3. FEATURE FEATURE
earthing point prior to starting remit covering bulk complies or can be economically altered to
the discharge process. Care is handling it is a matter comply.
needed to ensure that the tank- that requires attention Good housekeeping, regular inspection
er only couples up to the correct whether it is for con- and maintenance, plus an awareness of
intake line feeding the designat- ditioning of mashes for potentially hazardous processes or areas, are
ed bin and that intake lines are direct sales or for pel- a requisite for trouble free operation. The
of correct diameter, earthed and leting. The three essen- Directives combine these aspirations into
routed with minimum horizontal tials being moisture, requirements and apply not only to the sup-
length and number of bends in temperature and time, pliers of equipment but, in particular, to the
order to reduce the pressure whilst bearing in mind users themselves.
needed to carry out the convey- the heat sensitivity of
ing operation. some ingredients. For Dust classification
As referred to earlier, the configuration pelleting, correct conditioning is necessary to It is the obligation of the user to sat-
of hopper design and type of discharge is ensure starch gelatinisation and pellet quality. isfy himself as to the class or classes of the
all-important in ensuring the free flow of An efficient cooling system is essential materials to be handled and to provide this
materials from the bins to the blending prior to finished products being conveyed to information to the designer or manufacturer
operation. For accuracy this will include one packing or bulk out loading bins, the latter of equipment.
or more main weigh hoppers, a small weigh discharging either directly to bulk vehicle or These are defined under four Kst classifi-
hopper for minerals, and a smaller one for via a travelling weigher. cations (K staube = Class of dust), and relate
micro ingredients. Most of the materials processed in the to rate of pressure rise.
animal feed, pet and fish food, North America
grain, flour, bakery, sugar,
Kst. 0 = Non-explosive
starch and fertiliser industries
Kst. 1 = Weak to moderate
are subject to the DSEAR/
Kst. 2 = Strong
ATEX Explosion Regulations
Kst. 3 = Very strong
that came into operation on
July 1, 2003. There are many
misconceptions and confusion Most materials used in feed mills are
as to the requirements of the covered under Kst. 1 but there are a few to
Directives. It is timely to reiter- which Kst. 2 could apply.
ate the general principles relat- Asia Europe
ing to the regulations, particu- Zoning
larly for those who have only In addition to dust classification, the
recently become involved in user is required to carry out a survey and
one or other of the industries to designate plant and buildings into zones
in which potentially explosive which will be appropriately signed at points
The blended batch is fed to the grinding
plant preceded by a screen to allow meals
materials are handled. of entry. Zones 20, 21 and 22 are the most
likely to apply to feed milling and associated Silo Construction & Engineering
and minerals to bypass the grinder before The Directives industries.
Modular square bins
re-joining the ground materials and passing The Directives apply from July 1, 2003, Zone 20 covers an area in which an explo-
to a three tier mixing assembly consisting of to all new equipment and any existing that sive atmosphere consisting of combustible
pre-mix bin, mixer and dump bin. Molasses is modified or relocated after this date. This dust in air is present frequently for long
and fats are added at the mixer. has particular relevance in ensuring that, if periods or continuously.
Although heat treatment is outside this purchasing any second-hand equipment, it Zone 21 is where an explosive atmosphere
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14 | January - february 2012 Grain &feed millinG technoloGy &feed millinG technoloGy
Grain 21589 Westeel International 2012 G&FMT.indd 1 January - february 2012 | 15 5:03:02 PM
1/11/12
Westeel: International 2012
Publication Grain & Feed Milling Techniques: 90 mm x 270 mm"
4. FEATURE
is likely to occur occasionally in normal Very often a primary explosion in itself the following may be found helpful as an “aide
operation. is not dangerous but the vibration it sets memoire” towards good housekeeping;
Zone 22 is where an explosive atmosphere up disturbs any dust lying on floors, beams, • Enforce a strict no-smoking rule, on pain
would not normally occur but, if it does, it rafters, into the atmosphere. If a source of of dismissal.
would only be for a short period. ignition is present it can result in a secondary • Ensure that all electrical equipment,
Obviously it is the duty of manage- and devastating explosion. So, cleanliness cabling and control panels conform to
ment to ensure that standards of operation throughout the plant is of first priority with relevant standards and regulations, and
and cleanliness are maintained to meet the particular attention being paid to “out of are kept free of dust.
requirements of Zone 22 as far as is practical. sight” areas and cleaning up spillages imme- • Use only totally enclosed, fan-cooled
In carrying out risk assessments it is diately – using one of a variety of vacuum motors, ensuring they are adequately
natural to concentrate on major processing cleaning systems available. Brushing up is earthed.
equipment such as silos, grinders, elevators, definitely out – it only disperses the dust • Ensure light fittings are dust-proof.
dust collectors, etc., and to overlook the elsewhere. • Test cables and wiring regularly.
myriad range of smaller ancillary items that Despite taking all precautions that one • Locate switchgear and process control
also need to be checked. Typical items can think of, it is sod’s law that incidents still panels in dust-free rooms under light
include lighting, electrical fittings, motors, take place – thankfully not too often in view negative pressure.
level indicators, solenoid valves, control of increased awareness of the dangers that • Inspect liquid lines regularly for leaks.
panels. In fact, anything that can generate are always present. Ensure that insulation, if used, has not
a spark. become impregnated, as this could be
It is well known that three elements are Examples ignited by electrical trace heating.
required to cause an explosion – dust in Three examples illustrate the variety of • Bund walls around main storage tanks
suspension at a critical level, oxygen, and a incidents that can happen. should be sized to suit.
spark or hot surface. The first two are always The first resulted from smoulder- • Check that bearings, particularly those
there, so it is against the third item that every ing material entering a bin, setting off fitted to elevators and grinders, are not
precaution must be taken, including satisfac- a primary explosion which ruptured the over-heating.
tory earthing throughout the plant. explosion panel as it was designed to do. • Detect belt slip and misalignment on
Bear in mind that dust in suspension Unfortunately the escaping gases caused a elevators – a major source of fires – by
appearing as a light fog provides the condi- secondary explosion which devastated the rotation and side alignment sensing, and
tion in which a spark can cause an explosion. top floor of the building. As it was impracti- anti-run-back protection.
The finer the dust particles the greater the cal to vent every bin to atmosphere it was • Check for possible temperature rise in
danger because of the increased surface area subsequently agreed with HSE that the top stored bulk materials, which could result
exposed to atmosphere. floor over the bins would in future be a “no in spontaneous combustion.
The duties of the user having been go” area whilst the plant was in production • Inspect bin interiors using only battery-
described in general terms, what about the and for ten minutes afterwards. A lockable operated, non-glass, flameproof
supplier of the equipment? Firstly, he has to gate was fitted to the access stairs and a inspection lamps, which are suitably
satisfy himself that the user has provided him warning notice affixed. secured and never allowed to be in
with all the necessary details concerning clas- The second resulted from a hot spot due contact with the product. (In the past
sifications of materials to be processed and to a malfunction in the motor of a dust unit it was not unusual for naked electric
the areas in which equipment is to be located, fitted on top of a grinder expansion hop- bulbs to be lowered into bins – at best
Heat Recovery Dryer
30%
together with any other relevant information. per. The explosion panel ruptured but had protected with a wire guard).
The supplier then has to ensure that the not been vented to atmosphere through a • Ensure hot work is carried out only on
equipment he supplies is designed, manufac- nearby wall. isolated, cleaned-out plant, against Work
tured and installed to satisfy requirements Unfortunately, two employees were Permit issued by management, and
by taking all precautions to prevent an standing nearby at the time and were badly provision of fire blankets, extinguishers, etc.
explosion but also, and most importantly, to burned. It was interesting to note that a Many fires have occurred during periods
mitigate against an explosion should such an choke had been fitted to the bin discharger of repair, renovation or plant modification Up to
event occur. beneath the expansion hopper and pre- (as in the case of Windsor Castle a few
Equipment can be manufactured in such
heavy construction that an explosion would
be contained but this is so expensive as to be
vented the explosive gases passing into
a subsequent elevator and storage bins,
otherwise the result would have been even
years ago). During these special periods, in
addition to taking fire precautions, it is advis-
able to inspect the area closely for at least an
Energy Savings
impractical. The alternative is to fit a certi- more serious. hour at the end of each working day. With our Heat Recovery Dryer you get the
fied explosion panel vented to atmosphere The third was caused by welding being Adherence to these principles will ensure
lowest energy consumption on the market.
through a nearby wall or roof. carried out on the side of a silo, one of not only a pleasant environment in which to
Due to the location of plant within a build- several such accidents over the years, in work but also one that is as intrinsically safe TORNUM Grain Coolers – The eco-friendly
ing venting may not be practical and so the which the operator was injured. In this as possible. way of preserving grain quality.
fitting of expensive flame quenching or explo- case it was not the result of negligence.
sion suppression equipment may be required. The silo had been isolated from its feeding
To prevent the propagation of an explo- conveyor, cleaned down internally and the
sion, items of equipment should be isolated subject of a work permit. Unfortunately, About the author:
from each other. An example being to incor- a small amount of material had remained Mr Alf Croston is managing director
porate valves or slides so that only one bin in an inaccessible spot and on being of Croston Engineering, at Tarvin, near
can be filled and exhausted at a time. Bin disturbed created the conditions for an Chester, which was founded in 1976.
dischargers and screw conveyors can be explosion to take place. His company specialises in the design
designed with chokes incorporated. The foregoing describes in broad outline and building of bulk storage, handling
The installation of a rubble separator on the rationale behind the ATEX Directives. and process plants throughout the UK
the intake system and magnets at appropri- Many of the requirements are common sense, and Ireland for many household names TORNUM AB Box 100, SE-535 22 Kvänum, SwEdEn
ate points throughout the plant are obvious but common sense has to be backed up with in industry. Phone +46 (0)512–291 00 E-mail: info@tornum.com
precautions. documentation in this day and age. However, www.tornum.com
16 | January - february 2012 Grain &feed millinG technoloGy &feed millinG technoloGy
Grain January - february 2012 | 17
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January - February 2012
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Energy saving in flour milling storage
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Preservatives are a recurring
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