BRITISH SOCIETY OF SUGAR TECHNOLOGISTS ATM 16 th OCTOBER TECHNICAL PAPER No. 2. AVOIDING IMPERIAL SUGAR 2 One approach to Preventing Recurrence

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1 BRITISH SOCIETY OF SUGAR TECHNOLOGISTS ATM 16 th OCTOBER 2014 TECHNICAL PAPER No. 2 AVOIDING IMPERIAL SUGAR 2 One approach to Preventing Recurrence Introduction The multiple explosions and fires at Imperial Sugar refinery in Savannah, Georgia in on February 7 th 2008, which caused massive damage and loss of life, sent waves of concern across the sugar industry world. Nothing as serious as this had been seen before and the realisation that equipment that had behaved perfectly well for many years could suddenly cause such major issues rang alarm bells within the major sugar manufacturers and traders. Following this incident with white sugar handling, there have also been at least six major fires in raw sugar storage and handling systems in the last three years and a fire at the top of a white sugar silo in Germany this year, all of which have heightened awareness of the need for systems to prevent explosions and fires. Many will remember also grain silo explosions, particularly in USA in the past. This paper looks at one approach to minimising the risks to production and products in the future from sugar dust explosions (SDE). 1. Principles In order to experience an SDE, the conditions within sugar handling plants and specific equipment have to meet all the following criteria, known as the Fire Pentagon, which is a more detailed version of the older Triangle of Fire: 1. Fuel. A dry, combustible material with a particle size small enough to provide a high surface area to volume ratio to facilitate rapid combustion. 2. Oxidising Substance. There must be oxygen present in that concentration of sugar dust. If the space is filled with an inert gas, there is no risk of an explosion. 3. Dispersion. The dust in question must be of a combustible nature and be able to become airborne. BSST ATM 2014 Paper 2 Page 1 of 8

2 4. Confinement. The concentration of dust in any enclosed space, whether a room or the space within a piece of equipment, must exceed a 20% concentration of airborne particles. This is easily measured in a room by holding your arm out at full stretch with the fingers turned 90 degrees. If you are not able to see them, the concentration is above 20%. Also, the atmosphere inside the enclosed space must be dry. If the space is humid or damp, the risk of an explosion is greatly reduced. High atmospheric humidity does not necessarily reduce the risk within sealed equipment, but may reduce the risk in the room area. 5. Ignition. There must be a source of ignition available within that enclosed space. The source of ignition can be anything from an electrical switch (light, power or sensor), to an overheating bearing or melted grease, and/or an abraiding conveyor belt or metal-to-metal contact producing heat or sparks. The removal of any one of these five conditions will eliminate the risk of fire and explosion. Sugar, being a combination of Carbon and Hydrogen and, in its dry state, having very little moisture in it, is a perfect material for susceptibility to explosions. Further, the conveying of sugar in manufacturing facilities and other handling plants almost always involves transfer points, which encourage the development of sugar dust. As the particle size would be typically between 0 and 100 micron, the dust is easily airborne and able to spread widely, settling on any horizontal surfaces nearby. The dust layers build up gradually, almost invisibly to those working every day in the environment, producing a latent fuel source waiting for a suitable air disturbance. The conveying system is normally a combination of screw conveyors, belt conveyors, elevators and screening equipment, depending upon the layout of the building and the requirements of the particular process involved. All of these items play their part either in increasing or reducing the sugar dust being conveyed: Screw conveyors inevitably grind some of the sugar crystals into smaller pieces and the smaller pieces into dust. This can be minimised by running the screw slowly, but then the size of the screw has to be increased to achieve the same throughput. Belt conveyors are the kindest to the sugar and limit the damage to the crystal, but, in transferring to and from a belt conveyor, any dust in the sugar stream becomes airborne, as the minimum transfer drop is around 500mm. Elevators are reasonably gentle with sugar crystals, depending upon their design, but some breakage of sugar crystals is inevitable at the point where the elevator buckets pick up the sugar and dust clouds are created and the transfer points into and out of the elevator. Screening equipment is designed to separate the sugar dust from the crystals and to grade the crystals into product requirements. As these are almost all vibratory machines, a dust cloud is created within the unit, coating the inside of the casings. The graded crystals coming from the screen contain much less dust and are inherently safer in terms of SDE, as long as scree conveyors are not used again after screening. The sugar dust removed by the screening equipment is, by definition, the most hazardous and must be handled with care. So now we have created sugar dust inside the conveying equipment, layers of dust on horizontal surfaces outside the equipment, what more do we need? BSST ATM 2014 Paper 2 Page 2 of 8

3 First, we need Oxygen. This is simple! As far as anyone is aware, no plant encloses their sugar handling area in an inert gas, such as nitrogen. Whilst this would be extremely effective in eliminating the risk of an explosion, it tends to be life-limiting for the personnel working in the plant and authorities such as the Health and Safety Executive in the UK would consider such an approach as injurious to health. So, the inside and outside of sugar handling equipment is contained in an atmospheric environment with an approximately 21% O 2 content. Second, we need a source of ignition or an adequate heat source. This can be achieved in a number of ways, each of which would produce the necessary conditions for a first explosion: 1. A fire. This may be an open heater, such as a 5 bar electric unit, or someone smoking or lighting matches, or cutting and welding work being carried out. Many enlightened sugar companies have already banned smoking, matches and open heaters and have controlled carefully the cutting and welding work activity. Although, as recently as 2013, the author witnessed workers preparing to carry out some welding work adjacent to a running sugar belt conveyor that was already exuding sugar dust. It took some persuasion to prevent them from starting work! 2. Metal-to-metal contact. In screw conveyors and elevators particularly, there is a mechanical possibility of metal rubbing against metal creating heat. If the screw of the conveyor is in contact with the casing at any time, then heat will be created and the possibility of sparks cannot be ruled out. In elevators, the buckets can be contact with the casing, either because the belt to which the buckets are attached has moved to one side or because the belt (or chains) has become slack and the buckets are grinding along the bottom. There was evidence in 2012 in a new installation where the belt had moved sideways and the buckets were rubbing against the casing so much that the outside of stainless steel casing was hot to touch and had changed colour! 3. Bearings overheating. This applies to all the equipment, but, on modern screw conveyors and elevators, the main bearings are outside the casing and separated from the sugar stream. On belt conveyors, whilst the main drum and drive bearings are outside the sugar stream, each of the idlers supporting the belt is within the sugar stream and is likely to become coated with sugar dust over time. Each idler has two bearings. On a troughed belt, the troughing idler set has three idlers and thereby six bearings and the return idlers each have two bearings. On one 20 metre long inclined belt, the author counted 160 bearings! If sugar and sugar dust is allowed to build up on and around these bearings and the bearings become hot, it is possible for a small fire to develop. Now, all the conditions to permit sugar dust to start burning have been met. Once the fire starts, a primary explosion will soon follow. This may be a small, local bang, but the shock wave produced will cause other sugar dust nearby to become airborne and, if the concentration is high enough, a secondary, larger explosion will take place. The secondary explosion may blow covers off the equipment and the shock wave will then cause more sugar dust both inside and outside the equipment to become airborne and explode and so a series of explosions will follow, resulting in Imperial Sugar 2! BSST ATM 2014 Paper 2 Page 3 of 8

4 2. ATEx explained Simply The European system, known as ATEx, is actually two European Union directives, 94/9/EC, principally for manufacturers, and 1999/92/EC for operators of plant and these have given rise to the many harmonized (and yet to be harmonized) standards to enable this law to be applied. In the UK, the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) implement the directive 1999/92/EC, often called the user s directive. Directive 94/9/EC is implemented in the UK by the Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 1996 (EPS Regulations). The regulations do not only apply to sugar, but also to many other equally and more hazardous materials and is, thereby, a blunt instrument, which has all the usual ambiguities of EU legislation within it. The ability to interpret the rules is an art form of its own! For instance, and of interest to those existing sugar handling facilities, there is a section on how to manage the risks on equipment that has been installed since before July 2003 and not significantly altered. An extract from the guidelines for UK legislation is given in Appendix 1. For all equipment, there are requirements through Risk Assessment, to define risk zones on conveying equipment, which invokes certain quality of component parts and the monitoring requirements. The typical zoning is indicated in the diagrams below and the zones are defined as: Zone 20 (Dust) A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is present continuously, or for long periods or frequently. (typically >1000 hr/year) Zone 21 (Dust) A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is likely to occur in normal operation occasionally. (typically hr/year) Zone 21 (Dust) A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is not likely to occur in normal operation, but, if it does, will persist for a short period only. (typically < 10 hr/year, max 1 hr/release) BSST ATM 2014 Paper 2 Page 4 of 8

5 3. How to Prevent a Recurrence? There are three possible approaches, only one of which will come close to guaranteeing that a recurrence will not occur, but each of the approaches relates in some way to an interpretation of the ATEx regulations: Approach 1 Do nothing. After all, the plant is more than ten years old, has not be significantly altered in that time and there has not been an explosion so far! In reality, actually doing nothing is not an option. There must, as a minimum, be a Risk Assessment carried out and regularly updated. But, before this approach is taken, it would be wise to read and understand the full EU regulations and, for the UK, their legislation. An extract of the guidelines for UK legislation relating to existing equipment is given in Appendix 1. Approach 2 Carry out a finance-limited upgrade to improve housekeeping and move towards some Food Safety standards. Retro-fitting of suppression and mitigation equipment may be an option, but installing modern systems into old conveying equipment may have limited effect. The guidelines in Appendix 1 also apply to this approach. Approach 3 - Carry out a full replacement and upgrade to meet all the requirements to ATEx standards and, at the same time, meet modern Food Safety standards. The cost of this exercise will be high, but it will ensure that the plant is as safe as it can be. 4. Actions needed to improve, upgrade or replace Whichever approach is taken, there are a number of actions that sugar manufacturing and handling facilities need to take to minimise the risk of a recurrence. In an ideal world, all of these need to happen and is not an either/or approach, but a total assessment of the facility and a review of its management, including: A. Housekeeping. There are several strands in this section, all of which need to be addressed and monitored to ensure that the conditions for an explosion cannot develop: a. Preventing the build-up of sugar dust on horizontal surfaces on and around the sugar handling equipment requires routine and regular removal of dust deposits, which is most effectively achieved using vacuum systems and collection of the dust for re-processing. Sweeping the dust has a very limited effect and usually results in the dust being moved from one place to another. If a dust extraction system has been installed, it is simple to connect vacuum hoses and heads to that system, so that all the dust is collected in one place. b. Preventing the sugar dust from escaping into the plant rooms through a combination of sealing the casings of the conveyors and elevators and the installation of dust extraction systems at the transfer points. Good design od equipment is needed, but, if existing, older plant is being upgraded, then attention is needed to sealing the casings and transfer points effectively. c. Routine cleaning of the inside of conveyors, screens and elevators during operational shutdown periods to remove the dust build-up. The dust builds up to form a loose cake on screw shafts, casing surfaces not swept by sugar, conveyor belt idlers and supporting structures, elevator casings and buckets. This cake can break free during an explosion and break up to become part of the dust cloud. BSST ATM 2014 Paper 2 Page 5 of 8

6 B. Control of personnel. The ability for human involvement in sugar handling areas should be minimised to essential personnel only, because more staff accessing the area inevitably increase the risk that a source of ignition will be generated. Minimising personnel entry also reduces the Food Safety risk to the product. The strategy needed to minimise these risks includes: a. Access control both so that only authorised staff enters the area, but also that it is clear who is in the controlled area at any moment. This should ideally be a computer-controlled card system, but, in some plants, security staff manage access, b. Prohibition of smoking by staff, including taking matches or lighters into the area, taking tools and equipment that might cause sparks, unless previously authorised, including mobile phones, c. Safety systems and permitting to ensure that nay necessary engineering work, particularly that involving hot working, with grinders, oxy-acetylene cutting and/or welding, is carried out securely and that the risk for fire is minimised. d. As much for Food Safety as anything, appropriate clothing with no pockets above the waist, hair and beard coverings, rubber soled safety shoes and the removal of watches, rings with stones and other jewellery. C. Monitoring of equipment in use. Not least because of the access control put into place (detailed above), but also because of the remoteness of some of the equipment, at the top of sugar silos for example, regular inspections of the conveying equipment during use are rarely carried out. The greatest risks come from overheating bearings and metal-to-metal contact in operating equipment and it is essential that, as a minimum, daily checks are carried out on the system. This needs to be carried out by a competent engineer using a combination of visual and audio inspection, supplemented by temperature monitoring of the bearings. Modern Watchdog systems are available to monitor temperatures and alignment sensors automatically and can be connected to the main control room to warn of developing issues, but these do not remove the need for an engineering daily inspection. D. Assessment of existing equipment. The majority of sugar handling facilities has existing operating equipment which meets the need in terms of throughput and required product outcomes, but may not be up to modern standards for SDE. This equipment will need to be reviewed and risk-assessed, leading to either: a. Upgrading existing equipment, including process simplification where possible, or b. Replacing with new equipment, which may also include simplification, and c. Installation of preventive systems, and d. Establishing regular monitoring. 5. The Selection of Approach In one particular project, which was developed and refined over two years to produce the most costeffective and rational reconstruction of the sugar handling facility, the result was a significant rationalisation of the process, eliminating the need for one sugar elevator, simplification of the conveyors to and from silos and to the packing areas and the installation of high specification handling equipment and suppression systems. BSST ATM 2014 Paper 2 Page 6 of 8

7 1. Elimination of a dry sugar elevator. Through some adventurous design and utilising the maximum possible conveying angles for troughed belts, together with innovations from the conveyor suppliers that permitted some gaining of height clearances, it was possible to remove the sugar elevator after the sugar cooler and still get the sugar to a conveyor belt to the sugar silos, including going through a scalping screen. Otherwise, this elevator was in the middle of the building, where a full suppression system would have been needed. 2. Conveying to and from silos. The previous system only had one reversible conveyor to take production sugar to the silos and bring sugar back for packing. Clearly, during production operations, this meant that sugar from production had to be sent directly to packing - not an ideal situation to minimise cake formation. The new layout included a double-deck conveyor, housed in the same gallery, with production sugar going to silos on the bottom unit and sugar to packing on the top belt. In addition, there had been an elevator at the silo end of the gallery and another lifting the sugar to packing; the latter was also eliminated. 3. Elevator to silos. The suppliers were able to supply a single elevator to raise the sugar the 45 metres, rather than the two lifts that had been previously installed. The design of the elevators was crucial to the overall system and the ones selected were designed with the capability of resisting a 9 bar shock pressure, due to their tubular construction, thus eliminating the need for venting or suppression at intervals up the entire length of the elevators. These improvements not only saved on motors and gearboxes, but also reduced the overall quantity of suppression and isolation systems required. 4. Preventive and migration systems. With all the design and layout changes, the reduction of the number of elevators from 5 to 2 and the simplification of the conveying system, the need for automatic suppression in case of an explosion and for preventive systems to prevent migration of that explosion was reduced to systems around the two remaining elevators. Each elevator has suppression cannon at the head and the boot, isolation cannons at the top and the bottom of each leg of the elevator and isolation cannons on the inlet and discharge as well as on the dust extraction ducts. In all, there are 5 cannons at the head of each elevator and 5 at the boot, but nothing is required up the length. Piers Bostock 6 th October 2014 Acknowledgements Robson Handling Equipment for the Fire Pentagon and zoning diagrams Various sugar factories for the photographs of their installations (good and bad!) BSST ATM 2014 Paper 2 Page 7 of 8

8 APPENDIX 1 Extract from ATEx Guidelines The extract below relates to existing equipment and defines, however ambiguously, what steps need to be taken, depending upon when the equipment was first installed and how much, if any modification and alteration has been made to it since installation. 3.0 SPECIAL REQUIREMENTS RELATING TO PRE-EXISTING INSTALLATIONS, MODIFICATIONS and EXTENSIONS 3.1 Equipment installed before July It is not the intention of the Health & Safety Executive (HSE) to condemn well-maintained, safe equipment merely because it was installed before July However, a risk assessment must be made to comply with the provisions of DSEAR as it reflects 1999/92/EC to ensure that the equipment is still safe to operate. Compile a risk assessment and keep records of the above checks plus safety and technical data. All Machinery Directive declarations that apply to the installation should be available as required. Check that the operation and maintenance manual is complete and up to date. In the event of a claim arising from a subsequent explosive event, the operator s legal defence will rely heavily on the quality of the Risk Assessment. For hazardous area classification in existing workplaces, the risk assessment should be completed well before the transition date of 1 st July 2006 to enable any changes and upgrades to be in place and operational by that date. DSEAR reg. 17 deals with the transitional provisions. 3.2 Equipment installed before July 2003 but modified or extended after July 1st It is quite likely that older serviceable equipment may have been or will be modified or extended. It will not always be necessary to replace the whole machine or installation, but it is essential that any mandatory ATEX compliant additions are compatible with the existing equipment and that the integrity and safety in the event of an explosion are not compromised. A risk assessment should be made for the original equipment following the plan in the previous section Equipment installed before July This should then be extended to cover the added features and components. Additional points to consider include the following: The strength of the housing that may contain the explosive atmosphere must be assessed. Any extension to this housing must not weaken the structure. The reduced explosion pressure (Pred) for the extended vessel or housing should be assessed. This can be done by making a direct comparison with a vessel or housing of known strength. All explosion ventilation measures and other equipment that may act as a barrier between the zoned hazardous atmosphere Check that the equipment or installation complied with regulations and best practice at the time of its installation. Check that the surroundings of the equipment have not changed in a way that may prejudice safety. For example, other equipment, work stations or rights of way may have been established within range of an explosion relief panel. Check that the products handled and equipment duty cycle have not changed in a way to enhance possible risk. Check that the equipment is in good condition and not worn or weakened in a way that may enhance risk. Check that maintenance records are complete and correct and that inspection and maintenance schedules are followed correctly. The following checks should be made and action taken to support the risk assessment: Compliance with legislation implementing the ATEX Directives and a safer or safe area are regarded as protective systems under 94/9/EC. If these have been installed after 30th June 2003 they must be manufactured by a manufacturer with current ATEX EC-type examination certificate and a Production QA notification or verification certificate included in the application of standard EN These documents must be obtained from a Notified Body. These items include explosion relief devices and material discharge devices such as rotary valves. If the modified or extended vessel or housing incorporates explosion relief or suppression devices, these must be compatible. In the case of explosion relief panels, it would be wise to replace them all with panels of equal rating, to prevent possibly destructive effects in the event of an explosion. These can occur if, for instance, explosion panels open at different pressures. Any new protective system, including replacements for existing devices must be externally certified to the protective system requirements of 94/9/EC 3.3 Equipment installed after 30 June All equipment installed after 30 June 2003 must fully comply with the provisions of ATEX Directive 94/9/EC. It is therefore essential to know the zone under the ATEX guidelines where the equipment is to be sited. There is sometimes a tendency to over-zone; this can lead to extra difficulty and cost without enhancing safety. In outdoor locations equipment will normally be installed in a safe area for dust hazards, with Zone 22 applying at most to the immediate area around discharge or loading points. The internal ATEX Zone within housings, enclosures and machines handling dust will apply to all electrical devices and all mechanical devices that could be a possible ignition source. The housing itself, for example a hopper, silo or dust collector is not directly affected by the provisions of ATEX. However, it may be possible for this enclosure to import an ignition source from elsewhere and therefore an explosive event could occur. It would then be necessary to minimise the risk. This could be by methods such as spark detection and quenching or explosion pressure relief devices. If such devices are to be employed the strength of the vessel must be established, to ensure that the suppression or venting takes place without the possibility of a dangerous rupture. The reduced explosion pressure (Pred) for the vessel or housing is a parameter used to determine the design and size of the protective device. BSST ATM 2014 Paper 2 Page 8 of 8