Risk assessment and explosion risk assessment

An integral part of each explosion risk and risk assessment, which in turn is often an element of the Explosion Protection Document, is a summary containing conclusions regarding the recommended changes and procedures in conditions of explosion hazard. On the example of one of the explosion risk assessments prepared for the mill together with the grain transporting and storage installation, you can create a set of recommendations assigned to the so-called layers of prevention and protection. These are sets of factors limiting, on the one hand, the possibility of an explosive atmosphere, and on the other - in a situation where it fails - minimizing the risk of an effective ignition source.

Organizational and technical measures to prevent explosion - recommendations and suggestions for the mill installation and the transport and storage of grain and flour.

The technology used in grain granaries and mills promotes the possibility of explosive atmospheres and explosions of cereal-flour mixtures - especially in the processes of cleaning, hulling or grinding cereal grains. Dust concentration within the explosive limits occurs inside all transport devices (bucket elevators, screw and chain conveyors), in many cleaning machines (peelers, brushing machines, scrubbing machines) and grinding machines (grinders, roller millers, bran projectors) and in grain and flour chambers during filling them.

We divide the dangerous places with a direct dust explosion into two groups.

Group 1

The first group includes all the above-described internal spaces of machines and devices in which dust is generated, transported or processed. In the event of an explosion inside the device, its casing is usually more or less ruptured, and flames and dust are projected into the immediate vicinity of the failure site.

Group 2

The second group includes virtually all spaces and surfaces outside production equipment on which dust can deposit, e.g. floors, window frames, window sills, beams, girders, walls, ceilings, surfaces of machines and devices, as well as electrical installations and devices. If the deposited dust is sprayed and ignited, an explosion with catastrophic consequences occurs.

Since the necessary condition for an explosion to occur - apart from the required dust concentration - is the creation of an ignition source with a heat value sufficient to ignite it, therefore the basic and essential activities must focus on counteracting and preventing the possibility of simultaneous emergence of such conditions in granaries and mills.

Actions taken should be primarily aimed at:

1. Technology activities:

• a) grain receiving devices should have an efficient aspiration installation,
• b) all transport devices (lifts and conveyors) and cleaning machines (trieres, brushing machines, scrubbing machines) must be dust-proof and aspirated at the places where dust is generated,
• c) the aspiration installation should also be applied to machines such as grain and mill cleaners, crushing and screening machines,
• d) aspiration installations should be turned on before starting machines and technological devices and stopped only after these machines and devices have been previously excluded from work,
• e) aspiration installations should be constantly operational and under review,
• f) gravity transport conduits should be sealed, and all controls and visors should be closed during the process,
• g) in the technological process, permanent magnets or electromagnets for catching ferromagnetic contamination should be installed in front of the following machines:
• peelers, scrubbing machines,
• grinders, first shot grinders.

The efficiency of the installed magnetic dams should be monitored on an ongoing basis.

• h) compliance with fire instructions when operating grain dryers, in particular:
• sending only dried grain to the dryer,
• not exceeding the permissible grain drying temperatures,
• maintaining the efficient operation of the launchers,
• preventing the accumulation of dust inside the dryers,
• constant supervision of the operating dryer.
• i) compliance with the OHS instructions for the operation of machines and devices displayed at workplaces.

2. Technical and conservation activities.

• a) electrical equipment must be uniformly earthed at the plant,
• b) in accordance with the requirements of the applicable regulations on the operation of energy devices, the required periodic visual inspections, inspections, maintenance, repairs and measurements should be carried out:
• power networks,
• electric propulsion devices,
• electrical equipment in potentially explosive atmospheres.
• c) maintenance, repair and overhaul works of electrical power equipment should be performed after the equipment is disconnected from the voltage by persons with the required qualification certificates,
• d) electric motors and electrical equipment, depending on the place of hazard, should be used with the required IP protection class. Hazardous places and the propulsion devices present there are specified for each object - "classification of the premises of the facility into the appropriate fire hazard category and explosion hazard category and determination of fire zones",
• e) in the case of pneumatic transport devices:
• keep pipelines tight on an ongoing basis,
• maintain an efficient equipotential installation,
• perform at least once a year the required periodic measurements of earthing and resistance of the equipotential installation,
• f) all internal transport devices (elevators and conveyors) are regularly inspected for their technical condition and proper operation on a daily basis,
• g) devices of aspiration (dedusting) installations are subject to daily operational efficiency checks, in particular:
• sleeve filters - in terms of the efficiency of sleeve shock devices,
• filter cyclones - in terms of the efficiency of sleeve cleaning devices,
• cyclones - in the regulation of cleaning irregularities,
• aspiration tubes - in terms of tightness and patency,
• h) regularly and at required intervals monitor the condition and lubrication of bearings in operated machines and devices,
• i) maintain permanent dust-tightness of lighting fixtures, plug-in sockets, junction boxes and other devices and elements of electrical installation in production and storage rooms.

3. Technical and organizational activities and recommendations.

• a) fire hazardous works (welding, heating, cutting, soldering, etc.) within silos and mill rooms must be performed only with a permit, storage of wet grain in chambers (silos) is particularly dangerous;
• b) it should be observed that during long-term storage the grain moisture content should not exceed 14%,
• c) store grain in piles up to a height depending on the degree of moisture and the strength of the ceilings,
• d) periodically carry out organoleptic checks of the quality and condition of stored grain; if necessary, apply maintenance procedures by means of ventilation - metastasis in order to remove excess moisture and lower the temperature of the grain,
• e) protect the existing technological openings, e.g. after finishing work, cover the silo openings with metal covers, cover the covers of grain, flour and bran chambers,
• f) for lighting cereal, flour and bran chambers, it is allowed to use:
• fixed lamps located outside the chambers in hermetic version,
• portable lamps located outside the chamber with a construction preventing the penetration of the dust atmosphere inside,

These lamps should be supplied with a voltage not exceeding 24 V, dustproof and protected against mechanical damage.

• g) electric flashlights with a voltage of up to 6 V,
• h) maintain law, cleanliness and order in all production and storage rooms. Do not allow dust to accumulate on mechanical and electrical devices and in structural parts of buildings (ceilings, walls, pillars, girders, etc.),
• i) cleaning works (dust sweeping) at power equipment in mills and granaries are performed by:
• electricians - inside closed switchboards and distribution boxes - direct operational service (users),
• outside distribution boxes, switches, plug sockets, lighting fittings and electric motors - under the constant supervision of an electrician.

Cleaning work should be done systematically.

• have handheld firefighting equipment technically efficient and in the amount consistent with the number of employees for each room,
• j) maintain constant access to fire-fighting equipment and main power switches,
• k) comply with the fire regulations in force at the mill and the provisions of the Fire Safety Instruction of the plant.

The above list applies to a specific industry, in this case it is the food industry, but it gives an idea of what factors must be taken into account when preparing recommendations summarizing methods of keeping the explosion hazard at an appropriate low level.