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5. Key hazards in the operation of thermal spray equipment

5.1 Introduction

5.1.1 In this section, the key hazards associated with thermal spraying are described. The section includes a brief description of the hazards and their possible consequences. In any risk assessment for a thermal spray process a consideration of all relevant hazards will need to be included. Ancillary activities that are likely to be undertaken, such as grinding, grit blasting, solvent cleaning, machining, etc, are not included in this booklet.

5.2 Compressed Gases Key Hazards: Unintended pressure release, manual handling, toxicity, fire and explosion

5.2.1 Most thermal spray operations involve the use of compressed gases, even if only to the extent of using them as a carrier medium for powder. With the exception of acetylene, gases are supplied at pressures up to 300 bar. The uncontrolled release of gases at such pressures can lead to injuries of the eyes, injection of gas into the blood stream and damage to hearing. The use of regulators or other equipment not rated for the service pressure can lead to catastrophic failure of these items. Fatal accidents are possible in these circumstances.

5.2.2 The manipulation of the containers of compressed gases and cryogenic gases can give rise to manual handling injuries. Cylinders of compressed gas can become rocket propelled if they are knocked over with the regulator fitted and the main valve open - it is said that a full cylinder can accelerate from 0 to 34 mph in a tenth of a second. Since they weigh 60 kg or more, measures must be taken to avoid this.

5.2.3 Many gases have inherently dangerous properties, which are outlined below. The fuel gases are in the main extremely flammable, and are capable of forming explosive mixtures with air. The Dangerous Substances and Explosive Atmospheres Regulations (DSEAR)¹¹ require that a risk assessment shall take into consideration the dangerous properties of materials that can lead to fire and/or explosion, and that measures shall be taken including, where appropriate, hazardous area classification to reduce these risks. The Regulatory Reform Fire Safety Order 2005¹², which replaced the Fire Precautions Act, mirrors these requirements. The 'carrier' gases are generally inert - for instance argon, but the build up of such a gas in a poorly ventilated area can lead to oxygen depletion and a risk of asphyxiation.

Acetylene
5.2.4 Acetylene is classed as an explosive at pressures above 0.62 bar, and the maximum pipeline working pressure for this gas is 1.5 bar. As little as 2.5% in air will burn. It smells a little like garlic, is lighter than air, and tends to collect in roof spaces. Mechanical shock to the cylinder, or overheating, may cause decomposition within the cylinder, which may result in high temperatures inside, and possible detonation, see also section 6.6.

Hydrogen
5.2.5 Hydrogen is colourless, odourless and non-toxic. It is lighter than air, so may collect in roof spaces. The flame from hydrogen burning in air is difficult to see. As little as 4% hydrogen in air will burn; it is an explosion hazard, and extremely flammable.

'Inert' gases - e.g. nitrogen, argon
5.2.6 The 'inert' gases used in thermal spraying, such as nitrogen and argon, are non-toxic. However, in quantities sufficient to displace a significant volume of air, they present an asphyxiation hazard.

LPG, e.g. propane
5.2.7 These gases are often deliberately odourised. They are non-toxic, but gases such as propane are extremely flammable. LPG and propane are heavier than air, and can collect in drains and trenches. They are a potential fire and explosion hazard and are asphyxiants. As little as 2% in air will burn.

Oxygen
5.2.8 Oxygen, while being fundamental for survival, is dangerous if present to excess. In an oxygen-enriched atmosphere materials burn very fiercely, to the extent that such fires are extremely difficult to extinguish. Oils and greases can react violently or spontaneously explode on contact with pure oxygen. Most metals, especially in powdered form, will burn in oxygen.

5.3 Noise
Key Hazards: Stress, difficulty in communication, long term hearing loss, tinnitus

5.3.1 Exposure to excessive noise can cause stress, difficulty in communication and lack of concentration. The physical effects can include tinnitus, and a progressive loss of hearing due to the damage of the sensory mechanism in the ear. This loss is especially marked in the frequencies that are crucial to the understanding of speech, and thus noise induced hearing loss is a distressing condition. It is permanent, and cannot be helped by hearing aids.

5.3.2 Legislation ⁹ now sets three standards by which noise is to be assessed:

5.3.3 All spraying processes are noisy and may generate noise levels in excess of the lower exposure action values. In particular, the High Velocity Oxy-Fuel Spraying (HVOF) process emits noise in excess of 130 dB(A) at source. Users of thermal spray equipment are responsible for conducting a risk assessment, which must be based on the exposure of the persons in the workplace, and of which a noise survey is only part. The risk from exposure to noise must be either eliminated at source or reduced to as low a level as is reasonably practicable. If the noise exceeds the upper exposure action value a programme of measures, excluding the provision of personal hearing protectors, is required to reduce the noise to as low a level as is reasonably practicable. Hearing protection is to be provided on request at the lower exposure action values and becomes mandatory at the upper exposure action value. Employers must ensure that their employees are not exposed above the exposure limit value or, if this limit is exceeded, they must take remedial measures to prevent it from happening again. The regulations also specify requirements for demarcation of hearing protection zones, health surveillance and the provision of information, instruction and training. See section 6.8 for more detail about noise control measures.

5.4 Radiant Energy
Key Hazards: Cataract, burns, arc-eye

5.4.1 The flame spray processes emit light within the visible and infrared regions. Exposure to excessive radiation in these regions over a long period of time can cause cataracts.

5.4.2 The arc and plasma processes emit quantities of ultraviolet light, in addition to visible and infrared radiation. Ultraviolet light can give rise to arc-eye and skin burns. See section 6.8 for further information about controlling the risks from radiant energy.

5.5 Fume and Dust
Key Hazards: Fire and explosion, toxic effects by inhalation, toxic effects by skin contact

5.5.1 Powdered metals, particularly titanium, aluminium and magnesium, give rise to a fire and explosion risk, depending on the circumstances. Powdered materials can collect in ducting, on filters and around the spray booth. If these accumulations are heavy, and they are disturbed, they can be ignited.

5.5.2 An explosion may result from the ignition of a dust cloud inside or outside the equipment. A fire may follow from an explosion and its fireball, or may result from self-ignition of layers of accumulated dust on hot surfaces, including some electrical equipment. Where this is a risk, the employer must carry out the risk assessment specified by DSEAR ¹¹ and the Regulatory Reform (Fire Safety) Order.¹²

5.5.3 Many of the materials that are routinely sprayed have the potential to be hazardous to health due to their intrinsic properties. Even the least toxic materials, if present in the air in sufficient quantity, can be hazardous to health. Powders for spraying are frequently supplied in size ranges that can be inhaled, and the spray processes all produce airborne particulate matter in size ranges that can be inhaled. Both inhalable and respirable matter may be produced. Respirable particles are more hazardous because they can be breathed deeply into the lungs.

5.5.4 The user must therefore identify the hazards, assess the risks and prevent or, where not reasonably practicable, adequately control them in accordance with the requirements of the Control of Substances Hazardous to Health Regulations (COSHH).¹⁰ This assessment must take into account all occasions where exposure may occur - which include loading the hoppers, spraying, cleaning, disposal and maintenance.

5.5.5 Several commonly sprayed substances are subject to statutory exposure limits, which are listed and updated annually in the Health and Safety Executive's publication Occupational Exposure Limits, EH40.¹⁵

Workplace Exposure Limits (2007) (units mg m^-3)
Note: these limits can change - always check with the most recent EH40.

5.5.6 The first requirement is to prevent exposure by substitution or by complete enclosure. Control of exposure is not deemed to be adequate unless the eight general principles of good practice set out in Schedule 2A to the Regulations have been applied, and any defined Workplace Exposure Limit has not been exceeded. In addition, exposure to substances that:

• carry risk phrases R45,R46 or R49, or
• are specified in Schedule 1, or
• have risk phrases R42 or R42/43, or
• are listed in the HSE publication 'Asthmagen? Critical assessments of the evidence for agents implicated in occupational asthma',¹⁶

Aluminium
5.5.8 Long term inhalation of aluminium powder or aluminium oxide may cause scarring of the lungs. Aluminium powder is highly flammable, can form explosive mixtures with air, and reacts with water to form hydrogen.

Chromium
5.5.9 Prolonged exposure to chromium metal dust may give rise to lung fibrosis. It is highly toxic. Chromium oxide and chromium carbide feedstock are both chromium (III), the less toxic form of chromium, however spraying may convert chromium (III) compounds to chromium (VI) compounds, which are suspected human carcinogens, and would normally carry risk phrases R45 or R49. Chromium (VI) compounds are also capable of causing sensitisation. Exposure to chromium (VI) compounds must therefore be reduced to as low a level as is reasonably practicable.

Cobalt
5.5.10 Cobalt is moderately toxic. Cobalt and its compounds may cause sensitisation by inhalation and skin contact, resulting in occupational asthma and allergic dermatitis, and hence exposure must be reduced to as low a level as is reasonably practicable. Cobalt powder is highly flammable.

Copper
5.5.11 Copper fume may give rise to metal fume fever. Inhalation may cause muscle weakness and headache. Some compounds of copper are highly toxic and may cause long term effects.

Iron
5.5.12 Inhalation of iron oxides over a long period may cause scarring of the lungs without physiological symptoms. Iron powder is highly flammable.

Molybdenum
5.5.13 Occupational ill-health from exposure to molybdenum is unlikely. It is highly flammable.

Nickel
5.5.14 Nickel is harmful. Repeated skin contact may cause allergic contact dermatitis. Some of its compunds are suspected human carcinogens, and exposure must be reduced to as low a level as is reasonably practicable. The powder is highly flammable.

Tin
5.5.15 Tin powder may cause irritation. Inhalation over a long period may cause scarring of the lungs without physiological symptoms. It is highly flammable.

Titanium
5.5.16 Occupational ill-health from exposure to titanium is unlikely. Titanium powder is highly flammable.

Tungsten
5.5.17 Ingestion of soluble tungsten compounds may cause illness but occupational ingestion of tungsten other than in hard-metal alloy is not known to cause long-term ill health. The powder is highly flammable.

Zinc
5.5.18 Zinc can cause metal fume fever, but is otherwise only moderately toxic. The dust is highly flammable, pyrophoric, and can form explosive mixtures with air. In addition, it can react with water to form hydrogen.

5.6 Water-Borne Diseases
Key Hazard: Legionellosis

5.6.1 If a wet collector is installed to capture the oversprayed material, it is likely to become contaminated with the particulate matter in the over spray. For at least part of the year, the temperature of the water in the UK will be within the range where bacteria grow freely, with ideal growing conditions provided by the particulate matter. Therefore there is a risk of colonisation by bacteria, including legionella pneumophila.¹⁷

5.6.2 During the spraying process the wet collector has the potential to produce airborne droplets, which may remain within the spray booth, or be carried out with the extracted air from the booth. This may expose the operators, other workers, and members of the public to legionellosis, including Legionnaire's disease, a type of pneumonia that can be fatal.

5.7 Mechanical Hazards
Key Hazards: Entanglement, being struck by machinery, trips and falls

5.7.1 If equipment has been installed to manipulate the component and/or spray gun, it is important that it is designed to protect the operator from mechanical hazards. It is possible for the operator to be struck by a moving table or manipulator, to be crushed against the wall of the booth, or to become entangled in rotating machinery. Serious injury could result.

5.7.2 In the relatively confined area of a spray booth, trailing cables and other obstructions present a tripping hazard, and because of the close proximity to machinery, the consequences may be serious.

5.8 Electricity
Key Hazards: Electric shock, explosion, fire, burns

5.8.1 Electricity can give rise to electric shock, fire, explosions and burns. It can also lead to falls or muscular injury. The risk of death from electric shock is related to the current that passes through the body, and the path that it takes. Voltages below 50V ac or 120V dc are considered safe except in conducting environments. A typical spray booth is a conducting environment. However, the power supply for the equipment will usually be fed from a higher voltage supply, typically 240 or 415 V, which must always be considered to be hazardous.

5.8.2 Other related causes of electrical accidents include poor placement of cables. They may become trapped in booth doors, or damaged while they are on the floor, leading to exposure of the live conductors. Electrical apparatus can also be the source of sparks and, like electrostatic discharges, can ignite explosive mixtures, additionally they can cause a fire which, with the gases present, could be dangerous.

5.9 Cryogenic Gases
Key Hazards: Asphyxiation, burns

5.9.1 Gases from cylinders or from cryogenic stores can build up in relatively confined spaces, giving an asphyxiation hazard due to oxygen depletion. It is important to judge whether a gas stored in such a space could do this by calculation of the volume. Fatal accidents have occurred from such causes. Cryogenic liquids, such as liquid nitrogen can also give rise to severe cold burns. If liquid nitrogen is decanted from one container to another through a pipe, the liquid that forms on the outside of the pipe, and runs off it, is very rich in oxygen. This can pose a fire hazard, particularly if the liquid soaks into clothing.

5.10.1 In the table below, a very approximate rating of the hazards in typical circumstances has been given. The references are to the installation stage, section 6, and the operation stage, section 7. It must be stressed that the hazard ratings may change in individual circumstances, for example the rating for toxic consumables would relate directly to the consumable in use.

Hazard	Flame Spray	Arc Spray	Plasma Spray in Air	Plasma Spray in Vacuum	HVOF	Paragraph No.
Compressed Gases	*	*	*	*	*	6.2 to 6.7, 7.2
Highly flammable or extremely flammable gases, hydrogen, propane, acetylene, LPG and/or oxygen (fire and explosion risk)	*		*** Hydrogen	***	***	6.2 to 6.7, 7.2
Asphyxiant gases	*	*	*	*	*	6.6, 7.2
Noise	**	**	***	**	***	6.8, 7.3
UV radiation		*	**	*		6.8, 7.4
Fine dusts, fume and powders (explosion risk)	* to ***	* to ***	* to ***	* to ***	* to ***	6.8, 7.5
Potentially toxic consumables	* to ***	*to ***	* to ***	* to ***	* to ***	6.8, 7.5
Water borne disease (if water curtain fitted	*	*	*		*	6.9
Mechanical hazards (automated equipment)	*	*	*	*	*	6.10, 7.6
Electric shock	*	*	*	*	*	6.11, 7.7
Cryogenic gases (where used)			*		*

The more stars, the greater the severity of the hazard highlighted for this process.

*** very high hazard
** medium level of hazard
* lower level of hazard