Dust: Ten zoning myths Bulk solids: Explosion · PDF fileDust: Ten zoning myths Bulk solids: ... in areas where dust explosion hazards are present, ... ¥ Mixers used for makin g water-wet

July

20

15

the journal for hazardous area environments hazardexonthenet.net

Plant safety: Incident reporting

Standards: Economic benefi ts

Dust: Ten zoning myths

Bulk solids: Explosion protection

Process safety: The ‘Trouser model’

3928 - HazardEx July15 Edn.indd 13928 - HazardEx July15 Edn.indd 1 16/06/2015 14:3216/06/2015 14:32

The top ten myths of dust zoningOver 70% of powders handled in

industry are capable of giving

rise to dust explosions under the right

conditions. Many of these powders

can be found in the food and beverage

sector, including sugar, sweeteners,

starch, fl our, grain, vitamins,

amino acids, resins, gums, fl avour

ingredients, caffeine, and many others.

A number of these materials have

been involved in some of the most

devastating dust explosion incidents

that have occurred in industry. For

example, the 2008 Imperial Sugar dust

explosion incident in Port Wentworth,

Georgia, was responsible for 14 deaths

and 42 serious injuries.

In this article, Simon Gakhar of

Chilworth Technology examines some

myths that have become established

in areas where dust explosion hazards

are present, and shares some of his

company’s experiences advising

clients on mitigating risk.

In order to effectively control potentially

devastating dust explosions, it is fi rst

necessary to identify where fl ammable dust

clouds might arise and how frequently.

This exercise is known as Hazardous Area

Classifi cation (HAC) or ‘zoning’ and in the

European Union under ATEX Directive

1999/92/EC (ATEX 137), it is a legal

requirement. Whilst HAC has established

roots in industries that handle fl ammable

liquids and gases, its application to dusts

is less widespread and prior to ATEX, to

fi nd a thorough and well-structured dust

HAC assessment would have been the

exception rather than the norm.

Since the ATEX Directives fi rst came

into legal effect in 2003, Chilworth has

undertaken many HAC studies and carried

out training for companies handling

fl ammable powders, a number of which

have been for food and beverage sector

clients. In addition, we have reviewed

many HAC assessments that had been

carried out internally by our clients. From

this, we can share some of our Process

Safety experience with you, in the form of

ten ‘Myths’ which have been assembled

from a combination of observations made

when conducting HAC assessments at

our clients’ sites, and comments made by

our clients and delegates at our training

courses. Although we are discussing ten

myths in this article, it should be noted

that this is by no means intended as an

exhaustive list. There are many more!

As we discuss these myths it will become

clear that excellence in process safety

performance is not about ticking boxes

and simple compliance. The focus should

be on practical solutions that are based

on appropriate data, relevant expertise,

extensive experience, and a supportive

organisational culture.

Excellence in process safety is not about ticking boxes and simple compliance

www.hazardexonthenet.net

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15Dust

Myth 1 – Over zoning is OKWe fi nd just as many powder handling

facilities where large (blanket) areas are

classifi ed as hazardous as ones where

insuffi cient provision has been made. This

approach is often conservative; and hence

acceptable as long as the company is

willing to also accept the costs associated

with it - equipment in hazardous areas is

expensive to procure, install and maintain.

In our experience, many large hazardous

areas are unnecessary and can lead to

problems such as:-

1. Substantial expense in procurement of

Ex rated equipment

2. Challenges in identifying and removing

other ignition sources in the extended

zones

3. Diffi culties in inspecting and maintaining

a large inventory of Ex equipment

4. An increased maintenance burden

on non-electrical equipment – and

potentially more plant “down-time”

than would be necessary for a correctly

zoned facility

5. As a result of the above and limitations

on resource, a more diffuse focus on

equipment in the ‘true’ hazardous areas

Where large areas are designated as

hazardous due to a signifi cant dust release,

there is a tendency to think: “We have the

area zoned so there’s no need to improve

dust control”. ATEX 137 has a hierarchical

risk reduction requirement and at the top

of the list is avoidance of the hazard, i.e.

remove the dust.

Minimising dust releases and avoiding

dust accumulations should therefore

be the primary objective, rather than

accepting large hazardous areas. Even

with correctly specifi ed and well maintained

Ex equipment, ignition prevention

cannot be guaranteed as there are other

potential ignition sources to consider

as well. Furthermore, presence of dust

accumulations outside the processing

equipment creates potential for a

devastating secondary explosion.

Myth 2 – I can see somedust, therefore I must zonePeople more familiar with dust control for

occupational health reasons often don’t

realise the vast differences in concentration

required to create fl ammable dust clouds.

The Minimum Explosible Concentration

for a dust is typically over 3 orders of

magnitude higher than the acceptable

concentration to prevent health issues

(specifi c material hazard dependent, of

course).

We therefore often see large hazardous

areas designated for small leaks, for

example from seals on rotary valves

or mixers. Whilst these leaks are not

desirable, they do not normally lead to

hazardous fl ammable dust clouds at the

leak source. It should be noted though,

that allowing such small leaks to continue

unchecked can lead to hazardous

accumulations of dust. Therefore, with

reference back to Myth 1, eliminating

the leak at source should be the key risk

reduction focus, rather than accepting this

accumulation and zoning for it.

Myth 3 – I cannot see any dust so do not need to zoneThis is the converse of Myth 2. There

is a common tendency to zone only for

dust you can see in the workplace and

not for what that could be released.

Taking the leaking seal example in Myth

2, a ‘fl ammable’ dust cloud is very

unlikely to form provided hazardous dust

accumulation is prevented. However, a

fl exible joint on a sieve may not leak dust

normally; but if the joint breaks, large

amounts of dust can be released. This is

precisely the type of event that the Zone 22

designation should cover.

Myth 4 - I don’t have toworry about HAC insideequipmentMany types of dust handling equipment

will have a potentially explosive dust

atmosphere internally. Examples include,

mixers, mills, screw conveyors, rotary

valves and sieves. However, the vast

majority of dust HAC reports and drawings

that we come across have only considered

the external workplace and not what the

zoning should be inside equipment. Both



Dust16

are important for correct ATEX equipment

selection, although since the cost of

ATEX certifi ed equipment is dependent

on the internal zoning assigned, care is

needed to ensure the internal zoning is

realistic and not overly pessimistic. Whilst

not zoning correctly inside equipment

can lead to an increased explosion risk,

over zoning can also lead to the need

for complex equipment which may be

diffi cult and require specialist skills to

maintain and manage. This in itself can

lead to an increased explosion risk if such

maintenance is not carried out correctly by

competent personnel.

Myth 5 – Dust handlingequipment should all beZone 20 internallyThere are many cases where dust handling

plant will operate with a continuous

fl ammable atmosphere inside and a

Zone 20 is hence justifi ed: for example,

inside dust extraction fi lter units which are

frequently cleaned by mechanical shaking

or by reverse compressed air pulses.

However, there are also plant items where,

internally, a Zone 20 may not be needed,

including for example:

• Slow speed screw conveyors – these do

not tend to stir the dust up and produce

continuous dust clouds unless the dust

is very fi ne and lightweight

• Drag link conveyors – powder is often

dragged along in bulk with little dust

internally, unless the dust is very fi ne

and lightweight, except at feed and drop

points

• Mixers operating more than 70% full

– a fl ammable dust cloud will be less

frequent during normal operation as the

fi ll level increases beyond 70% as the

powder is more likely to operate above

the Maximum Explosible Concentration

• Cone mills where the mill is often fl ood

fed

• Mixers used for making water-wet

products such as dough, where the mix

is only ‘dry’ for a short time before the

water is added

Note that this does not eliminate the need

to zone in the above situations, but often

a Zone 21 is acceptable, which would

also cover for example fi lling and emptying

operations with mixers.

There are of course exceptions and each

plant item needs to be assessed on a

case by case basis taking into account

factors such as powder properties which

may increase chances of more frequent

fl ammable dust clouds.

The key message is not to assume that the

fl ammable dust cloud frequency will always

be suffi ciently high to warrant a Zone 20

inside. As mentioned in Myth 4 above, this

this can have implications on equipment

cost and complexity in managing and

maintaining the equipment safely.

Also, ATEX Category 1D equipment

(needed for Zone 20) often has to be

supplemented with other mitigation

measures, such as inerting or explosion

protection.

Myth 6 – I only use theequipment for 9 hours ayear so it can be Zone 22insideWell established guidance for deciding

whether to designate a particular location

as Zone 20, 21 or 22 is based on the

Sugar distribution warehouse

Eliminating the leak at source should be the key risk reduction focus, rather than accepting an accumulation and zoning for it


estimated duration of the fl ammable dust

cloud, as follows:-

• >1,000 hours per year indicates Zone 20

• 10-1,000 hours indicates Zone 21

and……

• < 10 hours indicates Zone 22.

However, it is very easy to misinterpret the

intention of this guidance. We once were

asked to re-assess the internal zoning for

a mixer that had been classifi ed as Zone

22 on the inside because it operated

continuously for only 9 hours per year!

This is not correct; as the important

defi nition of a Zone 22 in this context is

that a fl ammable dust cloud should not be

expected as part of normal operation.

Clearly the 9 hours per year operation is an

intended i.e. ‘normal’ operation; and is in

fact continuous operation for the time the

equipment is running so it actually meets

the requirements for Zone 20. Additionally,

consideration needs to be given to the

coincidence of the presence of the dust cloud

and potential ignition sources. Whilst this

‘time based’ guidance is benefi cial, misuse

of it can seriously underestimate the hazard

and hence common sense should prevail.

Myth 7 – I can use the samezoning as our sister plantCompanies often construct identical plants

in different parts of the world and at the

design stage it is common to fi nd the exact

same zoning from an existing operational

unit copied across to the new design.

Whilst making use of experiences gained

in an operational plant is perfectly valid, the

practical zoning requirements for the new

plant could different, for example due to:-

• Differences in operator techniques and

skill level

• Differences in the skills of local

engineering resources and quality of

fabricating materials

• Differences in maintenance practices

• Dust extraction effi ciency

• Local conditions e.g. humidity

• Differences in raw material suppliers and

properties of raw materials

• Differences in product specifi cation e.g.,

particle size.

• Different local/national requirements and

standards

These must be factored in to the zoning at

the design stage. In addition, the zoning

assumptions should be validated during

commissioning and re-validated after

a period of operation (typically within 6

months). This “read-across” approach

to zoning can often lead to hazardous

conditions.

Myth 8 – Zoning only needsto cover routine operationsThis is a common misconception and one

which can easily lead to an underestimation

of the overall explosion risk of a process. It

is essential to consider all operations which

might be required to operate the process

effectively, including ancillary routine

operations such as cleaning, maintenance

and sampling, and even foreseeable non-

routine activities.


Dust18

The focus should be on practical solutions that are based on appropriate data, relevant expertise, extensive experience, and a supportive organisational culture


Non-routine activities might include

rodding to clear blockages or emergency

repairs. These operations can release

large amounts of dust and many incidents

occur during such non-routine activities.

However, designated hazardous zones

for potential dust releases for non-routine

activities can sometimes be large and

diffi cult to manage, therefore may not

be the most appropriate explosion risk

reduction approach for such low frequency

activities.

For example, the explosion risk from

activities such as fi lter bag cleaning could

be covered by suitable controls, such as

those required by a permit to work. The

key message here is that the specifi c

hazards from infrequent and non-routine

activities need to be identifi ed, risk

assessed and appropriate precautions

taken.

Myth 9 - My plant is inertedso it is non-hazardousinsideThis myth relates to how much credit you

can take for inert gas blanketing or purging

in order to modify a hazardous area inside

process plant. Although inerting in powder

handling plant is not as common as with

handling of fl ammable liquids and gases,

we do come across it from time to time –

especially for ignition sensitive powders or

where powders are handled together with

fl ammable liquids.

Whilst inert gas blanketing is and has

been an effective and reliable basis of

safety for many hazardous operations,

careful consideration needs to be given

to the design and reliability of the system

when assessing the overall explosion risk.

This could include assessing the zoning

requirements (and the subsequent need for

Ex rated equipment) inside vessels which

are inerted taking into account factors

which might result in inerting being lost.

At any time when inerting is lost i.e. oxidant

concentration rises above the Limiting

Oxygen Concentration (LOC), the basis

of safety has to default to ignition source

avoidance until a suffi cient oxygen defi cient

atmosphere is re-established.

Typical reasons why an inert atmosphere

might not be present continuously include:-

1. Some inerting systems may have not

been assessed for reliability and have no

effective means of monitoring the inert

atmosphere e.g. no oxygen monitoring,

inert gas pressure or fl ow indication with

alarms and trips. The inert atmosphere

may therefore be lost temporarily without

this being revealed to the operator.

2. Instruments for monitoring the inert

atmosphere such as oxygen analysers

have a limited reliability and this has to

be taken into account when deciding the

overall reliability of the inerting system.

Often too much credit is given to the

presence of instruments and analysers

without a full understanding of their

limitations in use. An unrevealed failure of

an oxygen analyser may indicate a safely

inerted plant when in fact it is not.

Hazardous area classifi cation for fl ammable dust locations must be tailored to the particular plant and process, taking into account both normal and foreseeable abnormal circumstances


19Dust


3. Often equipment is opened during

operation for sampling and inspection.

This can allow air to enter and raise

oxygen levels to above the LOC.

4. Some powder handling equipment is

rarely gas tight. This can also allow air in

as above.

Zoning inside inerted plant must therefore

be done only after careful consideration

of the inerting system reliability and

operations which might allow introduction

of oxygen.

Myth 10 – Hazardous areadrawings are suffi cient on their ownIt is not uncommon for us to visit sites,

ask for any supporting documentation on

their existing HAC and be presented with

a drawing of hazardous areas and nothing

else. Although it is necessary to have HAC

drawings, generating a detailed report is an

important part of the exercise. The report is

essential for:

1. Justifying all assumptions made

when arriving at the zoning, including

fl ammability data

2. Describing the plant and processes at

the time the HAC was carried out

3. Keeping a record of who carried out and

led the exercise and their competence

4. Describing both horizontal and vertical

extents of the hazardous areas – often

sites only create plan view drawings

showing horizontal extents and

exclude elevation drawings. Zone

heights can therefore only be deduced

from a detailed description in the

report.

5. Defi ning when the next review should

be carried out

HACs should also be reviewed as part of

the company’s management of change

process, both when process/plant changes

are being made and at regular intervals

regardless of changes. This is very diffi cult,

if not impossible, to do without clear

documentation.

Concluding RemarksIt is important to ensure that hazardous

area classifi cation for locations where

fl ammable dusts are present is tailored

to the plant and process under

consideration, taking into account

both normal and foreseeable abnormal

circumstances. Additionally, HAC

must be based on the appropriate

fl ammability and physical property data

of the dusts present. The HAC must

be performed by competent individuals

and must be documented in a clear

and comprehensive manner. Finally,

HAC must be reviewed at appropriate

intervals, especially after any change

to materials, equipment or process

conditions.

Simon Gakhar is Technical Manager

and Process Safety Specialist at

Chilworth Technology, with special

responsibility for developing and

maintaining technical capability within

the consulting team. He has over 25

years’ process industry experience,

15 years of which have been in

process safety.

About the author:


Dust20

Dust collector outside plant


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Dust: Ten zoning myths Bulk solids: Explosion · PDF fileDust: Ten zoning myths Bulk solids: ... in areas where dust explosion hazards are present, ... ¥ Mixers used for makin g water-wet