6) Intrinsic Safety and Dust

The topic links on this page include:

6.1 General  6.2 Intrinsic safe apparatus and dusts  6.3 Risk analysis  6.4 Why use intrinsic safety?

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6.1 General

A number of finely divided materials can be ignited to create an explosion when they form a cloud in air. Almost all organic and food product dusts together with metallic dust can readily be ignited. Dust explosions are generally more difficult to initiate than gas/air explosions but can be devastating. The initial explosion frequently disturbs and entrains layered dust to create one or more secondary explosions, thus creating a rolling explosion and extensive damage.

Dust explosions can be initiated by electrical sparks or by hot surfaces. There are numerous factors, which influence ignition energy and temperature of a particular material. For example the air to particle ratio, the particle size, humidity, and the melting temperature of the material.

Note: For those requiring a comprehensive reference ‘Dust explosions in the process industries’ by Rolf K. Eckhoff published by Butterworth Heinemann. ISBN 0 7506 3270 4 is recommended.

The ignition energy of a dust/air mixture is high compared with that of a gas/air mixture. For example, some sensitive materials such as rubber, sulfur and fine wood dust require 1 to 10 mJ while less sensitive materials, such as coffee, require more than 500 mJ.

There is some concern that some very finely divided particles, for example those associated with nano-technology, may have even lower ignition energies. Consequently, the decision has been made to use the IIB gas as the test mixture (ignition energy 80µJ) for intrinsically safe apparatus for use in dust atmospheres. This is a very conservative decision but presents very little operational difficulty. The current state of knowledge on the spark ignition characteristics of dusts and the difficulty of creating a satisfactory test apparatus for dust atmospheres justifies a cautious prudent decision.

The major problem in dust atmospheres is the possibility of thermal ignition. There are two common mechanisms, one is the ignition of a dust cloud by a hot body and the other is the creation of smouldering in a layer of dust on a hot surface.

The minimum ignition temperature of the majority of dusts lies between 300°C and 600°C. Some dusts do ignite at lower temperatures, for example finely divided sulfur has a minimum ignition temperature of 240°C. It is quite difficult to generate these temperatures in a dust cloud with the power levels permitted by a IIB gas classification and hence the probability of ignition of a dust cloud by intrinsically safe apparatus is quite low and not the major problem.

The principal difficulty is the possibility of causing smouldering within a dust layer, which when disturbed bursts into flames and initiates an explosion. The mechanism of causing smouldering is complex but can be simplified into keeping the dust below its ‘glow temperature’. The majority of materials have a glow temperature, ranging from 250°C to 500°C, that is lower than the minimum ignition temperature of the corresponding dust cloud. Some flammable dust layers have the fortunate characteristic of melting before attaining their theoretical glow temperature and consequently they do not create this ignition risk (for example polystyrene).

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6.2 Intrinsically safe apparatus and dusts

Intrinsically safe apparatus certified for use in hazardous gas atmospheres has been used to ensure safety in dust atmospheres for many years. Currently a great deal of activity is taking place to formalise the requirements for apparatus specifically for use in dusts. An apparatus standard IEC 61241-11 is in the final stages of preparation. The ultimate intention is to amalgamate the dust and gas requirements within the relevant IEC standards but this will take a number of years (five?). Eventually there will be three levels of protection ‘iaD’, ‘ibD’, and ‘icD’ corresponding to the equivalent gas levels of protection. The intention is that ‘iaD’ equipment will achieve the ‘very high’ level of protection required by equipment designated as ‘EPL Da’ (where EPL means ‘Equipment Protection Level’ as defined in IEC60079-0). ‘ibD’ with a ‘high’ level of protection will achieve an ‘EPL Db’ and ‘icD’ with an ‘enhanced’ level of protection will be ‘EPL Dc’.

Table 6.1 Comparison of different levels of risk

The risk of spark ignition is avoided by satisfying the requirements for apparatus intended for use in IIB gases

To avoid the risk of thermal ignition the preferred technique for apparatus, which is intended to be located in the hazardous area, is to exclude the dust by using an IP 6X enclosure or by encapsulation. This involves determining a maximum temperature rise of the exposed surface, which in the case of most intrinsically safe apparatus will be very small.

The preference for a dust tight enclosure is because the ‘dust fraternity’ has implicit faith in this technique. It can be argued that the restriction of the available power is a more reliable technique as it is less prone to maintenance errors.

There is an exemption to the enclosure rule for apparatus that is difficult to operate inside an enclosure, such as some sensors. In these circumstances the power level is restricted, to avoid the possibility of temperature ignition (750mW at 400°C).

In practice all intrinsically safe associated apparatus such as barriers and isolated interfaces, which are IIC or IIB certified for gases are suitable for use in intrinsically safe systems. It is has been common practice for several years for interfaces to be certified for both gas and dust applications. For example, the current MTL range of barriers (MTL7700) and isolators (MTL5000) are certified for both gas and dust applications in accordance with the requirements of the ATEX Directive and FM standards.

The design of intrinsically safe apparatus for use in dusts is the subject matter of Part 11 of IEC 61241.

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6.3 Risk analysis

Analysing the risk associated with a flammable dust differs from the analysis of a gas risk largely because dust does not disperse in the same way as a gas, it has to be removed.

A decision was made some years ago to only area classify dust clouds and to treat the possibility of a smouldering dust layer as a source of ignition. (a decision largely influenced by the ATEX Directives).

The area classification of dust clouds follows the pattern of gas clouds. Zone 20 corresponds to Zone 0 (where the hazard is present continuously or for long periods) Zone 21 to Zone1 and Zone 22 to Zone 2 as the probability of the dust cloud being present reduces.

Area classification of dusts is the subject matter of Part 10 of IEC 61241. If the combination of area classification and sources of ignition is pursued too diligently this can create some tortuous thinking. Fortunately, the application of a little pragmatic common sense solves most instrumentation problems.

For example, if a temperature sensor is buried in a mound of grain for a considerable length of time, then it is reasonable to use a level of protection ‘iaD’ since deciding the area classification is difficult and if the grain is smouldering it will probably burst into flame when disturbed and could possibly explode. As it is not expensive to make the system ‘ia D’, this becomes the obvious solution. However if a temperature monitor is measuring temperature in a location where it is infrequently covered by dust and can be readily and frequently cleaned then a level of protection ‘icD’ is adequate. It might still be expedient to use ‘ia D’ equipment but it is not essential to do so.

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6.4 Why use intrinsic safety?

The principal reason for using intrinsic safety is because it is essentially a low power technique. Consequently, the risk of ignition is minimised, and adequate safety can be achieved with a level of confidence that is not always achieved by other techniques.

It is difficult to assess the temperature rise, which can occur if equipment is immersed in a dust because of the many (frequently unpredictable) factors, which determine the temperature rise within the dust layer. The safest technique is therefore to restrict the available power to the lowest practical level. A major factor in favour of intrinsic safety is that the power level under fault conditions is controlled by the system design and does not rely on the less well-specified limitation of fault power.

Intrinsic safety also has the advantage that the possibility of ignition from immersed or damaged wiring is minimised.

It is desirable to be able to do ‘live maintenance’ on an instrument system, and the use of the intrinsically safe technique permits this without the necessity of special ‘dust free’ certificates. There is a need to clear layers of dust carefully and to avoid contamination of the interior of apparatus during maintenance but this is apparent to any trained technician. (There is no significant possibility of a person in a dust cloud, that can be ignited, surviving without breathing apparatus)

To summarise, intrinsic safety is the preferred technique for instrumentation where dust is the hazard because:

• the inherent safety of intrinsic safety gives the greatest assurance of safety and removes
  concern over overheating of equipment and cables
• the installation rules are clearly specified and the system design ensures that all safety aspects
  are covered
• live maintenance is permitted
• equipment is available to solve the majority of problems
  

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