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The top ten myths of dust zoning

21 July 2015

Over 70% of powders handled in industry can cause dust explosions under the right conditions. Many of these powders can be found in the food and beverage sector, including sugar, sweeteners, starch, flour, grain, vitamins, amino acids, resins, gums, flavour ingredients, caffeine, and many others. In this article, Simon Gakhar of Chilworth Technology examines some myths that have become established in areas where dust explosion hazards are present

In order to effectively control potentially devastating dust explosions, it is first necessary to identify where flammable dust clouds might arise and how frequently. This exercise is known as Hazardous Area Classification (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 flammable liquids and gases, its application to dusts is less widespread and prior to ATEX, to find a thorough and well-structured dust HAC assessment would have been the exception rather than the norm.

Since the ATEX Directives first came into legal effect in 2003, Chilworth has undertaken many HAC studies and carried out training for companies handling flammable 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.

Myth 1 – Over zoning is OK

We find just as many powder handling facilities where large (blanket) areas are classified as hazardous as ones where insufficient 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.  Difficulties 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 significant 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 specified 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 some dust, therefore I must zone

People more familiar with dust control for occupational health reasons often don’t realise the vast differences in concentration required to create flammable 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 (specific 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 flammable 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 zone 

This 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 ‘flammable’ dust cloud is very unlikely to form provided hazardous dust accumulation is prevented. However, a flexible 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 to worry about HAC inside equipment

Many 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 are important for correct ATEX equipment selection, although since the cost of ATEX certified 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 difficult 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 handling equipment should all be Zone 20 internally

There are many cases where dust handling plant will operate with a continuous flammable atmosphere inside and a Zone 20 is hence justified: for example, inside dust extraction filter 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 fine and lightweight

•  Drag link conveyors – powder is often dragged along in bulk with little dust internally, unless the dust is very fine and lightweight, except at feed and drop points

•  Mixers operating more than 70% full – a flammable dust cloud will be less frequent during normal operation as the fill level increases beyond 70% as the powder is more likely to operate above the Maximum Explosible Concentration.

•  Cone mills where the mill is often flood 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 filling 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 flammable dust clouds.

The key message is not to assume that the flammable dust cloud frequency will always be sufficiently 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 the equipment for 9 hours a year so it can be Zone 22 inside

Well established guidance for deciding whether to designate a particular location as Zone 20, 21 or 22 is based on the estimated duration of the flammable 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 classified as Zone 22 on the inside because it operated continuously for only 9 hours per year! This is not correct; as the important definition of a Zone 22 in this context is that a flammable 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 beneficial, misuse of it can seriously underestimate the hazard and hence common sense should prevail. 

Myth 7 – I can use the same zoning as our sister plant

Companies often construct identical plants in different parts of the world and at the design stage it is common to find 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 efficiency

•  Local conditions e.g. humidity

•  Differences in raw material suppliers and properties of raw materials

•  Differences in product specification 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 needs to cover routine operations

This 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.

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 difficult 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 filter bag cleaning could be covered by suitable controls, such as those required by a permit to work. The key message here is that the specific hazards from infrequent and non-routine activities need to be identified, risk assessed and appropriate precautions taken.

Myth 9 - My plant is inerted so it is non-hazardous inside

This 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 flammable liquids and gases, we do come across it from time to time – especially for ignition sensitive powders or where powders are handled together with flammable 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 sufficient oxygen deficient 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 flow 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.

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 area drawings are sufficient on their own

It 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 flammability 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.  Defining 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 difficult, if not impossible, to do without clear documentation.

Concluding Remarks

It is important to ensure that hazardous area classification for locations where flammable 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 flammability 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.

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