Dust: Ten zoning myths Bulk solids: Explosion · PDF fileDust: Ten zoning myths Bulk solids:...
Transcript of Dust: Ten zoning myths Bulk solids: Explosion · PDF fileDust: Ten zoning myths Bulk solids:...
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’
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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
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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
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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
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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.
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The focus should be on practical solutions that are based on appropriate data, relevant expertise, extensive experience, and a supportive organisational culture
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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
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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:
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Dust collector outside plant
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