Doubling the Risk ?
The ATEX Directives and recent IEC standards have increased awareness of the risk associated with both dust and gas hazards. However none of the existing documents cover what should be done when both dust and gas is present. This combination frequently occurs in the storage of agricultural products, the pharmaceutical industry and in some parts of the petrochemical industry and hence a solution is required. Fortunately for low power requirements such as instrumentation, intrinsic safety can usually achieve an acceptable level of safety.
Historically the solution was to choose equipment, which was considered acceptable in the gas when, considered on its own, and then consider whether this equipment was acceptable for use in the dust atmosphere. If the equipment satisfied both criteria it was considered acceptably safe. Recently the possibility that there may be interaction between the gas and dust which adversely affect the ignition properties of the combination have been aired by the cognoscenti who are well known for predicting ignition by the impact of butterfly wings. In practice there are so many unknown and unpredictable variations in the ignition mechanisms of dusts that the possibility of there being some combination which is more sensitive than the separate entities is high. However the probability of the combination occuring is likely to be low. Unfortunately there is no definitive data on what the likely increase in sensitivity will be, and a research programme to investigate all the possible variations on the combinations is impractical and consequently unlikely to take place in the foreseeable future.
The available solution follows the path beloved by the medical profession of following the ‘precautionary route’, which can roughly be translated as ‘ we do not know the answer so increase the safety factor’. Using IIC intrinsically safe equipment can largely eliminate the spark ignition risk. The inherent factors of safety required by the intrinsic safety standard and built into the ‘test apparatus’ ensure that even if the gas is hydrogen or acetylene the possibility of ignition is acceptably small. It can be argued that a IIB classification is more than adequate for most circumstances, but if a IIC solution exists this has a larger comfort factor and should be the preferred solution. If by chance you have a combined carbon disulfide and aluminium dust risk then you have my sympathy and a suggestion that a change of employment is desirable, but no practical solution exists as far as I know.
The significant risk from instrumentation in most dust locations is that of temperature ignition. If the equipment is certified for use in dusts the avoidance of high temperatures will have been established. A major advantage of intrinsically safe apparatus is that the power levels are low and consequently the temperatures created are less of a problem. The majority of field mounted apparatus [for example dp transmitters] are mounted in substantial enclosures to keep the dust out for both operational and reasons and hence there is no problem. Some sensors cannot be enclosed for operational reasons and these should be operated at a very low power level to ensure safety. A figure of 750mW is used for a dust hazard alone and this is considered safe even in extreme circumstances, but should possibly be lowered for the hybrid risk to something like 500mW. This appears to be severe but in practice is not difficult to achieve. For example an isolator for transferring temperature from an RTD [MTL 5074] has a matched power of 130mW and is adequate for all circumstances. Lower figures are achieved for switches and thermocouples.
Conclusion
The solution is usually to use IIC certified equipment in a dust tight enclosure. If there is an exposed sensor then make sure the possible fault power available is small. If when you have adopted this solution someone questions its adequacy, the response has to be ‘what would you recommend then, and what evidence do you have to support your recommendation ?’
