gaskets comply
with the material specifications.
Source:IChemE
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