The smallest-sized wet wood dust is just as explosive as dry wood dust according to a report prepared by FPInnovations for the sawmill sector. Source: Vancouver Sun
The surprise finding, which raises questions about the usefulness of misting at sawmills, was part of a first-of-its-kind study in Canada ordered after a pair of deadly sawmill explosions in the province last year that killed four workers.
“It was assumed moisture would be a bigger factor,” said Darrell Wong, one of the report’s authors. He is a manager of FPInnovations, the nonprofit forestry research centre at the University of British Columbia.
But Wong said more study must be done before sawmills should consider jettisoning misting systems. Misting systems have a secondary function of knocking wood dust out of the air.
Wood dust suspended in the air was confirmed as the fuel source for the two explosions by WorksafeBC, the province’s chief workplace safety agency.
As part of the new study, hundreds of dust samples from 18 sawmills were analyzed.
Ken Higginbotham, a spokesman for a group of 10 major lumber producers who also helped fund the study, said they were also surprised by the moisture findings.
Higginbotham noted the “ideal situation” may be to use equipment to suck dust directly out of the building.
The study also found there is not much difference among the explosiveness of various types of wood dust of timber, including type of wood (spruce, pine, fir, Douglas fir or cedar) and timber killed by the mountain pine beetle.
That suggests timber killed by the beetle has not had its properties changed to make it more explosive, said the report.
Among the factors needed to create a dust explosion is fine particles suspended in the air.
FPInnovations applied two criteria to determine which areas in the sawmills were at greater risk of an explosive hazard: the accumulation of wood dust at a rate of greater than one eighth of an inch in an eight-hour shift and samples that have more than 40% of particles that were 425 micrometres (just under half a millimetre) or less in size.
Just 20 wood dust samples met those criteria, with 14 of those from mills that were processing beetle-killed timber. A majority of these samples were collected from under or near conveyors and in basements.
This information can help sawmills determine areas of risk, and lead to fixes through design or maintenance, said Wong.
The accumulation can be checked initially by eye: if you can’t see the colour of a surface through the dust then it’s too much dust, he said.
It’s important for mills to verify the risk through testing by a lab after they have pinpointed areas where fine dust is accumulating. Recently, BC’s major forest companies completed the creation of a wood dust audit standard they promised last year after the two deadly sawmill explosions.
Wong said the audit process is a good starting point to help identify areas of wood dust explosive risk.