At 6:15 a.m, noises were heard and vibrations were felt at the black-liquor steam generator of a paper mill. The steam generator usually operates at 4 mbar negative pressure. However, excess pressure of between 20 and 30 mbar was registered inside it. After a few disturbances, the monitoring parameters went back to normal.
With the steam generator in operation, the site’s inspection department for pressure vessels (SIR) visually inspected the outside of the lower portion of the hearth, but found nothing out of the ordinary. Blocks of sodium sulphate are generated by the combustion of the black liquor. A falling block was suspected as the cause of the noises and vibrations. The fuel-oil supply was turned on to melt the deposits.
That night, a water leak was detected on the boiler (moisture on the nozzle-scouring bar, abnormally low temperature on the primary air inlet). The boiler was made ready (smelt drained) in case a quick blowdown was required.
The water level inside the boiler’s tank subsequently became unstable. At 1:30 a.m., the flow rate of the supply water suddenly increased. The order to drain the boiler was given at 1:35 a.m. The building was contained and all access to it was prohibited.
All the paper machines and pulp production facilities on the site were stopped. Several weeks were required to inspect, repair, and restart the boiler. Production losses were estimated at between €2m and €3m.
A complete inspection of the steam generator, with metallographic analyses and thickness measurements, was scheduled for 16 December to locate the leak and check the equipment’s integrity. The operator took advantage of the shutdown to inspect 45 other pressure vessels. The inspections found that a steel tube with a steel coating was punctured. The assumed cause was the combination of corrosion-erosion due to turbulence in the intake air and pitting corrosion due to the presence of chlorides (contaminants present in the fuel?).
The SIR intends to revise the boiler’s inspection plan, in particular by increasing the frequency of checks. This plan will be re-amended once the source of the corrosion has been identified.
