New hazardous zone classification for dust risks
Published: 02 April, 2009
Is your site up to the latest standards? Jonathan Gilbert of Tyco Safety Products explains the jargon and offers some solutions for dust protection
The ATEX directive became mandatory in 2003 and introduced new hazardous area zone classifications for dust risks. The ATEX Definition of a Hazardous atmosphere is:
“mixture with air, under atmospheric conditions, of flammable substances in the form of gasses, vapours, mists or dusts in which, after an ignition has occurred, combustion spreads to the entire unburned mixture.”
People are well aware of the dangers associated with flammable gases but in contrast dust risks may not be taken seriously, after all “its only dust!”. But in fact there are far more explosions attributable to dusts than explosions attributable to flammable gases, there are about 2,000 dust explosions in Europe alone each year. There are a large number of flammable dusts commonly encountered in many industries.
Some typical dusts risks are:
• Agricultural: corn, milk powder, sugar, milled barley, flour
• Carbonaceous: coal, peat, activated charcoal
• Chemical: adipic acid, sulphur, anthraquinone
• Pharmaceutical: aspirin, paracetamol
• Metal: aluminium, iron, zinc.
It is clear from the above that the numbers of industries with possible dust risks far outnumber those industries were gases and vapours would be the main danger. Safety officers should be continually reviewing the processes and materials being used on a site and advising the appropriate precautions needed.
Some of the terms explained
Zones 20, 21 and 22 correspond to Zones 0, 1 and 2 for gas risks (see Figure 1 below). If your process presents a dust risk then your fire detection system should be approved for the appropriate dust zone classification, this means that many older systems may not meet the latest safety requirements. Tyco Safety Products system 620 and system 800 are approved for both gas and dust applications.
What is “intrinsically safe” (IS)?
Intrinsic Safety Exi works by restricting the available energy to ensure there is insufficient energy to ignite the atmosphere under normal and certain foreseeable fault conditions.
• Exia: safety maintained with up to two faults (can be used in any zone including zones 0 & 20)
• Exib: safety maintained with up to one fault (only for use in zones 1, 2, 21 & 22)
• Group I – electrical equipment for mines susceptible to firedamp
• Group II – electrical equipment for places with a potentially explosive atmosphere, other than mines susceptible to firedamp
• Group II is sub-divided into gas groups: IIA, IIB and IIC according to the nature of the gas / vapour for which the equipment is certified; IIA: Propane; IIB: Ethylene; IIC: Hydrogen (Hydrogen being the most easily ignited).
The gasses mentioned in the table are representative gases of each of the respective groups. When designing a system the actual gas or gasses present must be considered. Typically a minimum of group IIB is required for dust risks. The maximum surface temperature of the electrical equipment must always be below the ignition temperature of the flammable atmosphere even if in fault. The classes reference a max of 40 0C.
Temperature Class Max Surface Temp. Deg C
T1 450
T2 300
T3 200
T4 135
T5 100
T6 85
Equipment that is T6 is safer than T1 equipment. This is because the maximum surface temperature is lower for T6 and hence less likely to heat the atmosphere to ignition point.
Careful design is required when planning an intrinsically safe system to ensure the energy levels in the risk area are not dangerous. This not only means selecting the correct equipment but the amount and type of cable as well as the standard of installation are also critical. Vital to an intrinsically safe system is the use of a safety barrier to prevent energy surges into the risk area. Barriers come in two types: the older more traditional type is the Zenner Barrier. This works by switching the power to ground if it detects a fault. The big disadvantage of this type of system is that a very high integrity earth is required along with a strict regime to ensure the integrity is never compromised during the whole life of the system.
The better approach is to use a Galvanic Isolator which does not require an earth connection but acts like an electronic switch to prevent any power surges. Tyco Safety Products recommend the use of galvanic isolators with both system 620 and 800 intrinsically safe equipment.
Why is there a risk with dust?
Normally most solid materials must undergo pyrolysis (heating) to produce flammable volatiles which can then ignite. 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. Heat transfer to an opaque material is very efficient.
• The finer the dust is the greater the danger. Small particles (typically less than 5 microns) will be vaporised 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 as a possible risk.
Tyco Safety Products offer two IS fire detection solutions.
The conventional system 620 for smaller applications and the MZX digital addressable system 800 for larger systems where specific data from each detector is require such as temperature or carbon monoxide levels. Both systems include:
• Photo-optical/heat multi-sensor smoke detector
• Rate of rise heat detector
• Fixed temperature heat detector
• Point IR flame detector
• Carbon monoxide/heat multi-sensor fire detector.
The wide range of detection options available means that you do not have to compromise your choice of detector in a gas or dust hazardous atmosphere.
How to stop dust affecting the fire detection performance.
If you are protecting a dust risk then it is inevitable that there will be airborne dust present at some time. The performance of an optical or ionisation smoke detector will most likely be adversely affected by the presence of dust. Heat detectors are often used in this situation but are likely to dramatically extend the time between an ignition and the reporting of a fire due to the need for heat to build up. Tyco Safety Products’ unique intrinsically safe carbon monoxide/heat multi-sensor fire detector is approved for gas and dust hazardous area applications. In most cases, CO-based fire detection is the preferred choice for dusty environments. CO detectors are unaffected by dust and detect slow smouldering fires before the situation becomes critical.
Tyco Safety Products Intrinsically Safe conventional and MZX addressable detectors are approved to ATEX and IECEx requirements by Baseefa. ATEX being the safety standard for Europe while IECEx is the international standard. The equipment is also LPCB and VdS approved for performance.
Both system 620 and 800 offer economical, practical and reliable fire detection solutions for hazardous areas. Both systems benefit from the inherent advantages of intrinsic safety and both employ galvanic isolation thus avoiding the need for a potentially expensive high integrity earth connection.
