Category: Hazardous Material

Flammable materials

Flammable and combustible liquids:

Flammable and combustible liquids are liquids that can burn. They are classified, or grouped, as either flammable or combustible by their flash points. Generally speaking, flammable liquids will ignite (catch on fire) and burn easily at normal working temperatures. Combustible liquids have the ability to burn at temperatures that are usually above working temperatures.

There are several specific technical criteria and test methods for identifying flammable and combustible liquids. Under the Workplace Hazardous Materials Information System (WHMIS) 1988, flammable liquids have a flash point below 37.8°C (100°F). Combustible liquids have a flash point at or above 37.8°C (100°F) and below 93.3°C (200°F).

Flammable and combustible liquids are present in almost every workplace. Fuels and many common products like solvents, thinners, cleaners, adhesives, paints, waxes and polishes may be flammable or combustible liquids. Everyone who works with these liquids must be aware of their hazards and how to work safely with them.

Flash point

the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid. The flash point is normally an indication of susceptibility to ignition.

The flash point is determined by heating the liquid in test equipment and measuring the temperature at which a flash will be obtained when a small flame is introduced in the vapor zone above the surface of the liquid.

Definitions

Combustible liquid: any liquid having a flash point at or above 100ºF (37.8ºC).

Combustible liquids shall be divided into two classes as follows:

1. Class II liquids shall include those with flash points at or above 100ºF (37.8ºC) and below 140ºF (60ºC), except any mixture having components with flash points of 200ºF (93.3ºC) or higher, the volume of which make up 99 percent or more of the total volume of the mixture.
2. Class III liquids shall include those with flash points at or above 140ºF (60ºC). Class III liquids are subdivided into two sub classes:
   • Class IIIA liquids shall include those with flash points at or above 140ºF (60ºC) and below 200ºF (93.3ºC), except any mixture having components with flash points of 200ºF (93.3ºC), or higher, the total volume of which make up 99 percent or more of the total volume of the mixture.
   • Class IIIB liquids shall include those with flash points at or above 200ºF (93.3ºC). This section does not regulate Class IIIB liquids. Where the term “Class III liquids” is used in this section, it shall mean only Class IIIA liquids.

When a combustible liquid is heated to within 30ºF (16.7ºC) of its flash point, it shall be handled in accordance with the requirements for the next lower class of liquids.

Flammable liquid: any liquid having a flash point below 100ºF (37.8ºC), except any mixture having components with flash points of 100ºF (37.8ºC) or higher, the total of which make up 99 percent or more of the total volume of the mixture. Flammable liquids shall be known as Class I liquids. Class I liquids are divided into three classes as follows:

1. Class IA shall include liquids having flash points below 73ºF (22.8ºC) and having a boiling point below 100ºF (37.8ºC).
2. Class IB shall include liquids having flash points below 73ºF (22.8ºC) and having a boiling point at or above 100ºF (37.8ºC).
3. Class IC shall include liquids having flash points at or above 73ºF (22.8ºC) and below 100ºF (37.8ºC).

It should be mentioned that flash point was selected as the basis for classification of flammable and combustible liquids because it is directly related to a liquid’s ability to generate vapor, i.e., its volatility. Since it is the vapor of the liquid, not the liquid itself that burns, vapor generation becomes the primary factor in determining the fire hazard. The expression “low flash – high hazard” applies. Liquids having flash points below ambient storage temperatures generally display a rapid rate of flame spread over the surface of the liquid, since it is not necessary for the heat of the fire to expend its energy in heating the liquid to generate more vapor.

The above definitions for classification of flammable and combustible liquids are quite complex. The diagram below should aid in their understanding.

Liquid itself burn:

Flammable and combustible liquids themselves do not burn. It is the mixture of their vapours and air that burns. Gasoline, with a flash point of -40°C (-40°F), is a flammable liquid. Even at temperatures as low as -40°C (-40°F), it gives off enough vapour to form a burnable mixture in air. Phenol is a combustible liquid. It has a flash point of 79°C (175°F), so it must be heated above that temperature before it can be ignited in air.

Flammable or explosive limits:

A material’s flammable or explosive limits also relate to its fire and explosion hazards. These limits give the range between the lowest and highest concentrations of vapour in air that will burn or explode.

The lower flammable limit or lower explosive limit (LFL or LEL) of gasoline is 1.4 percent; the upper flammable limit or upper explosive limit (UFL or UEL) is 7.6 percent. This means that gasoline can be ignited when it is in the air at levels between 1.4 and 7.6 percent. A concentration of gasoline vapor in air below 1.4 percent is too “lean” to burn. Gasoline vapor levels above 7.6 percent are too “rich” to burn. Flammable limits, like flash points however, are intended as guides not as fine lines between safe and unsafe.

Auto ignition Temperature:

A material’s auto ignition or ignition temperature is the temperature at which a material self-ignites without any obvious sources of ignition, such as a spark or flame.

Most common flammable and combustible liquids have auto ignition temperatures in the range of 300°C (572°F) to 550°C (1022°F). Some have very low auto ignition temperatures. For example, ethyl ether has an auto ignition temperature of 160°C (356°F) and its vaporous have been ignited by hot steam pipes. Serious accidents have resulted when solvent-evaporating ovens were heated to temperatures above the auto ignition temperature of the solvents used. Auto ignition temperatures, however, are intended as guides, not as fine lines between safe and unsafe. Use all precautions necessary.

Good ventilation important:

Well-designed and maintained ventilation systems remove flammable vapours from the workplace and reduce the risk of fire and health problems.

The amount and type of ventilation needed to minimize the hazards of flammable and combustible liquid vapours depend on such things as the kind of job, the kind and amount of materials used, and the size and layout of the work area.

An assessment of the specific ways flammable and combustible liquids are stored, handled, used and disposed of is the best way to find out if existing ventilation controls (and other hazard control methods) are adequate.

Some workplaces may need a complete system of hoods and ducts to provide acceptable ventilation. If flammable vapours are likely to condense, the ducts should have welded joints. Other workplaces may only require a single, well-placed exhaust fan. Use non-ferrous fan blades and shrouds (housing), and explosion-proof electrical equipment in ventilation systems for these liquids. Regular cleaning of the ducts, filters, plenums, etc. will decrease the severity of any fires and will reduce the likelihood of spontaneous combustion if some self-heating material is present. Ventilation equipment used to handle solvent vaporous should meet the relevant fire code requirements.

If the ventilation keeps vapour levels below the occupational exposure limit of a chemical, usually there is little risk of fire or explosion. Vapour levels harmful to people are, in most cases, much below the lowest concentration of vapour in air that can burn. For example, toluene has a workplace exposure limit of 20 ppm [50 parts of toluene per million parts of air or 0.005 percent] (ACGIH 2008 TLVs & BEIs) in many jurisdictions. This is far below the lower flammable limit (LFL) for toluene, which is 12,000 ppm (1.2 percent).

In baking and drying ovens, enclosed air-drying spaces, ventilation duct work or other enclosures where workers are not normally exposed to the vapour, keep vapour levels to 20 percent or less of the LFL.

Good storage area :

Store flammable and combustible liquids in areas that are:

• Storage of Flammable liquids shall be in NFPA approved flammable storage lockers or in low value structures at least 50 feet from any other structure. Do not store other combustible materials near flammable storage areas or lockers.

• Bulk drums of flammable liquids must be grounded and bonded to containers during dispensing.

• Portable containers of gasoline or diesel are not to exceed 5 gallons.

• Safety cans used for dispensing flammable or combustible liquids shall be kept at a point of use.

• Appropriate fire extinguishers are to be mounted within 75 feet of outside areas containing flammable liquids, and within 10 feet of any inside storage area for such materials.

• Storage rooms for flammable and combustible liquids must have explosion-proof light fixtures.

• Bulk storage of gasoline or diesel are kept in above ground tanks. Tank areas are diked to contain accidental spills. Tanks shall be labeled IAW NFPA guidelines. All tank areas shall be designated no smoking – no hot work – no open flame areas.

• No flames – hot work or smoking is be permitted in flammable or combustible liquid storage areas.

• The maximum amount of flammable liquids that may stored in a building are 20 gallons of Class IA liquids in containers100 gallons of Class IB, IC, II, or III liquids in containers500 gallons of Class IB, IC, II, or III liquids in a single portable tank.

• Flammable liquid transfer areas are to be separated from other operations by distance or by construction having proper fire resistance.

• When not in use flammable liquids shall be kept in covered containers.

• Class I liquids may be used only where there are no open flames or other sources of ignition within the possible path of vapor travel.

• Flammable or combustible liquids shall be drawn from or transferred into vessels, containers, or portable tanks within a building only through a closed piping system, from safety cans, by means of a device drawing through the top, or from a container or portable tanks by gravity through an approved self-closing valve. Transferring by means of air pressure on the container or portable tanks shall be prohibited.

• Maintenance and operating practices shall be in accordance with established procedures which will tend to control leakage and prevent the accidental escape of flammable or combustible liquids. Spills shall be cleaned up promptly.

• Combustible waste material and residues in a building or unit operating area shall be kept to a minimum, stored in covered metal receptacles and disposed of daily.

• Rooms in which flammable or combustible liquids are stored or handled by pumps shall have exit facilities arranged to prevent occupants from being trapped in the event of fire.

• Inside areas in which Class I liquids are stored or handled shall be heated only by means not constituting a source of ignition, such as steam, hot water or forces central systems located away from the area.

Important to practice good housekeeping and maintain equipment:

Good housekeeping and equipment maintenance are important wherever any chemicals, including flammable and combustible liquids, are used.

• Keep all areas where these liquids are stored, handled or used clear of burnable materials.

• Provide drip trays and empty them often wherever recurring leakages occur.

• Consider using splash guards to enclose machines or processes that eject flammable or combustible liquids.

• Clean up liquid spills immediately.

• Remove any obstructions that prevent containers with lids held open by fusible links from closing fully.

• Make sure that flammable and combustible liquids are not left where they could block or otherwise prevent people from escaping in case of a fire.

Regular equipment inspection and maintenance are important for controlling the hazards of flammable and combustible liquids.

• Ensure maintenance personnel know the hazards of the materials to which they might be exposed.

• Carry out repairs to equipment properly, including special equipment like explosion-proof fittings. Fires and explosions have resulted from the addition of non-approved parts or equipment to approved systems.

• Do not use safety containers that are damaged in any way. If repairs using approved parts cannot restore safety containers to a safe condition, discard the containers once they have been properly cleaned.

Cabinets

Not more than 120 gallons of Class I, Class II, and Class IIIA liquids may be stored in a storage cabinet.

Basic safety practices for flammable and combustible liquids:

Following these basic safe practices will help protect you from the hazards of flammable and combustible liquids:

• Obtain and read the Material Safety Data Sheets (MSDSs) for all of the materials you work with.

• Be aware of all of the hazards (fire/explosion, health, chemical reactivity) of the materials you work with.

• Know which of the materials that you work with are flammable or combustible liquids.

• Eliminate ignition sources (sparks, smoking, flames, hot surfaces) when working with flammable and combustible liquids.

• Use the smallest amount of flammable liquid necessary in the work area.

• Keep storage areas cool and dry.

• Store flammable and combustible liquids away from incompatible materials (e.g., oxidizers).

• Use approved containers for disposal of rags and other work.

• Store, handle and use flammable and combustible liquids in well-ventilated areas.

• Use approved equipment, including labelled safety containers, for flammable and combustible liquids.

• Keep containers closed when not in use.

• Bond and ground metal containers when transferring flammable and combustible liquids.

• Practice good housekeeping and equipment maintenance. Keep area clear of burnable materials.

• Wear the proper personal protective equipment for each of the jobs you do.

• Know how to handle emergencies (fires, spills, personal injury) involving the flammable and combustible liquids you work with.

• Follow the health and safety rules that apply to your job.

Responsibilities

Management

• Provide proper storage for flammable liquids

• Ensure proper training is provided to employees who work with flammable liquids

• Ensure containers are properly labeled

Supervisors

• Provide adequate training in the use and storage of flammable liquids

• Monitor for proper use and storage

• Keep only the minimum amount required on hand

• Ensure MSDS are current for all flammable liquids

Employees

• Follow all storage and use requirements
• Report deficiencies in storage and use to supervisors
• Immediately report spills to supervisors

Hazard Control

Engineering Controls

• Properly designed flammable storage areas

• Ventilated Storage areas

• Grounding Straps on Drums and dispensing points

Administrative Controls

• Designated storage areas

• Limiting amount of flammable liquids in use and storage

• Employee Training

• Limited & controlled access to bulk storage areas

• Posted Danger, Warning and Hazard Signs.

Handling flammable materials

Fire prevention

To prevent fires, flammable materials must be properly managed in the workplace. There are three main ways to prevent fires:

(1) Limit the amounts of flammable and combustible materials

• Keep only what you need on-site.

• Purchase materials in the smallest volumes necessary.

• At work locations, keep only those chemicals that are needed for the present task.

• Do not let hazardous wastes accumulate at the work site.

• Store products, including wastes, used at the work site in proper containers.

• Keep flammable materials separate from other processes and storage areas.

(2) Provide proper ventilation to ensure flammable vapours do not accumulate

• Install properly designed ventilation in storage areas.

• Ensure that processes that use or make flammable materials do not exhaust back in the work site.

• Ensure that equipment, such as spray booths, where flammable materials are used, are exhausted outside of the building, and away from air intakes.

• Ventilation systems must be properly maintained and comply with the Building Code.

(3) Control ignition sources

• Ground and bond all work and ignition-proof equipment

• Ensure that there is no smoking in work areas where flammable materials are stored or used

• Never store flammable materials near hot equipment or open flames

• Use intrinsically safe and non-sparking tools

It is important that the employer assess the work site and identify potential fire hazards. This will allow the employer to identify the best ways to control these hazards.

Flammable gases

Flammable gases stored in cylinders are usually at very high pressures, so their uncontrolled release can present both physical and flammability hazards. A small amount of the released gas can fill a large area with a potentially explosive concentration very quickly. This is particularly the case with liquefied gases such as Liquefied Petroleum Gas (LPG).

When storing flammable gas in the workplace:

• store flammable gas cylinders in a separate well ventilated room

• ensure that cylinders are properly secured so that they cannot fall over and valves are protected from damage

• always use the correct fittings and valves for the specific cylinder, do not mix and match fittings

• protect hoses, connections and containers from damage and inspect them regularly for signs of wear.

Static electricity hazard:

Flammable and combustible liquids can present a static electricity hazard depending on their ability to generate static electricity, how well they conduct electricity (conductivity), and their flash point.

Solvents and fuels produced from petroleum (e.g., benzene, toluene, mineral spirits, gasoline, jet fuel) can build up a charge when they are poured or flow through hoses. They tend to hold a charge because they cannot conduct electricity well enough to discharge when in contact with a conducting material, like a metal pipe or container, that is grounded. When enough of a charge is built up, a spark may result. If the vapour concentration of the liquid in air is in the “flammable range” and the spark has enough energy, a fire or explosion can result.

According to the NFPA (Code 77), solvents that are soluble in water (or can dissolve some water themselves) do not build up static electricity. Examples of such liquids include alcohols and ketones like acetone. However, when liquids are transferred into non-conductive containers (e.g., plastic, glass), even conductive solvents may build up a charge because the plastic or glass containers decrease the rate at which the charge in the solvent dissipates.

The flash point and vapour pressure of the liquid and the temperature are other factors to consider. The vapour levels will be higher in the air around the container if you are working outside on a hot summer day than in the winter when the temperature is below 0°C (32°F) or colder.

At higher elevations in the mountains, the air pressure is significantly lower and solvents boil at lower temperatures. Under these conditions, the flash point and the temperature for the optimal vapour/air ratio are lower and some “combustible” liquids can become “flammable”.

A liquid like hexanes has a low flash point and it is flammable when its temperature is in the range -33°C to -3°C (-28°F to +26°F) at sea level. At normal room temperatures, the vapour/air ratio at the surface of the solvent will be well above its upper flammability limit and would be “too rich” to burn. However, at some distance away from the solvent surface, there is a concentration of hexane vapour in the air that is in the flammable range.

A fuel like kerosene is a combustible liquid with a flash point above 38°C (100°F). Under hot weather conditions or if high flash point liquids are heated to temperatures around or above their flash points, a flammable vapour/air mixture will form.

Generally, the conditions for igniting a liquid are optimal when the liquid is used at a temperature that produces a vapour in air concentration (at the surface of the liquid) that is halfway between the upper and lower flammability limits. Recognizing that these conditions represent an “optimal” fire hazard, one has to take appropriate precautions.

Important to bond and ground containers:

Transferring a liquid from one metal container to another may result in static electrical sparks. To prevent the build up of static electricity and prevent sparks from causing a fire, it is important to bond metal dispensing and receiving containers together before pouring. Bonding is done by making an electrical connection from one metal container to the other. This ensures that there will be no difference in electrical potential between the two containers and, therefore, no sparks will be formed.

The best way to bond containers is to securely attach a special metal bonding strap or wire to both containers. Some liquid transfer pumps have self-bonding hoses. Bonding can also be done by keeping a solid metal-to-metal contact between the containers themselves or between a metal container and a conducting nozzle. These latter two methods are usually not reliable because a good electrical contact is often hard to make and maintain during the entire transfer.

In the flammable liquid storage and dispensing area, ground dispensing drums. Grounding is done by connecting the container to an already grounded object that will conduct electricity. This could be a buried metal plate, a metallic underground gas piping system, metal water pipes or a grounded, metal building framework. Bonding both containers and grounding one of them “drains off” static charges and prevents the discharge of sparks. All grounding and bonding connections must be bare metal to bare metal. Remove all dirt, paint, rust or corrosion from points of contact. Specially designed and approved bonding and grounding wire assemblies are available from safety equipment retailers.

Storage Inside Building

Egress

Flammable or combustible liquids, including stock for sale, shall not be stored so as to limit use of exits, stairways, or areas normally used for the safe egress of people.

Office Occupies

Storage shall be prohibited except that which is required for maintenance and operation of equipment. Such storage shall be kept in closed metal containers stored in a storage cabinet or in safety cans or in an inside storage room not having a door that opens into that portion of the building used by the public.

General Purpose Public Warehouses

There are tables in the standard summarizing the storage requirements applicable to “General Purpose Public Warehouses.” These tables refer to indoor storage of flammable and combustible liquids which are confined in containers and portable tanks. Storage of incompatible materials that create a fire exposure (e.g., oxidizers, water-reactive chemicals, certain acids and other chemicals) is not permitted.

Warehouses or Storage Buildings

The last type of inside storage covered by this paragraph addresses storage in “warehouses or storage buildings.” These structures are sometimes referred to as outside storage rooms. Practically any quantity of flammable and combustible liquid can be stored in these buildings provided that they are stored in a configuration consistent with the tables in this paragraph.

Containers in piles shall be separated by pallets or dunned where necessary to provide stability and to prevent excessive stress on container walls.

Stored material shall not be piled within 3 feet of beams or girders and shall be at least 3 feet below prinkler deflectors or discharge orifices of water spray, or other fire protection equipment.

Aisles of at least 3 feet in width shall be maintained to access doors, windows or standpipe connections.

Storage Outside Buildings

Requirements covering “storage outside buildings” are summarized in tables in this paragraph. Associated requirements are given for storage adjacent to buildings. Also included are requirements involving controls for diversion of spills away from buildings and security measures for protection against trespassing and tampering. Certain housekeeping requirements are given which relate to control of weeds, debris and accumulation of unnecessary combustibles.

Fire Control

Suitable fire control devices, such as small hose or portable fire extinguishers, shall be available at locations where flammable or combustible liquids are stored.

At least one portable fire extinguisher having a rating of not less than 12-B units shall be located:

• outside of, but not more than 10 feet from, the door opening into any room used for storage; and
• not less than 10 feet, nor more than 25 feet, from any Class I or Class II liquid storage area located outside of a storage room but inside a building.

The reason for requiring that portable fire extinguishers be located a distance away from the storage room is that fires involving Class I and Class II flammable liquids are likely to escalate rapidly. If the fire is too close to the storage area, it may be impossible to get to it once the fire has started.

Open flames and smoking shall not be permitted in flammable or combustible liquid storage areas.

Materials which react with water shall not be stored in the same room with flammable or combustible liquids. Many flammable and combustible liquid storage areas are protected by automatic sprinkler or water spray systems and hose lines. Consequently, any storage of water-reactive material in the storage area creates an unreasonable risk.

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Self-reactivity under conditions of shock or increase in temperature or pressure:

Materials in this group are chemically very unstable. Depending on the material, they can react vigorously and, in some cases, explosively under conditions of mechanical shock such as a hammer blow or even slightly elevated temperature or pressure. Materials in this category include:

• ammonium perchlorate

• azo and diazo compounds

• acetylides                                                                                 

• azides

• fulminates

• hydrogen peroxide solutions (91% by weight)

• many organic peroxides

• nitro and nitroso compounds

• nitrate esters

• perchloric acid solutions (over 72.5% by weight)

• picric acid

• picrate salts

• triazines

• some epoxy compounds

Vigorous reactivity with water release deadly gas

Some materials can react vigorously with water to rapidly produce gases which are deadly at low airborne concentrations. For example, sodium or potassium phosphide release phosphine gas when they contact water. Alkali metal cyanide salts, such as sodium or potassium cyanide, slowly release deadly hydrogen cyanide gas on contact with water. The cyanide salts of alkaline earth metals such as calcium or barium cyanide react at a faster rate with water to produce hydrogen cyanide gas. This can result in a life-threatening problem in confined spaces or poorly ventilated areas.

Large amounts of corrosive hydrogen chloride gas are rapidly released when water reacts with aluminum chloride, phosphorous trichloride, tin chloride and chlorosilane compounds. When water contacts thionyl chloride or sulphuryl chloride, they decompose rapidly giving off sulphur dioxide gas and hydrogen chloride gas.

Treat all unknown materials as very hazardous until they are positively identified.

Fire and explosion hazards of dangerously reactive chemicals:

Highly reactive chemicals may undergo vigorous, uncontrolled reactions that can cause an explosion or a fire, or rupture sealed reaction vessels or storage containers.

Even slow reactions can be hazardous if they involve large amounts of material or if the heat and gases are confined, such as in a sealed storage drum. Drums that are swollen and distorted from over-pressurization are potentially very dangerous. They may rupture at any time without warning and release their contents.

Some dangerously reactive liquids such as methyl acrylate and acrylonitrile, are also flammable liquids. They give off enough vapour at normal workplace temperatures to form flammable mixtures with air. They can be serious fire hazards at temperatures lower than those at which they would begin to polymerize or decompose.

Fires involving dangerously reactive materials can be more hazardous than normal. The heat from the fire can lead to violent, uncontrolled chemical reactions and potentially explosive ruptures of sealed containers.

Proper ventilation important:

Well designed and maintained ventilation systems remove airborne, dangerously reactive materials from the workplace and reduce their hazards.

The amount and type of ventilation needed depends on such things as the type of job, the kind and amount of materials used, and the size and layout of the work area. An assessment of the specific ways a workplace stores, handles, uses and disposes of its dangerously reactive materials is the best way to find out if existing ventilation controls (and other hazard control methods) are adequate.

Some workplaces may need a complete system of hoods, ducts and fans to provide acceptable ventilation. Others may require a single, well-placed exhaust fan. No special ventilation system may be needed to work with small amounts of dangerously reactive materials which do not give off airborne contaminants.

Make sure ventilation systems for dangerously reactive materials are designed and built so that they do not result in an unintended hazard. Ensure that hoods, ducts, air cleaners and fans are made from materials compatible with the dangerously reactive substance. Systems may require explosion-proof electrical equipment.

Ensure that the system is designed to avoid buildups of dusts or condensation of vapours. The vapours of inhibited liquids are not inhibited. When they condense, the liquid could polymerize or decompose easily.

Keep systems for dangerously reactive materials separate from other systems exhausting incompatible substances. Periodic inspection of ventilation systems will help maintain them in good operating condition.

Containers for dangerously reactive materials:

Inspect all incoming containers before storing to ensure that they are undamaged and properly labelled. Do not accept delivery of defective containers.

Store dangerously reactive materials in containers that the chemical supplier recommends. Normally, these are the same containers in which the material was shipped. Repackaging can be dangerous especially if contaminated or incompatible containers are used. For example, strong hydrogen peroxide solutions can decompose explosively if placed in a container with rusty surfaces. Bottles for light-sensitive materials are often made of dark blue or brown glass to protect the contents from light. Containers for water-sensitive compounds should be waterproof and tightly sealed to prevent moisture in the air from reacting with the material.

Make sure containers are suitably labelled. For materials requiring temperature control, the recommended storage temperature range should be plainly marked on the container. It is also a good practice to mark the date that the container was received and the date it was first opened.

Protect containers against impact or other physical damage that might cause shock. Do not use combustible pallets, such as wood, for storing oxidizing materials or organic peroxides.

Normally keep stored containers tightly closed. This helps to avoid contamination of the material or evaporation of solvents used to dilute substances, such as some organic peroxides, to safer concentrations.

Some dangerously reactive liquids, such as strong hydrogen peroxide solutions or certain organic peroxide products, gradually decompose at room temperature and give off gas. These liquids are shipped in containers with specially vented caps. These vent caps relieve the normal buildup of gas pressure that could rupture an unvented container. Check vent caps regularly to ensure that they are working properly. Keep vented containers in the upright position. NEVER stack vented containers on top of each other.

Storage area for dangerously reactive liquids and solids:

Store dangerously reactive liquids and solids separately away from processing and handling areas and from incompatible materials. Some dangerously reactive materials are incompatible with each other. Do not store these beside each other. Separate storage can minimize personal injury and damage caused by fires, spills or leaks.

Check the reactivity data and storage requirements sections of the MSDS for details about what substances are incompatible with a specific dangerously reactive material.

Construct walls, floors, shelving, and fittings in storage areas from suitable materials. For example, use non-combustible building materials in storage areas for dangerously reactive oxidizers or organic peroxides. Use corrosion-resistant materials for dangerously reactive corrosives.

Ensure that floors in storage areas are watertight and without cracks in which spilled materials can lodge. Contain spills or leaks by storing smaller containers in trays made of compatible materials. For larger containers, such as drums or barrels, provide dikes around storage areas, and sills or ramps at door openings.

Store smaller containers at a convenient height for handling below eye level if possible to reduce the risk of dropping them. Avoid overcrowding in storage areas. Do not store containers in out-of-the-way locations where they could be forgotten.

Store containers away from doors. Although it is convenient to place frequently used materials next to the door, they could cut off the escape route if an emergency occurs.

Store dangerously reactive materials in areas which are:

• Well ventilated.

• Supplied with adequate firefighting equipment including sprinklers (sprinklers may not be allowed in areas where materials that react dangerously with water are present).

• Supplied with suitable spill clean-up equipment and materials.

• Free of ignition sources such as sparks, flames, burning tobacco or hot surfaces.

• Accessible at all times.

• Labelled with suitable warning signs.

Storage temperature important:

Store dangerously reactive materials in dry, cool areas, out of direct sunlight, and away from steam pipes, boilers or other heat sources. Follow the chemical supplier’s recommendations for maximum and minimum temperatures for storage and handling. Higher temperatures can be hazardous since they can start and speed up hazardous chemical reactions. In many cases, inhibitors can be rapidly depleted at higher-than-recommended storage temperatures. Loss of inhibitor can result in dangerous reactions.

Some dangerously reactive materials must be kept at low temperatures in refrigerators or freezers. Use only approved or specially modified units. These are generally known as “laboratory safe”. Standard domestic refrigerators and freezers contain many ignition sources inside the cabinet.

It can also be hazardous to store dangerously reactive materials at less than the recommended temperature. For example, acrylic acid is normally supplied with an inhibitor to prevent polymerization. Acrylic acid freezes at 13?C (55?F). At temperatures less than this, it will partly solidify. The solid part contains little or no inhibitor; the inhibitor remains in the liquid portion. The uninhibited acrylic acid can be safely stored below the freezing point but it may polymerize violently if it is heated to warmer temperatures.

Some organic peroxides are sold dissolved or dispersed in solvents, including water, to make them less shock-sensitive. If these are cooled to below their freezing points, crystals of the pure, very sensitive organic peroxide may be formed.

Alarms that indicate when storage temperatures are higher or lower than required may be needed.

Follow the chemical supplier’s directions about inhibitors used in a particular product. Where appropriate, check inhibitor and oxygen levels and add more as needed according to the supplier’s instructions.

Do not keep a material for longer than the chemical supplier recommends.

Dispensing or using dangerously reactive materials:

Open and dispense containers of dangerously reactive materials in a special room or area outside the storage area. Do not allow any ignition sources in the vicinity. Take care that the dangerously reactive materials do not contact incompatible substances. Use containers and dispensing equipment, such as drum pumps, scoops or spatulas, that the chemical supplier recommends. These items must be made from materials compatible with the chemicals they are used with. Keep them clean to avoid contamination.

When transferring materials from one container to another, avoid spilling material and contaminating your skin or clothing. Spills from open, unstable or breakable containers during material transfer have caused serious accidents.

NEVER transfer liquids by pressurizing their usual shipping containers with air or inert gas. The pressure may damage ordinary drums and barrels. If air is used, it may also create a flammable atmosphere inside containers of flammable or combustible liquids.

Glass containers with screw-cap lids or glass stoppers may not be acceptable for friction-sensitive materials. Avoid using ordinary screw-cap bottles with a cardboard liner in the cap for moisture-sensitive chemicals. Airborne moisture can diffuse slowly but steadily through the liner. NEVER transfer materials stored in a vented container into a tightly-sealed, non-vented container. The buildup of gas pressure could rupture it.

Dispense from only one container at a time. Finish dispensing and labelling one material before starting to dispense another. Dispense the smallest amount possible, preferably only enough for immediate use.

Keep containers closed after dispensing to reduce the risk of contaminating their contents.

NEVER return any unused material, even if it does not seem to be contaminated, to the original container.

If a dangerously reactive material freezes, do not chip or grind it to break up lumps, or heat it to thaw it out. Follow the chemical supplier’s advice.

Avoid dropping, sliding or skidding heavy metal containers such as drums or barrels of friction- or shock-sensitive material.

What are safe techniques to use when handling dangerously reactive materials:

Make sure that all areas where dangerously reactive liquids and solids are used are clean and free of incompatible materials and ignition sources. Do not allow temperatures in these areas to become hot enough to cause a hazardous reaction.

Always:

• Inspect containers for damage or leaks before handling them.

• Handle containers carefully to avoid damaging them.

• Keep containers tightly closed except when actually using the material.

• Avoid returning used chemicals to containers of unused materials.

• Keep only the smallest amounts possible (never more than one day’s supply) of dangerously reactive materials in the work area.

• Return unopened containers to the proper storage area at the end of the day and opened containers to a dispensing area at the end of the day.

• Check that all containers are properly labelled, and handle containers so that the label remains undamaged and easy to read.

Regular workplace inspections can help to spot situations in which dangerously reactive materials are stored, handled or used in potentially hazardous ways.

Basic safe practices when working with dangerously reactive liquids and solids:

Following these basic safe practices will help protect you from the hazards of dangerously reactive liquids and solids:

• Read the Material Safety Data Sheets (MSDSs) and labels for all of the materials you work with.

• Know all of the hazards (fire, explosion, health, corrosivity, chemical reactivity) of the materials you work with.

• Know which of the materials you work with are dangerously reactive.

• Store dangerously reactive materials in suitable, labelled containers (usually their shipping containers) in a cool, dry area.

• Store, handle and use dangerously reactive materials in well-ventilated areas and away from incompatible materials.

• Follow the chemical supplier’s advice on maximum and minimum temperatures for storage and use.

• Follow the chemical supplier’s advice on checking and maintaining inhibitor and dissolved oxygen levels where appropriate.

• Eliminate ignition sources (sparks, smoking, flames, hot surfaces) when working with dangerously reactive materials.

• Handle containers carefully to avoid damaging them or shocking their contents.

• Keep containers closed when not in use.

• Keep only the smallest amount possible (not more than one day’s supply) in the work area.

• Dispense dangerously reactive materials carefully into acceptable containers, using compatible equipment.

• Do not subject dangerously reactive materials to any type of friction or impact.

• Be careful when performing operations such as separations or distillations, that concentrate dangerously reactive materials.

• Practice good housekeeping, personal cleanliness and equipment maintenance.

• Handle and dispose of dangerously reactive wastes safely.

• Wear the proper personal protective equipment for each of the jobs you do.

• Know how to handle emergencies (fires, spills, personal injury) involving the dangerously reactive materials you work with.

• Follow the health and safety rules that apply to your job.

Good housekeeping important:

Maintain good housekeeping at all times in the workplace:

• Clean-up any spills promptly and safely according to directions in the MSDS.

• Use suitable clean-up materials.

• Properly dispose of unlabelled or contaminated materials.

• Promptly remove combustible wastes, including wood, paper or rags from work area.

• Avoid any buildup of chemical dusts on ledges or other surfaces.

• Ensure that all waste containers used are compatible with the reactive materials, properly marked and located close to the job.

*Note: For example, some commercial sorbent materials used for spill clean-up may initiate polymerization in some monomers. Do not use sawdust or other combustible sweeping compounds to clean up spills of oxidizers or organic peroxides.

Personal cleanliness important:

Personal cleanliness helps protect people working with dangerously reactive materials:

• Wash hands before eating, drinking, smoking or going to the toilet.

• Remove contaminated clothing and footwear since they can be a severe fire or health hazard.

• Wash contaminated clothing and footwear thoroughly before rewearing or discarding. Check the MSDS or contact the chemical supplier for details.

• Do not wear or carry contaminated items into areas having ignition sources or where smoking is allowed.

• Store food and tobacco products in uncontaminated areas.

• Clean yourself thoroughly at the end of the workday.

What should do during emergency:

Act fast in emergencies like chemical leaks, spills and fires:

• Evacuate the area at once if you are not trained to handle the problem or if it is clearly beyond your control.

• Alert other people in the area to the emergency.

• Call the fire department immediately.

• Report the problem to the people responsible for handling emergencies where you work.

• Obtain first aid if you have been exposed to harmful chemicals and remove all contaminated clothes.

Only specially trained and properly equipped people should handle the emergency. Nobody else should go near the area until it is declared safe.

Planning, training and practicing for emergencies help people to know what they must do. Prepare a written emergency plan. Update it whenever conditions in the workplace change.

The MSDSs for the materials used are a starting point for drawing up an emergency plan. MSDSs have specific sections on spill clean-up procedures, first aid instructions, and fire and explosion hazards, including suitable fire extinguishing equipment and methods. If the directions in each MSDS section are not clear or seem incomplete, contact the material’s supplier for help.

It is very important to know the best ways to fight fires involving dangerously reactive materials. For example, using water on water-reactive chemicals can cause the rapid release of lethal gas or, in some cases, violent explosions.

There are numerous other sources to turn to for help in developing your emergency plans. The local fire department can provide assistance in this area, as well as training. You can also obtain useful information at little or no cost from environmental and health and safety enforcement agencies, provincial accident prevention associations, St. John Ambulance, insurance companies, occupational health and safety groups, unions, professional associations, certain colleges and universities, and CCOHS. Private consultants who specialize in these matters are also available.

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Corrosives are materials that can attack and chemically destroy exposed body tissues. Corrosives can also damage or even destroy metal. They begin to cause damage as soon as they touch the skin, eyes, respiratory tract, digestive tract, or the metal. They might be hazardous in other ways too, depending on the particular corrosive material.

Most corrosives are either acids or bases. Common acids include hydrochloric acid, sulfuric acid, nitric acid, chromium acid, acetic acid and hydrofluoric acid. Common bases are ammonium hydroxide, potassium hydroxide (caustic potash) and sodium hydroxide (caustic soda).

Other chemicals can be corrosive too. Check the supplier labels on chemical product containers.

It is wise to treat unknown materials as very hazardous until they are positively identified.

Corrosive materials are present in almost every workplace. Acids, bases (which include caustics or alkalis), and other chemicals may be corrosive. Everyone who works with corrosives must be aware of their hazards and how to work safely with them.

corrosives hazardous to my health?

Corrosives can burn and destroy body tissues on contact. The stronger, or more concentrated, the corrosive material is and the longer it touches the body, the worse the injuries will be.

Some corrosives are toxic and can cause other health problems. Check the MSDS and label on the container for warnings of other possible health effects.

An example is the chemical glutaraldehyde. It has been used as a disinfectant and sterilizing agent in medical and dental settings as well as other uses. It is harmful if inhaled or swallowed. Glutaraldehyde can be irritating or corrosive to the respiratory tract, eyes and skin. It may cause permanent eye injury. It is also a skin sensitizer as it may cause (severe) allergic skin reactions.

hazards associated with corrosives:

Some corrosives are also flammable or combustible and can easily catch fire and burn or explode.

Some corrosives are incompatible with other chemicals. They may undergo dangerous chemical reactions and give off toxic or explosive products if they contact each other.

The MSDSs and the labels on the containers should explain all of the hazards for the corrosive materials that you work with.

hazardous to my eyes:

Corrosive materials can severely irritate, or in come cases, burn the eyes. This could result in scars or permanent blindness. The stronger, or more concentrated, the corrosive material is and the longer it touches the eyes, the worse the injury will be.

hazardous to my skin:

Corrosives touching the skin can severely irritate or even badly burn and blister the skin. Severe corrosive burns over a large part of the body can cause death.

hazardous when I breathe them in:

Breathing in corrosive vapours or particles irritates and burns the inner lining of the nose, throat, windpipe and lungs. In serious cases, this results in pulmonary edema, a buildup of fluid in the lungs that can be fatal.

hazardous when corrosives touch metals:

Many corrosives attack and corrode metals. Contact with corrosives can damage containers, equipment, installations and building components made from unsuitable materials. The rate of metal corrosion is greater when the corrosive is stronger and the temperature is higher. When acids attack metals, hydrogen gas is often given off. This is a flammable gas which can burn or explode if an ignition source is present.

Common bases, such as sodium hydroxide and potassium hydroxide, can also attack some metals like aluminum, zinc, galvanized metal, and tin to produce hydrogen gas. The MSDS for a particular corrosive should explain which metals or other materials, such as plastics or wood, it will attack.

Ventilation wherever corrosives are present:

Well-designed and well-maintained ventilation systems remove corrosive vapours, fumes, mists or airborne dusts from the workplace and reduce their hazards.

The amount and type of ventilation needed to minimize the hazards of airborne corrosives depends on such things as the kind of job, the kind and amount of materials used, and the size and layout of the work area. An assessment of the specific ways corrosives are stored, handled, used, and disposed of is the best way to find out if existing ventilation controls (and other hazard control methods) are adequate.

Some workplaces may need a complete system of hoods and ducts to provide acceptable ventilation. Others may require a single, well-placed exhaust fan. Use corrosion-resistant construction in ventilation systems for corrosive materials. No special ventilation system may be needed when working with small amounts of corrosives which do not give off airborne contaminants.

Storage of corrosives:

In general, store corrosives separately, away from processing and handling areas, and from other materials. Separate storage can reduce the amount of damage caused in case of fires, spills or leaks. If totally separate storage is not possible, store corrosives away from incompatible materials.

Some corrosives are incompatible with each other. For example, acids and bases react together, sometimes violently. Do not store them beside each other.

Walls, floors and shelving in corrosive storage areas should be made from materials that resist attack by corrosives. Floors in areas where liquid corrosives are stored should not allow liquids to penetrate. Since many corrosive liquids flow easily, store them in corrosion-resistant trays to contain spills or leaks. For large containers, such as 250-litre (55-gallon) drums, provide dikes around liquid storage areas and sills or ramps at door openings.

Store containers at a convenient height for handling, below eye level if possible. High shelving increases the risk of dropping containers and the severity of damage if a fall occurs.

Store corrosives in areas which are:

• Well ventilated.

• Supplied with adequate firefighting equipment.

• Supplied with suitable spill clean-up equipment and materials.

• Labelled with proper warning signs.

At all times:

• Allow only trained, authorized people into storage areas.

• Keep the amount of corrosive material in storage as small as possible.

• Inspect storage areas regularly for any deficiencies, including corrosion damage, leaking containers, or poor housekeeping. Correct all deficiencies as soon as possible.

Storage temperature important:

Store corrosives in dry, cool areas, out of direct sunlight and away from steam pipes, boilers or other sources of heat. If a sealed full drum or carboy of a corrosive liquid is stored in direct sunlight or near other heat sources, vapor levels in the container can build up. This leads to an increase in pressure in the container. In severe cases, this could cause the container to rupture. A buildup of pressure might also result in the material shooting out into the face of the person opening the container.

Follow the chemical manufacturer’s or supplier’s recommendations for storage temperature. Where appropriate, store corrosive liquids at temperatures above their freezing (melting) points. Acetic acid, for example, has a freezing point of approximately 17°C (63°F) and can freeze in an unheated room. As it freezes, it expands and can crack a glass container.

Avoid rapid temperature changes in corrosive liquid storage areas. If a tightly-sealed corrosive liquid container is cooled suddenly, a partial vacuum could form inside it. In extreme cases, the container might collapse and leak.

Handling corrosive materials carefully:

Take care when dispensing or transferring corrosives from one container to another. Dispense from only one container at a time. Finish all the dispensing of one material before starting to dispense another. Be sure containers are closed after dispensing.

Handle corrosives so that dusts, mists, vapours, or fumes do not get into the air. Be very careful when transferring from larger containers into smaller ones. Many injuries have been caused by spillage from open, unstable, or breakable containers during material transfer.

If liquid corrosives are stored in drums, use a corrosion-resistant drum pump for transferring liquids into other containers. Pumps are also available for dispensing corrosive liquids from most sizes and types of the supplied containers. Do not transfer liquids by pressurizing their usual shipping containers with air or inert gas. Ordinary drums and barrels may be damaged by the pressure. Never pipette corrosive liquids by mouth. Use a pipette bulb or aspirator instead. Transfer corrosive solids using tools like scoops or shovels that are corrosion resistant.

Dispose of waste material safely:

Corrosive wastes are hazardous and must always be handled safely.

All containers for corrosive wastes must be made from corrosion-resistant materials. Identify the contents of these containers with suitable labels.

“Empty” drums, bottles and other containers often have hazardous corrosive residues inside them. Never use these “empty” containers for anything else, no matter how clean they seem to be. Treat them as corrosive wastes. It may be possible to safely decontaminate “empty” containers. The chemical manufacturer or supplier can give advice about this.

Never dispose of corrosives down sinks or drains that connect to sanitary or storm sewers. Dispose of them according to the manufacturer’s or supplier’s directions, or through hazardous waste collection and disposal companies. In all cases, dispose of corrosive wastes according to the environmental laws that apply to your jurisdiction. Contact the appropriate environmental officials for details about the disposal laws that apply for specific corrosives.

Good housekeeping, personal cleanliness and maintain equipment?

Good housekeeping, personal cleanliness and equipment maintenance are important wherever any chemicals, including corrosives, are used.

Maintain cleanliness and order at all times in the workplace:

• Clean up any spills and buildups of corrosives promptly and safely.

• Properly dispose of unlabelled or contaminated chemicals.

• Remove empty containers at once from work areas.

• Ensure that all containers for waste are properly marked and easily located.

Personal cleanliness is a very important way of protecting personnel working with hazardous chemicals.

• Wash hands before eating, drinking, smoking or going to the toilet.
• Remove and clean contaminated clothing before wearing it again, or discard it.
• Do not smoke, drink, chew gum or eat in any areas where hazardous chemicals are present.
• Store food and tobacco products in uncontaminated areas.
• Avoid touching yourself with contaminated hands.
• Clean yourself thoroughly at the end of the workday.

Regular maintenance of equipment is important in preventing leaks or emissions of corrosives into the workplace.

• Ensure maintenance personnel know the possible hazards of the materials they might be exposed to.

• Be sure they know any special procedures and precautions that might be needed before they begin to work on equipment.

Regular workplace inspections can help in spotting areas where health and safety problems may be developing.

Wear proper personal protective equipment:

If other methods, such as engineering controls, are not available or effective enough to control exposure to corrosives, wear suitable personal protective equipment (PPE). Choosing the right PPE to wear when doing a particular job is essential. MSDSs should provide general guidance. Selecting PPE for a specific job is best done with the help of someone who knows how to evaluate the hazards of the job and how to select the proper PPE.

Avoid Skin Contact

Wear protective gloves, aprons, boots, hoods, or other clothing depending on how much chance there is of skin contact. This clothing must be made of materials that resist penetration or damage by the chemical. The MSDS should recommend appropriate materials. If it does not, contact the chemical’s manufacturer or supplier for specific information.

Protect Your Eyes and Face

Always wear eye protection when working with corrosives. Although ordinary safety glasses provide some protection, chemical safety goggles are best. In some cases, you should also wear a face shield (with safety glasses or goggles) to protect your face from splashes. The current CSA Standard Z94.3, “Eye and Face Protectors,” provides advice on selection and use of eye and face protectors.

Avoid Breathing Corrosive Vapours, Fumes, Dusts or Mists

If respirators must be used for breathing protection, there should be a written respiratory protection program to follow. Guidance for developing a program can be found in the current CSA Standard Z94.4, “Selection, Care, and Use of Respirators.” Follow all legal requirements for respirator use and approvals. These may vary between jurisdictions in Canada.

Know and be familiar with the right PPE for emergencies, as well as normal operations.

You must wear the PPE needed for doing a particular job. PPE cannot protect you if it is not worn.

Basic safety procedures concerning corrosives:

Following these basic safe practices will help protect you from the hazards of corrosive materials:

• Obtain and read the Material Safety Data Sheets (MSDSs) for all of the materials you work with.

• Be aware of all of the hazards (fire/explosion, health, chemical reactivity) of the materials you work with.

• Know which of the materials you work with are corrosives.

• Store corrosives in suitable labelled containers away from incompatible materials, in a cool, dry area.

• Store, handle, and use corrosives in well-ventilated areas.

• Inspect containers for damage or leaks before handling. Never use containers that appear to be swollen.
• Handle containers safely to avoid damaging them.

• Dispense corrosives carefully and keep containers closed when not in use.

• Stir corrosives slowly and carefully into cold water when the job requires mixing corrosives and water.

• Handle and dispose of corrosive wastes safely.

• Practice good housekeeping, personal cleanliness and equipment maintenance.

• Wear the proper personal protective equipment for each of the jobs you do.

• Know how to handle emergencies (fires, spills, personal injury) involving the corrosive materials you work with.

• Follow the health and safety rules that apply to your job.

• Flush contaminated eyes or skin with water for at least 20-30 minutes, sometimes longer, in case of accidental contact. Call immediately for medical assistance.

• Know where to closest eyewash station and safety showers are located, and how to use them.

• Never return unused material to the original container. It may contain traces of contamination which may cause a chemical reaction.

• Do not reuse empty containers — the residue may be hazardous.

CHEMICAL IDENTIFICATION SIGNAGE

CHEMICAL STORAGE SIGNAGE

CHEMICAL WARNING SIGNAGE

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Tool box talk – Hazardous Communication

What are compressed gases:

Thousands of products are available which contain gases and mixtures of gases stored under pressure in cylinders. Most of these gases are classified as “compressed gases” according to WHMIS 1988 technical criteria. The Controlled Products Regulations contain details of these criteria.

three major groups of compressed gases:

There are three major groups of compressed gases stored in cylinders: liquefied, non-liquefied and dissolved gases. In each case, the pressure of the gas in the cylinder is commonly given in units of kilopascals (kPa) or pounds per square inch gauge (psig).

Gauge pressure = Total gas pressure inside cylinder – atmospheric pressure

Atmospheric pressure is normally about 101.4 kPa (14.7 psi). Note that compressed gas cylinder with a pressure gauge reading of 0 kPa or 0 psig is not really empty. It still contains gas at atmospheric pressure.

Liquefied Gases

Liquefied gases are gases which can become liquids at normal temperatures when they are inside cylinders under pressure. They exist inside the cylinder in a liquid-vapour balance or equilibrium. Initially the cylinder is almost full of liquid, and gas fills the space above the liquid. As gas is removed from the cylinder, enough liquid evaporates to replace it, keeping the pressure in the cylinder constant. Anhydrous ammonia, chlorine, propane, nitrous oxide and carbon dioxide are examples of liquefied gases.

Non-Liquefied Gases

Non-liquefied gases are also known as compressed, pressurized or permanent gases. These gases do not become liquid when they are compressed at normal temperatures, even at very high pressures. Common examples of these are oxygen, nitrogen, helium and argon.

Dissolved Gases

Acetylene is the only common dissolved gas. Acetylene is chemically very unstable. Even at atmospheric pressure, acetylene gas can explode. Nevertheless, acetylene is routinely stored and used safely in cylinders at high pressures (up to 250 psig at 21°C).

This is possible because acetylene cylinders are fully packed with an inert, porous filler. The filler is saturated with acetone or other suitable solvent. When acetylene gas is added to the cylinder, the gas dissolves in the acetone. Acetylene in solution is stable.

Pressure hazards associated with compressed gas cylinders:

All compressed gases are hazardous because of the high pressures inside the cylinders. Gas can be released deliberately by opening the cylinder valve, or accidentally from a broken or leaking valve or from a safety device. Even at a relatively low pressure, gas can flow rapidly from an open or leaking cylinder.

There have been many cases in which damaged cylinders have become uncontrolled rockets or pinwheels and have caused severe injury and damage. This danger has happened when unsecured, uncapped cylinders were knocked over causing the cylinder valve to break and high pressure gas to escape rapidly. Most cylinder valves are designed to break at a point with an opening of about 0.75 cm (0.3 inches). This design limits the rate of gas release and reduces cylinder velocity. This limit may prevent larger, heavier cylinders from “rocketing” although smaller or lighter cylinders might take off.

Poorly controlled release of compressed gas in chemical reaction systems can cause vessels to burst, create leaks in equipment or hoses, or produce runaway reactions.

Fire and explosion hazards associated with compressed gases:

Flammable Gases

Flammable gases, such as acetylene, butane, ethylene, hydrogen, methylamine and vinyl chloride, can burn or explode under certain conditions:

Gas Concentration within the Flammable Range: The concentration of the gas in air (or in contact with an oxidizing gas) must be between its lower flammable limit (LFL) and upper flammable limit (UFL) [sometimes called the lower and upper explosive limits (LEL and UEL)]. For example, the LFL of hydrogen gas in air is 4 percent and its UFL is 75 percent (at atmospheric pressure and temperature). This means that hydrogen can be ignited when its concentration in the air is between 4 and 75 percent. A concentration of hydrogen below 4 percent is too “lean” to burn. Hydrogen gas levels above 75 percent are too “rich” to burn.

The flammable range of a gas includes all of its concentrations in air between the LFL and UFL. The flammable range of any gas is widened in the presence of oxidizing gases such as oxygen or chlorine and by higher temperatures or pressures. For example, the flammable range of hydrogen in oxygen gas is 4 to 85 percent and the flammable range of hydrogen in chlorine gas is 4.1 to 89 percent.

Ignition Source: For a flammable gas within its flammable limits in air (or oxidizing gas) to ignite, an ignition source must be present. There are many possible ignition sources in most workplaces including open flames, sparks and hot surfaces.

The auto-ignition (or ignition) temperature of a gas is the minimum temperature at which the gas self-ignites without any obvious ignition sources. Some gases have very low auto-ignition temperatures. For example, phosphine’s auto-ignition temperature of 100°C (212°F) is low enough that it could be ignited by a steam pipe or a lit light bulb. Some compressed gases, such as silane and diborane, are pyrophoric – they can ignite spontaneously in air.

Flash-back can occur with flammable gases. Many flammable compressed gases are heavier than air. If a cylinder leaks in a poorly ventilated area, these gases can settle and collect in sewers, pits, trenches, basements or other low areas. The gas trail can spread far from the cylinder. If the gas trail contacts an ignition source, the fire produced can flash back to the cylinder.

Oxidizing Gases

Oxidizing gases include any gases containing oxygen at higher than atmospheric concentrations (above 23-25 percent), nitrogen oxides, and halogen gases such as chlorine and fluorine. These gases can react rapidly and violently with combustible materials such as the following:

• organic (carbon-containing) substances such as most flammable gases, flammable and combustible liquids, oils, greases, many plastics and fabrics

• finely-divided metals

• other oxidizable substances such as hydrazine, hydrogen, hydrides, sulphur or sulphur compounds, silicon and ammonia or ammonia compounds.

Fires or explosions can result.

The normal oxygen content in air is 21 percent. At slightly higher oxygen concentrations, for example 25 percent, combustible materials, including clothing fabrics, ignite more easily and burn much faster. Fires in atmospheres enriched with oxidizing gases are very hard to extinguish and can spread rapidly.

Dangerously Reactive Gases

Some pure compressed gases are chemically unstable. If exposed to slight temperature or pressure increases, or mechanical shock, they can readily undergo certain types of chemical reactions such as polymerization or decomposition. These reactions may become violent, resulting in fire or explosion. Some dangerously reactive gases have other chemicals, called inhibitors, added to prevent these hazardous reactions.

Common dangerously reactive gases are acetylene, 1,3-butadiene, methyl acetylene, vinyl chloride, tetrafluoroethylene and vinyl fluoride.

Health hazards associated with compressed gases:

Many compressed gases are toxic or very toxic. They could cause various health problems depending on the specific gas, its concentration, the length of exposure and the route of exposure (inhalation, eye or skin contact). Contact between the skin or eye and liquefied gases in liquid form can freeze the tissue and result in a burn-like injury.

the danger of an inert gas:

Inert gases, such as argon, helium, neon and nitrogen, are not toxic and do not burn or explode. Yet they can cause injury or death if they are present in sufficiently high concentrations. They can displace enough air to reduce oxygen levels. If oxygen levels are low enough, people entering the area can lose consciousness or die from asphyxiation. Low oxygen levels can particularly be a problem in poorly ventilated, confined spaces.

Cylinders and fittings:

Compressed gases are stored in heavy-walled metal cylinders designed, produced and tested for use with compressed gases. Cylinders are made in a wide variety of sizes and shapes. They range from small lecture bottles, often used for demonstration purposes, to large cylinders over 3 metres long. Cylinders for transportation must meet CSA standard CAN/CSA-B339 “Cylinders, Spheres and Tubes for the Transportation of Dangerous Goods”. This standard covers requirements for the manufacturing, inspection, testing, marking, requalification, reheat treatment, repair, and rebuilding of cylinders, spheres, and tubes (containers) for the transportation of dangerous goods. In addition, it includes the requirements for the qualification of new designs and registration requirements. You should also consult CAN/CSA-B340 “Selection and Use of Cylinders, Spheres, Tubes, and Other Containers for the Transportation of Dangerous Goods, Class 2”.

Cylinders that meet these criteria are often referred to as “TC approved” cylinders. Cylinders are permanently marked, typically on the shoulder or the top surface of its neck.

Usually, cylinders must be retested or inspected every five or ten years. The date of each test must be stamped on the cylinder.

Cylinder Valves and Connections

Compressed gas cylinders must be connected only to regulators and equipment designed for the gas in the cylinder. Since connecting the wrong equipment can be dangerous, a number of different standard cylinder valve outlets are available for different classes of gas. For example, these standard connections prevent the valve connection for a flammable gas from fitting the connections for an incompatible gas, such as an oxidizing gas.

Most compressed gas cylinders have valve caps or some other method of protecting the valve from damage during handling and transportation. A dust cap may be placed over the valve outlet itself to help keep it clean.

Cylinder safety devices:

Most cylinders have one or more safety-relief devices. These devices can prevent rupture of the cylinder if internal pressure builds up to levels exceeding design limits. Pressure can become dangerously high if a cylinder is exposed to fire or heat, including high storage temperatures.

There are three types of safety-relief devices. Each relieves excessive gas pressures in a different way:

• Safety- or Pressure-Relief Valves: These valves are usually a part of the cylinder. They are normally held closed by a spring. The force holding the valve closed is set according to the type of gas in the cylinder. The valve opens if the cylinder pressure exceeds the set safety limit. Gas is released until the cylinder pressure drops back to the safety limit. The valve then closes and retains the remaining gas in the cylinder.

• Rupture Discs (also known as frangible or bursting discs): These discs are usually made from metal. They burst or rupture at a certain pressure, releasing the gas in the cylinder. The bursting pressure is designed so that the disc ruptures before the cylinder test pressure is reached. These devices cannot be reclosed, so the entire contents of the cylinder are released.

• Fusible Plugs (also called fuse or melt plugs): Temperature, not pressure, activates fusible plugs. These safety devices are used where heat could initiate an explosive chemical reaction. A pressure-relief valve or rupture disc acts too slowly and too late to prevent rupture of the cylinder if an explosive reaction has already begun. The fusible plug releases the gas before the hazardous reaction can begin. Fusible plugs are made of metals that melt at low temperatures. For example, acetylene cylinders have a fusible plug which melts at about 100°C (212°F). This temperature is safely below the temperature at which hazardous polymerization may occur.

Not all compressed gas cylinders have safety devices. Some gases are so toxic that their release through a safety device would be hazardous. Cylinders for these gases are built to withstand higher pressures than normal cylinders. When these “toxic gas” cylinders are involved in a fire, the area must be evacuated.

Proper ventilation important:

Well-designed and well-maintained ventilation systems remove gases from the workplace and reduce their hazards.

The amount and type of ventilation needed depends on such things as the type of job, the kind and amount of materials used, and the size and layout of the work area.

Assess the specific ways your workplace stores, handles, uses and disposes of its compressed gases. An assessment can reveal if existing ventilation controls and other hazard control methods are adequate. Some workplaces may need a complete system of hoods and ducts to provide acceptable ventilation. Others may require a single, well-placed exhaust fan. Storage facilities for particularly hazardous materials such as chlorine, may require an additional emergency ventilation system, or continuous monitoring with appropriate alarms. Other workplaces using small amounts of inert gases may require no special ventilation system.

Make sure ventilation systems are designed and built so that they do not result in an unintended hazard. Ensure that hoods, ducts, air cleaners and fan are made from materials compatible with the gas used. Systems may require explosion-proof and corrosion-resistant equipment. Separate ventilation systems may be needed for some compressed gases to keep them away from systems exhausting incompatible substances.

Transport or move cylinders:

Always transport cylinders with valve caps or other valve protection in place. Pulling cylinders by their valve caps, rolling them on their sides or dragging or sliding them can cause damage. Rolling cylinders on their bottom edge (“milk churning”) may be acceptable for short distances. Never lift cylinders with magnets or chain or wire rope slings. Transport cylinders on specially built hand carts or trolleys or other devices designed for this. All transport devices should have some way of securing cylinders to prevent them from falling.

About the compressed gas storage area:

Store compressed gas cylinders separately, away from processing and handling areas, and from incompatible materials. Separate storage can minimize personal injury and damage in case of fires, spills or leaks. Many compressed gases can undergo dangerous reactions if they come in contact with incompatible materials (gases, liquids or solids), so store them apart from each other. For example, store oxidizing gases at least 6 metres (20 feet) away from fuel gases or other combustible materials, or separate them with an approved fire wall. Check the reactivity information and storage requirements sections of the MSDS for details about which materials are incompatible with a particular compressed gas. The National Fire Code addresses requirements for segregation of different gases in storage.

If compressed gas cylinders are stored outside, use a well-drained, securely fenced area. Keep them on a raised concrete pad or non-combustible rack. Protect cylinders from the weather and do not allow them to stand directly on wet soil as this can cause corrosion.

Indoor storage areas must have walls, floors and fittings made of suitable materials. For example, use non-combustible building materials in storage areas for oxidizing gas and corrosion-resistant materials in storage areas for corrosive gas. Make sure floors are level and protect cylinders from dampness. Avoid overcrowding in storage areas or storing cylinders in out-of-the-way locations.

Always chain or securely restrain cylinders in an upright position to a wall, rack or other solid structure wherever they are stored, handled or used. Securing each cylinder individually is best. Stacking of groups of cylinders together offers some protection, but if this is done improperly, the entire group or individual cylinders could fall.

Store compressed gas cylinders in areas which are:

• Well-ventilated and dry.

• Fire-resistant and supplied with suitable firefighting equipment including sprinklers, where appropriate.

• Away from electrical circuits and ignition sources such as sparks, flames or hot surfaces.

• Accessible at all times, but away from elevators, staircases or main traffic routes where cylinders may be dangerous obstacles.

• Labelled with suitable warning signs.

Always store full cylinders separately from empty cylinders.

compressed gas storage temperatures:

Store compressed gas cylinders in dry, cool areas, out of direct sunlight and away from steam pipes, boilers or other heat sources.

Follow the gas supplier’s recommendations for storage and use temperatures. To prevent excessive pressure buildup, never expose cylinders to temperatures above 52°C (125°F). Do not subject them to temperatures below -29°C (-20°F), unless they are designed for this. Cylinders that become frozen to a surface can be freed by using warm water (less than 52°C). Never apply direct heat to a cylinder.

Guidelines for safe handling and use

Use the smallest practical cylinder size for a particular job. Do not keep cylinders longer than the supplier recommends. Compressed gas cylinders are mainly shipping containers. They are built to be as light as possible while remaining safe and durable. Do not drop cylinders or otherwise allow them to strike each other. Rough handling, including using cylinders as hammers or as rollers to move equipment, can seriously damage them.

Do not strike an electric arc on a cylinder. Arc burns can make the metal brittle and weaken the cylinder.

Never tamper with cylinders in any way. Do not repaint them, change markings or identification, or interfere with valve threads or safety devices.

Apart from the fact that it is illegal, it can be dangerous for non-specialists to refill cylinders or to change their contents. Explosions, cylinder contamination or corrosion can result.

Special precautions for oxidizing gases:

Special cleaning procedures (equivalent to oxygen service) are required for all equipment to be used with oxidizing gases. There are several ways to do this. Contact your gas supplier for the best methods for specific systems.

Do not oil or grease any equipment that may contact oxidizing gases. Keep greasy hands, rags and gloves away from any part of the cylinder and fittings. Normal body oils are usually not hazardous, although it is a good practice never to touch any surface that may contact an oxidizing gas. Use lubricants and connection or joint sealants recommended by the gas supplier.

Only use oxygen for its intended purpose. Never use it to purge pipelines or to provide ventilation. Freshening the air with oxygen may make people more comfortable, but it also enriches the oxygen content in the area which can quickly create a major fire hazard. Serious accidents have occurred when oxygen was used to run tools designed for compressed air. High oxygen pressure can cause the lubricant in the tool to explode.

handle and store “empty” cylinders:

Non-Liquefied and Dissolved Gases

The amount of material remaining in a non-liquefied or dissolved gas (acetylene) cylinder is directly proportional to the cylinder pressure gauge reading. As the gas is used, the reading on the cylinder pressure gauge drops. When the cylinder pressure gauge reads zero, the cylinder is not really empty. The cylinder still contains gas at atmospheric pressure. Keep a slight positive pressure in the cylinder. Consider it “empty” when the cylinder pressure gauge reads about 172 kPa (25 psig) or when the cylinder will not deliver at least 172 kPa to the outlet pressure gauge.

Liquefied Gases

The pressure in liquefied gas cylinders remains constant at a given temperature as long as any liquid remains in the cylinder. The only way to know how much material remains in a liquefied gas cylinder is to weigh the cylinder. The empty (tare) weight of the cylinder is stamped on its neck or valve stem. Record the net weight of the cylinder contents on a card attached to it. As with non-liquefied and dissolved gases, never empty the cylinder completely. Keep a small amount of material in the cylinder to maintain a slight positive pressure.

equipment maintenance important:

Regular workplace inspections can help to spot situations where compressed gases are stored, handled, or used in potentially hazardous ways.

Regular inspection of equipment can provide a warning of potential hazards:

• Examine regulators, pressure relief valves and cylinder connections.

• Ensure that cylinders are free of corrosion, leakage, pitting, dents or gouges.

• Regular equipment maintenance can prevent hazardous conditions in the workplace.

Ensure that maintenance personnel:

• Know the possible hazards of the materials they may encounter and any special procedures and precautions before they begin to work.

• Carry out repairs to equipment properly, using equipment suitable for the contents of the compressed gas cylinder.

• Avoid forcing connections, using homemade adaptors or tampering with cylinders in any way.

• Comply with applicable regulations and contact the gas supplier for advice.

What should I do in an emergency:

Act fast in emergencies such as chemical fires or gas cylinder leaks.

• Evacuate the area at once if you are not trained to handle the problem or if it is clearly beyond your control.

• Alert other people in the area to the emergency.

• Call the fire department immediately.

• Report the problem to the people responsible for handling emergencies where you work.

• Obtain first aid and remove all contaminated clothes if you have been exposed to harmful chemicals.

Note: All major compressed gas suppliers have emergency response teams. These teams can be activated by calling the telephone number that is usually printed on the shipping documents and MSDSs.

Locate emergency eyewash stations and safety showers wherever accidental exposure to gases that can damage skin or eyes is possible.

Only specially trained and properly equipped people should handle emergencies. Nobody else should go near the area until it is safe.

Planning, training and practicing for emergencies help people to know what they must do.

The MSDSs for the gases used are a starting point for drawing up an emergency plan. MSDSs have specific sections on spill and leak procedures, first aid instructions, and fire and explosion hazards. If the directions in each MSDS section are not clear or seem incomplete, contact the gas supplier or manufacturer for help. Many other sources can also help develop emergency plans. Local fire departments can assist with fire emergency plans and training. Occupational health and safety and environmental enforcement agencies, provincial safety associations, Compressed Gas Association Inc., St. John Ambulance, insurance carriers, professional societies in occupational health and safety, labour unions, some colleges and universities, and CCOHS can supply useful information. Specialized private consultants are also available.

Basic safe practices when working with compressed gases:

Following these basic general safe practices will help protect you from the hazards of compressed gases:

• Read the MSDSs and labels for all of the materials you work with.

• Know all of the hazards (fire/explosion, health, chemical reactivity, corrosivity, pressure) of the materials you work with.

• Know which of the materials you work with are compressed gases and check the label, not the cylinder colour, to identify the gas.

• Store compressed gas cylinders in cool, dry, well-ventilated areas, away from incompatible materials and ignition sources. Ensure that the storage temperature does not exceed 52°C (125°F).

• Store, handle and use compressed gas cylinders securely fastened in place in the upright position. Never roll, drag, or drop cylinders or permit them to strike each other.

• Move cylinders in handcarts or other devices designed for moving cylinders.

• Leave the cylinder valve protection cap in place until the cylinder is secured and ready for use.

• Discharge compressed gases safely using devices, such as pressure regulators, approved for the particular gas.

• Never force connections or use homemade adaptors.

• Ensure that equipment is compatible with cylinder pressure and contents.

• Carefully check all cylinder-to-equipment connections before use and periodically during use, to be sure they are tight, clean, in good condition and not leaking.

• Carefully open all valves, slowly, pointed away from you and others, using the proper tools.

• Close all valves when cylinders are not in use.

• Never tamper with safety devices in cylinders, valves or equipment.

• Do not allow flames to contact cylinders and do not strike an electric arc on cylinders.

• Always use cylinders in cool well-ventilated areas.

• Handle “empty” cylinders safely: leave a slight positive pressure in them, close cylinder valves, disassemble equipment properly, replace cylinder valve protection caps, mark cylinders “empty” or “MT,” and store them separately from full cylinders.

• Wear the proper personal protective equipment for each of the jobs you do.

• Know how to handle emergencies such as fires, leaks or personal injury.

• Follow the health and safety rules that apply to your job.

Click here to download the safety guidelines and check sheet 

compressed-gas-guideline

compressor-gas-cylinder

Signage : Gas cylinder

Introduction:

This guidance document provides advice on the prevention and mitigation of dust explosions and fires. Many materials we use everyday produce dusts that are flammable and in the form of a cloud can explode, if ignited. Examples are:

• sugar;

• coal;

• wood;

• grain;

• certain metals; and

• many synthetic organic chemicals.

Quite generally, the advice applies to anything which can burn, and which exists in a fine powdered form, unless tests show that particular hazards are not present. In some cases, a very simple knowledge of chemistry can rule out the explosion risk, eg in the case of sand, cement and sodium carbonate (soda ash).

Dust explosions are not new and records from over 100 years ago exist of incidents that have resulted in large loss of life and considerable and costly damage to plant and buildings.

Objectives :

• outline legislation;
• illustrate the effects of dust explosions;
• show how to prevent dust explosions;
• explain how to protect plant and equipment if an explosion occurs; and
• give advice on the particular hazards of fires within dust handling plants.

Why does dust explode?

A dust explosion involves the rapid combustion of dust particles that releases energy and usually generates gaseous reaction products. A mass of solid combustible material as a heap or pile will burn relatively slowly owing to the limited surface area exposed to the oxygen of the air.

If you have the same solid in the form of a fine powder and you suspend it in air as a dust cloud the result will be quite different. In this case the surface area exposed to the air is much larger, and if ignition occurs, the whole of the cloud may burn very rapidly. This results in a rapid release of heat and gaseous products and in the case of a contained dust cloud will cause the pressure to rise to levels which most industrial plant is not designed to withstand.

Although a cloud of flammable dust in air may explode violently, not all mixtures will do so. The concentration of dust and air must be within the upper and lower explosive limits for the dust involved.Measurements of the lower explosive limits of many materials are available, and for many organic materials the limit is in the range 10 – 50g/m3. A dust cloud of this concentration resembles a very dense fog. Upper explosive limits are difficult to measure accurately, and have little practical importance.

Effects of a dust explosion:

A dust explosion can result in:

• death or serious injury to workers;
• destruction of plant and buildings;
• a large fireball;
• secondary explosions; and
• fire.

When a dust cloud ignites in an enclosed volume it results in a very rapid rise in pressure within the container. The container may be an item of plant or a room of a building. Typical peak pressures in laboratory apparatus are in the range 8 – 10 bar. In normal circumstances the plant or building will not be strong enough to withstand the pressure from the explosion and it will fail in a sudden and uncontrolled manner.
Anyone close to exploding plant, or inside a room where an explosion occurs is likely to be killed or seriously injured. The plant or building will only survive if the design or other protective measures deliberately allows for the high pressures.

Prevent or mitigate the effect of a dust explosion:

Assessing the risk:

This task should be your starting point, and it can be addressed under a series of questions.

Is my dust capable of exploding?

Where could dense dust clouds form?

What could ignite them?

How likely is this?

What would be the consequences?

Who would be at risk?

Can we prevent the risk of an explosion altogether?

If this is not possible, what can be done to protect people, and minimise the consequences of an explosion?

Following the risk assessment the options should be considered in this order:

• Eliminate the risk.

• Provide controls to minimise the risk.

• Provide supplementary controls to mitigate the consequences.

The great majority of dust explosions start inside the process plant, and most of the control measures concern conditions inside the dust handling system. They can be grouped under the headings of:

• controls over dust cloud formation;

• preventing the explosive atmosphere by inerting;

• avoiding ignition sources; and

• plant controls, which may have various purposes.

How to reduce the risks:

Dust control is the single most important factor in preventing fires or explosions in any dust-generating workplace. Effective dust control can reduce the risk of a catastrophic fire or explosion in the workplace. However, if a dust control system isn’t designed or working properly, it may create conditions that would support a dangerous fire or explosion.

The most effective way to reduce the risk associated with combustible dust is to eliminate its source. If that’s not possible, you can use other risk controls. When you’re choosing risk controls, start by asking yourself the questions contained in the following steps, which are listed in descending order of effectiveness.

Elimination or substitution:

Eliminate the hazard by substituting a safer process or material where possible, which is the most effective control. Some questions to consider:

• Can you use a different material that does not produce combustible dust?
• Can you use a different process that does not generate any dust?

Engineering controls:

Make physical modifications to facilities, equipment, and processes to reduce exposure. Some questions to consider:

• Has your dust collection system been properly designed by an engineer?
• Is your dust being captured at the source?
• Is your dust collector located outdoors?
• Does your dust collector have appropriate explosion venting?
• Are you able to minimize ignition sources and contain sparks or flames?
• Are dust and product conveyance and handling systems designed to minimize dust from spreading?
• Can you use misting to keep dust from dispersing?

Administrative controls:

Change your work practices and policies and provide awareness tools and training to limit the risk of combustible dust. Some questions to consider:

• Have you developed a dust management program, including a risk assessment of all work areas where combustible dust is produced or may accumulate, all potential ignition sources, and all means of dust dispersion?
• Do you properly inspect, measure, and monitor dust accumulations?
• Do you have effective manual clean-up and housekeeping programs?
• Have workers received suitable training and orientation?
• Do you have an emergency preparedness plan?
• Have you developed and implemented safe work procedures to minimize fire and explosion risks?

Personal protective equipment:

Personal protective equipment is not an effective control measure for combustible dust explosions. However, where dust is produced you may also have other hazards to assess. Some questions to consider:

• Do workers have the proper respirators, eyewear, and protective clothing?
• Has personal protective equipment been tested to make sure it is working properly?

Control over the formation of dust clouds:

Sometimes the process can be designed to prevent or minimise the formation of a dust cloud inside the equipment. If your product is available as a paste, in dampened or pelletised form instead of fine powder the explosion risk may be avoided completely. Any movement of pelletised or granular material is likely, however, to produce dust by attrition.

Many types of process plant inevitably contain explosible clouds of dust. Cyclones or dust filters provided as part of a ventilation system concentrate the dust and are likely to contain an explosive atmosphere somewhere within them, even if the dust concentration in the extract ducting is well below the lower explosive limit. In some cases there are alternatives. For example, tray driers create a smaller dust cloud than fluid bed driers. Wet dust collectors avoid the cloud that is formed regularly inside a reverse jet dry filter.

Inerting:

This is a way you can prevent explosions by preventing the formation of an explosive atmosphere. In a substantially closed system the oxygen content of the atmosphere within the plant can be controlled at a safe level. You will normally need to determine the maximum safe oxygen content experimentally. This will vary with the type of inert gas and the chemical reactivity of the material being processed.

Inerting is only likely to be effective in a system that is fully enclosed, with a minimum number of places where air can enter. You need to consider how process materials will be added to or removed from the system. If air enters at this point, a purge cycle is likely to be needed before the process restarts.

Many factors will influence the overall reliability of an inerting system. For example,

• the location and number of atmospheric sampling points;
• type of sensor head;
• frequency of calibration of the sensor;
• contaminants in the system that interfere with sensor readings;
• provision of safe means of control or shutdown, if the oxygen concentration exceeds a predetermined level;
• adequate supplies of inert gas for all foreseeable needs;
• the number of locations where air may enter the plant;
• the safety margin allowed when setting control levels for oxygen;
• the reliability of any electronic control system;

Where inerting is used as a means of preventing explosions, the overall reliability of the system should be assessed.

Control over sources of ignition:

Careless use of welding, flame-cutting equipment or other hot work has caused many incidents. It is essential that before hot work begins you isolate the plant effectively to prevent fresh material entering, and clean it thoroughly. After the work is complete, the site should be watched for at least an hour, for signs that fire is growing from a smouldering deposit.

Sparks from hot work may travel a considerable distance, particularly if you carry out the work at a high level. You can greatly reduce the risk of ignition by adopting cold cutting methods. Commonly accepted best practice for hot work requires a permit-to-work system, with the permit issued by a responsible person before work commences.

Such permits need to set out clearly:

• your arrangements for handover,
• the allowable range of work,
• time limits on when the work may be done; and
• the precautions required.

Common ignition sources include:

• hot surfaces;
• naked flames;
• faulty or unsuitable equipment;
• overheating of moving mechanical plant eg by friction;
• impact sparks;
• electrostatic discharges;
• spontaneous heating; and
• smoking materials.

You may require additional precautions where combustible dusts and flammable solvent vapours are present together, eg in some drying or mixing processes in the chemical industry.

Exothermic decomposition, air oxidation or biological action may cause spontaneous heating in many materials. Careful control of maximum temperatures is necessary when you handle such materials in a hot process, such as drying.

Plant design and controls:

Various types of plant design and control may be important in controlling the risk or consequences of a dust explosion. This section cannot be comprehensive but highlights the type of process deviations that you need to control, preferably by continuously monitoring the plant. Examples are:

• Extensive centralised dust collection systems create many links through which burning material can spread following an explosion in the filter. This can be controlled, but filters drawing dust from just one or two locations reduce the risk more simply.

• Overloading or blockage of the feed system may cause some process plant to overheat. If this is possible, reliance on visual indication may not be adequate.

• Large volumes of dust may escape if filters fail, relief panels become loose or sacks being filled fall off a collection point. You may need to monitor the air pressure at appropriate points within the plant to identify such an event promptly.

• Where you provide local exhaust ventilation to control the release of dust from an operation you may find it necessary to interlock the process so that it can only run with the ventilation operating properly.

• Detectors are available which continuously monitor the product from a grinding plant or similar unit for sparks or glowing material. They can then activate a water spray downstream from the detector and extinguish potential ignition sources before they reach a large dust cloud in other parts of the plant.

• High-level alarms on bins or hoppers may be useful in preventing material being spilt. Many reliable types are now available.

• Deviations from a safe condition should cause automatic plant shut-down or the raising of an alarm. In the latter case the follow up action needs to be preplanned.

Explosion relief venting:

A simple and common method of protecting process plant against the consequences of an internal dust explosion is to provide one or more deliberate points of weakness. We call these explosion relief vents. If they are of suitable size and in the right place, they will safely vent an explosion within the plant. The intention is to prevent injuries to persons nearby by avoiding uncontrolled failure of equipment.

Extensive research over the last 20 years has provided soundly based calculation methods to determine the vent area required. To design an explosion vent you require:

1. the volume of the equipment to be protected;
2. the properties of the dust measured in a 20-litre or larger apparatus;
3. an estimate of the strength of the plant involved; and
4. the opening pressure of the relief panels.

The plant user supplies information about the properties of the dust whilst the equipment manufacturer or installer supplies the calculation of relief areas. Some manufacturers test a complete assembly of, for example, a filter, with its vent panels. Others may calculate the equipment strength and fit vent panels from a specialist supplier that have been separately tested.

Explosion suppression and containment:

Although the provision of explosion relief vents is the most widely used technique for protecting process plant from dust explosions, suppression and containment are equally valid alternatives. The choice of technique will depend not only on safety considerations, but also issues like cost, reliability, continuity of operation and keeping a plant free from contamination. Explosion venting will be inappropriate if the material is too toxic or environmentally harmful to release to atmosphere, or if there is no safe place to locate the vent outlet.

Dust explosions typically produce maximum overpressures in the range 8 to 10 bar. It is not generally practicable to produce plant capable of withstanding such pressures unless it is of small volume and simple circular or spherical shape. Hammer mills and certain other grinding equipment are however, often strong enough to contain an explosion; you will need to consider protection of the ductwork leading to and from them unless it is of similar strength. Plant operating under a vacuum, eg some types of drier, may also be strong enough to withstand the low explosion pressures that would result.

Explosion suppression systems allow the control of a developing explosion by the rapid injection of a suitable suppressing medium into the flame front. They have been developed into reliable systems over years of testing and operating experience. They are classed as autonomous protective systems and need certification and appropriate marking under the EPS regulations.

Plant siting and construction:

Where some risk of a dust explosion remains despite a high standard of control over sources of ignition, and provision of protective measures, the siting of unit(s) in the open air may minimise the consequences of an explosion.

Open air siting of dust handling process plant is strongly recomended:

• where the scale of the operation is large, such as large silos;
• where substantial sized plant, such as a dust filter, has a flammable, dust cloud inside it constantly during normal operation; or
• where a particularly severe explosion is possible, as with metal powders.

Methods to separate items of plant and so restrict this possibility include the use of:

• rotary valves;
• a choke of material in an intermediate hopper;
• screw conveyors with a missing flight and baffle plate;
• explosion suppression barriers; and
• explosion isolation valves.

Human factors:

Fires and explosions can occur even in the best designed plant if the people involved do not understand the hazards of the dust and the controls provided. The Dangerous Substances and Explosive Atmospheres Regulations require you to provide information for employees about risks and safety measures provided, together with adequate health and safety training. You should give all people involved in plants handling explosible dusts training in general terms about the nature and hazards of dust explosions, typical sources of ignition, safeguards provided, precautions to take and any emergency procedures on their plant. Particular points you should cover in such training are: the importance of good housekeeping, the need to report promptly any substantial release of material, or any equipment malfunction that could be a source of ignition.

You may need to restrict access to some areas while the plant is operating. This is easier to achieve where there is clear marking of the areas concerned. This type of arrangement is sometimes used for areas at the top of storage bins, where it has not proved possible to duct explosion vents to the outside. DSEAR also requires the access points to zoned areas to be marked with a yellow and black triangular Ex sign (see below), where the risk assessment shows it will have some benefit. Signs might help remind employees where special rules apply, for example on the use of portable electrical equipment, or define parts of the premises where office staff are not intended to have access because they have not been trained.

Area classification where dusts are handled:

Area classification is a technique intended to help people decide where specific controls over all sources of ignition are needed. It was originally developed to help with the selection of fixed electrical equipment, but its use has now been extended to any equipment that has hot surfaces or generates other possible ignition sources. Parts of buildings or process plant may be described as zone 20, 21 or 22, depending on the amount of time that an explosible dust cloud may be present. Equipment installed in a zoned area should then be built to an appropriate standard.

The zone definitions are contained in regulations, and are repeated as follows. These regulations bring in a new legal requirement to carry out area classification, where dusts are handled in quantity. In most plant handling dusts the inside of the dust equipment will be zone 20 or 21. Rooms within the building, if they need to be zoned, should only be the less onerous zone 22. A few very small areas where dust escapes in quantity in normal operation might need to be zone  21. In the open air, dust clouds are unlikely to persist for more than a brief period, and any zoning is likely to be very limited in extent.

Where dust layers are often present, explosible dust clouds can be formed by any sudden movement of air, except with products like sugar, which quickly absorb moisture from the air. Experience shows however, that while fires may easily start in dust layers on hot surfaces, very few explosions are caused by hot surfaces outside the dust containment system.

Dusty areas may extend well away from sources of release of dust unless you install local dust extraction to prevent this. Air currents will carry the finest dust particles a considerable distance and allow them to settle at high levels within a building. Dust deposits on beams and ledges at high level create a secondary explosion risk, but you should also be aware that surface deposits of dust might ignite on equipment that is designed to run hot, or may block ventilation holes or otherwise interfere with the cooling of electrical equipment.

Where the interior of a plant item requires regular illumination, you can almost always do this with the light source outside the plant. Mains powered portable lights should not be lowered into storage bins. Even if the light unit is designed for an explosive atmosphere, the cable might be easily damaged, and the risk is high. If illumination from the inside is needed, and a dust certified lamp is not available, battery-powered lamps certified for use in gaseous flammable atmospheres are unlikely to cause ignition. If, however, they are dropped and buried in a heap of dust some high powered types could overheat and start a fire.

Frictional heating of moving parts of process plant may raise the temperature locally to the point where ignition of a dust occurs without any spark or flame. Bucket elevators have proved vulnerable to this problem, as have hammer mills and rotary atomisers on milk spray driers. Modern plant may have features designed to prevent or detect such problems eg ammeters on motors to indicate overloading. Inadequate maintenance can negate the effectiveness of these features.

Equipment used in classified areas:

Electrical and non electrical equipment supplied after June 2003 that creates a potential ignition risk and is designed for use in explosive dust atmospheres, is subject to specific regulations. Such equipment should be marked with the sign of explosion protection (see below), a category number (1,2, or 3) followed by the letter D for dust, a temperature rating and other codified identifying marks. The temperature rating may be expressed as a T class (eg T4 or T6) or an actual temperature. Details of the marking scheme are contained in standards.

There is rarely any need to site power-consuming electrical equipment inside an area classified as zone 20. If you need to install electrical equipment where it will be buried in dust (eg inside a storage bin) you should consult the equipment supplier.

It is preferable to site electrical equipment away from dusty areas, but where you install equipment close to sack tipping points, sanding machines, sampling points or similar foreseeable dusty areas that are classified as zone 21, new equipment should meet the requirements for ATEX category 2D. Existing equipment made to older standards such as BS 6467, or with a dust tight enclosure made to IP6X (see BS EN 60529) is still likely to be suitable.

You are likely to need ignition protected equipment in areas inside buildings around process plant handling flammable dusts which are classified as zone 22. In this situation new equipment built to ATEX category 3D requirements will be suitable. Older equipment made with a dust resistant enclosure to IP5X may remain in service.

Area classification, Zones definitions:

Zone 20

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.

Zone 21

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.

Zone 22

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 occur, will persist for a short period only.

Mitigation measures:

The most important mitigation measure is maintaining the process buildings in a clean condition. If you allow dust deposits to accumulate, they can provide the fuel for a secondary explosion. Dust deposits shaken into suspension from all the ledges within a room by a small primary explosion may then ignite. You only need comparatively small amounts, and a layer of flour 0.3mm thick on the floor can in principle fill a room with an explosible dust cloud up to 3m above floor level.

The first step towards preventing dust accumulations within a building is to maintain a plant in a leak-tight condition. Loosely bolted flanged joints, damaged flexible seals and ill-fitting or propped open access hatches are common sources of leaks. Some processes can be operated at slightly below atmospheric pressure to reduce the escape of dust.

Despite this, the building will require regular cleaning, and the preferred method is a vacuum system rather than brushes and shovels, which tend to raise dust clouds. You should avoid the use of compressed air lines to dislodge dust deposits, as this will cause unnecessary dangers by creating dust clouds. There is no general preference between mobile vacuum cleaners and a centralised system. Depending on the design of the building, both may have their place.

You can reduce the labour involved in cleaning by designing plants and buildings with the minimum number of horizontal ledges on which dust can settle, and sufficient access platforms to avoid the need for temporary platforms. Do not neglect the highest parts of buildings as these are the areas where the finest and most hazardous dust can be found.

Electrical apparatus may be particularly prone to overheating if dust deposits accumulate and the standards10 assume that dust deposits will never be more than 5mm thick. If you cannot control dust accumulations to this thickness, you should obtain special advice from the equipment supplier.

Where filtered dusty air is returned to a workroom, it is important to ensure that this does not significantly increase the exposure of an individual employee to the dust. Health limits for dusts are typically a thousand times less than explosion limits, and you should, therefore, consider the effect of recirculation in any assessment made under the Control of Substances Hazardous to Health Regulations. The failure or partial failure of a filter may greatly increase exposure to dust unless there is prompt detection of the fault. Dust filters may not remove volatile materials and where these are present a further assessment of the health risks is needed. A badly designed air recirculation system may also adversely affect worker comfort.

We can group more technical measures to mitigate an explosion into the following main categories:

• explosion relief venting;

• explosion suppression and containment; and

• plant siting and construction.

In 1981 an explosion at a plant in Banbury which manufactured custard powder injured nine men and caused substantial damage to an external wall of the building9. A fault in a pneumatic conveying system caused a holding bin to overfill and the air pressure caused the bin to fail. The released custard powder ignited as a dust cloud within the building.

An explosion initiated in the dust collector of a grain storage facility at Blaye in France. The towers contained elevators and the gallery over the 44 silos contained belt conveyors. All the areas were open allowing the spread of dust clouds and flames. Both towers, the gallery and 28 silos were completely wrecked with the loss of 11 lives.

One man died following an explosion in a plant that manufactured powdered aluminium. Part of the process used nitrogen to maintain an inert atmosphere but system controls were rudimentary and inadequate to detect blockages caused by powder collecting in the nitrogen supply pipework.

Magnesium Grinding and Polishing:

Magnesium is rated as an St 3 dust, which means that any explosion will be very severe. If you are involved in the special case of grinding or polishing Magnesium you should ensure that:

• None of the equipment has been used previously for abrading iron or other ferrous material.

• There is a dust extraction system leading to a scrubber where the dust-laden air is drenched with water. It is usual to provide a separate scrubber for each grinding or polishing device. The scrubber will need cleaning out at least once a week and tools containing iron or ferrous material should not be used. The scrubber should have a high level vent to avoid accumulations of hydrogen.

• Duct work carrying grinding or polishing dust is kept as short as possible, with few crevices to retain dust. It should also be possible to inspect and clean the inside surfaces.

• You dispose of any dust collected by removing from site or by burning in a controlled manner.

• Wet sludge is stored outside where gas evolved may disperse safely.

Open air siting of dust handling process plant is strongly recomended:

• where the scale of the operation is large, such as large silos;

• where substantial sized plant, such as a dust filter, has a flammable, dust cloud inside it constantly during normal operation; or

• where a particularly severe explosion is possible, as with metal powders.

Click the below link to know and download about combustible dust safety check sheet

comb-dust-safety-check-sheet

combustible_dust

combustible-dust-handout-tbt