An explosion occurred at the production plant’s concentrated malodorous gases collection system on November 7th, 2016, at 3:14 am. The workers were exposed to concentrated malodorous gases at 7:26 am when they were changing damaged rupture discs. An explosion occurred at the production plant’s concentrated malodorous gases collection system. The collection system was built in 1989. Malodorous gases are collected from different parts of the process and burned in recovery boiler. Rupture discs and water locks are used for the overpressure protection. Veitsiluoto Mill in Kemi is an integrated production facility manufacturing office papers, coated mechanical papers and sawn products. Annual production capacity of three paper machines, pulp mill, ground-wood mill, saw mill and sheeting plant is 810,000 tonnes of office and coated magazine papers. Saw mill’s capacity is ca. 160,000m3. Pulp mill’s yearly production of 375,000 tonnes is used as raw material for office papers. Veitsiluoto Mill employs about 1,000 people, of which around 620 are employed by Stora Enso. When a worker lost consciousness his colleaque noticed it immediately and called for emergency center at 7:29 am. Process operator from the control room went to the site and found hydrogen sulphide odors. He returned to the control room and donned the pneumatic equipment. Also another process operator with proper equipment went to the site and together they resqued the victim to the fresh air. During the rescue more than 100 ppm of hydrogen sulfide was measured.The patient was wearing contaminated clothing, which had not been removed. The first rescue department units arrived at 7:40 am and the first ambulance at 7:49 am. The victim’s contaminated clothing caused by primary care personnel exposure during transport. Concentrated malodorous gases contain hydrogen sulphide, other sulphuric compounds, methanol and turpentine. According to the safety data sheet of Stora Enso, malodorous gases are extremely flammable (H220), fatal if inhaled (H330) and toxic if swallowed (H301) or in contact with skin (H331). The explosion occurred when air entered the system through a dried water lock. The explosive air-gas mixture was probably ignited due to static electricity. Several explosions had occurred in the same system during 2016. Most of the earlier events had only damaged rupture discs that were used for overpressure protection, but an explosion on June 29th had caused more significant material damages. The explosions had led to decisions about corrective actions but on November 7th some of them had not yet been fully implemented. The accident investigation identified deficiencies in both the state of the concentrated malodorous gases collection system and procedures related to maintenance work.

In order to prevent similar accidents, the investigation team recommends the following measures. The recommendations are general, concerning the entire industry.
1. Equipment that have been added to ensure the safety of the process must be placed so that their visual inspection and maintenance can be performed safely. Safety equipment and devices should preferably have appropriate indicators or be connected to the automation system so that their actuation can be detected.
2. The reasons for the actuation of the safety devices/equipment must be identified. Running the production against the safety controls must not be accepted as a normal procedure.
3. The risks related to the aging of equipment must be assessed. Special attention must be paid to process safety critical devices and their preventive maintenance, condition inspections and need for renewal.
4. The operator must ensure that the workers are aware of the hazards of different tasks and work areas as well as the related safety and work instructions. The instructions must include procedures on how to bring the process back into a safe state after a fault situation. Management must go through the essential safety instructions with the workers and supervise that instructions are followed.
5. Adequate communication between shifts must be ensured. Appropriate shift change and communication methods must be defined for operator-based maintenance workers.
6. Operator-based maintenance workers must receive sufficient guidance concerning the work area and the  tasks, including related hazards, before starting the work.
7. Liabilities and obligations concerning activities performed after fault situations (including shutdowns and startups) must be clearly defined.
8. Management of change must also include the assessment of the safety effects of smaller changes. The up-todateness of risk analyses, instructions and P&ID must be checked regularly.
9. The maintenance responsibilities for installations that are situated across several departments and units must be clearly defined.
10. Also when handling and storing chemicals that are formed during the production process, regulations and guidance concerning the handling and storage of hazardous chemicals must be followed. Internal audits and inspections must also take into account the failure mechanisms of process safety critical systems as well as procedures for corrective actions concerning fault situations, including the responsibilities and tasks of the operating and maintenance staff.