December 01, 2004
by World Grain Staff
by Paolo Panzavolta
In recent years, the importance of quality of life has greatly increased. In mills, this has generated some changes: more hygiene, traceability of food products and better protection for employees, through noise prevention and safety measures.
In Europe, there has been the added requirement to unify the various national standards into a new European set of norms valid across the European Union that will allow products to be interchangeable between countries. The process of creating common rules and regulations is still in progress. Since last year new rules that have come into force include those that pertain to explosion prevention in mines and other work places.
New regulations cover all possible activities, from milling plants to the leather industry to small car body shops, where the danger can come from liquid, gas, dust or a combination of these components. Unlike the United States, the E.U. has no specific standards for the prevention of dust explosion in agricultural and food production facilities (the NFPA 61), instead each plant must be considered on a case-by-case basis: for the moment, specific, more precise standards cover just petrol or gas stations.
In accordance with these new European rules, a factory must be divided into different zones, depending on the potential for explosion — from non-classified places, such as a control room, where there will never be fuel for an explosion, to places in which they are always present, like inside a feed silo or an empty gas tank. Precautions appropriate to each zone must be taken to eliminate any potential source of ignition.
Other standards indicate how to make or test the equipment designed to operate in the different zones: for example, codes limit the use of light metals (aluminum, manganese, titanium and zirconium) in places with a high risk of explosion, with a maximum limit of 10% of mass on the exterior of any equipment in a constantly explosive ambience.
There are also regulations for the thermal resistance of plastics, both in hot and cold ambient temperatures, resistance of equipment to impact, which can apply for some spouting applications and falling tests for portable equipment.
If explosive atmospheres and potential ignition sources are unavoidable, specifically approved explosion resistant equipment, explosion suppressors and explosion relief panels must be used to ensure that the workplace is safe. Damage to property and especially injury to people must be prevented.
There are also standards for the laboratory tests used to obtain all relevant data about potentially explosive materials. Parameters that can be obtained for dust are the minimum and maximum explosive concentration of dust clouds; minimum ignition temperature of the dust cloud and of a dust layer; maximum explosion pressure; limiting oxygen concentration (LOC) or the concentration of oxygen necessary for an explosion; maximum rate of explosion pressure rise; and minimum ignition energy (MIE) of dust clouds.
QUANTIFYING THE RISK
The first thing to be investigated in a plant is what flammable substances are present. In a mill the most evident is the powder generated by grain treatment.
The chemical composition of powders is not uniform (for example, gluten and starch have different characteristics). Their size and moisture also is not uniform — for this reason, data differs depending on the process.
A considerable amount of data is available from literature, based on tests made on dry material: for the minimum explosive concentration the range is between 50 and 100 grams per cubic meter (grs/cu.m); the maximum is about 800 grs/cu.m; maximum explosion pressure is between 8 and 10 bar (it is possible to make equipment explosion resistant); minimum ignition temperature is from 400°C to 450°C, MIE about 100 Millijoules.
As the particle size and moisture increase, there is less chance of explosion, because a big particle can burn but not explode, and water helps render an explosive or flammable atmosphere inert. It is considered that particle sizes above 1 mm cannot create an explosion.
Other fuel sources are present in a mill that have to be taken into consideration, such as liquid propane gas in the laboratory, solvents for glue and paints and gasoline for motor vehicles.
It can be seen that explosive conditions only exist during transition periods, like start up or shut down of the plant. During operation dust concentrations inside machines and fittings are generally over maximum explosive levels, while outside the equipment, explosive concentrations are at a safe level, even without taking into consideration that the moisture content, normally above 14%, requires much more power for ignition.
Another consideration is that a dust/ air mixture usually is not homogeneous; it is quite easy to have dangerous concentration near openings but not in a big area: in fact dust normally deposits within a meter of its source. It is a different story for all the places in which we separate conveying air and dust, such as charging bins or filters: here we create just the right conditions for an explosion.
Dust deposits pose another danger: draught can blow the dust, making a mixture, or the layer can have an insulating effect on a hot surface, resulting in a much more dangerous temperature. For these reasons equipment should be easy to clean or, better, self-cleaning: in any case deposits must be no more than five mm thick.
EXPLOSION RISK ZONES
In the U.S. there are two NEMA (National Electrical Manufacturers Association) zones that relate to explosions: the NEMA 9, in which there is a higher risk of explosion, and the NEMA 12, with lower risk. In Europe, there are three zones for gas and three for dust: the three for dust are Zones 20, 21 and 22, in order of decreasing probability of having an explosive atmosphere.
This means that Zone 20 is a place in which a cloud of combustible dust is frequently present, like in a container, hopper, pipe or filter. Zone 21, is one in which this occurs occasionally, like in an emptying bag hopper; Zone 22, is one in which it isn’t likely to occur, but if it does, it will persist for a short period. An example of the three zones can be seen in Figures 1 and 2 (on page 28), in which we have a cyclone, a filter and a fan. Equipment to be installed in one of these zones requires certification. For Zone 22 this can be self-certification.
In general we need to avoid ignition sources in all situations. However, this is especially true in Zones 20 and 21. In Zone 20, we need to have a redundant or fail-safe system to avoid ignition sources in the rare event of malfunctions.
We must put in to practice all the guidelines mentioned above but the best way is always to study analytically the entire plant. First of all we must reduce the risk by, for example, segregating as much as possible the dangerous areas; have a clean plant by means of generous aspiration, easy to clean equipment and accessories and a good cleaning program; avoid sources of ignition, like ungrounded equipment or lack of electric continuity. It is safer, for example, to use anti-static clothes or plastic hammers: a spark from our trousers or our personal equipment can be as dangerous as from our hammermill.
This means spending a lot of time, classifying the areas and making the necessary changes: in Europe millers are obliged to comply with these safety measures before July 2006, but, in any case, avoiding the risk of accidents must always be the highest priority in our plants. WG
Paolo Panzavolta is a consultant in the field of milling and baking plants. For more information, contact him at: firstname.lastname@example.org