Abschnitt 3.2 - 3.2 Hazards associated with heat treatment in industrial furnaces employing air or process gas atmospheres; corresponding measures
3.2.1 Workrooms and work areas
The media and equipment used also have an impact on work premises and workplaces. Heat treatment plants may pose specific requirements upon the equipment of their workplaces. Ventilation and extraction systems are often required.
Figure 17
Extraction at a multi-purpose chamber furnace
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In heat treatment plants, the type of the plant, the process media used and the tasks to be performed give rise to the following aspects to which particular attention should be paid, over and above the general requirements for workplaces.
Elevated thermal load, caused for example by residual heat from hardening material, flares, pilot flames, heat radiation from the plant, especially on older, poorly insulated plants
Health hazards presented by occupational exposure limits not being continuously observed. The primary cause in this case are leaks from the furnace plant of process atmosphere containing acutely toxic gas components. Carbon monoxide (CO), which is present in many process atmospheres, is a particularly critical gas.
Danger of falling when work is performed on the furnace plant
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The following general measures have proved effective in heat treatment shops.
Shop ventilation system:
The shop ventilation system has the purpose of collecting leaks from the furnace plants that are not captured by the exhaust collection system, other atmospheric pollutants, and the thermal load from the heat treatment operation, and discharging them from the shop. Aspects relevant to the design of a shop ventilation system include the following:
Maximum thermal input from the outside, for example due to solar radiation
Emissions of thermal radiation from the plant
Fluctuations due to seasonal influences (e.g. seasonal variation in weather, capacity utilization)
The exhausted air volume, for example at leakage points, must be balanced by a corresponding supply of fresh air. Ancillary rooms, such as break rooms and similar areas, must also be taken into account in a shop ventilation concept.
A major problem relating to the operation of shop ventilation systems is irregular maintenance of the systems or its complete absence. The performance of ventilation systems deteriorates considerably when they are not maintained regularly. Causes include soiled filters, clogged pipes and broken fan blades. Ensure therefore that your ventilation systems are regularly maintained.
Extraction at leakage points
(see also: Discharge of waste gases):
In order to reduce to a minimum the danger presented by acutely toxic gas components in leaking process atmospheres, ensure that gas from more major leakage points is exhausted as completely as possible. An extraction system is recommended at the following points:
Above furnace doors
Pilot flames on flares
Above gas burners
Furnace doors, especially when they are opened
Pressure relief valves
Intake and discharge points on conveyors and similar systems
Flame curtains
Observance of the occupational exposure limits:
Determine whether the existing shop ventilation and extraction systems are adequate for compliance with the occupational exposure limits. This can be determined by workplace measurements. Workplace measurements can be taken by permanently installed measuring equipment or regular discrete measurements (for example by mobile instruments or portable gas warning devices). The instruments must be maintained, inspected and calibrated in accordance with the manufacturer’s instructions.
Should compliance with the occupational exposure limit values not be possible, further measures are necessary, such as:
Checking of existing equipment (shop ventilation and extraction systems)
Searches for unknown leakage points
Reduction of the leakage rate at the leakage points
Improvement of the shop ventilation and extraction systems
Access to workplaces
Around a fifth of occupational accidents in plants are caused by falling, slipping and tripping hazards. The effects of these accidents should not be underestimated. Safe routes to the place of work are therefore of great importance. In heat treatment shops, access to workplaces on older plant, for example for maintenance work, is often unsafe.
Access points to workplaces:
Figure 18
Retrofitted facility for access to a furnace roof
Figure 19
Stair for access to the furnace roof
Define traffic routes on the furnace plant and safeguard them, for example by means of fixed barriers to protect against falls. Should this not be possible, you must specify or create attachment points for PPE against falls from a height.
| Carbon monoxide |
|---|---|
| Carbon monoxide is a colourless, odourless and flammable gas. Its toxic effect arises from its disruption of oxygen transport by the blood. Carbon monoxide’s capacity to bind to red blood cells is approximately 300 times that of oxygen. As a result, many red blood cells form a bond with carbon monoxide molecules even at very low concentrations of carbon monoxide in the breathing air. This bond is very difficult to break. Symptoms of carbon monoxide poisoning are headaches, reddening of the skin, dizziness, impaired vision, unconsciousness and in the worst case respiratory arrest. The use of respiratory protection filters is not recommended, as penetration of the filter cannot be detected by the wearer. Since the usual furnace atmospheres with a carbon monoxide component are lighter than air, the highest concentrations are found above furnace systems or close to the shop ceiling. Increased caution is required when accessing a furnace system, since the lining and the oil bath can store the furnace atmosphere. Occupational exposure limit: 30 ppm Explosion limits 12.5 - 74 % by volume (in air) | |
Further information on this hazardous substance can be found in the DGUV’s GESTIS database of substances.
Figure 20
Effect of carbon monoxide
3.2.2 Supply of energy and media
Heat treatment cannot be performed without energy and media. Measures are required to ensure that the provision of energy and media does not give rise to a hazard.
Figure 21
Marking of pipelines and fittings
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Marking of pipelines and containers:
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Hazards presented by the supply of energy and media may arise in particular as a result of:
Confusion of media lines due to inadequate marking, resulting in shut-off, isolation or opening of the wrong medium
The location of the shut-off device is such that critical media (such as the natural gas supply) cannot be isolated without hazard in the event of an accident.
Damage to media lines (for example due to collision with an industrial truck)
In the event of an accident, important media are not available, as a result of which it is not possible to place the furnace plants in a safe operating state without risk.
Formation of dangerous explosive atmospheres owing to leaks on shut-off devices in media lines
Violation of the occupational exposure limit (OEL) owing to leaks from media lines
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You must therefore implement the following measures for the energy and media supply:
Mark pipelines and containers with the name of the substance and its hazardous substance symbol or pictogram. On pipelines, the direction of flow of the substance must also be marked. On shut-off or control devices, indication of the use or final appliance is also recommended. This reduces the likelihood of operating errors.
Protect media lines against being damaged by external hazards, such as collisions with industrial trucks. This can be attained by routing away from traffic and transport routes. If this is not possible, media lines can also be protected by mechanical obstacles, such as collision guards.
Also consider the issue of corrosion and, where media lines are routed outdoors, weather conditions.
Provide facilities by means of which, in the event of a hazard incident (such as fire), critical media such as flammable gases or liquids can be isolated safely from the equipment.
Conduct regular drills with your employees of how to respond in the event of substances being released from the storage facility, a fire breaking out or any other emergency occurring, and of how to escape to safety.
Should nitrogen be required for safe operation of the furnace system (for example for purging), ensure that the required nitrogen supply is permanently available, including in the event of a power failure. This can be attained by the use of normally open solenoid valves in lines supplying nitrogen for safety purposes, and manual valves with safeguards to prevent incorrect operation. In addition, the minimum quantity of nitrogen required for safe shutdown (emergency purging) of all supplied appliances in combination must be available at all times. Level monitoring systems that automatically request and initiate supply have proved very effective.
To avoid operator errors, marking of the normal display range on instruments and the normal operating position of valves is recommended.
Filters should preferably be installed in gas and fluid supply lines in order to prevent mechanical contamination. This measure maintains the shut-off capacity of valves for longer and prevents it from gradually deteriorating. Where gases are heavily contaminated, the installation of parallel interchangeable filters may be necessary: these allow the filter media to be changed with the system running.
Check what equipment is still required to function in the event of a power failure in order to prevent danger to persons in the production area. This equipment must be connected to an emergency power supply. Extraction and gas detection systems and the cooling water supply must often be connected to a standby power supply.
Should an accident involving persons and hazardous substances (such as ammonia) occur, the fire service must also be alerted in order for the area to be safeguarded and cleared for access by the ambulance service.
To keep leakage in the pipeline system as low as possible, you must observe at least the following minimum measures:
The materials and components used must be suitable for the pressure stage and temperature and for the medium.
The components and connections used must be technically leakproof.
Check the technical leakproofing after installation and at regular intervals. (Refer to Table 3 for the inspection intervals.) This is particularly important for components carrying acutely toxic or flammable gases. In addition to the outward leak-tightness of pipelines, the internal leak-tightness of shut-off devices (solenoid valves, gate valves, etc.) is also important for plant safety.
To ensure that safety equipment intended to prevent the release of gases and formation of dangerous explosive atmospheres functions reliably, you must ensure that it is maintained and checked.
| Piping system - leak testing | |||
|---|---|---|---|---|
| Reason | Source | Comment | ||
| Gas distribution up to the first shut-off valve in the system | 6 years after completion; thereafter at shorter intervals as appropriate | DVGW rules for natural gas | ||
| Removal/installation of elements (valves, pressure gauges, isolators, etc.) | On restoration to service | |||
| Gas distribution in the furnace plant | Annually | Recommendation | Determined by the operator; local circumstances (stationary monitoring, air exchange, confined spaces) must be taken into account | |
| Distribution of gas from gas generator (e.g. endothermic gas generator, ammonia evaporator) to furnace plant | Annually | Recommendation | Determined by the operator; local circumstances (stationary monitoring, air exchange, confined spaces) must be taken into account | |
Note:
The leak-tightness of pipelines can be tested only on lines under operating pressure (or higher pressures). Note therefore when testing that not all pipe sections are under pressure in every operating state. The tests must be performed by competent persons in accordance with the German Ordinance on industrial safety and health (BetrSichV).
Table 3
Leak-tightness of piping systems
3.2.3 Methanol tank
Methanol is used in heat treatment mainly for the production of process gases. In this process, methanol is injected into the furnace plants, either directly or following evaporation or cracking in separate plants. Methanol is usually stored in tanks located outside buildings.
Figure 22
Methanol tank
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The following hazards in particular may arise during operation of a methanol tank:
Leakage of methanol
Leaks must be anticipated on the tank, the supply lines to the plants, and the pump set, and also during the refuelling process.
Leaking methanol gives rise to the following hazards for employees and the environment:
Formation of a dangerous explosive atmosphere
Combustion of leaked methanol
Contamination of groundwater and soil
Poisoning of workers through inhalation and skin absorption when present in the area affected by the leak, and during maintenance work
Bursting of the tank
Accidents, such as damage to the tank caused by vehicles
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For the safe erection and operation of a methanol tank, take at least the following measures:
Erection:
Prior to erection and commissioning, contact an approved inspection body and the local authority regarding approval and operation of the plant.
As a general rule, notification is mandatory in Germany by way of a simplified approval procedure in accordance with the 4th Ordinance governing implementation of the German federal pollution control act (14th BImSchV) where the total stored quantity exceeds 3,000 kg of combustible gas and liquid. Notification in the case of methanol is mandatory at 5,000 litres and over in some of the German regions and at 10,000 litres and over in others.
A site inspection by an approved inspection body is required at commissioning.
The base/foundation must guarantee stability and be made of non-combustible material.
Lightning protection and potential equalization must be assured.
Methanol tanks must be protected against fire loads, such as flammable substances (petrol, fats, oils), wooden sheds, wood stacks. This is usually achieved by a clearance of at least 5 metres or for example by a protecting wall.
The filling line and safety valves must be at least 5 metres clear of shafts, ducts and windows.
Adequate circulation of air around the methanol tank must be assured. The tank must be accessible for operation, service and maintenance. This is generally assured by a clearance of at least 1 m from buildings, walls etc.
Collision protection for the tank and the pipes is required.
Access by unauthorized persons must be prevented by fencing or an enclosure.
The base of the discharge area must be protected against penetration by methanol. Facility must be provided on the discharge area for rainwater shafts to be closed off.
The tank must be equipped with overfill protection.
A dehydration filter must be fitted to prevent atmospheric humidity from arising in the plant.
A detonation flame arrestor with venting to a safe area must be provided.
Underground tanks and underground supply lines must be double-walled and monitored for leaks.
Fire safety must be ensured by alarm and extinguishing equipment if necessary.
Installation of a leak monitor in the pump cabinet is recommended.
The following areas must normally be of explosion-proof design:
For above-ground methanol tanks, a zone of 5 metres around the tank
For underground methanol tanks, a zone of 0.5 metres around the manhole shaft and a zone of one metre around the vent opening
Operation:
Organizational arrangements are required in order to ensure that only authorized persons have access to the installation.
If necessary, rainwater shafts must be closed off during refuelling.
During filling of the methanol tank, a nitrogen-inerted recovery line is used to displace the atmospheric oxygen in the filling line and tank system and to transport the methanol from the tanker vehicle into the storage tank; a vent line for excess pressure must be provided.
Should skin contact with methanol be possible, gloves resistant to methanol must be worn (see separate methanol information box).
The tank must be serviced annually.
The system must be inspected every three years by an approved inspection body.
Tanks must be inspected every 10 years (in accordance with the German Water resources act, WHG) by an inspection body or specialist company.
| Marking of chemical protective gloves |
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A glove is deemed resistant to chemicals when a protection index of at least class 2 is achieved for three test chemicals (from the specified list of 12 chemicals).  Figure 23 Marking of chemical protective gloves | |
| Methanol | |
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| Methanol is a colourless, highly flammable liquid with a pleasant to pungent odour.
It is acutely toxic if swallowed or inhaled or in the event of skin contact. Complaints
such as dizziness, headaches, nausea or impaired vision may arise only hours or days
later. Methanol has a high permeation ability; gloves of common materials such as latex, nitrile rubber or polyvinyl chloride are penetrated in less than an hour. The most suitable glove materials are butyl rubber, FKM and neoprene. | ||
| Occupational exposure limit: | 200 ppm | |
| Explosive range (vapours) | 6 - 50% by volume (in air) | |
| Boiling temperature | 64.5°C | |
Further information on the hazardous substance can be found at: http://www.gischem.de | ||
3.2.4 Supply of ammonia
Ammonia is used in heat treatment processes primarily for the extraction of process gases. In nitriding processes for example, it serves as a source of atomic nitrogen. The ammonia used is usually stored in separate containers. The containers house the compressed gas tanks and the discharge equipment, including heating and gas monitoring.
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Figure 24
Ammonia tank
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The following hazards in particular may arise during operation of an ammonia supply system:
The pressurized gas tanks contain liquid ammonia. The pressure is highly dependent on the temperature; excessive heating can be caused for example by direct sunlight or fire. This may lead to the permissible operating pressure being exceeded, resulting in gas escaping or the tank bursting.
Where materials are used that are not resistant to ammonia, corrosion may result in the escape of gas.
During maintenance work or exchange of the compressed gas tanks, the release of ammonia constitutes an elevated risk potential.
The following relevant properties give rise to hazards when ammonia is released:
Ammonia is:
Acutely toxic when inhaled, and causes severe skin burns and serious eye damage.
Classified as a flammable gas, and together with air is capable of forming an explosive mixture, especially in enclosed spaces.
Classified as hazardous to water in German water hazard class 2.
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The following measures are commonly taken for safe erection and operation of an ammonia supply system:
Erection:
Contact the gas supplier whilst the plant is still at the planning stage. The supplier will advise you on selection of the storage system and on the technical requirements to be met and the necessary permits, as these may differ between the different German regions.
Rooms in which an ammonia supply system is located must feature fire-retardant separation from adjacent rooms. Should the adjacent rooms be occupied permanently by persons, the form of separation must also be gas-tight and free of apertures. The rooms require adequate natural or forced ventilation.
Owing to the flammability of ammonia gas, it must be determined whether rooms housing an ammonia supply system are to be classified as potentially explosive atmospheres. In the field, these rooms are often classified as zone 2 potentially explosive atmospheres.
Erection of ammonia supply systems in break rooms or at permanently manned workplaces is prohibited.
Pipes carrying ammonia should be welded and detachable unions kept to the minimum necessary.
All packing materials used must be resistant to ammonia. Non-ferrous metals such as copper or its alloys are not suitable.
Pressurized gas tanks and valves of approved types must be used.
The safety concept must include a gas detection system for monitoring the ammonia supply system.
An emergency stop facility for manual shutdown of the system must be provided outside the potential danger zone. The gas detection system must also be capable of shutting down the system automatically.
Access by unauthorized persons must be prevented by fencing or an enclosure.
Spill containment facilities must be designed in accordance with the provisions of the German Water resources act (WHG) and the German Ordinance on facilities for handling substances hazardous to water (AwSV)
Operation:
Organizational arrangements are required in order to ensure that only authorized persons have access to the installation.
As part of planning for a disaster scenario involving a large-scale ammonia leak, the means of coordination with emergency services must be determined, and the measures required in order for the surrounding area to be protected and informed (other buildings and the neighbourhood).
Compressed gas tanks may be connected and disconnected only by employees who have been instructed in the task and specifically assigned to it.
Exchanging of compressed gas tanks must always be performed by two persons (dual control principle).
Figure 25
Personal protective equipment for activities involving ammonia
Figure 26
Eyewash station
Personal protective equipment must be worn during connection and disconnection of the compressed gas tanks (at least tightly fitting safety goggles and chemical protective gloves).
Ensure that facility for a comprehensive eye wash is provided.
Respiratory masks with a type K gas filter have proved effective for brief repairs of minor accidents (such as closing of valves). At high concentrations, under uncertain conditions or in confined spaces, only self-contained breathing apparatus may be used.
The ammonia supply system must be maintained regularly in accordance with the manufacturer’s instructions.
The ammonia supply system must be inspected periodically in accordance with the provisions of the German Ordinance on industrial safety and health (BetrSichV) with regard to the risk of explosion and hazards posed by pressure.
The compressed gas tanks must be inspected every five years (in accordance with the ADR 4.1).
The requirements to be met at the site deriving from the regional water resources legislation must be observed.
| Ammonia | |
|---|---|---|
| Ammonia is a colourless, flammable and acutely toxic gas. It has a pungent odour and
a strong caustic effect on the skin, mucous membranes and respiratory tract, even
in strongly diluted form. It is extremely soluble in water and therefore acutely hazardous
to water (German water hazard class 2). Owing to its very low odour threshold, it
is detectable by smell significantly below the occupational exposure limit. In the event of skin or eye contact, the affected parts of the body must be washed carefully under running water for at least 10 minutes. Contact with the eyes necessitates immediate further treatment by a specialist following the administration of first aid. | ||
| Occupational exposure | ||
| limit: | 20 ppm | |
| Explosive range: | 15.4 - 33.6 by volume (in air) | |
| Boiling temperature | -33.4 °C | |
| Further information on the hazardous substance can be found at:http://www.gischem.de | ||
Table 4
Effect of ammonia
| NH3-concentration in ppm | Effect upon the unprotected human organism | Duration of exposure |
|---|---|---|
| < 5 | Perception by odour | Unlimited |
| 20 | Occupational exposure limit Initial exposure gives rise to slight irritation | 8-hour working day |
| At 100 | Unpleasant, but no permanent health impairment | Leave the area as swiftly as possible |
| At 300 | Not bearable, irritation of the eyes, nose and respiratory organs | Leave the area as swiftly as possible; no serious long-term injuries |
| ≥ 1700 | Asphyxiation, paralysis, acute risk to life | Leave the area immediately, lethal within minutes |
3.2.5 Liquefied petroleum gas tank
Liquefied petroleum gas is used in heat treatment processes as a heating fuel gas and as a process gas with a high carbon content. Liquefied petroleum gas includes propane, propene, butane and mixtures containing these substances. It is heavier than air and highly flammable. Liquefied petroleum gas increases considerably in volume (by a factor of 260) when the gas vaporizes.The lower explosion limit is extremely low.
Owing to these properties, liquefied petroleum gas has a high hazard potential.
Figure 27
Marking of a liquefied petroleum gas system
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Operation of a liquefied petroleum gas tank and the associated distribution system may give rise in particular to the following hazards:
Leakage and operational escape of liquefied petroleum gas
Escape must be anticipated on the pressure vessel, on lines supplying the plants, and during the refuelling process.
Escaping liquefied petroleum gas presents the following hazards for workers:
Formation of a dangerous explosive atmosphere
Combustion of escaping liquefied petroleum gas
Displacement of oxygen
Cooling by evaporation
Note:
Dispersal in shafts or sewers may result in the hazard arising some distance away from the leakage point.
Tank burst
Frostbite caused by contact between the skin and cryogenic liquefied petroleum gas or cryogenic plant components
Accidents, such as damage to the tank caused by vehicles
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To ensure that liquefied petroleum gas tanks are erected and operated safely, the TRBS 3146/TRGS 746 Technical rules concerning stationary pressure systems for gases must be observed. Measures commonly applied include:
Erection:
Contact the gas supplier whilst the plant is still at the planning stage. He will advise you on selection of the storage system and on the technical requirements to be met, in particular in accordance with the TRBS 3146/TRGS 746 technical rules, and on the necessary permits, as these may differ between the different German regions.
The base/foundation must be stable and made of non-combustible material.
Liquefied petroleum gas tanks must be protected against fire loads, such as flammable substances (petrol, fats, oils), wood sheds and wood stacks. This is usually achieved by a clearance of at least 5 metres or for example by a protecting wall.
Air must be able to circulate adequately around the liquefied petroleum gas tank and the tank must be adequately accessible for operation, maintenance and servicing. This is generally assured by a distance of at least 1 m from buildings, walls etc. (50 cm in the case of tank walls without apertures).
Filling lines and safety valves must be at least 5 metres clear of open ducts, shafts, openings to rooms at a lower level or air intake vents.
Liquefied petroleum gas tanks and their lines must be protected against mechanical damage. Should the clearance not be sufficient, collision protection may be required, depending upon the situation on site.
Access to the valves by unauthorized persons must be prevented. This can be achieved for example by means of fencing or an enclosure.
The base of the discharge area must be protected against penetration by liquefied petroleum gas. Facility must be provided on the discharge area for rainwater shafts to be closed off.
The tank must be equipped with overfill protection.
Underground tanks must be protected against corrosion, for example by a particularly effective external coating of epoxy resin.
Large discharge quantities and underground tanks may necessitate installation of an evaporator.
Fire safety must be assured; this can be achieved for example by alarm and extinguishing equipment.
Areas at risk of explosion on/around the tanks must be identified as shown in Figures 29 and 30.
Figure 28
Dimensioning of the area at risk of explosion (zoning) for a liquefied petroleum gas
tank installed above ground in the open air
Figure 29
Dimensioning of the area at risk of explosion (zoning) for an underground liquefied
petroleum gas tank
Operation:
Organizational arrangements are required in order to ensure that only authorized persons have access to the installation.
If necessary, rainwater shafts must be closed off during refuelling.
The tank and all plant components requiring regular inspection must be inspected in accordance with the German Ordinance on industrial safety and health (BetrSichV), Annex 2. The remaining parts of the plant must be inspected in accordance with Annex 3, Section 2 of the BetrSichV, and maintained in accordance with the specific requirements of the plant. The scope of inspection and the persons responsible for carrying out the inspections prior to commissioning, together with the maximum intervals for periodic inspections, are determined in accordance with Section 4 of the BetrSichV by the size of the tank.
| Propane | |
|---|---|---|
| Propane is an odourless, colourless, highly flammable gas which is heavier than air. An odourant is added to it to enable gas leaks to be detected. The scale of the hazard cannot be inferred from the strength of the odour. The gas is usually stored in liquid form (tank, bottle). The lower explosion limit of propane is among the lowest. | ||
| Occupational exposure limit: | 1800 ppm | |
| Explosive range: | 1.7 - 10.8% by volume (in air) | |
| Ignition temperature | 470 °C | |
| Boiling point | -42 °C | |
| Density with respect to air | 1.55 | |
3.2.6 Liquid nitrogen tank
Nitrogen is used in heat treatment processes in particular as a safety gas for purging processes, besides its use as a process gas. Where it is used as a safety gas, pertinent issues include the minimum quantity that must be kept available for safe operation, and whether a safety supply to the plants is still assured in the event of a power failure. Nitrogen is stored in liquid form in tanks outside buildings. The evaporator station is located directly adjacent to the tanks.
Figure 30
Nitrogen tanks with evaporators
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The following hazards in particular may occur during operation of a nitrogen tank and the associated plant:
Leakage of nitrogen
Leakage must be anticipated both on the tank and the supply lines to the plants, and during refilling.
Nitrogen leaks give rise to the following hazards for workers:
Risk of suffocation due to the displacement of breathing air
Cooling by evaporation
Frostbite caused by skin contact with cryogenic nitrogen or cryogenic plant components
Tank burst
Accidents, such as damage to the tank caused by vehicles
Insufficient supply of nitrogen and thus insufficient safety nitrogen for furnace operation
Possible causes include:
Insufficient evaporator capacity
Manual valve shut off
Tank level too low
Bursting of pipes and tanks owing to embrittlement of material caused by non-vaporized cryogenic nitrogen downstream of the evaporator
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For the safe erection and operation of a nitrogen tank, take at least the following measures:
Erection:
The base/foundation must guarantee stability and be made of non-combustible material.
Lightning protection and potential equalization must be assured.
Air must be able to circulate adequately around the nitrogen tank and the tank must be adequately accessible for operation, maintenance and servicing. This is generally assured by a clearance of at least 1 m from buildings, walls etc.
Collision protection must be provided for the tank and the lines.
Access by unauthorized persons must be prevented by fencing or an enclosure.
The tank must be equipped with overfill protection.
Operation:
Organizational arrangements are required in order to ensure that only authorized persons have access to the installation.
Inspection must be performed annually by a competent person, and maintenance conducted every six years.
The pressure vessel must be inspected every 10 years by a specialist company or an approved test body.
Should nitrogen be required for safe operation of the furnace system (for example for purging), ensure that the required nitrogen supply is permanently available, including in the event of a power failure. This can be attained by the use of normally open solenoid valves in the supply lines carrying nitrogen for safety purposes and manual valves with safeguards to prevent incorrect operation. In addition, the minimum quantity of nitrogen required for safe shutdown (emergency purging) of all supplied appliances in combination must be available at all times. Level monitoring systems that automatically request and initiate supply have proved very effective.
Elevated concentrations of oxygen may arise in the air in the immediate proximity of cryogenic plant components. This is associated with a significantly increased fire hazard.
The formation of ice in the vicinity of liquid nitrogen tanks and associated evaporators can increase the risk of slipping. Gritting materials should ideally be kept available throughout the year.
Figure 31
Formation of ice on nitrogen tank pipelines
3.2.7 Storage and refilling of quenching oils
Should hardening oils be stored improperly or an accident occur on the oil bath, for
example due to confusion of hardening oils or to operating errors, specific hazards
and even fires may occur.
Quenching oils are liquids that are hazardous to water. They are normally assigned
to German water hazard class 1, and in the case of emulsified quenching oils to water
hazard class 2.
Figure 32
Hardening oil tank
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Hardening oil released as a result of leaks or accident may give rise to the following hazards:
An increased fire load (particularly by contamination of combustible materials such as rags, wood, cardboard, packaging material)
A danger to water should it enter the sewerage system or seep into the soil
Slipping hazards
An increased fire hazard caused by confusion of inadequately marked containers or drums (for example containing waste oil contaminated with water)
Should hardening oil be stored outdoors without being covered by a roof, a risk exists of it being contaminated by water by "breathing" of the containers as a result of large differences in temperature (causing condensation) or the ingress of rainwater.
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Coordination with the emergency services and measures for protection of the surrounding area must be set out as part of disaster planning for the quenching oils present.
Drums must be stored on their sides in closed rooms, or at least under a roof.
During transfer by pump, ensure that the equipment used (pump, hoses, etc.) is free of contaminants (such as water, fuels, etc.). Use of a transparent hose enables contamination of the oil being pumped to be detected easily by visual inspection (for example by cloudy, milky discolouration in the event of water contamination).
Containers or drums used during filling of the oil bath must be clearly marked. Setting containers aside exclusively for use for the temporary storage of quenching oil, for example for repairs, has proved effective. This prevents product contamination/mixing.
Allowance must be made for the expansion in volume of hardening oil by approx. 4 % (0.00075 %/°C) when it is heated to its operating temperature. The oil bath should not therefore be immediately filled with "cold" oil up to the set filling level.
Although quenching oils are not classified as hazardous substances under the GefStoffV, they should nevertheless be stored in accordance with the requirements of the TRGS 510 Technical rules concerning the storage of hazardous substances in transportable containers and the TRGS 509 Technical rules concerning the storage of liquid and solid hazardous substances in stationary containers and filling and discharge points for transportable containers. Owing to their flash point of approximately 160 - 310 °C, quenching oils must be assigned to storage class 10 as flammable liquids.
The local requirements of the German Water resources act (WHG) must be observed.
Figure 33
Oil foam formation on hardening oil contaminated with water (simulated in the laboratory)
3.2.8 Discharge of waste gases
Depending on the process engineering method used and the point of discharge, waste gases are contaminated with different hazardous substances (residues of carbon monoxide, residues of ammonia), oily vapours and pyrolysis products of organic materials occurring in the process. These waste gases must be reliably exhausted from the shop and if necessary purified.
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| Hazards |
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Waste gases are produced in plants wherever combustible materials are intentionally combusted, for example at flame curtains, pilot flames, burn-off on pressure-relief flaps, flares.
Should the waste gases not be discharged in a controlled manner, the following hazards may arise:
The occupational exposure limit is not reliably observed and the requirement for hazard minimization as set out in the German Ordinance on hazardous substances is not met.
Deposits of oily vapours present a fire hazard and an elevated fire load.
The accumulation of unburned flammable gases can give rise to a risk of fire or explosion.
| Measures |
|---|
The discharge of waste gases forms part of the process engineering system and must therefore be integrated into the process. The exhaust system must be planned as part of the process engineering plant.
Where waste gases cannot be discharged through an exhaust line with natural draught, forced exhaust systems must be employed. In the interest of reliable and safe design of the forced exhaust systems, observe the following points:
Key components such as fans and filters should be of redundant design (this permits maintenance and replacement without interrupting operation).
Oily and damp waste gases should be discharged separately from "dry" waste gases.
"Dry" waste gases should be purified by means of dust filters.
Oily/humid waste gases should be purified by means of wet scrubbers (wet scrubbing generally eliminates any remaining fire hazard in the exhaust system).
Up to the wet scrubber, extinguishing and cleaning equipment should be integrated into the forced exhaust tracts through which oily waste gases are discharged.
Wall and ceiling entries must be of fireproof design.
The control systems of the forced exhaust system and the furnace plant must be coupled. Should the forced exhaust system fail, the furnace plant must automatically assume a defined operating state with low waste gas volumes, or be shut off completely.
Operation of the forced exhaust system must be guaranteed even in the event of a power failure. This is important in order to maintain discharge of the elevated quantities of hazardous substances from the furnace plant. The arrangement need not be uninterruptible.
Extraction can be on demand according to the furnace processes; lower demands are placed for example upon extraction at a closed furnace door than that at an open furnace door.
Example of a fail-safe waste gas/waste air forced exhaust system of redundant design
Intended for moist exhaust air containing oil vapour/mist
Figure 34
Diagram showing an example of a forced exhaust system
The performance of forced exhaust systems deteriorates considerably if they are not regularly maintained. Causes include fouled filters, clogged pipes and broken fan blades. Ensure therefore that your exhaust systems are checked and maintained regularly in accordance with the German Ordinance on hazardous substances (GefStoffV).
3.2.9 Operation of furnace plants
Modern furnace plants employ highly automated control systems; these do not however cover all eventualities. Take account of the following aspects in order to avoid hazards arising from the combination of the furnace system and the human factor.
Figure 35
Installation comprising tempering furnace, cleaning plant, two hardening furnaces,
tempering furnace
| Hazards |
|---|
The following hazards may arise from the combination of the control system and the human factor:
Where multiple plants are operated: confusion of the operating sequences or the functions of the controls, resulting in plant malfunctions presenting a hazard to persons.
Incorrect operating procedures when the plants are operated in manual rather than automatic mode, resulting in plant malfunctions presenting a hazard to persons.
Where plants are coupled within the same process but their control systems are coupled inadequately or not at all, a system does not respond automatically when an incident occurs on the system with which it is coupled. Further faults presenting a hazard to persons may occur.
Intervening in operation without at the same time interrupting automatic mode may result in safety equipment being defeated, particularly on fully automatic, fenced systems. Protection against automatic movements is then no longer assured.
| Measures |
|---|
To prevent the operation or automation of furnace plants from giving rise to hazards, take the following measures:
Ensure that the safeguards present are used by the employees, are functional, and cannot easily be manipulated or defeated.
In order to prevent employees who operate multiple furnace plants or carry out maintenance and servicing tasks on multiple plants from inadvertently performing incorrect operations, the operating procedures and control elements of furnace plants should, as far as possible, be of uniform design. This is particularly important with respect to safety equipment. Take this aspect into account when procuring new plants and converting legacy plants. The importance of this is particularly evident in the case of emergency stop controls. It is imperative that dangerous movements be stopped when the emergency stop buttons are pressed. On some installations however, gas supply and heating of the furnace plants are also interrupted.
Where your installations employ different operating concepts, you must provide employees who operate multiple plants with regular instruction on the differences between them, particularly with regard to the safety elements.
Figure 36
Multi-purpose chamber furnace line comprising tempering furnace, cleaning plant and
multi-purpose chamber furnace
When faults occur on installations, they must often be operated in manual mode in order for the fault to be cleared. This requires sound knowledge of the process as a whole and of the function of the furnace plant. Determine which employee is authorized for this purpose for each furnace plant. Select only sufficiently qualified employees. Ensure during shift planning that qualified personnel are present at all times.
In order for operating errors to be prevented, marking the normal state of displays and the normal position of controls has been shown to be good practice.
Ensure that controls and workstations are not located in front of or in the immediate proximity of hazardous areas, such as automatically opening furnace doors.
Ensure in particular when designing fully automatic plants that routine operating interventions are possible without interruption of automatic operation. Where this is not possible on your plants, consult with the manufacturer or your accident insurance institution if necessary in order to determine what modifications are needed. Should technical modifications not be possible or be too complex, determine in consultation with your employees how the operating interventions should be performed. The defeating of safeguards is not permitted under any circumstances.
| Emergency stop |
|---|---|
| Machines must be equipped with an emergency stop device. The emergency-stop device
must act upon components of the machine from which an imminent danger may arise and
which can be brought to a normal standstill more quickly by the emergency stop device.
The imminent danger is usually caused by mechanical movements. Emergency stop devices must be provided on each control station and at other locations at which initiation of an emergency stop may be necessary. An emergency stop must be reset manually on the emergency stop device which was actuated. It must not be possible for the machine to restart or be restarted before the emergency stop is reset. The emergency stop reset must not be coupled to the machine restart command; restarting of the machine must require a separate action by the operator. | |
| Requirements for shifts with unmanned operation |
|---|---|
| Owing to their long process times, heat treatment plants are often run overnight or
over the weekend. Personnel are often not required for normal operating procedures
during these periods. Some plants are therefore operated unmanned in these shifts.
Before unmanned operations are introduced, a safety concept must be drawn up based
upon a comprehensive risk assessment. The safety concept must specify both technical
and organizational measures to assure safety in compensation for the absence of operating
personnel. The manufacturer of the plant can provide useful assistance in the development
of a safety concept. The aspects to be considered in the safety concept should include the following:
| |
| Lone working |
|---|---|
| DGUV Regulation 1, Principles of Prevention, Section 8 and DGUV Regel 100-001 concerning
principles of prevention In general, lone working is associated with the normal risk to life and does not require special occupational safety measures. As part of their general duty of care, many employers nevertheless seek to avoid requiring their employees to work alone, keep the duration of lone working as brief as possible, or use organizational or technical means to detect as swiftly as possible when an employee working alone is in an emergency situation. The situation is different when lone working includes dangerous work. As a rule, dangerous work should not be performed alone. Where it is necessary in exceptional cases, you must implement additional monitoring measures over and above the general protective measures necessitated by the nature of the work itself. Monitoring can take the form of technical measures (such as personal alarm systems) or organizational measures (patrols by a second person, systems for reporting by telephone/radio alarm at agreed times, or permanent camera surveillance). Dangerous work is work that is associated with increased risk because adequate protective measures cannot be taken. Examples of dangerous work include:
The risk presented by the work and the necessary protective measures and measures for monitoring lone working are determined as part of the risk assessment. | |
3.2.10 Handling of hardening material and fixtures
Fixtures are manufactured from heat-resistant (e.g. steel) alloys. They are custom-designed for each furnace plant and can be adapted to different charges or charge structures. Unlike the actual charge itself, the fixtures pass through heat treatment processes multiple times, which is not without consequence. Avoid hazards caused by wear and ageing behaviour of the fixtures.
| Hazards |
|---|
The following particular hazards arise on fixtures due to wear and ageing:
Dangerous faults caused by the fixture jamming or catching owing to changes in dimensions or geometry during the transport process
Assembly of the fixtures made more difficult, resulting in a higher risk of crushing, and of flaking/breakage during straightening
Mechanical failure of fixture elements; falling of parts of the charge off the fixture. The danger is particularly great when fixtures are transported suspended from the crane. This can lead to the entire batch breaking of. Processes involving high carbon contents in the furnace atmosphere present greater risks. When quenching is performed in aqueous polymer solutions, the fixtures may be damaged after only a few heat treatment cycles.
Figure 37
Cracking on a fixture
Figure 38
Cracking on a fixture
| Measures |
|---|
Take the following measures to reduce the risk of faults being caused by wear and ageing of fixtures:
Select a material for the fixture that is suitable for the intended heat treatment process.
Fixtures that are intended for transport by crane and as an integral part of the load constitute load handling attachments as governed by the 9th Ordinance under the German Product Safety Act (ProdSG). Test these fixtures in the same way as other load handling attachments, and document the test.
Examples of fixtures that are intended as an integral part of the load are fixtures for use in shaft furnaces. Other charging equipment, such as wire baskets, do not constitute load-handling attachments.
Like other load handling attachments, fixtures that are deemed to be load handling attachments must be marked by the manufacturer with the CE mark, the manufacturer’s details and the maximum load capacity.
The accompanying documents include a declaration of conformity and an instruction handbook describing use and testing of the fixtures. The information contained in the manufacturer’s instruction handbook must be observed during use and testing of fixtures.
For fixtures that are pushed or conveyed on rollers, the use of templates or contour patterns has been shown to be an effective means of checking the fixtures. If these checking elements are installed in the transport system upstream of the furnace, damaged fixtures can be detected before they are transported into the furnace plant.
Check the fixture for wear and ageing and take defective elements out of use.
To determine when a fixture is due for inspection, proven good practice is to record the number of heat treatment cycles through which the fixture or element has passed.
| CFC fixtures for oil quenching: |
|---|---|
| Where CFC charging equipment is used in oil quenching processes, particular consideration must be given to the possibility of the quenching oil being carried over down the production line. This must be checked for the application under consideration. Owing to its structure, CFC is porous. Treatment of the material by infiltration can reduce porosity, but not eliminate it entirely. In tempering furnaces for example, this additional oil load can lead to increased concentrations in the process chambers and insulation of the heat treatment plant. This in turn gives rise to an increased fire load and risk of explosion. Rinsing of the CFC materials, for example with modified alcohols or hydrocarbons, reduces oil carry-over but does not eliminate it completely. The best results can be attained by thermal cleaning or drying in a vacuum chamber in the cleaning plant. | |
3.2.11 Formation of explosive atmospheres
Flammable gases, which can form dangerous explosive atmospheres when mixed with air (oxygen), are often used in heat treatment shops. Since explosions can cause considerable injury and damage, measures to prevent the formation of dangerous explosive atmospheres are particularly important.
Figure 39
Gas detector
| Statutory references |
|---|---|
| |
| Further information |
|---|---|
| |
| Hazards |
|---|
Dangerous explosive atmospheres may form as a result of:
Leaks on plant or pipelines carrying gas
Malfunctioning of safety equipment
Incorrect operating interventions in response to faults on the plant, owing to inadequate training of operating personnel
Evaporation of hardening oil or other flammable liquids and mixture of the vapours with air
This occurs for example in tempering furnaces when the hardening oil from the quenching process has not been sufficiently removed in the cleaning plant.
Even where the formation of a dangerous explosive atmosphere is not possible, a hazard to employees cannot be excluded, owing to the toxicity of the gases. Monitoring and observance of compliance with the occupational exposure limit values is therefore absolutely essential.
| Measures |
|---|
As a general rule, the risk assessment must also consider the formation of dangerous explosive atmospheres. You must define the necessary measures and document them in the explosion protection document.
To keep the leakage on pipelines and plant components to a minimum, implement the following measures:
The materials and components used must be suitable for the pressure stage and temperature and for the medium.
The components and connections used must be technically leakproof.
Check the technical leakproofing after installation and at regular intervals. In addition to the outward leak-tightness of pipelines, the internal leak-tightness of shut-off devices (solenoid valves, gate valves, etc.) is also important for plant safety.
To ensure that safety equipment intended to prevent the release of gases or formation of dangerous explosive atmospheres functions reliably, you must ensure that it is maintained and checked.
Incorrect operating interventions following malfunctions can be avoided when it is established which employees are authorized to correct malfunctions, and ensured that these employees possess the necessary knowledge and practice.
The necessary knowledge includes detailed knowledge on:
Functioning of the plant
The dangers presented by the plant
The dangers presented by the process media
The plant’s safety concept
Besides these general measures, the following specific measures are recommended:
Propane has a density 1.5 times that of air. It also has a lower explosion limit of 1.7 % by volume in air. Consequently, propane accumulates in depressions or pits and may form explosive mixtures even at very low concentrations. Static monitoring is therefore recommended of pits, open cellars, outlets, etc. in the vicinity of pipes or unions carrying propene.
The smaller the room volume, the quicker critical concentrations of flammable gases are reached. Where the equipment installed in a small room presents a possibility of uncontrolled gas leakage, you must therefore implement static measures for monitoring these rooms, or take organizational measures defining the conditions under which the rooms may be entered.
| Explosive ranges of gases |
|---|---|
| Further important parameters for combustible gases are their lower explosion limit (LEL) and upper explosion limit (UEL). Mixtures of gases with air are explosive in the concentration range between the LEL and the UEL. | |
Figure 40
Explosive ranges of flammable gases/vapours in air
3.2.12 Hot surfaces and cryogenic gases
Unprotected contact with hot or cryogenic surfaces and cryogenic liquefied gases can cause major injury to the skin and the underlying tissue. You must take technical or organizational measures to prevent such injuries.
Figure 41
Temperature-sensitive warning sign
| Further information |
|---|---|
| |
| Hazards |
|---|
Hot solids, liquids or gases can cause severe burns or scalding:
When hot surfaces are touched unintentionally
(e.g. pipes, furnaces, containers).
When hot surfaces are touched intentionally
(e.g. handwheels, valves, handles).
When direct contact is made with hot materials
(e.g. liquids, superheated steam, hot air).
When contact is made with open flames.
When contact is made with spatter of hot media.
Table 5
Burn thresholds T0 in the event of contact with hot surfaces of various materials (EN ISO 13732-1)
| Material | T0(°C) for 1 minute contact duration | T0(°C) for 10 minutes contact duration | T0(°C) for 8 hours contact duration |
|---|---|---|---|
| Uncoated metals | 51 | 48 | 43 |
| Coated metals | 51 | 48 | 43 |
| Ceramic, glass and stone-like materials | 56 | 48 | 43 |
| Plastics | 60 | 48 | 43 |
| Wood | 60 | 48 | 43 |
Contact with cryogenic surfaces can cause pain, numbness or localized frostbite on exposed areas of skin.
Where contact with cryogenic liquefied gases is possible, the following hazards are particularly relevant:
Severe frostbite or cold burns caused by direct contact
Embrittlement of materials (affecting many plastics, structural steel) with resulting loss of strength
Oxygen deficiency caused by evaporation of liquid nitrogen in small rooms
| Measures |
|---|
Hot surfaces on plant:
Where contact with hot plant surfaces is possible, ensure that contact protection is provided in order to prevent burns. Where contact with hot surfaces is not possible even during infrequent work, contact protection is not required. Where technical measures to prevent contact with hot surfaces are not possible, organizational measures (such as marking of the hot surfaces) and personal measures (such as the wearing of insulating, temperature-resistant clothing and personal protective equipment) must be taken.
Hot surfaces on workpieces:
Where sufficient residual heat is still present in workpieces, it can cause major burns. Ensure that whilst these workpieces are cooling down, they are stored in such a way that contact with them is not possible. Where such measures are not possible, take organizational measures (e.g. barrier tape, warning signs). Ensure that employees who have contact with these workpieces wear personal protective equipment, such as insulating protective gloves.
Cryogenic liquefied gases:
Where parts of plant, work equipment and tools come into contact with cryogenic liquefied gases, the temperature of these gases must not cause them to become brittle. Copper, austenitic steels and certain aluminium alloys are suitable materials. Of the plastics, PTFE is suitable under certain conditions.
Protection similar to that for hot surfaces must be provided against contact with cryogenic surfaces on plant.
Surface temperature of the skin in °C
Figure 42
Severity of burns
- (1)
No change in the tissue
- (2)
First-degree burns (reddening of the skin, painful swelling)
- (3)
Transition zone
- (4)
Second-degree burns (blistering, partial destruction of the skin) and third-degree burns (complete destruction of the skin)
Source: Figure 6.1-1. Severity of burns as a function of skin temperature and exposure duration (SKIBA, 1979)
Should your employees be at risk of contact with non-insulated cryogenic surfaces or with cryogenic liquefied gases, they must wear personal protective equipment. When cryogenic liquefied gas is transferred to containers at ambient temperature or objects at ambient temperature (or higher) are immersed into cryogenic liquefied gas, spatter - possibly violent - must be anticipated. The likelihood of contact must be considered during selection of personal protective equipment.
 | Personal protective equipment |
|---|
Observe the following when selecting personal protective equipment:
Clothing should be clean, dry and made of natural fibres, and should completely cover the arms and legs.
Protective gloves must insulate well and be made of material that does not embrittle (e.g. leather).
Both clothing and protective gloves should fit loosely to allow swift removal in the event of wetting or ingress of cryogenic gas.
Spectacles do not provide sufficient protection; face protection should therefore be worn.
Ensure that cryogenic liquids cannot escape during transport, for example by using suitable sealable containers that prevent an impermissible build-up of pressure.
Evaporation of one litre of liquid nitrogen results in approximately 700 litres of gaseous nitrogen being formed. Should cryogenic liquefied gases escape in rooms of low volume, the atmospheric oxygen can thus rapidly fall below 17 %, resulting in an oxygen deficiency. You must therefore ensure adequate ventilation, room air monitoring or equivalent measures in rooms of low volume.
3.2.13 Operation of oil baths
Oil fires are among the most frequent causes of damage in hardening shops. What action must you take to keep these and other hazards low when operating oil baths?
| Hazards |
|---|
Hardening oils are flammable liquids. Fires often cause only material damage. Persons are also frequently injured however, for example during fire-fighting or evacuation of the affected rooms. The material damage may cause long disruptions to production or production processes.
Contamination of the oil bath with water is the most frequent cause of oil bath fires. Owing to the very high volumetric expansion of water (1 litre of water produces approximately 1,700 litres of water vapour), very violent reactions may occur during the quenching process. The consequences are the formation of oil foam, overflowing of the oil bath and in extreme cases the ejection of oil from the bath.
Oily deposits in the lines of the waste gas extraction system also give rise to an elevated fire hazard, since with the exception of the ignition source, all conditions for a fire (presence of air, oil vapours from low-boiling substances) are almost always met.
Further possible causes of fires on oil quenching facilities:
Severe local overheating of the oil bath caused by bulk material charges containing small, thin parts (large surface area in relation to the weight)
Failure of the oil bath’s cooling or circulation systems
Heating running continuously owing to failure of the control system
Level in the oil quench bath too high or too low
Oil bath not rated for the weight of the charge, resulting in the oil bath temperature being too high during quenching
Incomplete insertion of the charge into the oil bath (malfunction/operator error)
Fires also occur in the tempering furnace area. The most frequent reason is carry-over of oil into the tempering furnace, caused by problems during cleaning of the oily charge after quenching.
A further issue are deposits of burnt hardening oils. Incomplete combustion of quenching oils in the absence of atmospheric oxygen can produce fumes, aldehydes, soot and polycyclic aromatic hydrocarbons (PAHs), the latter being clasified as carcinogenic.
Figure 43
Charge immersed in an open oil bath
| Measures |
|---|
Take the following measures to reduce the risk of oil fires:
Where waste gases contain oily vapours, the extraction equipment, particularly the pipelines, must be cleaned regularly.
On water-cooled systems, check the water content of the quenching oil: it must not exceed 0.1 % by weight.
When hardening oil is topped up or refilled, check that the oil to be used is free of water.
Oil baths must not be filled above the maximum permissible capacity; should the bath be overfilled, the excess oil must be drained off. The maximum oil bath level must allow for the oil displacement caused by the charge.
Where bulk material charges contain small, thin parts, the charge weight must be reduced. Generic recommendations for the magnitude of this reduction are not possible.
An additional manual check of the oil bath temperature should be performed, for example at each change of shift. The operating temperature of the oil bath should be at least 60 °C below the flash point of the hardening oil.
An additional manual check of the oil bath level should be performed, for example at each change of shift.
Proper functioning of the oil bath circulation system and other plant components with a bearing on safety must be ensured, for example by weekly checks of the temperature characteristics in the oil bath.
Following the cleaning process, the hardening material must be checked for residual oil.
The installation of stationary extinguishing equipment has proved effective on open oil baths.
Systems employing water as the operating medium (e.g. cleaning plant) should not be erected in the vicinity of open oil baths.
Should the charge be lowered into an open oil bath by crane, observe the following points:
Swift lowering with a minimum quenching rate of 20 cm/s must be possible.
It must be possible for the charge to be lowered even in the event of a power failure; this can be assured for example by means of a brake lifter operating independently of the power supply, or an emergency power supply.
The cranes should feature facilities by which the crane can be positioned quickly above the oil bath, such as position switches or markings on the craneway.
Figure 44
Oil air cooler
Operation of the crane must be possible without danger to the operator even when the surface of the bath is on fire. This can be achieved by remote control, suitable heat shielding on the controls, and if necessary fire-retardant overalls.
Further problems may be presented by the following situations:
Malfunction of the oil bath elevator on multi-purpose chamber furnaces during the quenching process
Jamming of the oily charge in the flame curtain area
In continuous systems in which the hardening material is dropped into the oil bath via a chute, jamming or back-ups in the chute can cause oil bath fires. Ensure therefore that discharge is continuous.
Leaks on the suction side of the recirculating pump in the oil cooling circuit, for example on the seals, may enable small quantities of air to enter. The leaks are often so minor that no oil escapes when the pump is at a standstill, and they cannot be detected.
Should the hardening material not be adequately cleaned in the downstream cleaning plant, hardening oil may be carried over. This then evaporates during tempering.
Should larger quantities evaporate within a short space of time, a risk exists of a dangerous explosive atmosphere forming. At the very least, the hardening oil is deposited in the insulation of the tempering furnaces and in the exhaust lines, where it forms a potential fire load. Ensure therefore that the cleaning plants are working effectively.
Unauthorized persons present in the proximity of the oil baths during quenching. The presence of persons below ground level is particularly critical; additional hazards may arise here, for example owing to automatic CO2 extinguishing systems or overflowing oil baths.
Figure 45
Crackle test
| Oil sample |
|---|---|
| The water content in the oil may vary widely from one part of the oil bath to another. Sampling should be carried out at operating temperature and with circulation activated. Passing a container (with a volume of 1/2 to 1 litre, open at the top) through the chamber furnace together with the charge has proved to be an effective method of obtaining the most representative oil sample possible. As soon as the charge has left the chamber furnace, the container can be removed and its contents transferred to a suitable vessel for further analysis. Since water is heavier than oil, a sample should be taken from the bottom of open oil baths. For this purpose, the sampling container should be provided with a lid that can be opened when the container is at the bottom of the oil bath. A simpler solution is for a filter with drain cock to be connected, the intake point of which is at the lowest point in the oil bath. The water content in the hardening oil can be ascertained with relative ease by the crackle test. For this purpose, 4 to 5 cm3of the hardening oil to be tested is heated in a test tube over a Bunsen burner. The presence of water is revealed by crackling, shocks and strong formation of foam at levels as low as 0.05 % by weight. Larger quantities of water are indicated by the oil assuming a cloudy, coffee-brown appearance. Personal protective equipment must be worn during performance of the crackle test; the opening of the test tube must be directed away from the body and not towards other persons. | |