What image does this heading bring?
For many it may be the image of a ruptured gas line under significant pressure rapidly spreading and forming the monster of all vapour clouds creeping up on those inconspicuous effective ignition sources. For others it may be the thought of a dense liquefied gas which is speedily rolling down slopes and filling service ducts and gullies lying in wait for the mayhem of evacuees to snatch a static discharge from and engulf the unfortunate initiator within a rapidly expanding fireball.
The dangers of fuel leaks are well known and all precautions are taken to ensure that fuels are always safely contained. This is mainly achieved by sound engineering, rigorous inspection/ maintenance and certified installers such as the UK Gas Safe Scheme which replaced Corgi.
Why is it then that leaking combustible dusts from process equipment is not given a second thought?
This same process dust in most instances is more violent than the above mentioned flammable gases and is often already present in large quantities due to the acceptance of previous migration throughout the whole facility, on ledges; suspended ceilings; floors etc.
It has been proven that 1mm of dust lying on the floor can generate a concentration of 60g/m3 when dispersed as a dust cloud 5mtrs high, and a concentration of 400-500g/m3 when dispersed as a dust cloud 1mtr high. The majority of organic combustible dusts have a minimum explosible concentration of 60g/m3 and optimum of 400-600g/m3.
Although the probability of an effective ignition source for the majority of combustible dusts may be considered low, the consequence of a secondary dust explosion or indeed a series of secondary dust explosions are often catastrophic and lead to the total loss of facilities and often include multiple fatalities. The Chemical Safety Board have many excellent videos which demonstrate these significant hazards, one of which is titled ‘’Combustible Dust – An Insidious Hazard’’ and can be found at www.csb.gov/videoroom
Containment of dust within the process is therefore of paramount importance, not only do robust containment techniques reduce the risk of secondary dust explosions, it also reduces the external hazardous zone extent (Hazardous Area Classification) in the essence of DSEAR & ATEX 137, which ultimately reduces the cost of compliance by keeping the inventory of equipment falling within a designated hazardous area down to a minimum.
Managers, operators and technicians must be educated in respect of combustible dusts to ensure that leaking combustible dust is immediately removed and equipment rectified in the same manner one would do if a flammable gas or liquid was leaking from the process.
When designing new plant or assessing current installations careful consideration should be made to the containment of dust, ie keep it inside the process equipment. Remove it at source, ie use effective local exhaust ventilation. Another technique quite often overlooked is to consider handling the bulk material with more care. Limit the amount of turnover points from conveyors and reduce the heights which bulk solids will fall within the process, mitigating additional attrition and reducing fines liberation.
Some processes I have seen often just move dust around, extracting it from the primary location only to deliver it back into the process very close to where it was removed or even upstream hence the dust is just circulating around within the process. The less fines you continue to move throughout the process the lower probability there is for potential primary and secondary releases, therefore less hazardous zones and lower risk.
Having a good understanding of explosion science is important to achieving sound measures so that solutions are based on scientific fact and not on such criteria as (an often seen phrase) “anecdotal evidence”. It is important that all safety systems are fully designed and justified and the same applies for changes to zone classification and extent.
Remember over zoning is always false economy and only leads to the recommendation for more certified ATEX Ex safe equipment. Risk is a product of probability and consequence, ATEX certified equipment only reduces the probability of an effective ignition source being present. Other factors such as reducing the likelihood of a flammable atmosphere or reducing the consequence of an event by some form of mitigation may also prove to reduce the risk to an acceptable level without the replacement of existing equipment.
Every effort to contain the combustible dust within the process can always be economically justified by the offset of reduced cleaning costs, reduced maintenance, improvements in working conditions and not least efficiency of the process and reduction of material waste.
The UK has not had any major secondary dust explosions recently. This would suggest the law of probability is now tipping the scales in favour of a significant event very soon.
Don’t let it be on your facility! Take measures now to reduce dust levels within your process.
It is very important to understand that an appreciation of DSEAR / ATEX 137 and the science involved via a short course is not the same as gaining an in depth knowledge of the subject.
Assessments and therefore risks will vary according to the level of knowledge, training, experience and hence competency possessed by the individual completing the tasks. In many large plants and complex cases it may be wise and often prove cost effective to seek advice and assistance from experienced professionals.
For further information please contact;
PG Dip Eng, FIFSM, SFIIRSM
GexCon UK Ltd
Tel : +44 (0) 1695 726565