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6. Design,
construction and installation of equipment
6.1.1 At the outset, the user of the equipment should conduct a risk assessment to ensure that the equipment will be sited in a suitable area, and that as many hazards as is reasonably practicable are avoided. Those that remain should be controlled by engineering means if feasible, and this will form a large element in the design, construction and installation of the equipment. The user must also consider all phases of the life cycle of the equipment - the design, construction, installation, commissioning, use, maintenance, repair and decommissioning/disposal.
6.2 Hazardous Area Classification
Definition of Terms 18
6.2.1 Hazardous area: An area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus. Hazardous areas are classified into one of three zones, 0, 1 and 2, depending on the frequency and duration of an explosive gas atmosphere occurring.
6.2.2 Zone 0: An area in which an explosive gas atmosphere is present continuously or for long periods (this is defined as more than 1000 hours per year). Example: At the spray gun tip before/after ignition or when the flame goes out.
6.2.3 Zone 1: An area in which an explosive gas atmosphere is likely to occur in normal operation (this is defined as less than 1000 hours, but more than 10 hours per year). Example: Inside the spray booth. The immediate area around the flammable gas stores.
6.2.4 Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it does occur, is likely to do so only infrequently and will exist for a short period only (this is defined as less than 10 hours per year). Example: The area around a flange, connection or other pipe fitting that is not expected to release flammable material during normal operation.
6.2.5 Any thermal spray/metal deposition process that uses liquid or gaseous fuel will have the potential to generate a flammable/explosive atmosphere. Manufacturers of equipment have a legal obligation to ensure that their equipment is built to safe standards. Gas suppliers are equally obliged to ensure that their products and equipment are supplied safe for use. Users of this type of equipment have a responsibility to ensure that it is safely installed within their premises in order to minimise the potential for explosion, and thereafter used and maintained in a safe condition. Explosions can also arise from the dusts generated in thermal spraying processes.
6.2.6 Users must therefore assess their installations to determine whether there are potential sources of release of flammable/explosive materials, e.g. from joints in pipework, valves, connections, fittings, etc, in accordance with the requirements of DSEAR 11and the Regulatory Reform (Fire Safety) Order12. Any part of the equipment that contains mechanical joints as opposed to welded joints will eventually leak. The recommended method for doing this assessment is called "Hazardous Area Classification" (HAC). HAC helps the user to locate the equipment appropriately and determines, by a system of zoning, the separation required from other potentially hazardous activities, and the equipment that may be safely used in the area. Zones should be clearly defined with appropriate signs, etc.
6.2.7 In general, HAC will be required wherever liquid or gaseous fuels are used in fixed installations. The European Standard BS EN 60079-10: 1996, "Electrical apparatus for explosive gas atmospheres, Part 10. Classification of hazardous areas" is the applicable standard.18 Both electrical and non-electrical sources of ignition should be considered as part of the overall assessment process. BS EN 60079-10: 1996 defines certain key terms, which are to be found in the section on definition of terms, and reiterated at the start of this section.
6.2.8 The zone designation determines the standards to which apparatus used in the various zones must conform. For existing installations this requirement will be retrospective and must be strictly applied.
6.2.9 Dusts can also create an explosion hazard, and BS EN 5028119 relates to hazardous area classification for these materials.
6.2.10 Hazardous area classification is a time consuming and specialised process. It requires a multi-disciplinary team led by someone fully trained and competent in hazardous area classification. Specialist help may be required.
6.2.11 It should be stressed that HAC is only part of the risk assessment process that is required to design and install thermal spray equipment.
6.3.1 New equipment supplied for use in Europe, must have a CE mark and a Declaration of Conformity, or a Declaration of Incorporation20 stating compliance with the various Directives. Used equipment older than the date of CE introductions is exempt so long as it is not modified in any way. To justify CE marking the equipment, the manufacturer will have to ensure that it has been designed with safety in mind, according to either generic or specific standards, e.g. references 21-29. The manufacturer's declaration will indicate to which European Directives the equipment conforms. These will include the Machinery Directive, the Low Voltage Directive, the Electromagnetic Compatibility Directive and the ATEX Directive (Explosive Atmospheres Directive). Note that a manufacturer's declaration is not a guarantee of safety, but it is an indication that he has done a risk analysis on the product which can be reviewed by the authorities (not by potential or existing customers). Nevertheless, the responsibility for safety lies with the user. (Gas equipment does not yet fall within the scope of CE marking.)
6.3.2 Manufacturers of thermal spraying equipment have a legal duty to ensure that the equipment they supply is safe for its intended use. This will include compliance with the 'Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres Regulations 199630 which implements the requirements of the ATEX Directive from June 2003. Some parts of the equipment will not be able to conform to the ATEX Directive, because of the flame or arc being present. Ultimately the design standards to be applied will be determined, where relevant, by the Hazardous Area Classification (HAC). It is important that users consult with manufacturers at the earliest opportunity to determine the recommended conditions of installation, including the location.
6.3.3 A major consideration in all but the arc spray process is the safe handling of flammable gases. The spray area must be designed and installed so as to avoid so far as is reasonably practicable the possibility that an explosive or flammable mixture will exist in the workplace. Measures should also be taken to minimise the possibility of the ignition of such a mixture if it did arise due to a failure. The HAC must take into account the hazards already present in the workplace as well as those brought in by spraying equipment. It is important that a suitable and sufficient fire risk assessment is conducted. 11, 12, 31
6.3.4 The HAC will enable the user to zone the work area, to define the engineering controls that are necessary, and the activities and ancillary equipment that are to be permitted in each part of the work area. Potential sources of ignition in the area could include the spray equipment, uncertified electrical equipment, defective or poorly maintained electrical equipment, unsafe working practices, electrostatic discharges, hot surfaces and might include operations such as grinding etc.
6.3.5 Only competent, suitably qualified/trained personnel should install thermal spray equipment. Users will generally be buying equipment from more than one source, and if this is the case they take the responsibility for safely integrating the items into a whole. This must be incorporated into their risk assessment. They need to satisfy themselves that they have obtained sufficient information from the suppliers of the major components of the system (e.g. the spray equipment, booth manufacturer, extraction system manufacturer), that it can all be integrated to form a safe piece of equipment, with adequate safeguard against mechanical and operator failure. The user should ensure that all aspects of the installation conform to the Provision and Use of Work Equipment Regulations.5
6.4 Location of Thermal Spraying Equipment
6.4.1 Spraying equipment must not create hazards for nearby processes nor must it be put at risk by nearby processes. It should be located against an outside wall to facilitate easy gas delivery. This will also make the installation of the ventilation and extraction easier. The building should be single storey, to avoid gas accumulating in occupied or at-risk areas above or below the process.6.4.2 The location should be free from unventilated areas overhead, such as voids above false ceilings where lighter than air gases could accumulate; and from voids below floor level, such as pits, service trenches, gullies or underground rooms, where gases that are heavier than air could accumulate.
6.4.3 The area should be as spacious as possible, with a high ceiling and good natural ventilation. There should be plenty of air inlets, roof vents and opening doors and windows. The design ventilation rate, for example five volume changes of air per hour needs to be specified. Five volume changes can normally be obtained by ensuring that 2 to 3% of the wall and roof area is ventilated, distributed between high and low level. Mechanical ventilation may need to draw from low level for heavier than air gases (such as LPG and propane, argon).
Figure 5: A spray booth set up inside an existing workshop (Picture courtesy TWI)
6.4.4 The HAC will determine the allowable proximity of sources of ignition. Possible sources would include machinery, ovens, furnaces, boilers, heaters, and operations such as grinding, cutting, welding, etc. The HAC will determine the extent of the area that should be free of any electrical equipment, e.g. wall sockets, isolator boxes, junction boxes, control panels, which have not been designed for use in the classified areas.
6.5.1 It is advised that, wherever possible, the stores for bulk supply of gases are outside the work area, in purpose-built bays that can be locked. For thermal spraying, gases may be purchased as single cylinders, manifolded cylinder banks or as cryogenic liquid vessels. Access should be designed with care, since the gas cylinders will be delivered to the site from lorries, and the gases will be probably be removed from the store either using fork lift trucks (if they are in banks) or using trolleys if single gas cylinders.
6.5.2 Fuel gases and oxygen must be segregated, either by a distance of not less than three metres, or by a wall that can resist fire for a minimum of 30 minutes. It is permissible to keep LPG and propane with acetylene provided that the quantity of LPG and propane does not exceed 50 kg, otherwise they must be segregated. Hydrogen must be segregated from other fuel gases by a distance of at least one metre. The store should be well ventilated and, if it is outdoors, the cylinders should be protected against rain and direct sunlight. Storage areas should be kept clean and only used for the storage of cylinders.
Figure 6: Purpose built stores for oxygen and hydrogen (Pictures courtesy BOC gases)
6.5.3 Empty gas cylinders should be segregated from full or partially used cylinders. The store must be marked with relevant signs - e.g. 'flammable gas', 'no smoking'. For more details on gas storage see BCGA Guidance Note GN2.32
6.6 Fuel Delivery to the Spray Equipment High Pressure Gas Equipment
6.6.1 The Pressure Systems Safety Regulations33 stipulate the legal requirements for design, periodic examination, maintenance and repair of gas systems. Gas supply systems should be designed and installed in accordance with BCGA Codes of Practice CP7 for single cylinders34, CP4 for manifold systems35, CP6 for acetylene36 and CP5 for acetylene manifolds.37 Note: the design and construction requirements of transportable pressure receptacles (i.e. gas cylinders) used for the carriage of gas to fixed installations are covered by The Carriage of Dangerous Goods (Classification, Packaging and Labelling) and Use of Transportable Pressure Receptacles Regulations.386.6.2 Acetylene is subject to specific legislation, but prior approval is not required for an installation between 0.62 and 1.5 bar, provided that the conditions of Certificate of Exemption No. 2 1989, made under the Explosives Act (Exemption) Regulations39, are complied with. The user should refer to the Health and Safety Executive, Explosives Inspectorate, Bootle, before proceeding.
6.6.3 Fixed installations, which may include plasma, high velocity oxy-fuel spray (HVOF) and automated flame spray will require fixed piping from the gas storage area to the working area and the design of the system has to take into account that sufficient flow is available. It is strongly recommended that the advice of a reputable manufacturer or preferably a gas supplier is taken in the design of this system, since adequate provision must be made for the protection of the system against such faults as flashback, by the inclusion of flashback arrestors and non-return valves. The use of flexible piping should be kept to a minimum.
6.6.4 The design and integrity of the gas delivery system, taken in conjunction with control measures, such as gas detection and interlocking of the thermal spray equipment, forms a crucial part of the HAC. It is advised that competent advice is sought.
6.6.5 Manifolds and piped systems should be installed in accordance with the relevant BCGA codes of practice by competent installers who have been trained in the hazards of the gases being used. Care should be taken in the routing of gases. Pipework should be routed on the outside of the building for as much of its run as is reasonably practicable. The pipework should have the minimum number of connections, in order to minimise possible sources of leaks. Once pipework enters the building it should be a single section of pipe until it connects with the process. If this is not practicable then all necessary connections must either be welded or brazed to eliminate/minimise potential sources of leaks within the building. Soft soldered joints are not acceptable.36
6.6.6 The systems should be installed using solid pipework as far as possible; the use of rubber hoses, even to the appropriate standard, should be limited to reduce the risk of damage and gas leaks. The pipework should be visible and accessible for inspection and maintenance purposes hence installation in ducts, roof spaces and enclosed spaces should be avoided. All pipework should be suitably marked, indicating the type of gas within the system and its direction of flow.
6.6.7 Once installation is complete, the system shall be visually inspected by a competent person and pressure tested. The pressure test should be carried out at 1.5 times its maximum working pressure using an inert gas with similar properties to the fuel gas used in the system (e.g. helium can be used to test a hydrogen system). The pressure should be held in the system for a period not less than 30 minutes without a drop. The competent person should issue an appropriate certificate of testing. Appropriate safety precautions should be taken during gas testing, see Health and Safety Executive guidance note GS4.40
6.6.8 Equipment for use with oxygen must be certified as clean for oxygen service. It is essential to ensure that no organic materials such as oils or greases come into contact with oxygen, otherwise a fire or explosion may result.
6.6.9 The hazardous area classification must be carried out on an oxy-fuel installation to identify the areas that need zoning.
Low Pressure Gas Equipment
6.6.10 Flexible hoses for low pressure delivery to equipment (less than 20 bar) should be correctly colour coded, and manufactured to the correct specifications (BS EN 559, 200341 or equivalent ISO or DIN standard):
Acetylene, hydrogen
Red
Propane and LPG
Orange
Oxygen
Blue
Non-combustible gases
Black
6.6.11 Connections should be by means of crimp type, non-adjusting hose clips.42 Pieces of twisted wire, or the screwdriver adjustable bands should not be used.
Kerosene
6.6.12 For systems fuelled by kerosene, the possibility of producing an explosive mixture is much reduced, since the flash point of kerosene is higher than fuels such as acetylene or propane (the precise temperature depends on the kerosene type). However, an atomised spray could explode. The fuel may be fed by a pressurised line to the console, leakage from which could therefore create a risk of explosion. Thus on the high pressure side an all-metal pipe should be fitted. A metal-braided hydraulic hose should be used to connect the console to the gun.
6.7 Gas Detection
6.7.1 There is the potential for gas leaks to occur, even in the best installed and maintained plant. Small leaks may be dispersed by an adequate general ventilation system. If a large leak should occur, greater than the capacity of the ventilation system, it is important that the leak is detected quickly, so that the fuel gas can be isolated and the equipment shut down safely.6.7.2 Gas detection will not prevent gas releases, but will detect them before they reach explosive levels, and detection systems can be connected to initiate gas shut-off, isolation of electrical supply and can be used either to maintain ventilation or to activate emergency ventilation systems. Gas detection relies on accurate calibration, appropriate positioning and regular inspection and testing for its effectiveness - advice on this and on the type of gas detector required will be given by the supplier. It is important that everyone involved with this type of equipment fully understands the purpose of gas detection and what to do if it alarms. Complacency must be guarded against.
6.7.3 The decision to install gas detection should be based on a suitable and sufficient risk assessment, taking account of the existing controls, age, condition and layout of the process. This will require advice and assistance from a safety practitioner, and possibly a supplier of gas detection equipment. The places most likely to require gas detection will be confined spaces on processes where there is a potential for creating an explosive gas atmosphere. Some manufacturers of metal spray equipment now fit gas detection within spray booths and control panels as standard. However, for existing equipment the necessity for gas detection needs to be evaluated against the adequacy of the ventilation levels achieved in these areas - a suitable and sufficient risk assessment will help to determine this.
6.7.4 If gas detection is deemed necessary, it is vital that it is interlocked to the ventilation system and the gas and electricity supplies. This is so that:
- On start-up of the equipment, the interlocked system will not allow the fuel supply to be opened until the ventilation system has removed any flammable gas and is fully operational.
- If the ventilation system fails, then the gas and electricity supplies are isolated ·
- No-one is allowed to enter the room until the ventilation system has removed any flammable gas and it is fully operational.
In the event of an alarm, the following should all happen simultaneously:
- The electricity supply is isolated, to prevent ignition sources
- The ventilation system continues to operate and/or an emergency ventilation system is activated, to provide increased levels of ventilation
- The fuel gas supply is rapidly shut off at source.
The integrity of the electrical systems should be appropriate for the risks involved. 25,29
6.8.1 Many thermal spray installations are installed in permanent enclosures. The enclosure will house the spray gun itself, the manipulation system(s) for the components and/or gun and any cooling system for the components. It will also contain the equipment for the removal of overspray and fume and dust. The control console should be sited outside the enclosure, so that the operator is not exposed to the hazards within. The powder hopper should be outside the enclosure if possible, but may be sited inside.
6.8.2 Any extraction system fitted to the enclosure should not be disabled or shut down by the equipment 'emergency stop'. In addition, the extraction system should be interlocked with the spray equipment in order that the operation of the spray equipment is disabled if the extraction system fails, or is not running.
Protection From Noise
6.8.3 Plasma and high velocity oxy-fuel spray (HVOF) systems are usually installed in purpose-built enclosures, designed to control the exposure of the operator and others to noise.
6.8.4 In controlling exposure to noise, the first priority is to choose equipment that is less noisy, and thus to solve the problem at source.9 Good design is also important; designing out noise should be considered by manufacturers. Where this is not practicable, the noisy equipment should be installed in a sound attenuated booth designed to reduce the noise levels to below 80 dB(A). If enclosures are to be built around equipment for safety and containment purposes, consideration should be given to their acoustic properties and where possible they should be used as acoustic enclosures.
6.8.5 If noise levels associated with the operation of the equipment cannot be brought down below 80 dB(A), further measures must be taken. If employees and others are liable to be exposed to a daily personal noise exposure of 80 dB(A), or more, The Control of Noise at Work Regulations9 require the employer to carry out an assessment of the employees' (and any others who may be affected) exposure to noise. The results of this assessment will show what further measures must be taken. The exposure action values and limit values are tabulated in section 5.3.
6.8.6 In particular, any area where an employee is likely to be exposed at or above the upper exposure action value of 85 dB(A) is designated a hearing protection zone where ear protection must be worn. The requirement for personal protective equipment is described in the Personal Protective Equipment Regulations.43
Protection From Fume and Dust
6.8.7 The hierarchy for control of exposure to dust and fume is first to enclose the process, but if this is not reasonably practicable, to extract it, and as a last resort to issue personal protective equipment, in accordance with the Regulations.43 Workplace exposure limits must be met, and where there are substances that are potential sensitisers or carcinogens the exposure must be reduced to the lowest level reasonably practicable.
6.8.8 Since the operator will need to enter the enclosure, it must be fitted with an extraction system to remove dust, gas and fume. The extraction system should be interlocked with the thermal spray equipment to ensure that if the system is not operational then the gun cannot run. The flow rate for extraction must be adequate to ensure that exposure of the operators is controlled according to the requirements of the Control of Substances Hazardous to Health Regulations.10
6.8.9 In general, where the spray process is operated within an enclosure, with the operator situated outside, the extraction system will reduce the exposure of the operator to fume and dust to a low level, provided he or she remains outside.
6.8.10 Entering the booth could potentially lead to exposure to fume and dust, and for this reason consideration should be given to designing the system so that the operator is not able to enter until some time has elapsed from the end of spraying operations. This will allow the extraction system to clear the fume and dust from the atmosphere. However efficient the air extraction equipment may be, operators who enter the enclosure during setting up or spraying operations must wear personal protective equipment, which may include clean-air fed respirators, if conditions warrant it.
6.8.11 The exposure of the operator must also be controlled while undertaking such operations as filling the powder hopper.
Dry Extraction Systems
6.8.12 Dry extraction systems were originally brought into use in the form of multi cell cyclones but these, due to their design, only effectively capture dust down to around 5 µm. Since a large amount of the particulate released during spraying is below this level this material passes straight through the cyclone.
6.8.13 The most common form of dry collector used in thermal spraying today is the dry cartridge filter. This comprises of a series of filter elements folded to form a pleated material and provides the filter area required for the correct filtration of the process. The cartridges are housed within a steel casing mounted, preferably, outside the building. Cleaning of the filters is carried out by reverse pulsing of the cartridges with compressed air. This is a totally automated process carried out whilst spraying is in operation with no operator intervention required. Note that a fire trap, a box with baffle on the booth side, is required where sparks or hot metal may be a problem, e.g. in arc spray, to prevent hot particles getting into the filter.
6.8.14 Capture of the particulate can then be carried out in a variety of different methods from a basic hood behind the gun to a more technically advanced 'floorwash' system providing a much higher level of cleanliness and dust collection in the booth. This type of system provides more space for spraying within a current booth or a smaller area used within the factory.
6.8.15 The method of filtration, the filter speed and cleaning of the cartridges are all dependent upon the spray process being undertaken and the run times required for the process.
Explosion Risks
6.8.16 The filters used in a dry extraction system should be fitted with suitably designed explosion relief panels to vent a pressure build up should it occur. These should vent to a safe place. It should be noted that most dry cartridge filters will only operate up to 60oC, which limits their usage.
6.8.17 Where metal dusts, such as titanium and aluminium are present, the fire risks are considerable. MCrAlY dusts can also catch fire, especially in VPS systems if insufficient ventilation is available before the loading door is opened. The installation should have smooth-walled, round-sectioned ducting with no dead spots where dust can accumulate, short run lengths and preferably only one or two machines to each collector and access points for cleaning and inspection. Dust collectors should normally be outside, as there is no safe place inside for the opening of an explosion vent.
6.8.18 Special techniques should be used when tackling metal fires, for example do not use water. Dry sand and long shovels or special extinguishers should be used. No one should tackle a metal fire unless they have been trained to do so.
Control of Emissions to Atmosphere
6.8.19 The vent to the extraction system will generally be outside the building, and filtration will be required to remove the particulate matter from the exhausted air before it is released. There are limits on the particulate content of the exhaust air laid down in Environmental Legislation, summarised in the table below. For further information on the control of emissions, reference should be made to the Secretary of State's Guidance.44
Emission limits44 (units mg m-3)
|
Total particulate |
Chromium |
Nickel |
Copper |
Cobalt |
|
50 |
15 |
15 |
7.5 |
3 |
Protection From Radiation
6.8.20 To shield operators from ultraviolet, infrared and intense visible light in the plasma and arc spray processes, the windows in the booth should be glazed with suitable filtering material. There is a standard for semi-transparent welding screens that may be appropriate to this application, BS EN 1598.45 See references 46-48 for further guidance on controlling the risks from radiant energy.
6.9.1 The material that fails to hit the component and stick to it may be collected by a wet collector. Some wet wash systems are unable to adequately abate the emissions produced during thermal spray processes, although there are new types that may be as effective as dry collection.
6.9.2 Installation of a wet extraction system must take into account the possible risk from legionellosis, and the engineering control measures that may reduce the risk, such as suppression of the formation of droplets, and the siting of the outlet from the extraction system.17
6.9.3 The water wash is run in addition to an air extraction system from the booth, and it is important to ensure that the air speed is not greater than 9 m s-1, to avoid the entrainment of water droplets into the air extract.
6.10.1 In operations carried out in an enclosure, machinery to manipulate the gun and/or the component will be installed. This is basically of four types:
- Slow speed traverse
- High speed traverse
- Rotating tables
- Robot.
6.10.2 Measures will need to be taken to prevent machinery that is capable of rapid movement from hitting the operator. High speed traverse systems are fitted with a barrier which is interlocked. The operator sets the 'stop' points with the equipment running slowly.
6.10.3 If a robot is installed it is essential that it has hardware stops to limit its movement where the enclosure is smaller than its potential range, or where there are objects within range that it must not reach. Software stops are not sufficient to do this, since they may fail. The equipment should be interlocked such that the robot is disabled if the door is opened. Provision for 'teaching' the robot must be in accordance with the guidance. The user is referred to the Health and Safety Executive guidance on Robots, HSG 43.49
6.11 Electrical Safety
6.11.1 All equipment should be connected to the supply in the building by a competent electrician to ensure the integrity and capability of the wiring. Wiring should conform to the requirements for electrical installations in BS 7671.506.11.2 Electrical apparatus is a prime source of ignition of explosive atmospheres. In locations where the zone classification indicates a flammable atmosphere could exist, suitably certified equipment should be employed, see EN 60079-14 51, 52, EN 50281-1-2.19 The extraction equipment for the enclosure should also be appropriately approved.
6.11.3 For spraying any metal, e.g. zinc, titanium and aluminium, the risk assessment may show that the equipment needs to be ingress protected (IP) to a specific standard.53 The BS EN 5028119 is a standard applicable to electrical equipment for use in dust laden atmospheres.
6.11.4 Cables should always be placed in a safe position, to avoid trip hazards, damage and being trapped in doors. All conductors should be suitably protected from identifiable mechanical damage.
6.11.5 For all control systems, an appropriate level of integrity, determined from the risk assessment, should be provided. This includes the use of safety related equipment, e.g. interlock switches. For non-software applications refer to BS EN 95429 and for software applications to BS EN 6150854.