Fire broke out in the gas exhaust chimney at an oil refinery. The unit involved was a vacuum distillation column equipped with an economizer device to allow preheating the load before reaching the oven by means of hot smoke stemming from combustion inside the same oven. Around 11 pm, a technician on his rounds detected a leak on the economizer: the refinery operator proceeded by decreasing the unit load, bypassing the economizer and injecting steam into the equipment. 40 mins later, incandescent particles were observed at the chimney, which led to shutting it down, isolating it and inerting the oven. Shortly thereafter, fire broke out inside the chimney. The internal emergency plan was activated; in-house fire-fighters controlled the blaze in just a few minutes and set up a protective water curtain. The unit was taken offline at 12:50 am.

The operator anticipated the following scenario: atmospheric residue leak on a pin in the economizer, carrying the product by smoke into the chimney flue at 200°C; increase in smoke temperature after bypassing the economizer (350°C) and partial residue combustion (incandescent particles); intake of air while the oven was off and complete self-ignition of the product. The initial leak occurred subsequent to the perforation of 2 tubes by a pin located around an idle ‘chimney sweep’, a device that served to clean the exchanger by injecting steam. An analysis revealed that the steam injection valve upstream of the sweep device was leaky, thus causing slow lamination around the tubes, with the sweep turned off.

The operator implemented the following measures: verifications of the chimney (brickwork, steel lining), ducts, instrumentation, economizer (tubes verified using gammagraphy, with replacement of damaged tubes, sweeps verified and a hydraulic test conducted). As for experience feedback, a preventive chimney sweep verification programme was undertaken, along with a study to install both a double valve system with a drain on the steam injection line (closed in 2004) and deflectors at the sweeps. Moreover, the unit’s emergency shutdown procedure was revised to incorporate a sequence corresponding to losing the economizer, thereby avoiding self-ignition conditions at all times inside the chimney.