Fatal accident on a heating plant

06 May 2012
Two workers, one for the heating plant and the other one a truck driver, were emptying a load of stumps into a sieve when they heard a loud bang coming from the hammer crusher. The worker of the heating plant shut down the crusher and went inside to check what had caused the sound.

​After a while, the truck driver followed in after him to find the visibility poor, the maintenance hatch above the crusher missing and the other worker not to be seen. He went to a break room nearby to seek for help and came back with the plant manager who discovered that the worker had fallen into the crusher from the open maintenance hatch not leaving much behind.

What exactly happened?

The crusher was equipped with a heavy steel shield bolted into the frame. It was well known that sometimes the stump load could include metal particles, which can cause malfunctions in the crusher. This time the metal piece was big enough to cause a strong tension against the protective shield and the bolting gave up. The shield flew up hitting the maintenance hatch above so hard that it flew away.

The visibility inside the plant was poor because of the dust rising from the shieldless crusher. The worker probably didn’t see that the hatch was misplaced or he lost his balance for some reason watching down from the open hatch. The type of the crusher was such that it didn’t have brakes to stop the movement when turning the power off, so it was still rolling when the worker went inside to inspect the situation.

What caused the accident?

The direct reason for why the worker fell down from the open maintenance hatch is not known, but there are several reasons that more or less had an effect on the accident.

The metal pieces in the load apparently broke some blades of the crusher and they wedged against the steel shield. Due to tremendous forces the bolting of the shield broke and it flew rapidly against the maintenance hatch above. The bolting was strong but fatigue of the metal might have had an effect.

The maintenance hatch above was quite heavy (80kg), but it wasn’t connected in any way. The weight itself wasn’t able to keep it in place.

Increased amount of dust was able to spread in the air when the steel shield disconnected so the visibility inside the plant was really poor compared to a normal situation. Because of this, the deceased worker might not have seen that the maintenance hatch was misplaced or he might have even tripped and fallen through the hatch.

The type of the crusher was such that it didn’t stop instantly when shutting the power down. Especially in that kind of bad visibility, it would have been reasonable to wait until the crusher stops before going in.

Technical solutions to prevent similar accidents

  • In the heating plant where the accident occurred, the crusher was removed from use and replaced by a slow rotating crusher. Crushers with slower rotation speed are safer in case of similar wedging situations.
  • The machines should always have a control panel to allow complete shutdown. The access to danger-zone should be completely blocked until the movement has stopped.
  • The structure and connections of machines and protective shields should be sized so that they are strong enough to endure the forces generating in abnormal situations as well.
  • All maintenance hatches regardless of size and weight should be connected somehow to prevent unexpected detachment.

Besides technical solutions or if technical solutions are not applicable or reasonable, work procedures must be defined so that safe working conditions are guaranteed. The procedures must be instructed to all workers and following of safety rules must be supervised. It is also vital that workers have clear and simplistic guidelines for cases of malfunction.

Ville Niemelä, If