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Grain Storage & Processing

INTRODUCTION


The main hazards arising from the storage and processing of grain are those of dust explosion, together with the potential for spontaneous combustion of long term storage in silos and bins, particularly if the grain contains high moisture content, fire hazards associated with grain drying, and hazards to health.


Grain dust is the dust arising from the harvesting, drying, handling, storage or processing of barley, wheat, maize, oats and rye (cereal crops commonly referred to as “grain”) and other materials such as soya beans, cocoa beans, rapeseed and flaxseed, and includes any contaminants or additives within the dust. Grain passes through numerous handling operations and the generation of dust is widespread throughout agriculture, transportation, storage, the food and drinks industry, and in the production of animal feeds. Processes that create grain dust include:


  • Harvesting grain and transferring grain from combines into trailers;

  • Cleaning, dressing and drying grain;

  • The movement of grain in a grain store;

  • Grain transportation from grain stores or terminals;

  • Milling and mixing dry grain;

  • Feeding dry milled grain;

  • Cleaning and maintenance of buildings, plant, equipment.


Grain dust can be both highly explosive and hazardous to health; hence it falls firmly within the scope of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR) and the Control of Substances Hazardous to Health Regulations 2002 (COSHH).


Consequently, ensuring that effective risk assessments in compliance with these regulations have been conducted and the appropriate prevention and mitigation measures implemented are key underwriting and risk control considerations. It is essential that such risk assessments are conducted by competent persons and should include a detailed analysis of the entire plant design and safety controls.


The following represents a summary of the main prevention and mitigation measures that, depending on the circumstances, may apply. Whilst this bulletin is specifically focused on grain, much of the content applies to combustible and hazardous dusts in general.

 

 EXPLOSION PREVENTION AND MITIGATION


Although an explosive concentration of dust within an open work area during normal conditions cannot be ruled out, the great majority of dust explosions start with a primary explosion within an item of plant, which can often lead to the dispersion of a dust cloud and a secondary explosion in the surrounding area. Secondary explosions are generally more destructive than a primary explosion. 


Explosion prevention and mitigation measures are many and varied and generally fall into the categories outlined below. In the majority of cases, a range of techniques will be employed in combination, providing a risk control programme commensurate with the risk.


1.      Elimination of Ignition Sources

It is vitally important that close attention be given to the elimination of all potential ignition sources in areas where concentrations of grain dust are generated, with particular reference to the following:

 

  • Plant correctly earth bonded and other appropriate measures taken as a precaution against static electricity (RiskSTOP Technical Bulletin 35 refers);

  • All electrical and other equipment in areas where a concentration of grain dust is likely to occur installed in accordance with the “Atex Directive” and the prescribed principles of electrical zoning (Zones 20, 21 & 22) (RiskSTOP Technical Bulletin 23 refers). Particular care needs to be exercised over the use of inspection lamps and other portable tools; 

  • Hot work and other maintenance/contractor activities strictly controlled by effective permit to work systems;

  • Avoidance of heat and sparks caused by friction or impact, including the provision of magnetic and/or pneumatic separators on the material feeds to machinery;

  • All plant and services, including explosion protection systems and equipment, subject to appropriate planned preventative maintenance;

  • Safe installation, operation and maintenance of grain dryers in accordance with RISCAuthority RC10 – Fire safety in agricultural and horticultural premises;

  • Strict enforcement of smoke-free legislation;

  • Installation and use of space heating compatible with the environment.

 

2.     Explosion Containment

In some cases, such as with hammer mills and other grinding equipment, it may be possible for the equipment to be designed to withstand and contain an explosion. In these circumstances, precautions will need to be taken to ensure that the equipment is suitably separated from connecting ductwork and other items of plant, unless of similar strength, by the installation of rotary valves or similar barriers.

 

Where containment is not possible, attention should be given to the installation of explosion venting, gas inserting and explosion suppression, either individually or combined.

 

Irrespective of the extent of protection installed, it is important that the plant is monitored and interlocked in an appropriate manner, whereby deviations from a safe condition result in automatic plant shut-down and/or an alarm from which pre-planned emergency procedures are instigated preventing, amongst other things, the onwards transmission of burning material.

 

3.     Explosion Venting

A common method of protection of process plant and, in some cases the fabric of the building, is to provide one or more deliberate points of weakness in the form of explosion relief vents. Vents normally take the form of bursting panels or explosion doors, the design and size of which is a specialist discipline. Since June 2003, newly supplied explosion vents are required to be manufactured and tested in accordance with the Equipment and Protective Systems for Use in Potentially Explosive Atmospheres Regulations (EPS Regulations – “ATEX Certified”)


The design of explosion venting is covered under BS EN 14491:2012 - Dust explosion venting protective systems. This standard, together with BS EN 14797:2006 - Explosion venting devices and BS EN 14460:2018 - Explosion resistant equipment, represent the concept of dust explosion venting.


Venting should be to a safe place, normally to the open air, either directly or via a strong straight duct of limited length, taking account of people and property. Where panels could become dangerous missiles, these will commonly be restrained by the use of chains or cables, but of sufficient length to allow the panels to open fully. Self-re-closing doors are manufactured, which prevent secondary fire damage from oxygen intake by rapidly re-closing after the explosion is vented. Proprietary flameless venting devices that quench the flames and catch the burning dust are also available where an internal venting application is required.


It is important to recognise that all types of explosion vents require routine maintenance to ensure that seals remain in good condition, there is an absence of dirt and corrosion, hinges operate easily etc.


Typical explosion vent panels


A self-re-closing explosion door


  An internal flameless explosion vent


4.      Explosion Suppression

In some cases, explosion venting will be inappropriate, or unable on its own to provide the level of protection needed. In these circumstances an explosion suppression system will commonly be employed. In their simplest form, these systems comprise of a rapid-acting pressure sensor linked to a number of pressurised containers of suppressant, designed for rapid injection into the protected plant, mitigating the flame/pressure damage to an acceptable level. The suppressant is normally a dry powder, although superheated, high pressure water mist suppression systems are also available. As well as being activated by pressure, suppression systems may also be operated by the use of advanced carbon monoxide sensors.

 

Requirements for the design and application of suppression systems are covered under BS EN 14373:2021 – Explosion suppression systems.


Suppression combined with venting is typically used on applications where the plant cannot be protected by either suppression or venting techniques alone. In cases where the primary protection means is explosion suppression, the explosion venting system shall be set to activate above the explosion suppression activation pressure - typically twice (2×) as high. It is important that the efficacy of the combined system is validated by testing of the particular application.


In addition to providing entire plant protection, suppression systems can also be employed as an explosion barrier between interconnected items of plant.


 Typical suppression equipment


5.      Gas Inerting

In a substantially closed system, explosions can be prevented by controlling the oxygen content of the atmosphere within the system to a safe level by the use of gas inserting, commonly employing nitrogen or carbon dioxide. Where gas inerting is used, it is essential that the risk of asphyxiation during maintenance and from accidental leakage are assessed and properly controlled.


6.     Explosion Isolation

Where dealing with a series of interconnecting items of plant such as grinders, elevators, cyclones, filters and silos, precautions should be taken to prevent an explosion propagating from one unit to another via connecting pipes and ducts, reducing the risk of widespread damage.  This will normally be achieved by one or more of the following methods:


  • Rotary valves which have been type tested and ATEX certified. In cases where rotary valves are intended to act as an explosion choke, they are required to have rigid metal blades that will not deform under pressure and which have as small a clearance as practicable from the casing (see figure below);

  • A choke of material in an intermediate hopper;

  • Modification of screw conveyors to act as a choke by the removal of a section of the screw helix and insertion of a baffle plate;

  • Explosion suppression barriers;

  • High speed explosion isolation valves activated by detection of a flame or rise in pressure and passive barrier valves which are closed by the pressure wave of an explosion.

 

The general requirements for explosion isolation systems are covered under BS EN 15089:2009 – Explosion isolation systems. In addition, all equipment for use in potentially explosive atmospheres should be ATEX certified under the EPS Regulations.


Important dimensions of rotary valves used as chokes


A modified screw conveyor acting as a choke


An explosion suppression barrier


High speed sliding isolation (slam) valve


A passive barrier valve


7.      Spark Detection and Suppression

Spark detection and suppression systems may be employed on grinders and other plant where there is a risk of spark generation, with the objective of extinguishing sparks and glowing materials before reaching a large dust cloud or other items of equipment (RiskSTOP Technical Bulletin 16 refers).


Spark detection and suppression


8.      Plant Location and Construction

Locating large, hazardous items of plant such as grain silos and dust filters in the open, whereby the consequences of an explosion may be minimised will always be the preferred option.  Where this is not possible, consideration needs to be given to building design and the provision of features such as open louvres or lightweight wall and roof panels lightly attached, via which an explosion may be vented, and the risk of major structural damage reduced.

 

9.     Management

It is critical to ensure that high standards of management are maintained across all process buildings and plant. In particular, attention should be given to the following:


  • Maintaining plant in a leak–tight condition via close inspection of items such as flanged joints, flexible seals and access hatches;

  • Maintaining buildings in a clean condition avoiding accumulated dust deposits on walls, floors and ledges, etc. which can provide fuel for a secondary explosion. Vacuum cleaning equipment should be used rather than brushes and shovels, which will tend to generate dust clouds. This may be either from a segregated, fully earthed, centralised vacuum cleaning system or portable vacuum cleaning equipment. If the area in which a portable vacuum cleaner is to be used is defined as a hazardous area, an ATEX certified vacuum cleaner will be required. Otherwise the guidance published by the HSE at www.hse.gov.uk/food/dustexplosionapp1.htm should be followed;

  • Material collected during cleaning should be immediately placed in external metal containers with metal lids; 

  • Ensuring that motors and other electrical equipment and all equipment with the potential to generate heat are kept dust free;

  • Ongoing planned preventative maintenance programme and safety audits in relation to all plant and services, including explosion protection systems;

  • The provision of appropriate training and instruction of all employees involved in plants where grain is handled, and the essential safeguards and emergency procedures in place.


Also, the need for vigilance and the immediate reporting of any substantial release of material or equipment malfunction.


It is important to recognise that under the DSEAR Regulations, any explosion protection measures on plant newly brought into use after June 2003 are required to be verified by a person competent in the field of explosion protection which, in many cases, will require the appointment of a specialist consultant.

                                                          

SPONTANEOUS COMBUSTION


Care is required to be taken over the long term storage of grain in silos and bins (particularly wet material) and the potential for spontaneous combustion. Non-combustible construction for silos and bins is to be preferred, in addition to which alarm monitored temperature probes/heat detectors should be installed, commensurate with the risk.  Grain heaps in storage buildings should be aerated and/or monitored for temperature as required.

Prior to any further processing or storage, dried grain should be allowed to cool to ambient temperature as a precaution against spontaneous combustion.

 

GRAIN DRYING


Grain drying is commonly encountered on farm premises and will also be found as a key function of “cooperative” grain handling and storage facilities. Dryers vary significantly from small batch dryers to large continuous mixed flow dryers. These are normally oil or gas fired and comprise both of fixed installations and transportable equipment. Grain drying is also accomplished by the use of ducted on-floor or in-bin drying systems with a variety of fan and heater options.


Fires involving grain drying are a common occurrence and are often the result of poor housekeeping and/or plant maintenance. Consultants should exercise particular caution when encountering old grain drying installations, which may have received little maintenance over the years. Similarly, old grain dryers can be encountered which are of inferior design or, in some cases, have been inappropriately modified. 


Key risk control considerations include ensuring that:


  • Grain drying operations are included within the DSEAR risk assessment;

  • The grain dryer is appropriately installed, either in the open, in a detached building of non-combustible construction, or suitably fire compartmented;

  • The grain dryer is indirectly fired via a heat exchanger;

  • Where direct firing is employed, suitable baffle plates are installed between the furnace and the drying chamber;

  • Air intakes to the dryer located in a dust free area and protected by course screens to keep out debris;

  • Explosion venting incorporated where appropriate;

  • Dryers equipped with automatic temperature controls (preferably duplicated) arranged to shut down the heating supply and fans in the event of excessive temperature in the dryers, stoppage of the hot air or cooling air fans, or burner flame failure;

  • In circumstances where automatic temperature controls are not incorporated, the dryer should not be left unattended. Where automatic controls are provided, the plant should be visited hourly;

  • Equipment is operated and maintained by competent persons in accordance with the manufacturers’ instructions (annual inspection and maintenance prior to harvesting is the normal recommendation);

  • Robust housekeeping procedures are in place (see above);

  • Operatives suitably instructed and trained in emergency procedures.


The above provisions are in addition to the standard fire safety arrangements such as automatic fire valves on oil fired equipment, clearance around flue pipes and other common precautions appertaining to heating installations in general. 


Malting kilns which require special consideration have, as such, been excluded from this Technical Bulletin.

 

SPRINKLER PROTECTION


The installation of sprinklers in all major grain storage and process buildings should be considered, particularly where the buildings and/or plant are of combustible construction. Interior protection should be included in large processing equipment such as the lowest dry level of malting kilns, dust collectors and enclosed elevators. In areas where combustible dusts are present, care should be taken not support sprinkler pipe work on any wall, ceiling or roof that is expected to be displaced by the pressure of an explosion.

 

HEALTH HAZARDS


Exposure to grain dust can present serious health risks arising not just from the virgin grain, but also from numerous contaminants such as bacteria, fungal spores, insects, storage mites, etc. Consequently, the potential exists for a variety of effects on heath in the form of:


  • Eye and skin effects, such as conjunctivitis, itchy skin and skin rashes;

  • Reparatory effects, such as coughing and breathing difficulties, asthma, bronchitis, chronic obstructive pulmonary disease (COPD), extrinsic allergic alveolitis (for example, farmer’s lung) and organic dust toxic syndrome (for example, grain fever).


Key to managing the health risks associated with grain dust is the requirement under the Control of Substances Hazardous to Health Regulations 2002 (COSHH) as amended, to carry out a suitable and sufficient risk assessment, arising from which the exposure of employees is prevented or, if this is not reasonably practicable, for the exposure to be adequately controlled. This must include identifying all the potential sources of exposure, who is likely to be exposed and for how long. Measurements to exposure to dust by air sampling are likely to be required.


In conducting the risk assessment, all areas in which grain is handled or processed should be considered and the assessment extended to individual components or additives to grain, such as pesticides during storage and medicated products employed in animal feed production. In view of the hazard profile, it is vitally important that the risk assessment is conducted by a competent person, which for some businesses is likely to require the appointment of a consultant with expertise in this field.

 

Health hazards associated with grain dust are the subject of HSE Guidance Note EH66 which can be downloaded at www.hse.gov.uk/pubns/eh66.pdf. This provides full guidance on the health hazards which could result from exposure to grain dust, and advises on the precautions which may be necessary, under the following categories:


  • Risk assessment;

  • Prevention and control of exposure;

  • Maintenance, examination and testing of control measures;

  • Safety representatives;

  • Information, instruction and training;

  • Monitoring exposure;

  • Personal protective equipment and respiratory protective equipment;

  • Washing facilities;

  • Health surveillance;

  • Notification of reportable disease.


There is also a useful appendix providing details of common tasks generating airborne grain dust. Reference to this document should be made for further information.

 

ADDITIONAL INFORMATION


Freely available publications on the subject of dust explosions include:


 

Various articles published by ATEX Explosion Hazards Ltd - www.explosionhazards.co.uk 

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