Dust environments may not be an obvious risk for fire detection but they are involved in many more explosions than those containing flammable gases. So it can be just as vital to equip them with the right intrinsically safe detection systems, as Charles Gormley explains.
Dust risks have always been the ‘poor relation’ of flammable gases and vapours, in terms of the amount of suitable fire detection equipment available. This is also true for other types of equipment suitable for hazardous dust atmospheres. There are, however, far more explosions attributable to dusts than to flammable gases – some 2000 dust explosions in Europe each year. Furthermore, dust explosions can be just as devastating as those from flammable gas: the recent explosion at the Imperial Sugar refinery at Port Wentworth, Georgia in February 2008, for example, killed 13 people and injured many more.
The reason for the traditional lack of fire detection equipment suitable for dust environments was that until fairly recently, the only method of protection that was generally accepted for dust risks was that of enclosure in an IP5X/6X housing. Whilst this approach is suitable for some electrical equipment (switches etc), it is completely unsuitable for fire detectors which, by definition, must be able to sample the air in the protected space.
This is why the publication of the relatively new EN 61241 series of standards has proved to be so important. This new series allows, for the first time, other protection approaches to be taken in dust risk environments. For example, the new standard allows the concept of intrinsic safety to be extended to equipment to be used in dust environments. This makes a lot of sense because fire detection equipment using this form of protection for gas risks has been around for a long time and is well understood.
Hence we now have a situation in which manufacturers have been given an extended range of possible solutions, and in which there is a recognised framework for the classification and protection of dust risks. Furthermore, the existence of a recognised document for the protection of dust risk areas has given customers and specifiers the tools to request protection of these areas.
What is the risk?
While it may be tempting to lump together gas risks and dust risks, they are definitely not the same. People are intuitively aware of the risks associated with flammable gases and vapours, while dust risks can be perceived as fairly benign – after all, its only dust! Add to this the fact that dust spreads around and is not limited to or contained in obviously high hazard areas. But therein lies the problem; there are a large number of flammable dusts commonly encountered in many industries. The dusts most commonly involved in explosions include:
– cocoa powder – aluminium flakes – milled barley – coal – flour – paper – sugar – yeast – soap – polystyrene.
These can be present in and around a large array of machinery including cyclones, filters, blenders, dryers, bucket elevators, storage and grain silos. It should be fairly obvious from the above that the numbers of industries affected by possible dust risks far outnumber those industries were gases and vapours would be the main concern. They include the food and pharmaceutical industries and a great many others.
Potentially explosive
Under normal circumstances, most solid materials must undergo pyrolysis (heating) to produce flammable volatiles. Dusts are different. When a finely divided solid is in suspension in the air a number of factors become significant:
– Each grain of dust has a very low thermal mass; as a result, it can be heated very rapidly
– The surface area of the fuel, as represented by the dust particles, is very large
– The supply of air (oxygen) is normally abundant
– Dusts are opaque and heat transfer to an opaque material is very efficient
The finer the dust the greater the danger. Small particles (typically less than about 5 microns) will be vapourised by radiated heat and as a result will burn in very much the same way as a gas vapour/air mixture. However, any dust up to a particle size of approximately 420 microns (0.42mm) should be considered a possible risk.
It should be borne in mind that in comparison to flammable vapours, higher levels of energy are normally required to ignite a dust cloud. However, the risk is still very real and the standards for intrinsically safe equipment in dust areas call for a minimum level of spark protection equivalent to apparatus group IIB for gases.
We must also be concerned with the restriction of temperature from any surface to below that capable of igniting a dust cloud directly. This is analogous to the temperature class system used for flammable vapours.
Then, uniquely to dust risks, we must also consider the effects of hot surfaces upon which dust layers have accumulated. The risk is one of the possibility of smouldering combustion taking hold in a dust layer that has accumulated on a piece of electrical equipment. Smouldering combustion can be initiated at temperatures lower than those required to ignite a dust cloud directly; the thicker the dust layer the greater the risk, as smouldering depends on a sufficient quantity of dust being present to retain heat during the smouldering process.
Primary and secondary explosions
A major concern in premises subject to dust risks is that of a secondary explosion. Primary explosions are those that occur within a piece of dust handling equipment (cyclone, blender etc) or other confined area. The point is that such confinement produces a pressure wave which is then capable of dislodging accumulated dust in other areas.
Remember, accumulated dust is not an immediate explosive hazard when it is in the form of a layer on floors, roof beams and other building surfaces. However, this changes markedly if significant quantities of dust are dislodged and mix with air. In fact, most serious explosions which affect dust handling plants are as a result of a primary explosion which is then followed by a (devastating) secondary explosion. The major prevention against secondary explosions is good housekeeping: surfaces must routinely be kept clear of accumulations of dust and this must be part of the ongoing management programme of the plant.
Standards and certification
The protection of gas and dust risks does have many aspects in common. As a result there are ongoing discussions to standardise as much as possible between the two relevant standards (EN 60079 and EN 61241) in the future. This can already be seen fairly clearly in the area classification systems used, in which the area classification zone numbers are already closely aligned. For dusts the relevant existing zone number, relating to gases, is simply prefixed with the number 2:
– Zone 20 (Zone 0): Continuous source of release
– Zone 21 (Zone 1): Primary source of release
– Zone 22 (Zone 2): Secondary source of release.
IECEx versus ATEX
Third party certification of Ex equipment is a very important area, with IECEx certification also being increasingly sought.
There is often some confusion about what IECEx approval is and how it differs from ATEX approval. ATEX approval is a minimum legal requirement for any Ex equipment sold within the European Union. It sets minimum requirements know as Essential Health and Safety Requirements which must be met by the equipment. The most common way to demonstrate conformity to these requirements is to show that your equipment meets the current European standards (either EN 60079 or EN 61241) which are considered to be the current ‘state of the art’. However, you can achieve compliance with the requirements in other ways, and an important point is that the involvement of a third party notified body (such as Baseefa) is not always required. The only legal requirement is that the manufacturer makes available a Document of Conformity which states that the product complies, as far as the manufacturer is concerned. Hence ATEX ‘approval’ can be claimed for a device with no third party involvement.
With IECEx approval, on the other hand, you must have the involvement of a third party notified body to achieve accreditation. The accreditation is to a definite set of standards (EN 60079 and EN 61421 as appropriate) rather than looser health and safety requirements, so it is a ‘pure’ third party accreditation and thus gives a large measure of confidence in the product. The certificate is owned and issued by the third party certification body. Hence it can be viewed that IECEx certification is a worldwide product certification, whilst ATEX certification is a mandatory legal requirement within the European Union.
Application considerations
Dust can be a significant false alarm source with certain types of smoke detector. Many of the current installation standards (e.g. BS 5839-1:2002 in the UK or VdS 2095 in Germany) make a great deal of the requirement to keep false alarms to an absolute minimum. This leaves a duty on designers, installers and end users to ensure that any detector that is fitted is suitable for the area and environment in which it is placed. Further, the requirement to keep false alarms to an absolute minimum can be especially important in hazardous areas, where responses to alarms can include equipment shutdowns and mobilisation of emergency response teams!
It should be remembered that the requirements of the appropriate hazardous atmosphere standard (EN 60079 or EN 61241) take precedence within the confines of the hazardous area itself. This could be the justification for using certain detection types when perhaps, the local installation standard calls for another type as the first choice.
The Tyco range of Ex fire detectors and associated fire controllers, for example, have a number of features that can help with false alarm reduction and maintenance. Points to consider include:
– what zone are we to protect – zone 20, 21 or 22?
– how extensive are the zones?
– will the dust reach a high level/ceiling level (typically where the detectors are)?
– what colour is the dust (white, dark, reflective, metal)?
– how long does the dust take to settle onto surfaces?
– what is the standard of housekeeping in the plant?
It is unlikely that fire detectors would be installed in zone 20 areas, as these would normally be contained within process machinery. In zone 21 areas where white/reflective dust is present frequently, the use of optical smoke detectors may give rise to an unacceptable risk of false alarms. Other types of Ex detector from the range would have to be considered for these areas.
In zone 22 areas any type of detector could be used, including optical smoke detectors subject to a reasonable level of housekeeping in the area. In fact, it could be argued that an optical smoke detector being activated by excessive dust could indicate poor housekeeping or the fact that dust has been dislodged in high quantities – a very serious danger in itself.
In areas were optical smoke detectors would potentially have false alarm issues one very strong alternative would be to use a carbon monoxide (CO) detector or combined CO and heat. Carbon monoxide (CO) detection is immune to dust and is a proven detection technology especially when combined with heat detection.
A further benefit of using carbon monoxide technology would be the possible early detection of a smouldering fire due to a hot surface in any accumulated dust layer. Carbon monoxide is normally one of the gases generated, due to the limited oxygen available during the smouldering combustion process.
Very long-term contamination of certain detector types (primarily optical smoke) by dust is an ongoing issue in all environments, not just hazardous areas. As a result, Tyco Safety Products have built a range of measures into their detector ranges and fire controllers to assist with getting the maximum lifetime and false alarm immunity from these devices. For example, all optical detectors incorporate specially designed optical chambers to ensure the maximum immunity to dust deposits and the maximum period between cleaning and/or replacement.
In hazardous dust environments there will normally be an increased level of dust present. As a result, the time period over which dust accumulations within affected detectors can become significant will be accelerated. This must be taken into account in the frequency of maintenance.
If the addressable series detectors are used in the hazardous area together with an appropriate controller, then a further range of monitoring and maintenance facilities become available.
In addressable systems the detectors can be continually polled by the fire controller, which can track the amount of background dust contamination in the detector over an extended period. The controller can then alter the alarm threshold levels of the device, within tightly controlled limits, to ensure the maximum time between servicing and maximisation of false alarm performance.
If a detector becomes overly contaminated by dust while it is still fully operational, it can be automatically flagged as requiring maintenance. This allows for easy ongoing maintenance and monitoring and directs maintenance staff by address to the affected detector. If required, a visible warning can also be displayed on the controller for end user intervention.
It should be remembered that intrinsically safe fire detectors are not used in isolation – they are the major component in a complete intrinsically safe system. This system will comprise the detectors, barrier or isolator and the interconnecting cable. Gas and dust approved call points are also available, as well as an intrinsically safe system for sounder circuits.
Charles Gormley is UK training manager at Tyco Safety Products. He will be making a presentation on fire detection in hazardous dust risks on Thursday 14 May at International Firex at the NEC. For details of the company’s intrinsically safe detectors and systems go to: www.tycoint.com
Dust environments may not be an obvious risk for fire detection but they are involved in many more explosions than those containing flammable gases. So it can be just as vital to equip them with the right intrinsically safe detection systems, as Charles Gormley explains.
Dust risks have always been the ‘poor relation’ of flammable gases and vapours, in terms of the amount of suitable fire detection equipment available. This is also true for other types of equipment suitable for hazardous dust atmospheres. There are, however, far more explosions attributable to dusts than to flammable gases – some 2000 dust explosions in Europe each year. Furthermore, dust explosions can be just as devastating as those from flammable gas: the recent explosion at the Imperial Sugar refinery at Port Wentworth, Georgia in February 2008, for example, killed 13 people and injured many more.
The reason for the traditional lack of fire detection equipment suitable for dust environments was that until fairly recently, the only method of protection that was generally accepted for dust risks was that of enclosure in an IP5X/6X housing. Whilst this approach is suitable for some electrical equipment (switches etc), it is completely unsuitable for fire detectors which, by definition, must be able to sample the air in the protected space.
This is why the publication of the relatively new EN 61241 series of standards has proved to be so important. This new series allows, for the first time, other protection approaches to be taken in dust risk environments. For example, the new standard allows the concept of intrinsic safety to be extended to equipment to be used in dust environments. This makes a lot of sense because fire detection equipment using this form of protection for gas risks has been around for a long time and is well understood.
Hence we now have a situation in which manufacturers have been given an extended range of possible solutions, and in which there is a recognised framework for the classification and protection of dust risks. Furthermore, the existence of a recognised document for the protection of dust risk areas has given customers and specifiers the tools to request protection of these areas.
What is the risk?
While it may be tempting to lump together gas risks and dust risks, they are definitely not the same. People are intuitively aware of the risks associated with flammable gases and vapours, while dust risks can be perceived as fairly benign – after all, its only dust! Add to this the fact that dust spreads around and is not limited to or contained in obviously high hazard areas. But therein lies the problem; there are a large number of flammable dusts commonly encountered in many industries. The dusts most commonly involved in explosions include: cocoa powder – aluminium flakes – milled barley – coal – flour – paper – sugar – yeast – soap – polystyrene.
These can be present in and around a large array of machinery including cyclones, filters, blenders, dryers, bucket elevators, storage and grain silos. It should be fairly obvious from the above that the numbers of industries affected by possible dust risks far outnumber those industries were gases and vapours would be the main concern. They include the food and pharmaceutical industries and a great many others.
Potentially explosive
Under normal circumstances, most solid materials must undergo pyrolysis (heating) to produce flammable volatiles. Dusts are different. When a finely divided solid is in suspension in the air a number of factors become significant:
– Each grain of dust has a very low thermal mass; as a result, it can be heated very rapidly
– The surface area of the fuel, as represented by the dust particles, is very large
– The supply of air (oxygen) is normally abundant
– Dusts are opaque and heat transfer to an opaque material is very efficient
The finer the dust the greater the danger. Small particles (typically less than about 5 microns) will be vapourised by radiated heat and as a result will burn in very much the same way as a gas vapour/air mixture. However, any dust up to a particle size of approximately 420 microns (0.42mm) should be considered a possible risk.
It should be borne in mind that in comparison to flammable vapours, higher levels of energy are normally required to ignite a dust cloud. However, the risk is still very real and the standards for intrinsically safe equipment in dust areas call for a minimum level of spark protection equivalent to apparatus group IIB for gases.
We must also be concerned with the restriction of temperature from any surface to below that capable of igniting a dust cloud directly. This is analogous to the temperature class system used for flammable vapours.
Then, uniquely to dust risks, we must also consider the effects of hot surfaces upon which dust layers have accumulated. The risk is one of the possibility of smouldering combustion taking hold in a dust layer that has accumulated on a piece of electrical equipment. Smouldering combustion can be initiated at temperatures lower than those required to ignite a dust cloud directly; the thicker the dust layer the greater the risk, as smouldering depends on a sufficient quantity of dust being present to retain heat during the smouldering process.
Primary and secondary explosions
A major concern in premises subject to dust risks is that of a secondary explosion. Primary explosions are those that occur within a piece of dust handling equipment (cyclone, blender etc) or other confined area. The point is that such confinement produces a pressure wave which is then capable of dislodging accumulated dust in other areas.
Remember, accumulated dust is not an immediate explosive hazard when it is in the form of a layer on floors, roof beams and other building surfaces. However, this changes markedly if significant quantities of dust are dislodged and mix with air. In fact, most serious explosions which affect dust handling plants are as a result of a primary explosion which is then followed by a (devastating) secondary explosion. The major prevention against secondary explosions is good housekeeping: surfaces must routinely be kept clear of accumulations of dust and this must be part of the ongoing management programme of the plant.
Standards and certification
The protection of gas and dust risks does have many aspects in common. As a result there are ongoing discussions to standardise as much as possible between the two relevant standards (EN 60079 and EN 61241) in the future. This can already be seen fairly clearly in the area classification systems used, in which the area classification zone numbers are already closely aligned. For dusts the relevant existing zone number, relating to gases, is simply prefixed with the number 2:
– Zone 20 (Zone 0): Continuous source of release
– Zone 21 (Zone 1): Primary source of release
– Zone 22 (Zone 2): Secondary source of release.
IECEx versus ATEX
Third party certification of Ex equipment is a very important area, with IECEx certification also being increasingly sought.
There is often some confusion about what IECEx approval is and how it differs from ATEX approval. ATEX approval is a minimum legal requirement for any Ex equipment sold within the European Union. It sets minimum requirements know as Essential Health and Safety Requirements which must be met by the equipment. The most common way to demonstrate conformity to these requirements is to show that your equipment meets the current European standards (either EN 60079 or EN 61241) which are considered to be the current ‘state of the art’. However, you can achieve compliance with the requirements in other ways, and an important point is that the involvement of a third party notified body (such as Baseefa) is not always required. The only legal requirement is that the manufacturer makes available a Document of Conformity which states that the product complies, as far as the manufacturer is concerned. Hence ATEX ‘approval’ can be claimed for a device with no third party involvement.
With IECEx approval, on the other hand, you must have the involvement of a third party notified body to achieve accreditation. The accreditation is to a definite set of standards (EN 60079 and EN 61421 as appropriate) rather than looser health and safety requirements, so it is a ‘pure’ third party accreditation and thus gives a large measure of confidence in the product. The certificate is owned and issued by the third party certification body. Hence it can be viewed that IECEx certification is a worldwide product certification, whilst ATEX certification is a mandatory legal requirement within the European Union.
Application considerations
Dust can be a significant false alarm source with certain types of smoke detector. Many of the current installation standards (e.g. BS 5839-1:2002 in the UK or VdS 2095 in Germany) make a great deal of the requirement to keep false alarms to an absolute minimum. This leaves a duty on designers, installers and end users to ensure that any detector that is fitted is suitable for the area and environment in which it is placed. Further, the requirement to keep false alarms to an absolute minimum can be especially important in hazardous areas, where responses to alarms can include equipment shutdowns and mobilisation of emergency response teams!
It should be remembered that the requirements of the appropriate hazardous atmosphere standard (EN 60079 or EN 61241) take precedence within the confines of the hazardous area itself. This could be the justification for using certain detection types when perhaps, the local installation standard calls for another type as the first choice.
The Tyco range of Ex fire detectors and associated fire controllers, for example, have a number of features that can help with false alarm reduction and maintenance. Points to consider include:
– what zone are we to protect – zone 20, 21 or 22?
– how extensive are the zones?
– will the dust reach a high level/ceiling level (typically where the detectors are)?
– what colour is the dust (white, dark, reflective, metal)?
– how long does the dust take to settle onto surfaces?
– what is the standard of housekeeping in the plant?
It is unlikely that fire detectors would be installed in zone 20 areas, as these would normally be contained within process machinery. In zone 21 areas where white/reflective dust is present frequently, the use of optical smoke detectors may give rise to an unacceptable risk of false alarms. Other types of Ex detector from the range would have to be considered for these areas.
In zone 22 areas any type of detector could be used, including optical smoke detectors subject to a reasonable level of housekeeping in the area. In fact, it could be argued that an optical smoke detector being activated by excessive dust could indicate poor housekeeping or the fact that dust has been dislodged in high quantities – a very serious danger in itself.
In areas were optical smoke detectors would potentially have false alarm issues one very strong alternative would be to use a carbon monoxide (CO) detector or combined CO and heat. Carbon monoxide (CO) detection is immune to dust and is a proven detection technology especially when combined with heat detection.
A further benefit of using carbon monoxide technology would be the possible early detection of a smouldering fire due to a hot surface in any accumulated dust layer. Carbon monoxide is normally one of the gases generated, due to the limited oxygen available during the smouldering combustion process.
Very long-term contamination of certain detector types (primarily optical smoke) by dust is an ongoing issue in all environments, not just hazardous areas. As a result, Tyco Safety Products have built a range of measures into their detector ranges and fire controllers to assist with getting the maximum lifetime and false alarm immunity from these devices. For example, all optical detectors incorporate specially designed optical chambers to ensure the maximum immunity to dust deposits and the maximum period between cleaning and/or replacement.
In hazardous dust environments there will normally be an increased level of dust present. As a result, the time period over which dust accumulations within affected detectors can become significant will be accelerated. This must be taken into account in the frequency of maintenance.
If the addressable series detectors are used in the hazardous area together with an appropriate controller, then a further range of monitoring and maintenance facilities become available.
In addressable systems the detectors can be continually polled by the fire controller, which can track the amount of background dust contamination in the detector over an extended period. The controller can then alter the alarm threshold levels of the device, within tightly controlled limits, to ensure the maximum time between servicing and maximisation of false alarm performance.
If a detector becomes overly contaminated by dust while it is still fully operational, it can be automatically flagged as requiring maintenance. This allows for easy ongoing maintenance and monitoring and directs maintenance staff by address to the affected detector. If required, a visible warning can also be displayed on the controller for end user intervention.
It should be remembered that intrinsically safe fire detectors are not used in isolation – they are the major component in a complete intrinsically safe system. This system will comprise the detectors, barrier or isolator and the interconnecting cable. Gas and dust approved call points are also available, as well as an intrinsically safe system for sounder circuits.
Charles Gormley is UK training manager at Tyco Safety Products. For details of the company’s intrinsically safe detectors and systems go to: www.tycoint.com
