Risk assessment and explosion risk assessment

An integral part of every risk and explosion hazard assessment

An integral part of every risk and explosion hazard assessment, which are often elements of the Explosion Protection Document, is a summary containing conclusions regarding recommended changes and procedures in conditions of explosion hazard. Using an example of an explosion hazard assessment prepared for a mill with grain transportation and storage installation, a set of recommendations assigned to the so-called prevention and protection layer can be created. These are sets of factors that limit, on one hand, the possibility of creating an explosive atmosphere, and on the other – in situations where this fails – minimize the risk of the occurrence of an effective ignition source.

Organizational and technical measures for explosion protection

Recommendations and suggestions for the mill installation and grain and flour transportation and storage

The technology used in grain silos and mills encourages the formation of explosive atmospheres and explosions of grain-flour mixtures – especially in cleaning, hulling, and grinding processes of grain. Dust concentrations within explosion limits occur inside all transport devices (bucket elevators, screw and chain conveyors), in many cleaning machines (hullers, brushers, scrubbers), grinding machines (grinders, roller mills, bran cleaners), and in grain and flour chambers during filling.

Hazardous places

Group 1

The first group includes all the above-described internal spaces of machines and devices where dust is generated, transported, or processed. In case of an explosion inside the device, it usually results in more or less rupture of its casing and the ejection of flames and dust to the immediate vicinity of the failure site.

Group 2

The second group includes practically all spaces and surfaces outside production devices where dust can settle, such as floors, window frames, sills, supporting beams, girders, walls, ceilings, machine surfaces, and electrical installations and devices. In the case of dispersed settled dust and its ignition, a catastrophic explosion occurs.

Actions taken should primarily aim at

1. Technological actions

• Grain intake devices should have a functional aspiration system.
• All transport devices (elevators and conveyors) and cleaning machines (triers, brushers, scrubbers) must be dustproof and aspirated at dust-generating points.
• Aspiration should also be applied to such machines as grain and mill winnowers, grinding and sifting machines.
• Aspiration systems should be turned on before starting the machines and devices and turned off only after these machines and devices have been previously turned off.
• Aspiration systems must be constantly functional and subject to inspections.
• Gravity transport pipes should be sealed, and all inspection holes and viewports should be closed during the process.
• In the technological process, permanent magnets or electromagnets should be installed to capture ferromagnetic contaminants before the following machines:
  • Hullers, scrubbers.
  • Grinders, first-grind roller mills.
• The condition of installed magnetic barriers should be regularly checked.
• Observe the fire safety instructions when operating grain dryers, especially:
  • Direct only dried grain to the dryer.
  • Do not exceed permissible grain drying temperatures.
  • Maintain the efficient operation of ejectors.
  • Prevent the accumulation of dust inside dryers.
  • Maintain constant supervision of the operating dryer.
• Observe the occupational health and safety instructions for operating machines and devices posted at workplaces.

2. Technical and maintenance actions

• Electrical devices in the plant must be uniformly grounded.
• In accordance with the requirements of applicable energy device operation regulations, required periodic inspections, maintenance, repairs, and measurements must be performed:
  • Power networks.
  • Electrical drive devices.
  • Electrical devices in explosion hazard areas.
• Work on maintenance, repairs, and overhauls of power devices should be performed after disconnecting these devices from the power supply by persons with the required qualifications.
• Electric motors and electrical equipment, depending on the hazard location, should be used in the required degree of protection IP. The hazard locations and the equipment used are specified for each facility by the "classification of facility rooms to the appropriate category of fire hazard and explosion hazard category and determination of fire zones".
• For pneumatic transport devices:
  • Maintain pipeline tightness continuously.
  • Maintain a functional equipotential bonding system.
  • Perform required periodic grounding and equipotential bonding resistance measurements at least once a year.
• All internal transport devices (elevators and conveyors) are subject to constant daily inspections of technical condition and proper operation.
• Aspiration (dust removal) devices are subject to daily functional inspections, particularly:
  • Bag filters – functional inspection of shaking devices.
  • Filter cyclones – functional inspection of cleaning devices.
  • Cyclones – adjustment of cleaning irregularities.
  • Aspiration pipes – tightness and patency.
• Regularly inspect and lubricate the bearings of machines and devices as required.
• Maintain constant dust-tightness of lighting fixtures, socket outlets, junction boxes, and other devices and elements of the electrical installation in production and storage rooms.

3. Technical and organizational actions and recommendations

• Fire-hazardous work (welding, heating, cutting, soldering, etc.) in the vicinity of silos and mill rooms should only be performed with permission; storing wet grain in chambers (silos) is particularly hazardous.
• During long-term storage, ensure grain moisture does not exceed 14%.
• Grain storage in heaps should be based on the degree of moisture and the strength of ceilings.
• Periodically conduct organoleptic quality control and state of stored grain; if necessary, perform conservation treatments by ventilation – turning over to remove excess moisture and reduce grain temperature.
• Secure existing technological openings, e.g., cover metal lids over silos after work, cover grain, flour, and bran chamber covers.
• For lighting grain, flour, and bran chambers, use:
  • Fixed lamps placed outside the chambers in hermetically sealed enclosures.
  • Portable lamps placed outside the chamber designed to prevent dust atmosphere penetration inside.

These lamps should be powered by current not exceeding 24 V, dust-tight, and protected against mechanical damage.

• Electric flashlights up to 6 V.
• Maintain order and cleanliness in all production and storage rooms. Do not allow dust accumulation on mechanical and electrical devices and structural parts of buildings (ceilings, walls, columns, girders, etc.).
• Cleaning (dusting) around power devices in mills and silos is performed by:
  • Electricians – inside closed switchboards and distribution boxes – direct operational maintenance (users).
  • Outside switchboards, switches, socket outlets, lighting fixtures, and electric motors – under constant electrician supervision.

Cleaning work should be performed systematically.

• • Keep portable fire-fighting equipment technically functional and in quantities compliant with the allocation for individual rooms.
• Maintain constant access to fire-fighting equipment and main power switches.
• Follow the fire safety regulations in the mill facility and the provisions of the Fire Safety Instructions of the plant.

Summary

The above list concerns a specific industry, in this case, the food industry, but provides an overview of the factors that must be considered when making recommendations summarizing methods for maintaining explosion hazards at an appropriately low level.