A gas boiler exploded during a restart test phase in a steel plant at 11:13 p.m. The site’s firefighters were able to secure the facility. Although the boiler was severely damaged, production operations were not affected.
The boiler had just been put back into service after an extended maintenance shutdown period following a series of events in early 2016 (ARIA 47992 and 48395). For several weeks, the plant’s operation was crippled and relied on just one of the site’s four boilers.
During the afternoon, the shift foreman attempted to light the boiler’s first burner a total of three times, but was unsuccessful. The night shift foreman also tried to light the boiler nine times, without success. The ignition sequence was followed from the beginning after each attempt. On the tenth attempt to light the burner, he decided to shunt the flame detection automaton of the 1st burner, then the 2nd, and initiated the 3rd so as to bypass the pre-ventilation phases in the case of non-detection. The gas injection flow rate on the first two burners thus created an estimated volume of 90 m³ of propane when the third burner ignited, forming an ATEX (potentially explosive areas) zone which resulted in the explosion. The facility operator identified several root causes of this accident:
- the shift foreman was alone when the decision was made: night shift, assistant in training;
- a significant level of stress in the workplace owing to degraded operation of the facilities in recent weeks;
- under-estimation of the risk: new safety guidelines prohibit the presence of the technician in charge of monitoring the pilot flame and lighting the main flame;
- the shunting of the flame safety devices was not sufficiently secure; only the procedure prohibited it, knowing that another procedure authorised the shunt of this safety device for the pilot flames.
To avoid this type of accident, the operator:
- modified the igniter test procedures;
- installed flame cameras on the boilers;
- secured access to the key used to shunt flame detection at the burners;
- studied how to secure the automata;
- reminded employees that it is prohibited to shunt the main flame of the 1st burner;
- organised annual safety training refresher courses for the boiler technicians.