INTRODUCTION
The working of metal is ubiquitous but the heavier and dirtier processes tend to predominate in the older industrial areas. Some processes have changed little over the years but modern technology, particularly computer aided, has introduced the potential for large losses to occur. The industry includes many entirely different processes and the Consultant needs to have a broad knowledge of each to understand the potential fire hazards involved.
COMMON PROCESSES
Some of the more common manufacturing processes and their related fire hazards are briefly described as follows:
1. Foundries
Melting. Metal is smelted from the ore, often including scrap, in furnaces heated by oil, electricity or gas. The material is loaded into the top of the furnace (cupola) and is generally fed slowly into the main heating area where it becomes molten. Additives are included to control the final properties of the material or to assist the process before it is tapped off either directly into moulds via channels or into a container (ladle) for transfer to separate areas for casting. Melting can also be done in smaller tilting furnaces which may be operated by hydraulics.
Due to environmental requirements filters have to be incorporated to remove particles from the flue gases. This is especially prevalent where reprocessing of scrap material occurs. Fires have occurred within these filters and special protection considerations may apply, including the installation of spark detection and suppression systems – Technical Bulletin 16 refers. Furnaces are maintained at high temperature at all times to prevent thermal cracking of the brick linings during cooling or heating and commonly will be running 24/7. The furnace can be water cooled to help protect the lining. The melting of some metals can be very hazardous in which case the process is carried out under inert atmospheres or vacuum.
Casting. When the metal is molten it is cast into moulds which are generally made of sand or steel. Sand casting usually involves a wooden, plastic or metal pattern around which sand is forced. The pattern is removed leaving the shape of the item to be cast. To ensure that the sand remains in the required shape and that the mould releases properly, various liquid additives or resins can be added, some of which are highly flammable.
Sand cores (used to produce holes in the casting) are manufactured by mixing special sand with flammable resins and liquids (often alcohol based).
Large stocks of wooden dies and patterns can be found in foundries and are generally of high value.
For intricate casting, an expanded polystyrene mould is used around which the sand is compacted. In this instance the polystyrene is left in the mould, vaporising on contact with the molten metal.
Investment (lost wax) casting. This consists of a precision casting technique with considerable use of wax patterns and highly flammable liquids. Traditionally, a hazardous process requiring special fire safety considerations.
Die casting. Frequently used for low melting point metals and alloys such as aluminium and zinc and carried out by pressure or gravity. Pressure die casting is the more hazardous in that molten metal is injected under pressure into a metal mould, where it cools and is then ejected (similar to plastic injection moulding). The pressure is applied using hydraulic oil. The process, particularly for fancy goods manufacture, is frequently accompanied by lacquering using highly flammable solutions and extensive packing operations.
Potential Hazards
Highly flammable liquids and gases
Hot metal
Over heating
Hydraulic oil
Pattern making and storage
Transportable heating commonly employed
Extraction systems
2. Forging
This essentially consists of the shaping of metals by hammering or squeezing, whether as part of the initial refining and working process or for the manufacture of semi-finished components is carried out in a forge. Metal is heated in a furnace and transferred to an anvil located in a hammer press where it is struck by a large weight (tup). The striking action can be either by gravity (drop forging) or under power. In either case the weight is lifted and or lowered by electric, pneumatic, steam or hydraulic power. Presses are usually of massive construction.
Where manufacturing semi-finished components various types of presses and reheating furnaces will be encountered.
Forging can be done cold, for example by extrusion forging where metal is forced through a die under extremely high pressures.
Potential Hazards
Vulnerable fuel lines requiring emergency shut-off procedures
Hydraulic oils
Hot metal
Transportable heating commonly employed
3. Rolling Mills
Converting large ingots of metal to bars or beams of various cross section is carried out in rolling mills. Rolling is usually done hot; the ingot is heated to above red heat in a furnace and is transferred to the mill where is passes between water cooled rollers to progressively reduce the cross section. The rollers can be shaped to produce the different profiles.
Machines in rolling mills are of massive construction and can be hydraulically or electrically powered. Control of the mill is from a centrally computerised operations room and the position of this and its fire protection is of importance.
Non-ferrous metals are often cold rolled, sometimes with the use of light grade cooling and lubrication fluids.
Potential Hazards
Hydraulic oils
Hot metal
Flammable oils
Transportable heating
4. Pressing and Stamping
Metal can be shaped whilst at room temperature providing sufficient force is used. Presses vary from manual fly presses to power presses applying thousands of tonnes pressure used, for example, in the motor industry. In drop stamping, the upper part of the die is dropped by gravity in a similar fashion of drop forging.
In hot stamping, small ingots are heated in furnaces and placed in the die for shaping.
Potential Hazards
Hydraulic oils
Vulnerable fuel lines requiring emergency shut-off procedures
5. Sheet Metalworking
Fabrication. The production of items from sheet metal involves essentially simple processes such as cutting, folding and welding. Guillotines of varying sophistication are used from simple hand operated ones to fully computerised equipment that cut and fold to pre-set sizes. Thicker sheets of metal can be cut using oxy-acetylene or oxy-propane. Where large quantities of a particular shape are required a profile cutter is used which has a number of gas jets mounted on to a frame. Control can be mechanical following a template or may involve sophisticated computer numerically controlled (CNC) equipment.
Increasing use has been made of laser cutting which, whilst being no more hazardous than conventional means, can involve very expensive equipment and control plant.
Fabrication will often be followed by spray painting/stove enamelling or electro –static powder coating of components or finished products.
Metal spinning. Metal can be formed into hollowware by spinning. A mandrel is mounted on a modified lathe; a disc of metal is held between the mandrel and the chuck and is rotated at high speed. The disc is drawn over the mandrel by the use of levers. Materials up to a quarter of an inch thick can be used to producing anything from kettles to cement mixer barrels. There is frequently considerable use of combustible packaging.
Welding and brazing. Metals can be welded by use of gas flames or electric arc. In gas welding the fuel is usually acetylene but others such as hydrogen or various proprietary mixtures can be used. In electric arc welding an inert shielding gas such as argon or nitrogen is frequently used. Brazing and soldering is done at lower temperatures but frequently involves the use of flammable fluxes. In some systems the fuel gas is passed through a liquid flux entraining it for delivery to the burner. This precludes the need for flux to be applied to the joint prior to brazing or soldering.
Potential Hazards
Highly flammable liquids and gases
Combustible packaging
Welding and cutting processes
Spray painting/powder coating.
6. Machining
The machining of metal using a cutting head or tool is carried out on a wide variety of equipment from a simple lathe to a multi-headed CNC machining centre. Consultants when reporting should clearly differentiate between "low-tech" and "hi-tech" plant.
Various cutting fluids can be used to lubricate and cool the tool. "Suds" oil is predominantly water and presents little hazard, but others may be neat mineral oils. In some processes there can be considerable splashing or leakage on to floors necessitating the use of metal drip trays.
Aluminium machining can involve the use of light grade coolants such as kerosene with the associated fire risk. Floors can get soaked providing a considerable risk. Ancillary machining of plastics can be encountered.
Some metal swarf such as titanium and magnesium turnings, can become ignitable if machined finely enough so coupled with inadequately sharpened or worn cutting tools and insufficient coolant, this can lead to fires developing within the machining area and subsequently into swarf collection compartments.
Precision engineering will frequently involve the use of spark erosion machining for which special fire safety considerations apply.
Potential Hazards
Reactive and/or ignitable metals
Machining of plastics
Friction caused by blunt or worn cutting tools
Oil based coolants
Spark erosion machining
Degreasing
Heat treatment
Unattended processes
7. Heat Treatment
This process is carried out to relieve stresses and strains imparted during the working of the metal or to impart certain properties such as hardness to the metal. It involves heating and then cooling under controlled conditions, frequently by quenching in oil.
It is sometimes carried out "in-house" in a separate heat treatment department but more often sub-contracted to specialist heat treatment companies. Heat treatment can involve some potentially extremely hazardous operations and a separate section of the Manual is in course of preparation.
8. Polishing and Grinding
Metal goods can be finished by polishing. A greasy cutting paste is applied to a rotating cloth mop and the item held against it manually. The process can be automated in which case the machinery is enclosed within a booth. A considerable amount of greasy fluff is created which should be removed by an extraction system.
The process is frequently sub-contracted to specialist polishers who often operate on low budgets in poor buildings and with unfavourable fire safety management.
Metal components are often finished by a process of precision grinding which can be carried out on a variety of different manually or semi-automatic machines to include surface grinders (typically for flat workpieces,) cylindrical grinders (where the cylindrical workpiece rotates and is held at each end,) centreless grinders (using a slave or control wheel to rotate a cylindrical workpiece while a second grinding wheel machines the surface,) and internal grinders (where the workpiece is hollow and the internal surface is ground using a rotating grinding wheel.)
As with any grinding process, the grinding wheels are abrasive and will generate sparks when coming into contact with the metal component, therefore machining coolants are required.
Potential Hazards
Poor housekeeping
Combustible extraction ducting
Accumulation of fluff and fly
Sparks
Dust explosion
RISK ASSESSMENT AND CONTROL
The following features, amongst others, require consideration by Consultants as part of an effective risk assessment and control programme:
1. Construction
To prevent corrosion from the elements or from fumes given off in the process, many old foundries, drop forges and the like were built of bitumen coated metal, usually Robertson’s Protected Metal (RPM) providing the potential for rapid fire spread. These materials are unlikely to be around in any quantity today, although Consultants should be aware.
2. Management
Key considerations include the following:
Good organisation with separate storage areas for combustible packing, wood patterns, flammable liquids and any other combustibles. In larger risks separation should be by compartment walls of at least 120min fire resistance.
Good housekeeping standards are necessary to prevent accumulations of waste, spillages of oil etc, especially in the pits that surround larger presses and stamps. The use of sawdust for soaking up oil needs to be discouraged in preference to the use of proprietary oil absorbent granules.
Planned preventative maintenance systems are desirable in all cases but are essential where chances of metal break outs exist or where machines are run 24 hours per day. Specialist heat treatment facilities need high standards of management and maintenance, as do foundries and forges where in both cases high temperatures are used with their potential consequences.
If water comes into contact with molten metal severe explosions can result. Water services need to be sited so that leaks or bursts will not present a hazard. Where scrap is fed directly into furnaces that already contain molten metal it should be checked for water contamination, especially when it has been stored in the open.
Where light grade oils such as kerosene are used for cooling and lubrication special care is necessary to control spillage and to provide the necessary fire protection. Cold rolling mills are normally protected by fixed gaseous systems.
Transportable heating appliances fuelled by paraffin or LPG are commonly encountered in foundries and similar risks for localised heating. Subject to these being correctly operated and maintained, Underwriters will often consider these to be tolerable.
3. Overheating
Within the foundry industry the metal melting pots are usually water cooled and it is essential to maintain a reserve supply of water in the event of pump or power failure. Should the cooling supply be interrupted the metal can break out with dire consequences. Overheating may be caused by a number of factors including:
Failure of cooling circuits.
Faulty heating arrangements or thermostats.
Operator failure - incorrect training or incompetent personnel.
4. Flammable Liquids
Wide range of flammable and highly flammable liquids are employed in many metalworking and engineering facilities, and the need to carry out an effective risk assessment in accordance with the Dangerous Substances and Explosive Atmospheres Regulations (DSEAR) is paramount to ensuring that these are stored and used in a safe manner. Technical Bulletin 21 refers.
Pattern making is usually done by specialist companies, but foundries will often have a small repair shop. If plastic patterns are employed, this may include the use of use of resins, catalysts, and solvents.
5. Reactive Metals
All metals, including iron are explosive in a very finely divided state. Certain metals, mainly magnesium, titanium and aluminium are particularly hazardous because their swarf or other small particles are combustible and once ignited are difficult to extinguish.
The machining of these metals must be conducted under carefully controlled conditions and it needs to be established that the operators are fully aware of the dangers and the importance of correct cutting speeds and tool maintenance. Swarf must be cleared regularly and stored separately in a safe area. Special dry powder extinguishers must be available.
Grinding and polishing of these metals is particularly hazardous and may lead to explosions in the extraction system. Wet systems are normally used, especially for titanium, but care is needed as certain metals may react with water to form hydrogen. Specialist advice may be necessary in these cases.
6. Hydraulic Fluids
Mineral hydraulic fluids can be used under considerable pressures usually in conjunction with flexible hoses where moving parts are involved. Bursts or leakages are likely to give rise to fine mists or sprays of oil which can be easily ignited. The problem is especially relevant where hot metal is being worked or where hot services are nearby and an effective planned maintenance programme is very important in these situations.
Large hydraulic presses and forges often have the pumps and reservoirs located in cellars below the machines. These would normally be critical areas, difficult to fight a fire in and where provision of fixed extinguishing systems would usually be considered essential.
Further guidance regarding hydraulic fluids can be found in Technical Bulletin 14.
7. Degreasing
Removal of oily films produced in machining operations is often carried out in vapour degreasing tanks. They are usually heated by electricity or gas and need to be kept clear of combustible materials.
Although the solvents used are normally non-flammable, fires can occur due to a build-up of oily sludge at the bottom on the tank. This results in a rapid boiling of the solvents and the evolution of acidic fumes which can cause severe corrosion problems. Regular cleaning as part of a planned maintenance programme is essential.
8. Welding and Cutting Equipment
Safe practice should include:
The provision of non-combustible or proprietary fire retardant/flame resistant welding screens as required.
Gas cylinders stored externally in a safe manner.
Gas cylinders in use mounted secure on a cylinder trolley or secured upright to a workbench or the building fabric.
Flashback arrestors fitted as required and all flexible welding hoses which can become perished or damaged, examined and replaced as needed. In respect of oxy-acetylene equipment, Technical Bulletin 53 refers.
9. Spark Erosion Machining (Electro Discharge Machining)
Technical Bulletin 10 refers.
10. Unattended Operations
With the development of automation many machines are capable of running for long periods without human supervision. Combined with the large capital sums invested in these machines, such as spark erosion machines, it is becoming common for these to be run unattended overnight. Technical Bulletin 15 refers.
These notes are intended to provide general guidance on the key trade specific, survey considerations across a wide range of metalworking and general engineering processes. For more information and advice concerning a specific case, Consultants should contact the Technical Helpline.
