chris bloore - dairy industry - checklist development as an aid to auditing industrial processes...

Dairy Industry Systems Consultant

Checklist Development as an

Aid to Auditing Industrial

Processes Involving

Combustible Dusts

Dr Chris Bloore


2014


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


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.



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.



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.



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.



The Milkpowder

Industry


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.



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.



Some drier designs with separate fluid beds.

Spray Driers



Some drier designs with integrated fluid beds.

Spray Driers



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

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

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.



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

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

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.



Dust

Concentration


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.



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




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




The previous analysis takes no account of any fines

being returned to the chamber.



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.



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).



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.



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.



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.



Ignition

Sources


• 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



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



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



Explosion Venting

(Accidental and

Deliberate)


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.



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



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!



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.



Explosion

Isolation


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.



Explosion Isolation

Slam-shut non-return or diverter valves may be

used to isolate dust collectors.



Examples


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.



Air

Heaters



Drying

Chambers



Baghouses



CONCLUSION



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.

chris bloore - dairy industry - checklist development as an aid to auditing industrial processes...

Engineering

explosion hazard auditing

australian dairy industry

milkpowder industry

explosion protective

spray driers checklists

aviation industry

dairy companies

explosion vents