“Unlikely fires” occur when several factors collide

Lessons from Losses 5/2013. Two large-scale conveyor fires have recently taken place in the Swedish pulp industry. The fires were substantially different in character and scope, but the lessons to be learnt from them are largely similar.

​Both fires confirm the theory that a fire is most often caused by two or more adverse factors happening simultaneously, resulting in an “unlikely fire”.

Fire no. 1 – wet wood can’t ignite

The first fire was “highly unlikely” in that it took place in a wet wood chip pile, from where it spread to the conveyor system and adjacent wood chip and bark piles. No one thought wet wood chip could ignite, unless intentionally lit.

The cause of the fire has not been established with certainty, but one possible scenario is that a hot particle or object may have been transported on the conveyor and fallen onto the wood chip pile. Thereafter, the very strong wind may have contributed to the wet wood chip igniting and the fire spreading.

Large-scale destruction

Because the fire spread through the wood chip and bark piles and ignited the conveyors from the outside, the conveyors’ built-in sprinkler system was unable to prevent the complete destruction of the entire conveyor system.

Due to the large quantities of wood chip and bark in the area, which were stacked high and have high energy contents, the fire was impossible to fight effectively.

The available fire-fighting resources proved insufficient and they initially had to be harnessed solely to preventing the spreading of the fire to the digester’s conveyor bridges and the screening plant.

The fire was extinguished with the help of external resources such as helicopters, extinguishing equipment from SMC and diggers. The extinguishing efforts took one week. For practical reasons, seawater was used in the extinguishing process. Unfortunately this made the soaked wood chips difficult to dispose of after the fire. Had freshwater from a lake, for example, been used instead, the soaked chips may have been worth more.

Preventing the fire from happening

Reconstruction began immediately after the fire had been extinguished. All those involved did their best to identify optimal fire safety solutions that could prevent a similar fire from happening again.

In summarised form, the fire safety measures taken in the new conveyors and the storage area were as follows:

  • Fire-resistant rubber conveyor belts were installed.
  • All conveyors were equipped with sprinklers. Fire alarms (based on capillary tubes) were installed in all conveyors.
  • Some degree of fire sectioning of the conveyors was achieved using partitions and guillotine hatches.
  • Mobile fire pumps were acquired.
  • Sprinkler pipes (open ones) were installed along the lower edge of conveyors passing over wood chip and bark piles in order to facilitate dousing of the top layers of the piles.
  • Manual temperature control of the conveyor rollers was made possible using infrared cameras. Measurements are taken once a week.
  • Cleaning routines were improved and conveyors were built in a way, which makes cleaning significantly easier.
  • Readiness against similar fires was improved. External resources have been listed so they can be summoned more quickly.
  • Protective measures are also being planned against cases of things falling onto piles from a conveyor. The installations would consist of spark indicators and an extinguishing system.

Fire no. 2 – friction in tight places

This fire started in the conveyor that carries wood chips from the wood room to the chip silos.

During the morning, maintenance was performed on a conveyor bridge (the partition conveyor between two chip silos) underneath the conveyor in which the fire started. In the maintenance actions, a new friction coating was applied to the drive roller.

The fire was caused by the conveyor belt having slid sideways so that it lay against the drive roller’s cover. Heat caused by friction ignited some wood chip or dust found between the cover and the conveyor wall, and the fire quickly spread from there due to the cold and windy conditions that evening.

The distance between the cover and the outer wall was very small, which meant that a quantity of woodchip and sawdust had gathered there. That space had been difficult to access and therefore clean in the weekly cleaning of the conveyors.

Sprinklers from auto to manual

On the day of the fire, a leak had been observed in the sprinkler system, which was causing pressure fluctuations in the system. Concern over the fact that the pressure changes could set off the sprinkler system (which is a dry pipe system) and therefore cause a risk of freezing of the pipelines resulted in the decision to turn the sprinkler system onto manual control with additional guard surveillance of the plant at night (one fire officer with a vehicle onsite overnight). There were plans to fix the water leak on the following day.

Upon detecting the fire alarm from the conveyor system, the fire officer was quickly able to confirm the presence of a fire in the conveyors and activated the sprinklers.

There was a wood chip pile directly under the burning conveyor, which could have been ignited by falling embers or materials, but such spreading was prevented by the pile being partly covered by snow. When the fire services arrived, they focused their efforts on preventing spreading of the fire to this wood chip pile and to nearby chip silos. The silos had relatively large vents at the top and there was an evident risk that glowing materials could enter through them and cause a fire.

How to prevent the fire from happening

The following lessons were learnt and fire safety measures taken from this incident:

  • Conveyor bridges were rebuilt with a significantly bigger distance between the belt and the wall, so that the gap is easier to clean.
  • Fibre-optic fire cables were installed in conveyors as a supplement to the existing, traditional smoke detectors.
  • Post-checks must always be carried out after maintenance work has been performed on conveyors.
  • Sensors will be installed, which will cause an alarm and stop the conveyor if the belt slides sideways.
  • Hoods have been installed over the vents on silos to prevent external fires from entering the silos.
  • The temperature of each of the conveyors’ supporting rollers is checked weekly using an infrared camera.
  • A security guard checks all the conveyors each night.

Other safety measures, which have been taken at similar plants include:

  • Belt turners in the conveyors, which mean that spillage is captured in just two places.
  • Placement of manual fire extinguishers in the conveyor systems to allow swift fire responses by personnel.
  • Installation of dry risers in central locations to allow for quick access to extinguishing water.
  • Use of “sealed” air-carried Tubulator rubber conveyors instead of traditional rubber belts.
  • Disaster drills with a focus on the wood chip and fuel stores and the conveyor systems.

In conclusion, it can be said that improved preventive measures when it comes to safety equipment and routines within and around conveyor systems could prevent fires and/or reduce the extent of damages.

Anders Tjärnberg