Wood processing and woodworking facilities
Protection
Important pneumatic dust-handling systems, which have a high production impact and need spark detection/extinguishing protection, should utilise a multi-zone spark detection scheme arranged as follows:
a) Zone 1: should activate a spark-extinguishing system on the blowpipe. The system will reset with no downtime, as long as no additional sparks are detected.
b) Zone 2: located just downstream from the spark-extinguishing nozzles, it should simultaneously shut down the process, isolate the affected equipment (e.g., stop rotary airlocks, reverse conveyors, etc.) and actuate a water spray system in important downstream equipment where appropriate (e.g., cyclones, bag filters). Automatic sprinklers are an acceptable alternative to a water spray system, but are less desirable when there is dust explosion potential.
c) When it is practical and cost-effective to do so, provide a high-speed abort gate triggered by Zone 2 in lieu of deluging the downstream equipment. This can save downtime such as might be incurred by having to dry out or replace wet bags in a large bag filter. If this is done, a third detection zone would be needed downstream from the abort gate to activate a bag filter deluge system.
d) In all cases, conveying fans or blowers should be left running. This will help purge burning material from the system and distribute water spray throughout the equipment. In the case of bag filters, this will also help prevent an explosion by keeping dust clinging to the bags.
e) Spark detection may also be used on enclosed process equipment other than pneumatic systems where it is beneficial to do so (e.g. drop points from screw conveyors, cyclones, pulveriser outlets).
Occupancy
A high level of housekeeping should be maintained throughout these facilities. On equipment which handles fine material and/or causes fugitive dust to be emitted, emphasis should be placed on making the equipment dust-tight or using negative air pressure dust-collection systems to help prevent dust from migrating and collecting on structural members, equipment etc. For inherently dusty processes (e.g. particleboard and fibreboard), provide central ‘‘vacuum’’ dust-collection systems to facilitate plant clean up and minimise dust clouds.
Cyclone Collectors
General
Cyclone collectors separate dust from a gas stream by creating a spiral flow within it. The centrifugal force on the dust particles results in their moving toward the wall of the cyclone. The gas stream carries the particles to the bottom of the collector. Dust separates from the stream when the gas turns upward at the base of the collector. The dust can be deposited from the base of the collector into a hopper, dustbin or screw conveyor.
Cyclone collectors are most efficient when handling dust in the 10 to 300 micron range. They are used widely for the collection of coarse dust generated in operations such as woodworking and metal grinding. They may also be used as rough filters to reduce dust-loading on bag-type dust collectors or electrostatic precipitators.
Conventional Cyclones
A conventional unit is the simplest of the cyclone collectors. One unit, usually a large diameter cyclone, is provided for the dust-laden air stream. The cleaned air is expelled into the atmosphere or to a more efficient collector for further treatment.
High-Efficiency Cyclones
High-efficiency cyclones consist of a number of small diameter cyclones, usually less than 250mm in diameter, arranged parallel with a common hopper and common inlet and outlet plenums. Separation forces increase with the radial velocity of the particle. Radial velocity in turn increases with the decreasing diameter of the cyclone tube. High-efficiency cyclones may make further air treatment unnecessary.
People in Risk
Marcel Wood
Marcel Wood heads Etana’s risk management division. Before starting in insurance he worked for the fire department, rescuing people and (literally) putting out fires. He’s passed through Munich Re and Santam, and has designed the only risk management tool aimed specifically at the insurance industry – ARQ. Marcel is a mean guitarist, a brilliant vocalist and an accomplished soccer player with the personal motto: “Do your best all the time”.
