GFMT Article: Bulk storage and handling

We've dipped into GFMT archive and dug out this article on bulk storage and handling by Alf Croston, Managing Director, Croston Engineering, UK.

Read the full article as it appears in the Jan/Feb 2012 issue of the magazine here or scroll down for just the text.

Bulk storage and handling

by Alf Croston, Managing Director, Croston Engineering, UK

As with most things in life, the basics remain the same although they may become more sophisticated, or complicated, dependent on one’s viewpoint.

And so it is with this overview covering the bulk storage and handling of materials in the animal feed and human food industries, from the intake of raw materials through to the discharge of finished products.

It is barely 60 years since a very high proportion of the milling industry was located at the major ports with raw materials in sacks being transported from the docks by horse and cart and then hoisted up to the various floor levels for storage there to be cut and tipped into process bins as and when required.

Gradually, as more home grown grain became available, together with the advent of purpose-built bulk vehicles and an improved road network, there was a move to country mills more conveniently located to service the farming community by buying grain locally, processing it into feed and selling the resultant product back to the farmer.

The use of computers and automation throughout the milling process has reduced what was a labour intensive industry to one controlled by a few technically proficient operators, but to whom the basics of material handling must still apply, as do health and safety requirements, adherence to DSEAR/ATEX Explosion Regulations, plus health and hygiene control.

Hence this résumé.

Interruptions in production

The interconnection of process plant is designed to be fail-safe and so prevent chokes and interruption to production.

Intake capacity from bulk tankers has greatly increased over the years and is normally well in excess of 100 tonnes/hour via an intake hopper with safety grid located under cover plus an efficient dust extraction system, and discharging into a screw or chain type conveyor which may, if wished, be fitted with a variable speed drive so that the intake rate may be varied to suit the characteristics of the particular material being dealt with in order to prevent overloading subsequent equipment.

The conveyor should be fitted with rotation control and overfeed detection.

The intake bucket elevator, as with all similar units in the mill, must incorporate explosion relief panels at prescribed intervals, electrically linked to shut down the plant in the event of an explosion occurring.

Because of their inherent design, bucket elevators have a built-in explosion risk factor and, if located within a building, the explosion panels should be ducted to atmosphere. Elevators should also incorporate tensioning gear at the boot, anti-runback device to cater for a choke or power failure, rotation sensor to indicate if the belt is slipping and side alignment indication.

Intake points are frequently out of sight of the control room so, to avoid being allowed to run empty for long periods, and a procedure should be in place to shut down when not in use.

A rotary drum pre-cleaner located at the top of the Mill to remove foreign matter prior to the material being conveyed to raw material bins will protect subsequent equipment from being damaged.

The conveyors feeding silos and bins will have multiple outlets and the electrical control system must be designed so that only one slide is open at a time in order to prevent the propagation of an explosion from one bin to another.  As with the intake conveyor, all conveyors should incorporate overfeed detection and rotation sensing. 

To cater for dust laden air displacement at transfer points, small dust units with built-in exhaust fans at convenient locations will ensure a clean atmosphere. 

Storing different materials

The number, location and holding capacity of new material bins is determined by site conditions and the particular requirements of individual clients, bearing in mind the large number of different materials to be handled and stored in the feed industry.

Ranging from free flowing grains to a variety of meals and moisture content, the bins and discharge equipment should be designed to cater for those with the worst flow characteristics to ensure maximum flexibility so that individual bins can be used for the storage of any ingredients should the need arise.

Level probes are required to prevent overfilling, as are policed explosion panels.

Provision will be required for minerals and other ingredients that are delivered by bulk tanker and pneumatically conveyed to dedicated bins utilising either a blower mounted on the tanker chassis or, in some cases, by coupling up from a land-based blower. To prevent static electricity causing a spark, the tanker will be connected to an earthing point prior to starting the discharge process. Care is needed to ensure that the tanker only couples up to the correct intake line feeding the designated bin and that intake lines are of correct diameter, earthed and routed with minimum horizontal length and number of bends in order to reduce the pressure needed to carry out the conveying operation.

As referred to earlier, the configuration of hopper design and type of discharge is all-important in ensuring the free flow of materials from the bins to the blending operation. For accuracy this will include one or more main weigh hoppers, a small weigh hopper for minerals, and a smaller one for micro ingredients.

The blended batch is fed to the grinding plant preceded by a screen to allow meals and minerals to bypass the grinder before re-joining the ground materials and passing to a three tier mixing assembly consisting of pre-mix bin, mixer and dump bin. Molasses and fats are added at the mixer.

Although heat treatment is outside this remit covering bulk handling it is a matter that requires attention whether it is for conditioning of mashes for direct sales or for pelleting. The three essentials being moisture, temperature and time, whilst bearing in mind the heat sensitivity of some ingredients. For pelleting, correct conditioning is necessary to ensure starch gelatinisation and pellet quality.

An efficient cooling system is essential prior to finished products being conveyed to packing or bulk out loading bins, the latter discharging either directly to bulk vehicle or via a travelling weigher.

Most of the materials processed in the animal feed, pet and fish food, grain, flour, bakery, sugar, starch and fertiliser industries are subject to the DSEAR/ATEX Explosion Regulations that came into operation on July 1, 2003. There are many misconceptions and confusion as to the requirements of the Directives. It is timely to reiterate the general principles relating to the regulations, particularly for those who have only recently become involved in one or other of the industries in which potentially explosive materials are handled.

The Directives

The Directives apply from July 1, 2003, to all new equipment and any existing that is modified or relocated after this date. This has particular relevance in ensuring that, if purchasing any second-hand equipment, it complies or can be economically altered to comply.

Good housekeeping, regular inspection and maintenance, plus an awareness of potentially hazardous processes or areas, are a requisite for trouble free operation. The Directives combine  these aspirations into requirements and apply not only to the suppliers of equipment but, in particular, to the users themselves.

Dust classification

It is the obligation of the user to satisfy himself as to the class or classes of the materials to be handled and to provide this information to the designer or manufacturer of equipment.

These are defined under four Kst classifications (K staube = Class of dust), and relate to rate of pressure rise.

Kst. 0 = Non-explosive

Kst. 1 = Weak to moderate

Kst. 2 = Strong

Kst. 3 = Very strong

Most materials used in feed mills are covered under Kst. 1 but there are a few to which Kst. 2 could apply.

Zoning

In addition to dust classification, the user is required to carry out a survey and to designate plant and buildings into zones which will be appropriately signed at points of entry.  Zones 20, 21 and 22 are the most likely to apply to feed milling and associated industries.

Zone 20 covers an area in which an explosive atmosphere consisting of combustible dust in air is present frequently for long periods or continuously.

Zone 21 is where an explosive atmosphere is likely to occur occasionally in normal operation.

Zone 22 is where an explosive atmosphere would not normally occur but, if it does, it would only be for a short period.

Obviously it is the duty of management to ensure that standards of operation and cleanliness are maintained to meet the requirements of Zone 22 as far as is practical.

In carrying out risk assessments it is natural to concentrate on major processing equipment such as silos, grinders, elevators, dust collectors, etc., and to overlook the myriad range of smaller ancillary items that also need to be checked. Typical items include lighting, electrical fittings, motors, level indicators, solenoid valves, control panels. In fact, anything that can generate a spark.

It is well known that three elements are required to cause an explosion – dust in suspension at a critical level, oxygen, and a spark or hot surface. The first two are always there, so it is against the third item that every precaution must be taken, including satisfactory earthing throughout the plant.

Bear in mind that dust in suspension appearing as a light fog provides the condition in which a spark can cause an explosion. The finer the dust particles the greater the danger because of the increased surface area exposed to atmosphere.

The duties of the user having been described in general terms, what about the supplier of the equipment? Firstly, he has to satisfy himself that the user has provided him with all the necessary details concerning classifications of materials to be processed and the areas in which equipment is to be located, together with any other relevant information.

The supplier then has to ensure that the equipment he supplies is designed, manufactured and installed to satisfy requirements by taking all precautions to prevent an explosion but also, and most importantly, to mitigate against an explosion should such an event occur.

Equipment can be manufactured in such heavy construction that an explosion would be contained but this is so expensive as to be impractical.  The alternative is to fit a certified explosion panel vented to atmosphere through a nearby wall or roof.

Due to the location of plant within a building venting may not be practical and so the fitting of expensive flame quenching or explosion suppression equipment may be required.

To prevent the propagation of an explosion, items of equipment should be isolated from each other. An example being to incorporate valves or slides so that only one bin can be filled and exhausted at a time.  Bin dischargers and screw conveyors can be designed with chokes incorporated.

The installation of a rubble separator on the intake system and magnets at appropriate points throughout the plant are obvious precautions.

Very often a primary explosion in itself is not dangerous but the vibration it sets up disturbs any dust lying on floors, beams, rafters, into the atmosphere. If a source of ignition is present it can result in a secondary and devastating explosion. So, cleanliness throughout the plant is of first priority with particular attention being paid to “out of sight” areas and cleaning up spillages immediately – using one of a variety of vacuum cleaning systems available. Brushing up is definitely out – it only disperses the dust elsewhere.

Despite taking all precautions that one can think of, it is sod’s law that incidents still take place – thankfully not too often in view of increased awareness of the dangers that are always present.

Examples

Three examples illustrate the variety of incidents that can happen.

The first resulted from smouldering material entering a bin, setting off a primary explosion which ruptured the explosion panel as it was designed to do. Unfortunately the escaping gases caused a secondary explosion which devastated the top floor of the building.  As it was impractical to vent every bin to atmosphere it was subsequently agreed with HSE that the top floor over the bins would in future be a “no go” area whilst the plant was in production and for ten minutes afterwards.  A lockable gate was fitted to the access stairs and a warning notice affixed.

The second resulted from a hot spot due to a malfunction in the motor of a dust unit fitted on top of a grinder expansion hopper.  The explosion panel ruptured but had not been vented to atmosphere through a nearby wall.

Unfortunately, two employees were standing nearby at the time and were badly burned. It was interesting to note that a choke had been fitted to the bin discharger beneath the expansion hopper and prevented the explosive gases passing into a subsequent elevator and storage bins, otherwise the result would have been even more serious.

The third was caused by welding being carried out on the side of a silo, one of several such accidents over the years, in which the operator was injured. In this case it was not the result of negligence. The silo had been isolated from its feeding conveyor, cleaned down internally and the subject of a work permit. Unfortunately, a small amount of material had remained in an inaccessible spot and on being disturbed created the conditions for an explosion to take place.

The foregoing describes in broad outline the rationale behind the ATEX Directives. Many of the requirements are common sense, but common sense has to be backed up with documentation in this day and age. However, the following may be found helpful as an “aide memoire” towards good housekeeping;

Enforce a strict no-smoking rule, on pain of dismissal.

Ensure that all electrical equipment, cabling and control panels conform to relevant standards and regulations, and are kept free of dust.

 

Use only totally enclosed, fan-cooled motors, ensuring they are adequately earthed.

Ensure light fittings are dust-proof.

Test cables and wiring regularly.

Locate switchgear and process control panels in dust-free rooms under light negative pressure.

Inspect liquid lines regularly for leaks. Ensure that insulation, if used, has not become impregnated, as this could be ignited by electrical trace heating.

Bund walls around main storage tanks should be sized to suit.

Check that bearings, particularly those fitted to elevators and grinders, are not over-heating.

Detect belt slip and misalignment on elevators – a major source of fires – by rotation and side alignment sensing, and anti-run-back protection.

Check for possible temperature rise in stored bulk materials, which could result in spontaneous combustion.

Inspect bin interiors using only battery-operated, non-glass, flameproof inspection lamps, which are suitably secured and never allowed to be in contact with the product. (In the past it was not unusual for naked electric bulbs to be lowered into bins – at best protected with a wire guard).

Ensure hot work is carried out only on isolated, cleaned-out plant, against Work Permit issued by management, and provision of fire blankets, extinguishers, etc.

Many fires have occurred during periods of repair, renovation or plant modification (as in the case of Windsor Castle a few years ago). During these special periods, in addition to taking fire precautions, it is advisable to inspect the area closely for at least an hour at the end of each working day.

Adherence to these principles will ensure not only a pleasant environment in which to work but also one that is as intrinsically safe as possible.