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Chris Bloore delivered the presentation at the 2014 Dust Explosions Conference. The 2014 Dust Explosions Conference examined industrial hazards, the means to control or eliminate dust and analysed the latest technology to ensure the maximum protection and safety of organizations. The event also featured recent industrial case studies and new safety recommendations. For more information about the event, please visit: http://www.informa.com.au/dust14
Citation preview
Dairy Industry Systems Consultant
Checklist Development as an
Aid to Auditing Industrial
Processes Involving
Combustible Dusts
Dr Chris Bloore
Dairy Industry Systems Consultant
2014
Dairy Industry Systems Consultant
Summary
This presentation uses spray dried food products to
illustrate how systematically creating a checklist
allows potential safety hazards to be identified.
The result is a greatly simplified HAZOP procedure.
The real value of this approach does not lie in
"ticking boxes", but rather in the systematic process
of identifying boxes that may need ticking.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Introduction
There are obvious hazards from a fire or deflagration
within process equipment. These include primary
events such as:
• Flames emerging into occupied spaces.
• Loose items becoming missiles.
• Smoke filling buildings, impeding evacuation.
Checklists for Explosion Hazard Auditing
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Introduction
Secondary explosions are responsible for most
injuries and deaths, so it is important to prevent
deflagrations escaping from process equipment
and to minimise potential fuel within buildings by
good housekeeping.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Introduction
There are other hazards, however. These include:
• Falling from buildings while replacing explosion
panels.
• Explosion vent duct heaters igniting dust within
the vent duct.
• Explosion vents discharging on to catwalks and
stairs.
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Introduction
A final category of hazard is associated with fire
and explosion protective systems. These include:
• CO2 fire suppression systems which create a
dust cloud which then explodes.
• Dry powder or pressurised hot water explosion
suppression systems which activate while
personnel are inside process equipment.
• Mechanical failure of vessels caused by the
weight of firefighting water.
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The Milkpowder
Industry
Dairy Industry Systems Consultant
Milkpowder Plants
The New Zealand dairy industry spray dries over
1.2 million tonnes of milkpowder annually, earning
about 10% of its total export income.
The Australian dairy industry makes about half this
amount.
Parts of spray driers contain an explosible mixture
of powder and air whenever the plant is operating.
Fires and dust explosions are a real hazard in
milkpowder production, and several incidents take
place every year.
Checklists for Explosion Hazard Auditing
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Spray Driers
Dairy spray driers are basically large stainless steel
chambers supplied with large amounts of hot air.
Concentrated milk is sprayed into the air, where it
dries to a powder in about 30 seconds.
Most spray driers have one or more fluidised bed
secondary driers to complete the drying of the
powder over 15 to 30 minutes by blowing warm air
up through a layer of powder about 150 mm thick.
Some driers have such a bed built into the bottom
of their chambers.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Some drier designs with separate fluid beds.
Spray Driers
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Some drier designs with integrated fluid beds.
Spray Driers
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Checklists
Checklists are an invaluable safety tool in many
circumstances where a complex series of actions
and checks must be performed on a regular basis.
The classic case is the aviation industry. Aircraft
operation and maintenance involve repetitive
procedures which must be rigidly adhered to.
Can checklists be applied to fire and explosion
safety audits?
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Checklists
Safety audits or full-blown hazard and operability
(HAZOP) studies are carried out at the design
review stage of new process plant installations, and
when engineering or procedural changes are made
to existing installations.
Every process plant installation is unique. While
many features are similar to those of other
installations, each must be considered on its own in
some detail.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Checklists
In the course of carrying out many safety audits and
HAZOP studies in the milkpowder industry, I have
found the process of developing a checklist to be
very helpful.
Many dairy companies have multiple sites with
multiple dryers. Taking a checklist from one site to
another helps focus on the common hazards and
highlights the differences between dryers and sites.
These differences include staff attitudes and
experience.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Oxygen
Fuel Heat
Dispersion Confinement
Checklists
Deflagrations are powder fires with two extra
features, so the first step is to identify areas with
dispersed powder and potential ignition sources.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Checklists
This presentation illustrates my approach using
milkpowder plants as examples. A relatively small
number of common issues appear in various
combinations:
1. Dust concentrations in normal operation.
2. Abnormal events which can cause dust clouds.
3. Potential ignition sources, both external and due
to self-heating.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Checklists
4. Places where a deflagration could escape from
the process.
5. Isolation between various equipment items.
6. The adequacy of any venting or suppression
systems.
7. Means of egress for staff.
8. Hazards associated with firefighting.
9. Hazards associated with reinstatement of plant
following an incident.
Checklists for Explosion Hazard Auditing
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Dust
Concentration
Dairy Industry Systems Consultant
Dust Concentration
In many cases it is possible to calculate the
average powder concentration in spray dryers and
fluid bed dryers, and in dust collection equipment
such as cyclones and baghouses.
In other cases, experience allows a reasonably
accurate assessment of the normal concentration of
powder in the air within plant items.
In all these cases, normal operation is assumed.
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Calculating Dust Concentrations - Chambers
The average concentration of powder in the air
within a spray dryer chamber may be calculated by
a heat and mass balance:
1. The amount of energy needed to heat up and
evaporate 1 kg of water is:
Dhevap = 2256.7+4.219*(100-Tfeed) kJ/kg
2. The amount of energy released by 1 kg of air as
it cools from the dryer inlet temperature to the
outlet temperature is:
Dhair = Cp*(Tinlet -Toutlet) kJ/kg
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Calculating Dust Concentrations - Chambers
3. The amount of drying air needed to evaporate 1
kg of water is:
Air = Dhevap/Dhair kg
4. The amount of powder produced by evaporating
1 kg of water is:
Powder =TS/(100-TS-Moisture) kg
5. The amount of powder per kg of air is:
Concmass = Powder/Air kg/kg
6. The amount of powder per m3 of air is:
Concvolume = 1000*Densityair/Concmass g/m3
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Calculating Dust Concentrations - Chambers
The previous analysis takes no account of any fines
being returned to the chamber.
Checklists for Explosion Hazard Auditing
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Calculating Dust Concentrations - Fluid Beds
The average concentration of powder in the air
within a fluid bed secondary dryer chamber is very
low, but there are two places where it may become
significant:
1. A thin (10-15 mm) layer of powder in ballistic
trajectories above the bed. This represents less
than 1% of the volume of the bed headspace.
2. A dust cloud around 1 m3 in volume where fines
are returned to one or both ends of the bed. This
represents less than 10-15% of the volume of the
bed headspace.
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Calculating Dust Concentrations - Cyclones
Cyclones typically lose less than 1% of fat containing
powders and less than 1.5% of low fat powders.
Most of the powder in cyclones spirals down the
walls, leaving the bulk of the volume occupied by air
with at most 1.5% of the concentration in the air
entering the cyclone.
In many dryers much of the air falls on to a fluid bed
and does not even reach the cyclone(s).
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Calculating Dust Concentrations - Baghouses
Baghouses typically lose less than 20 mg/m3 to
comply with emission limits.
Powder concentrations in the inlet air will depend on
whether there are cyclones installed ahead of the
baghouse.
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Calculating Dust Concentrations - Baghouses
1. When there are cyclones upstream of
baghouses, the average powder concentration
within the baghouse will be very low.
Dust will be briefly present only when a row of
bags is pulsed, and then only near the bottom
powder outlet.
2. When all the air goes directly to a baghouse, the
average concentration will be similar to that in the
chamber exhaust.
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Calculating Dust Concentrations - Bins
Powder bins, silos and hoppers will contain airborne
powder only while being filled.
Lean phase pneumatic transport will create a
significant powder concentration in the headspace.
Gravity filling and the use of vacuum transport
systems will create much lower, but still potentially
hazardous concentrations.
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Ignition
Sources
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• Flames
• Hot Surfaces
• Mechanical Friction
• Impact Sparks
• Electrical Sparks
• Electrostatic Discharge Sparks (but not in milkpowder)
• Hot Work
• Self-Ignition
Ignition Sources
Each of the following ignition sources has set off
fires or dust explosions
Checklists for Explosion Hazard Auditing
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All of the external sources can be addressed by
administrative controls such as:
• no-smoking policies
• hot work permit systems
• good maintenance
• appropriate wiring practices
• appropriately rated tools and portable
equipment
• tight housekeeping protocols
Checklists for Explosion Hazard Auditing
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Self-ignition can be addressed by the use of:
• CO monitoring
• CCTV cameras looking at trouble spots in
dryers
• Routine inspections by operators
• Infra-red detectors
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Dairy Industry Systems Consultant
Explosion Venting
(Accidental and
Deliberate)
Dairy Industry Systems Consultant
Accidental Venting
Accidental venting can injure staff and trigger
secondary explosions in buildings. The strength
of the following items must be checked:
• Flexible connections (“socks” or “boots”)
• The ends of fluid beds
• All access hatches
Use explosion resistant couplings, blast shields
and reinforce vessels and hatches as required.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Accidental Venting
Accidental venting can also occur from:
• Gaps between explosion vent ducts and the
vessel they protect or the building wall.
• Poorly fitted doors and hatches.
• Unsecured disk atomisers
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Deliberate Venting
Deliberate venting must discharge safely.
Vents discharging close to stairways and catwalks
require shielding.
Vent ducts require heating to prevent cool spots
which increase the local relative humidity, attract
powder deposits and encourage self-heating.
Vent duct heating systems should not become
ignition sources!
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Deliberate Venting
Vent ducts must be as strong as the vessels they
protect.
Vent duct access doors must be secure and
interlocked to stop the plant if opened.
Signage must be appropriate, warning of a
confined space and an explosion hazard.
Handrails should protect staff inside the duct
replacing panels or shutting doors from falling out
of the building.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Explosion
Isolation
Dairy Industry Systems Consultant
Explosion Isolation
Where practicable, a deflagration in one part of a
process should not be able to propagate to other
parts.
Dry powder or pressurised hot water suppression
bottles installed in air ducts may be used to
isolate dryers from baghouses.
Rotary valves may be used to isolate powder
discharges from dryers, fluid beds, cyclones and
baghouses.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Explosion Isolation
Slam-shut non-return or diverter valves may be
used to isolate dust collectors.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Examples
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Spray Driers
In the following examples the checklists contain
different types of information:
1.Descriptions of the plant configuration.
2.Descriptions of any safety related equipment.
3.Descriptions of access controls and signage.
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Air
Heaters
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Drying
Chambers
Checklists for Explosion Hazard Auditing
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Baghouses
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
Baghouses
Checklists for Explosion Hazard Auditing
Dairy Industry Systems Consultant
CONCLUSION
Dairy Industry Systems Consultant
Checklists for Explosion Hazard Auditing
This presentation has only begun to examine
all the possible explosion hazards and
responses.
The systematic creation of a checklist allows
potential safety hazards to be identified in a
simple, comprehensive way.
The real value of this approach does not lie in
"ticking boxes", but rather in the systematic
process of identifying boxes that may need
ticking.
Dairy Industry Systems Consultant