Combustible Dusts A Design Guide

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Combustible Dusts A Design Guide

• 2008 Australasian
• Milling Conference

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Agenda

• Why me?
• Design Guide Project
• Regulatory Responsibility
• AS4360 approach to managing risk
• Summary of Key Control Measures

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Why me?

• RiskTech Pty Ltd
• Independent risk management consultancy
• Clients include
• Weston Milling
• Inghams
• Manildra Group
• Ridley

• Peter McGee
• BE (Env)
• Masters (FSE)
• Specialise in asset protection
• Focus on insurable risk

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Design Guide Project

• Inghams, Ridley, Weston Milling
• Confusion regarding selection of electrical
  apparatus
• Scope
• Design considerations for managing combustible
  dusts
• Not limited to selection of electrical apparatus
• Focused on a new mill, but can also equally be
  applied to an upgrade
• Also provides ability to audit against for
  non-conformances

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Design Guide Project

• Minimum Requirements
• Driven primarily by regulation
• Also includes common sense / easy to achieve
  controls such as maintenance and management
  controls e.g. housekeeping, no smoking rules and
  hot work permits.
• Best Practice requirements
• Driven primarily by insurance industry guides (FM
  Data Sheets, NFPA standards)

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Regulatory Responsibility

• Key requirement
• Electricity Acts ? AS3000 (Wiring Rules)
• ? appropriate electrical apparatus in hazardous
  areas ? relevant hazardous area standards
  (pre-curser to AS61241)

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Regulatory Responsibility

• Also OHS Acts and Duty of Care to provide safe
  workplace
• Only prescribed requirement in managing
  combustible dusts relates to electrical equipment

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Hazardous Areas

• Standards revised/updated
• Major change separation between combustible
  dusts and flammable vapours
• Complete new set of standards relating to
  combustible dusts AS61241 series

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AS61241

• Key changes
• Zoning 20, 21, 22
• Dust Layers
• Provides fire load
• Provides source for secondary explosion

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Electrical Apparatus

• Primary device Ex tD
• Also known as IP6X similar to DIP.
• Other devices include Ex mD, Ex pD, Ex iD.
• NB similar equipment in zone 20, 21, 22.

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Selection of Electrical Apparatus abridged
version

• Where
• Dust Layers lt 7.5mm
• Max surface temp of equip 75C lt minimum dust
  layer ignition temp
• Then
• Install IP6X, or IP5X for non-conductive dusts

• IP 5X 6X

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Combustible Dusts The Hazard

• Explosion can occur when
• Combustible dust present
• Suspended in atmosphere in correct proportions
  with air (O2) and
• Ignition source is introduced

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AS4360 Approach

• Risk Likelihood x Consequence
• Reduce Likelihood
• Avoid explosive atmosphere i.e. avoid dust
• Avoid ignition sources
• Reduce Consequence
• Control consequences i.e. manage explosion

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Design Guide Key Controls

• Plant Layout and Building Materials
• Ignition Source Controls
• Electrical apparatus (already addressed)
• Dust control
• Explosion Control
• Specific equipment
• Management controls and maintenance

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1. Plant Layout and Building Materials

• Limit the size of zones
• Prevent dust accumulation
• Exclude services (e.g. steam) from zoned areas
• Limit access to zoned areas
• Egress
• Non-combustible building materials

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2. Ignition Source Controls

• Hot work - more strict controls in zoned areas
• Static maintenance continuity testing
• Mechanical friction prevent product
  accumulation at bearings
• Foreign materials cleaning equipment
• Thermal decomposition
• Process heat failsafe thermostat controls
• Hot surfaces all surfaces lt 100C

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4. Dust control

• Reduce dust in suspension below the lower
  explosive limit (anywhere from 50-500g/m3)
• Design is crucial
• Volume sufficient to extract dust at pick up
  point
• Velocity avoid deposits in ductwork
• Air to cloth ration consideration for
  maintenance
• Safety factor
• Extract at every point of change in product flow
• Redesign when changes are made

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5. Explosion control

• Explosion venting should be primary method.
• Vent externally
• Ensure no structures/equipment exposed
• Suppression, containment, isolation, inerting
  other alternatives

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6. Specific Equipment

• e.g.
• Bucket Elevators
• Under-speed detection
• Track control
• Explosion venting
• Grinding
• Magnetic separation
• Air/gravity separation
• Monitoring for temperature and flow of air and
  grain

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7. Management Controls

• Housekeeping
• Maintain dust layers lt5mm at all times
• Best Practice audit program measuring layer
  thickness
• Management of change
• Training
• Maintenance
• Static earthing
• e.g. painting over explosion vents

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Summary

• Only regulatory requirement relates to selection
  of electrical apparatus
• Electrical apparatus are only one of a number of
  design issues to be considered when controlling
  combustible dusts
• Design changes provide the opportunity to
  consider these issues

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Lets help avoid this!