Common Industrial Processes PDF

**COMMON INDUSTRIAL PROCESSES** **9.1 INTRODUCTION** There is a wide range of industrial processes that use and produce hazardous substances. In the following sections a number of common processes are briefly described. The purpose of this is to illustrate typical exposure scenarios and ways in wh...

**COMMON INDUSTRIAL PROCESSES** **9.1 INTRODUCTION** There is a wide range of industrial processes that use and produce hazardous substances. In the following sections a number of common processes are briefly described. The purpose of this is to illustrate typical exposure scenarios and ways in which exposure to hazardous substances commonly occurs. It is not intended to be a comprehensive listing of industrial processes, nor is it intended to identify all the potential hazards likely to be present in the particular processes. The range of processes selected has been chosen to cover a wide range of industry sectors and should provide the student with an overview of many of the workplace hazards commonly encountered. **9.2 MATERIALS HANDLING** The following section looks briefly at a range of handling operations for both solids and liquids. These handling operations are found in many industries (e.g. paints, inks, pesticides, pharmaceuticals, polymers, chemicals, mining, agriculture etc) and are often the source of exposure to hazardous substances. In accordance with good industrial hygiene practice consideration should always be given to prevention of exposure. This may be by elimination or substitution of the substance with a less hazardous material. Where this is not practicable a combination of engineering, process and operational controls, together with instruction and training may be required to achieve adequate control. In many operations, local exhaust ventilation may be necessary and as a last resort all the above may need to be supplemented by the use of personal protective equipment. 128 **9.2.1 Handling of solids and powders** Dust generation under normal working conditions depends essentially on movement of some sort. This may occur at all stages of the process from delivery of the material to the factory through various transfers, weighing, sieving, filling and mixing operations. **Selection of materials** - In some situations, dust generation can be reduced or eliminated by using dust reduced forms of the substance which contain small quantities of a binder. In addition the material may be supplied in the form of pellets, granules or flakes which are less liable to generate airborne dust. Completely dust free forms such as pastes or powders dispersed in liquids may also be available. All of these options are dependant on whether the reduced dust form of the substance is compatible with the process requirements. **Bulk powder delivery and transfer -** Large quantities of bulk material may be supplied in bulk road or rail tankers or in tote bins, which are then discharged into silos. Delivery from tankers by pneumatic conveying is efficient and clean. However, it is reliant on the quality of the seals between the tanker and the discharge point, and between the tote bin and the silo. These seals and the flexible ducting need to be maintained in good condition to prevent leaks as the ducting may be at positive pressure. Direct feed of bulk materials from storage silos to mixing may avoid the need to handle the powder. While these systems are generally totally enclosed, there may be a need to incorporate vents to account for air displaced during silo filling and powder transfer. This displaced air which will contain high levels of airborne dust must be cleaned prior to discharge. Bulk powder transfer is often undertaken by moving belt conveying systems. These usually involve a number of transfer points at which dust can become airborne. The amount of dust generated can be minimised by reducing the height of the drop from the conveyor to a minimum and by 129 reducing any cross draughts. Conveyor transfer points may need to be partially enclosed with local exhaust ventilation incorporated. Bulk materials may also be delivered in paper or plastic bags; intermediate bulk containers (IBC's) such as aluminium totes or octabins or woven polyurethane sacks. Whenever dusty materials are transferred, handled or used, spillages may occur. It is important that there are good standards of housekeeping and cleanliness to minimise exposures and contamination. Wet wiping methods and use of industrial vacuum cleaners are preferred to the use of brushes which will render airborne settled dust deposits. (Source: Steve Bailey -- Reproduced with permission) ***Figure 9.1 -- Split bag containing sodium carbonate*** **Dust generation at powder handling processes -** Dust generation can occur at a number of different processes including weighing, sieving, charging (filling) of vessels and mixing (formulation). Whenever containers are charged or filled with powders air, which may be dust-laden, is displaced from the container. Depending on the amounts and toxicity of the powders being handled full or partial enclosure may be required incorporating local exhaust ventilation. 130 (Source: Steve Bailey -- Reproduced with permission) ***Figure 9.2 -- Manual loading of open mixer from drum*** A common potential problem is opening and emptying of bags containing powders. The use of enclosed and automated bag and powder handling plant can significantly reduce dust exposures. Systems are available that automatically open, empty, evacuate and bale empty bags. Weighing of mix ingredients may also be undertaken automatically in an enclosed system; however, if manual weighing is required the use of ventilated booths or laminar flow cabinets may be appropriate for hazardous substances. Sieving of some products may be required to break up agglomerates or ensure that only products of a certain size range are included. If the process is not fully enclosed, local exhaust ventilation is usually needed. Vibratory action is often used and planned maintenance of sieves, joints and connections is important. 131 **9.2.2 Handling of liquids** As with solids, exposures to liquids and their vapours can occur at a range of common operations. Bulk liquids may be supplied in bulk road or rail tankers or in drums which may be transferred to storage tanks. Transfer from tankers by enclosed transfer is efficient and clean. However, it is reliant on the quality of the seals between the tanker and the discharge point. These seals and the pipework and hoses need to be maintained in good condition to prevent leaks. Seals and joints on pipes, valves and pumps also need to be maintained in good condition to prevent leaks and fugitive emissions. (Source: Steve Bailey -- Reproduced with permission) ***Figure 9.3 -- Delivery of methanol from tanker*** Direct feed of liquids from storage tanks to point of use may avoid the need to come into contact with the liquid. While these systems are generally totally enclosed, there may be a need to incorporate vents to account for air displaced during tank or container filling. This displaced air may contain high levels of solvent vapour which should be recovered or captured using local exhaust ventilation. Exposure to liquids may also occur when pipework and vessels need cleaning or maintenance. Systems need to be in place to ensure they are effectively purged of hazardous materials prior to work. 132 Whenever containers are charged or filled with liquids such as solvents; air which may contain high levels of solvent vapour is displaced from the container. Depending on the amounts and toxicity of the liquids being handled full or partial enclosure may be required incorporating local exhaust ventilation. Precautions need to be in place whenever liquids are transferred to minimise the potential for spills and splashes. Other simple precautions such as use of lidded containers and closing containers can reduce exposure to vapours from evaporation of liquids. (Source: Steve Bailey -- reproduced with permission) ***Figure 9.4 -- Paint bench -- open tins and rags soaked in thinners can be a source of exposure*** **9.3 WORKING WITH METALS** There is a range of different techniques that may be used to process and fabricate metal products. This includes grinding, machining and welding of metals and metal alloys. **9.3.1 Grinding** Grinding involves the use of a bonded abrasive to wear away parts of a work piece to correct its dimensions, remove imperfections or increase the smoothness of a surface. Natural abrasives (e.g. diamond and sandstone) 133 have been largely replaced by artificial abrasives. These include aluminium oxide, silicon carbide (carborundum) and synthetic diamonds. (Source: HSE: Working with us on Noise and Hand-Arm Vibration 2008 -- Reproduced under the terms of the Click-Use licence) ***Figure 9.5 -- Grinding*** A variety of tools may be used to undertake grinding including grinding wheels as well as grinding discs and belts. Historically, a high incidence of silicosis occurred with the use of sandstone grinding wheels. Modern grinding wheels are usually made of corundum (an oxide of aluminium) and do not contain significant amounts of crystalline silica. However, silica dust may still be produced from materials being ground -- e.g. sand residues on sand castings. Local exhaust ventilation is required for most grinding, belt sanding and finishing operations. (Source: Steve Bailey -- Reproduced with permission) ***Figure 9.6 -- Grinding*** 134 In addition to the risks from inhalation of airborne dusts, grinding operations may also generate high noise levels and possibly significant levels of vibration leading to a risk of hand-arm vibration syndrome (HAVS), also known as "white finger". **9.3.2 Machining of metals** Machining of metal products is undertaken using a variety of machines including lathes, drills and saws. These processes generally produce relatively large metal particles and so airborne dust generation is low. Grinding processes produce much finer dust and are therefore of more concern in this respect. The main health hazard from machining is related to cuttings fluids which lubricate and cool the process. There are three main types of cutting fluids used; - Mineral oils -- mineral oils of various viscosities are used together with additives to provide specific characteristics - Soluble oils -- soluble water-in-oil cutting fluids are mineral oils that contain emulsifiers and additives including rust inhibitors and bactericides. They are diluted with water in varying ratios before being used - Synthetic fluids -- synthetic cutting fluids are solutions of non-petroleum based fluids, additives and water Exposure to cutting fluids during work or maintenance may cause contact dermatitis. Water-based cutting fluids may contain bacteria and cause infections, and the emulsifiers may dissolve fats from the skin. Oil folliculitis can occur with prolonged exposure to oil-based cutting fluids. Dermatitis is best controlled by good hygiene practices and minimising exposure. 135 1. A gas flame produced by the combustion of fuel gas such as acetylene or propane with air or oxygen An electric arc, struck between an electrode and a workpiece or between two electrodes Resistance to passage of electric current Occupational exposure to oil mists and aerosols may cause a variety of respiratory effects, including asthma, chronic bronchitis and impaired pulmonary function. **9.3.3 Welding and thermal cutting** Welding is a generic term referring to a process by which metals are joined together. This is often achieved by melting the metal in the area to be welded and adding a filler material to form a pool of molten metal which solidifies on cooling to form a strong joint. Common sources of heat are: **Gas welding --** this generally uses a combination of oxygen and acetylene. The gases are fed to a hand held welding torch to produce a flame. The heat melts the metal faces of the parts to be joined, causing them to flow together. A filler metal or alloy with a lower melting point than the parts to be joined is usually added. Chemical fluxes are used to prevent oxidation and facilitate the welding process. **Arc welding** -- an electric current is used to strike an arc between an electrode connected to an electric supply and the materials to be welded. The arc can generate temperatures up to about 4,000°C when the pieces to be welded fuse together. As with gas welding a filler metal or alloy is added to the joint. This may be achieved by melting the electrode (consumable electrode process) or by melting a separate filler rod which is not carrying the electric current (non-consumable electrode process). 136 To ensure a strong weld is achieved it is vital that the welding area is shielded from the atmosphere to prevent oxidation and minimise contamination. The two main ways that this protection can be achieved are by use of a flux or by use of an inert gas shield. (Source: Steve Bailey -- reproduced with permission) ***Figure 9.7 -- Arc welding*** In flux-shielded arc welding the consumable electrode consists of a metal core surrounded by the flux as a coating. The metal electrode core acts as the filler material for the weld. The flux coating also melts covering the molten metal with slag and by generating carbon dioxide envelopes the welding area with a protective atmosphere. In gas-shielded arc welding, a blanket of inert gas (e.g. argon, helium, nitrogen or carbon dioxide) seals off the atmosphere and prevents oxidation and contamination during the welding process. The two most common types of gas-shielded arc welding are metal inert gas (MIG) welding sometimes known as gas metal arc welding (GMAW) and tungsten inert gas (TIG) welding. 137 **Resistance welding** -- this uses the heat generated by the resistance to the passage of a high electric current through components to be welded. Heat generated at the interface between the components brings them to welding temperatures and small pools of molten metal are formed at these points. A common type of resistance welding is spot welding of thin metal sheets. Two electrodes simultaneously clamp the sheets together and electric current passes through the sheets. Weld strength is lower for this type of welding but it is very easily automated and is energy efficient. It is widely applied by industrial robots as in car manufacturing. **Health hazards of welding** -- airborne contaminants (including fumes and gases) from welding and flame cutting arise from a variety of sources: - the metal being welded, the metal in the filler rod or constituents of various types of steel (e.g. nickel or chromium) - any metallic coating on the article being welded (e.g. zinc and cadmium on plated metals) - any paint, grease or dirt on the article being welded (e.g. carbon monoxide, carbon dioxide and other irritant breakdown products) - flux coating on the filler rod (e.g. inorganic fluoride) - action of heat or ultraviolet light on the surrounding air (e.g. nitrogen dioxide, ozone) or on chlorinated hydrocarbons (e.g. phosgene) - inert gas used as a shield (e.g. carbon dioxide, helium, argon) Fumes and gases should be removed at the source by provision of local exhaust ventilation. Where there is still a risk to health from toxic fumes 138 and where local exhaust ventilation is not practicable a good standard of respiratory protective equipment is necessary. Additional hazards can occur when welding is undertaken in confined spaces such as tanks and vessels and ventilation of the confined space is important. This is required to remove contaminants produced by welding and also to maintain sufficient oxygen levels. Gas welding uses up oxygen as part of the process and gas shielded arc welding introduces inert gases that can displace the oxygen in the space. Entry should only be permitted after the area has been certified as safe as part of a permit to work system. Metal fume fever is associated with exposure to zinc oxide or copper oxide fumes which can occur in areas such as brass founding and welding of galvanized (zinc coated) steel. It is an acute effect that can occur within a few hours of exposure. Flu-like symptoms include cough, nausea and chills and fever. Recovery is usually complete within a day or two. It particularly affects people new workers and people returning to work after a break. In addition to the chemical hazards other physical hazards are likely to be present. High noise levels may be generated in several welding processes. Intense levels of ultraviolet light are emitted by electric arc welding. Even momentary exposure to ultraviolet radiation may produce a painful conjunctivitis (photokeratinitis) known as "arc eye". Exposure to the welder is prevented by use of visors or goggles that prevent passage of ultraviolet light and by use of screens to prevent exposure of others in the area. High levels of infra-red radiation may be produced from the hot surfaces leading to potential thermal stress issues. 139 (Source: Steve Bailey -- reproduced with permission) ***Figure 9.8 -- Arc gouging or cutting*** The following table summarises the main types of welding and any specific health hazards. 140 ----------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------- ***Table 9.1 Main types of welding and specific health hazards* Type of welding** **Description** **Specific health hazards** **Gas welding and cutting** Gas welding The flame melts the metal surface and filler rod to form a joint Metal fumes, nitrogen dioxide, carbon monoxide Gas cutting The metal is heated by a flame and is directed onto the point of cutting and moved along the line to be cut Metal fumes, nitrogen dioxide, carbon monoxide **Flux-shielded arc welding** Manual metal arc welding (MMA) Uses a consumable electrode consisting of a metal core surrounded by a flux coating Metal fumes, fluorides, ultraviolet radiation; ozone, nitrogen dioxide Submerged arc welding (SAW) Uses a consumable bare metal wire electrode. Granulated flux is deposited on the workpiece, which melts to produce a protective shield in the welding zone. Metal fumes, fluorides, ultraviolet radiation, ozone, nitrogen dioxide **Gas-shielded arc welding** Metal inert gas (MIG); gas metal arc welding (GMAW) Uses a consumable metal wire electrode of similar composition to the weld metal. It is fed continuously to the arc. Metal fumes, ultraviolet radiation, ozone, nitrogen dioxide, potential build up of inert gas Tungsten inert gas (TIG); gas tungsten arc welding (GTAW) The tungsten electrode is non-consumable, and filler metal is introduced as a consumable into the arc manually. Metal fumes, ultraviolet radiation, ozone, nitrogen dioxide, potential build up of inert gas **Electric resistance welding** Resistance welding (spot, seam, projection or butt welding) High electric current at low voltage flows through the components from electrodes generating heat at the interface. Heat and pressure by the electrodes produces a weld. No flux or filler metal is used. Metal fumes, ozone ----------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- ---------------------------------------------------------------------------------------------- to a product e.g. nickel to protect against corrosion, chromium to improve the surface properties. The product, wired as the cathode, and an anode of the metal to be deposited are immersed in an electrolyte solution (which can be acidic, alkaline or alkaline with cyanide salts) and connected to a direct current. (Source: Steve Bailey -- reproduced with permission) ***Figure 9.9 -- Electroplating bath with detergent foam*** 142 Positively charged metal ions migrate from the metal anode to the cathode, where they are reduced to the metal and deposited as a thin layer. This continues until the new coating reaches the desired thickness, and the product is then washed, dried and polished. (Source: Steve Bailey -- reproduced with permission) ***Figure 9.10 -- Electroplating bath fitted with local exhaust ventilation*** Galvanizing applies a zinc coating to a variety of steel products to protect against corrosion. Again, the product must be clean and oxide-free before processing in order that the coating adheres properly. This usually involves a number of cleaning or annealing processes before the product enters the galvanizing bath. **Chemical hazards of electroplating and galvanizing** -- the heated alkaline and acid solutions used in cleaning and treatments of metals are corrosive. They can cause burns and irritation to the skin and to mucous membranes and the eyes. Nitric and hydrofluoric acid are a particular problem if inhaled, as it may be many hours before the effects on the lungs become apparent. Pneumonitis and potentially fatal pulmonary oedema may appear later in a worker who suffered little initial effect from the exposure. Skin contact with hydrofluoric acid can cause severe burns without pain being noticed for several hours. 143 Baths containing cyanide solutions are frequently used in electrolytic degreasing and electroplating. If cyanides react with an acid hydrogen cyanide can be generated. Fatal exposures may also result from skin absorption or ingestion of cyanides. Chromic and nickel compounds are used in electroplating. Chromium compounds are usually present in the form of chromic acid, where the chromium is in its hexavalent state. This can cause burns and ulcers to the skin as well as perforation of the nasal septum. Nickel salts can cause allergic dermatitis and irritation. There is evidence that both chromium and nickel compounds are carcinogenic. **9.5 SOLDERING AND BRAZING** Soldering and brazing are similar processes where metal items are joined together by heating and melting a filler metal or alloy with a flux that flows into the joint. They differ from welding in that the metals being joined do not melt during the process as the filler metal has a much lower melting point. Soldering and brazing are distinguished from each other in that soldering is undertaken at temperatures below 450 0C, whereas brazing is undertaken at temperatures above 450 0C. Health hazards depend on the metal or alloys being melted and the type of flux used. Soft soldering using lead / tin alloy based solders is widely used in the electrical and electronics industries to make electrical connections. 144 (Source: Steve Bailey -- reproduced with permission) ***Figure 9.11 -- Soldering*** As soldering is undertaken at lower temperatures metal fumes are generally of little concern. The main health risk from soldering is from the breakdown products of the flux. The flux is commonly rosin based and the breakdown products from this are potent respiratory sensitizers. (Source: HSE Rosin-cored solder fume assessment: Health and Safety Laboratory -- reproduced under the terms of the Click-Use licence) ***Figure 9.12 -- Fumes from soldering*** Rosin is a naturally occurring, solid, resinous material obtained from pine trees. The flux helps the soldering by cleaning the surfaces to be joined, increasing the flow of the solder and preventing oxidation. 145 The main health effects occur from the breakdown at high temperatures (or pyrolysis) of the rosin. - When heated, particularly to temperatures above 200°C, rosin-based solder fluxes form fumes containing a range of resin acid particulates and other components as gases. Lower temperatures can significantly reduce the amount of fume produced. Between 250°C and 400°C particulate fume levels can triple. - When inhaled, rosin-based solder flux fume can lead to occupational asthma. Rosin-based solder flux fume is regarded as one of the most important causes of occupational asthma. The effects are permanent and irreversible. Continued exposure, even to very small amounts of fume, may cause asthma attacks and the person affected may not be able to continue work with rosin-based fluxes - The fume can also cause irritation to the eyes and upper respiratory tract. It has not been possible to identify a safe level of exposure below which occupational asthma will not occur. Exposure to all rosin-based solder flux fumes should, therefore, be avoided or kept as low as is reasonably practicable - Without effective control, solder fume rises vertically and for manual operations is likely to enter the breathing zone of the worker. If not captured at source other people may be affected by build up of background levels. Adequate control often requires the use of local exhaust ventilation; this may be conventional LEV or a low volume high velocity system directly applied near the soldering iron tip. Health surveillance may also be required - Soldering is also used to form joints in plumbing and pipe fitting. Use of a gas torch can raise temperatures high enough to produce lead fume. The use of lead free solders is sometimes required, but in either case, the risks from breakdown of the flux are still present 146 In general brazing uses borax or fluoride based fluxes. In 'silver soldering' -- a type of brazing -- the metal alloy or flux may contain significant quantities of cadmium. Cadmium free replacements are available with alloys containing silver, copper and tin. The fume can also cause irritation to the eyes and upper respiratory tract. It has not been possible to identify a safe level of exposure below which occupational asthma will not occur. Exposure to all rosin-based solder flux fumes should, therefore, be avoided or kept as low as is reasonably practicable o Without effective control, solder fume rises vertically and for manual operations is likely to enter the breathing zone of the worker. If not captured at source other people may be affected by build up of background levels. Adequate control often requires the use of local exhaust ventilation; this may be conventional LEV or a low volume high velocity system directly applied near the soldering iron tip. Health surveillance may also be required o Soldering is also used to form joints in plumbing and pipe fitting. Use of a gas torch can raise temperatures high enough to produce lead fume. The use of lead free solders is sometimes required, but in either case, the risks from breakdown of the flux are still present 146 In general brazing uses borax or fluoride based fluxes. In 'silver soldering' -- a type of brazing -- the metal alloy or flux may contain significant quantities of cadmium. Cadmium free replacements are available with alloys containing silver, copper and tin. 9.6 DEGREASING Many industrial products need to be cleaned at one or more stages of their manufacture to remove unwanted dirt and grease that would interfere with subsequent processes such as painting, plating or soldering where cleanliness of the article is essential. Solvents are used for degreasing a wide range of items from large metal panels to electronic components. Degreasing processes can be divided into two broad categories -- cold degreasing and vapour degreasing. Cold degreasing uses a solvent at ambient temperature. It includes hand cleaning by wiping and brushing as well as larger operations that use spraying of the solvent. In some cases immersion and use of ultrasonic cleaning is undertaken. Vapour degreasing involves immersing the article in solvent vapour that condenses on the article and runs off taking the grease and dirt with it. 9.6.1 Cold degreasing Dip cleaning is the simplest operation where the articles to be cleaned are immersed in the solvent. The cleaned articles are then removed by lifting out of the solvent (preferably in wire baskets so that they can freely drain excess solvent). Removal should be carried out slowly to allow sufficient drain time. Covers should be kept on dip tanks as far as possible to reduce emission of solvent vapours into the workplace. 147 Wiping is more applicable for cleaning large items of equipment but solvent use should be kept to a minimum. Used cloths containing solvent residues should be stored in lidded containers and disposed safely. Brush cleaning can be used to dislodge particles which are not effectively removed by dipping or wiping. The item to be cleaned is usually placed in a tray to collect the solvent. Care needs to be taken to minimise splashing of the solvent. Spraying is useful to clean areas that are inaccessible to wiping and brushing - it may avoid the need to dismantle components for cleaning. However, spraying can lead to high solvent vapour concentrations. 9.6.2 Vapour degreasing Vapour degreasing is an effective and widely used technique for cleaning of components. The vapour degreasing plant consists of a steel tank partly filled with solvent with a heater at the base. The solvent vapour rises to fill the tank to a height determined by a series of cooling coils in the upper part of the tank that act as a condenser. There is usually is a slotted duct (lip extraction) around the rim of the tank connected to an exhaust ventilation system to prevent escape of vapour from the tank. 148 (Source: Envirowise GG354 -- Surface cleaning and preparation: choosing the best option -- reproduced with permission) Figure 9.13 -- Conventional top loading vapour degreaser When a 'dirty' component is lowered into the vapour layer formed above the heated solvent, the vapour condenses on the cold surface of the component and dissolves any soluble contaminants. As the vapour condenses, the liquid drains back into the boiling solvent, carrying some of the 'dirt' with it. This process continues as more vapour condenses on the component. When the temperature of the component reaches that of the vapour, condensation on the component effectively ceases and the cleaning process stops. The cleaned components are then lifted slowly to drain fully while still within the tank. Various degrees of automation can be applied and plant can be integrated into conveyor production lines. It is then removed and allowed to cool to normal temperature. The soluble and insoluble dirt removed collects in the sump at the bottom of the plant. When the concentration of oil and dirt in the sump reaches a certain level, the solvent is recovered by distillation and may be used again. After cooling the oily residue can be removed and discarded. 149 (Source: Steve Bailey -- reproduced with permission) Figure 9.14 -- Large vapour degreasing tank fitted with cooling coils and slot extraction at the rim Degreasing solvents that are commonly used in industry include trichloroethylene and tetrachloroethylene. For specific applications, various products containing individual solvents, mixtures of solvents or emulsions are available. Poor operating methods and inadequate maintenance can lead to increased solvent emission in the working area. Common operating faults include: o poor stacking of hollow bodies -- e.g. tubes / cups should be stacked at an angle or rotated within the vapour zone to empty any solvents trapped in the article o insufficient drying time -- the components must be held in the 150 freeboard above the vapour layer for sufficient time to allow evaporation of solvent residues o lifting the components at too high a speed o excessive movement of components in the bath causing disturbance of the solvent vapour layer Procedures need to be in place during maintenance and removal of sludge from the base of the tank. The tank may be considered a confined space and high concentrations of solvent vapour may be present. Safe work procedures including a permit to work may be required to prevent potentially fatal exposure. It should be noted that chlorinated solvents are readily decomposed by hot surfaces and open flames. The toxic decomposition products include phosgene, hydrogen chloride and carbon monoxide. Smoking and hot processes such as welding should be prohibited in the vicinity of the degreasing operations. 9.7 PAINTING Paints and related products such as varnishes and lacquers are widely used in industry to provide a surface coating for corrosion protection, for appearance or other special purposes. Health risks can arise from solvents, additives or in particular respiratory sensitisation from isocyanates in polyurethane paints. The risks vary considerably in because of the variety of products and constituents, the range of situations in which they are used and the different methods of application. Solvent exposure is usually the most significant health risk during painting. The solvents used vary considerably between products and include aliphatic and aromatic hydrocarbons, ketones, alcohols, glycols and glycol ethers and esters. The use of low solvent content or water 151 based paints is becoming more widespread. (Water based paints still contain some solvents but these are at much lower concentrations). The amounts of solvent in paints used vary considerably (e.g. 5 - 50%). However, the paint may be thinned at the point of use, and the thinner may be of different composition to the primary solvent. Paint application methods fall into three main groups: o paint is applied to the surface by brush or roller o paint is applied by spray techniques o the surface is immersed in paint and allowed to drain, as in dipping and flow coating. In general, paint for spray application will contain more solvent than those for application by brush or roller. 9.7.1 Exposure to solvents in painting Painting with brush or roller involves relatively slow application of paints. The operator continually moves away from the most recently treated area and exposure largely depends on the general ventilation of the area. Spray painting generally involves higher rates of application, larger areas and higher solvent content. Also spraying of liquids containing solvents can produce high levels of solvent vapour as the fine spray produced allows rapid evaporation of the solvent. The use of respiratory protective equipment may be required if the area is enclosed or because of other constituents, e.g. applying anti-fouling paint containing pesticide to the hull of a ship in dry dock or spraying an isocyanate paint to vehicles in a closed booth or to an aircraft in a hangar. 152 Spray painting of smaller items on a production line may be done with an automatic spray gun or by an operator from outside a ventilated open-fronted booth. Conventional air spraying is the most commonly used method of painting in industry. A potential problem with air spraying is that large quantities of solvent vapour may be generated and overspray of the paint is common. (Source: HSE LEV Trainer Adviser Briefing Days -- reproduced with permission) Figure 9.15 -- Spraying isocyanate based paint in paint spray booth Electrostatic spraying; which has become more widespread places a charge on the paint mist particle so it is attracted to the part to be painted. This reduces rebound and overspray and consequently reduces mist and solvent exposure to the operator. In powder coating, the paint powder is conveyed from a powder reservoir to the spray gun. In the gun, an electrical charge is imparted to the paint particles. The powder is sprayed onto the electrically grounded item to be coated and the parts baked to fuse the powder into a continuous coating. Most industrial flow and spray painting operations require exhaust 153 ventilation for control of solvent vapors at the point of application and also during drying and baking operations. The use of automated spray painting allows for greater use of ventilated enclosures and minimises worker exposure. As stated earlier, particular care must be taken in the application of two-component urethane and epoxy paint systems that contain isocyanates which are potent respiratory sensitisers. This includes effective ventilation control and protective clothing. In addition where exposure cannot be effectively controlled by ventilation the operators should wear air-supplied respirators. Dermatitis due to primary irritation and de-fatting from solvents or thinners is common. Skin contact must be minimised, adequate washing facilities should be available, and suitable protective equip-ment used by the operator. (Source: Steve Bailey -- reproduced with permission) Figure 9.16 -- Paint mixing -- skin and airborne exposure can occur to solvents and pigments In addition to the hazards from the solvents, there may also be 154 hazards from pigments such as lead, cadmium and chromium compounds. Also some paints contain additives such as fungicides and pesticides.