DGUV Information 209-090e - Activities relating to Magnesium

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Abschnitt 2.1 - 2.1 Metal cutting machines: machine tools (e. g. machining centres, turning, milling, drilling and grinding machines)

2.1.1
Dry processing, single-part production (except for grinding machines)

During dry processing in single-part production, highly flammable chips have to be considered as a significant hazard.

The chips must be easy to remove in order to keep the fire load low in the machine area. Provisions for chip removal have to be specified in the cleaning schedule. Machining centres have to be thoroughly cleaned from other materials (especially ferrous materials) before machining.

Blowing off the chips in the interior of the machine by a compressed air pistol is generally not permitted due to the danger of whirling up the chips and must therefore be avoided.

The machine area and the surroundings of the machine have to be kept dry.

Ignition sources in the work area must be avoided. For this reason, for example tools are also checked for their condition and especially for wear. Furthermore, the working area has to be kept free from further ignition sources and marked with the prohibition sign P003 (no open flame; fire, open ignition source and smoking prohibited - source: ASR A1.3). The prohibition of open flames, fire, open ignition source and smoking must be implemented (safety marking according to ASR A1.3)

2.1.2
Dry processing in serial production and grinding, brushing, polishing

During dry processing in serial production as well as during grinding, brushing and polishing of magnesium components, dusts may, in addition to the chips, increasingly occur as significant hazard (hazard of a dust explosion).

The protective measures for dry processing in single-piece production apply to series production as well.

Besides the above mentioned measures, the following additional measures are required due to the generated dusts.

  • The dusts must be easy to remove so that no larger quantities can accumulate in the machine area. Provisions for dust and chip removal have to be specified in the cleaning schedule.

  • Generally, chips and dusts as well as magnesium waste have to be removed from the working area as fast as possible. For this reason, the (rather small) chip containers have to be cleaned regularly.

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Figure 4
Burning magnesium in the chip containerr

  • Removing dust from the interior of the machine by blowing off with a compressed air pistol has to be excluded due to the danger of whirling up the dusts. Cleaning the work clothes from dusts with compressed air must be prohibited. (TRGS 500).

Extractions, extraction systems as well as vacuum cleaners for cleaning have to be suitable and approved for magnesium dust with regard to fire and explosion hazards. (see clause "intended use" in the operating instructions of the relevant products).

This applies in particular to the

  • flow rate (min 20 m/s),

  • monitoring of the minimum air-flow rate,

  • prevention of dust deposits,

  • capture of dusts.

Generally, all systems, e. g. for grinding and brushing magnesium, are extracted via wet scrubbers. The extracted air flow is permanently running during machining and is monitored (flow rate: v > 20 m/s). The wet scrubber is equipped with vents so that unavoidably generated hydrogen can escape and cannot accumulate.

A continuous potential equalization (earthing from the grinding booth up to the wet scrubber including lightning protection according to EN 60204-1) is another precondition.

Depending on the risk assessment, in practice, e. g. in case of large chip accumulations, a fire detection system is installed in the interior of the machine. Outside the machine, an additional manual fire detection device (manual actuation) is installed. The signal is normally transmitted to the factory fire brigade or a central control station.

To enable access to manual fire suppression or elimination of the source of fire, interlocking guards with guard locking, e. g. with sealed emergency unlocking, are installed at access doors.

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Figure 5
Interlocking guard with guard locking/position switch with emergency unlocking by Bowden cable

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If a fire is to be suppressed manually, the machine door must only be opened by specifically instructed personnel or by the fire brigade. Whirling up due to application of the extinguishing agent must be strictly avoided (risk of dust explosion).

For information on instructions, see DGUV Leaflet Fachbereich AKTUELL FBHM-043 "Brand an Werkzeugmaschinen - Was ist zu beachten?" (English version: "Fire on machine tools - What has to be considered?")

Furthermore, regular maintenance and cleaning of the system must be ensured. The cleaning in the machine surroundings has to be specified in a cleaning schedule as well.

The capture of the magnesium containing dust deposits must only be carried out with approved industrial vacuum cleaners, if necessary with hydrogen venting. In case of moistened dusts, the free flow of generated hydrogen has to be ensured. Industrial vacuum cleaners without internal ignition sources are suitable (e. g. identification type plate: type 22, EPL Dc [1]). The dust containers should be emptied on every workday. Cleaning intervals and methods have to be documented in a cleaning schedule. For information on industrial vacuum cleaners and dust extractors, see DGUV Information 209-084 Industriestaubsauger und Entstauber".

Dusts and chips adhering to the work clothes represent a particular hazard. These dusts can be easily ignited and lead to a very fast and violent fire propagation with very high temperatures, if clothes are on fire.

In order to minimize the adherence of magnesium dusts during activities (e. g. deburring workpieces), suitable protective clothing with a smooth surface without pockets have to be worn (e. g. flame-resistant rubber or leather apron with smooth surface, see also Clause 3 Personal protective equipment).

Clause 6 describes the collection and storage of dry chips and dusts.

2.1.3
Machining with water-miscible metalworking fluids

Chips, hydrogen formation and deposits related to metalworking fluids present the hazards which have to be taken into account when machining with water-miscible metalworking fluids (MWF).

Protective measures

Chips have to be easy to remove in order to keep the fire load low in the machine area. They should be flushed by large quantities of water-miscible metalworking fluids (flush cleaning).

The entire machine interior should be provided with additional flushing nozzles to remove chips and avoid chip accumulation. For manual cleaning and removal of chips in the interior of the machine, an additional flushing hose with emulsion has proved to be useful.

A sufficient quantity of metalworking fluid at the machining point (flush cleaning) has to be ensured, e. g. by monitoring the MWF supply by pressure controls or flow monitors (for information on the design of the MWF circuits, see VDI 3035). This avoids "dry running" and the accumulation of chips.

Generally, MWF with additives (inhibitors) should be used, which largely minimize hydrogen formation.

In the course of machining, contents may be discharged; therefore, regular inspection and maintenance of the metalworking fluids is necessary, preferably in close cooperation with the MWF manufacturer.

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Generally, water-miscible metalworking fluids have to be regularly (e.g. weekly) monitored according to DGUV Rule 109-003 "Tätigkeiten mit Kühlschmierstoffen" (for pH-value, concentration, nitrite, see Clause 7 Inspection, Maintenance).

It is furthermore recommended to control the water hardness regularly to counteract an increase at an early stage and thus avoid massive deposits.

The reaction of magnesium with the MWF may lead to problems due to the formation of solid magnesium soaps and an increase of water hardness (hardening, salting). These deposits (crusts and layers as hard as glass) may lead to clogging of the pump and circuit systems and thus considerably impair the MWF supply.

A regular cleaning of the emulsion from magnesium particles is necessary. This can be achieved by filtering, e. g. by means of a filter fleece/pressure belt filter. Thus, "salting" by the solvation of magnesium ions can be minimized/delayed.

The filter fleeces covered with moistened, fine magnesium particles constitute a reactive fire load and must therefore be regularly removed from the working area and stored in suitable containers (e. g. non-combustible, closed with vent).

For eliminating the emulsion mist which is generated in the interior of the machine and to avoid accumulation of gaseous oxygen, an extraction system has to be provided.

The precondition for the start of the machine is an active extraction system maintaining the minimum volume flow/extracted air flow specified by the machine manufacturer (control e. g. by means of pressure or flow controls).

If the required extraction rate is not achieved or in case of failure, a signal has to be indicated by an automatic warning device or the machine must be stopped. After machining has been finished, the overrun of the extraction system has to be ensured.

Water accumulations have to be avoided even at a standstill of the machine and the extraction system. This requires the provision of vents in the top area inside the machine as well as in the extraction system (if possible at the highest point).

It has proven useful to design the top surfaces with an upward slope towards the opening. Large cavities in the machine foundation and in the enclosures have to be equipped with vents in order to prevent water accumulation.

Depending on the risk assessment, in practice, for example, if there is a hazard of increased chip accumulation, a fire detection unit in the interior of a machine and an additional manual fire detection device (for manual activation) outside the machine are installed. The signal is normally transmitted to the factory fire brigade or a central control station.

To enable access to manual fire suppression or to eliminate the source of fire, interlocking guards with guard locking, e. g. with sealed emergency unlocking are installed.

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Figure 6
Interlocking guard with guard locking/position switch with emergency unlocking by Bowden cable

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If a fire is to be suppressed manually, the machine door must only be opened by specifically instructed personnel or by the fire brigade. For guidance on instruction, see DGUV Leaflet Fachbereich AKTUELL FBHM-043 "Brand an Werkzeugmaschinen - Was ist zu beachten?" (English version: "Fire on machine tools - What has to be considered?")

Furthermore, an effective ventilation in the hall area has to be ensured.

It has to be taken care that the moistened sludge conveyed from the interior of the machine is removed and conveyed to a container of limited volume.

In order to keep the quantity of moistened chips on a low level (reactive fire load), non-combustible chip containers with vents are required for emerging hydrogen.

If possible, the transfer of large quantities of chips from the machine directly to briquetting has proven to be useful. Attention should be paid to a low residual moisture content.

The work area, the machines and the chip container have to be marked with warning signs (see Clause 8 "Marking ..."). The prohibition of open flames; fire, open ignition sources and smoking must be implemented (safety marking according to ASR A1.3).

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Figure 7
Briquetted chips

2.1.4
Machining with non water-miscible MWF (neat cutting oil)

2.1.4.1 Conventional wet processing

As for all conventional processings of materials with non water-miscible metalworking fluids (neat oil), the machining process leads to an atomization (MWF/air mixture) in the interior of the machine.

There is a risk of ignition of the MWF/air mixture in the work area due to hot surfaces or sparks. This may result in flashovers.

Fires in the interior of the machine (e. g. fire of the oil or of oil wetted chips) and flame ejections (e. g. from gaps, housing doors, loading and unloading openings) into the machine surrounding are the consequence.

In the further course of the fire, a fire may spread into the interior of the machine as a result of the ignition of oil-soaked magnesium chips. In the worst case, a secondary light-metal fire of the magnesium chips may develop.

Protective measures:

By selecting low-emission metalworking fluids, the formation of aerosols and vapours at the workplace can be reduced. Low-emission metalworking fluids are characterized by the following properties: (see DGUV Rule 109-003 "Tätigkeiten mit Kühlschmierstoffen", DGUV Information 209-026 "Brand- und Explosionsschutz an Werkzeugmaschinen" (English version: DGUV Information 209-027 "Machine Tool Fire and Explosion Prevention and Protection"), VDI 3802, sheet 2-03/2012 "Air conditioning systems for factories - Capture of air pollutants at machine tools removing material"):

  • formulated on the basis of low-evaporation mineral oils or synthetic esters or special liquids (e. g. polyalphaolefins),

  • addition of anti-mist additives.

TendencyViscosity grade acc. to DIN ISO 3448Viscosity at 40 °C acc. to DIN 51562Flashpoint acc. to DIN EN ISO 2592 (CoC)Evaporation losses at 250 °C acc. to DIN 51581-1, 2 (Noack procedure)Examples of machining processes
g_bu_948_as_10.jpgISO VG 54,14 - 5,06 mm2/s > 120 °C< 85%Honing, reaming
ISO VG 76,12 - 7,48 mm2/s > 145 °C< 80%Grinding, deep hole drilling
ISO VG 109 - 11 mm2/s > 155 °C< 60 %Turning, milling
ISO VG 1513,5 - 16,5 mm2/s > 190 °C< 25%Drilling
ISO VG 2219,8 - 24,2 mm2/s > 200 °C< 15%Threading
ISO VG 3228,8 - 35,2 mm2/s > 210 °C< 13%Thread rolling
ISO VG 4641,4 - 50,6 mm2/s > 220 °C< 11%Broaching

Table 1
Characteristics of combustible non-water miscible metalworking fluids (Source: DGUV Information 209-026 "Brand- und Explosionsschutz an Werkzeugmaschinen" (English version: DGUV Information 209-027 "Machine Tool Fire and Explosion Prevention and Protection"))

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Figure 8
Shut-off valve for air, extinguishing nozzle and flow sensor

It is principally recommended to select the MWF with the lowest vaporization losses and the highest flash point. If a viscosity range (viscosity index) is predetermined for a machining process, the MWF with the highest possible viscosity should be selected. See DGUV Information 209-026 "Brand- und Explosionsschutz an Werkzeugmaschinen" (English version: DGUV Information 209-027 "Machine Tool Fire and Explosion Prevention and Protection").

A sufficient MWF quantity at the machining point (flood lubrication) has to be ensured (for general information on the design of MWF circuits, see VDI 3035), e. g. by monitoring the MWF supply by means of pressure controls or flow monitors.

An extraction system has to be provided in order to remove the developing oil mist in the interior of the machine. The precondition for the start of the machine is an operating extraction system which maintains the minimum volume flow/ extracted air flow specified by the machine manufacturer (control e. g. by means of a pressure or flow control/sensor).

If the required extraction rate is not achieved or in case of failure, an indication is given by an automatic warning device and the machine shuts down.

In case the machine’s housing/enclosure is not sufficiently resistant, a pressure relief device has to be provided, preferably in the cover of the machine tool.

It should direct the flames and hot combustion gases which are generated as a result of an ignition into areas where they do not present a hazard to the operating personnel.

It is intended to ensure pressure relief due to a flashover of oil aerosols as directly as possible. Thus hazards to the machine personnel by ejecting flames would be reduced.

If the operation of a machine tool involves a high risk of fire and hazards of subsequent metal fires, integrated fire detection and fire suppression systems will have to be provided (see DIN EN ISO 19353).

The requirements for integrated fire detection and fire suppression systems as well as for the type of extinguishing agent also represent a decisive part of the safety concept. A fire which develops inside a machine must on no account spread over to the magnesium chips since this would cause a metal fire which is hard to control.

Therefore, increased requirements for a safe fire detection are specified, for example by means of monitoring with several combined fire sensors (flame and heat sensors). Furthermore, fire suppression should proceed fast in order to prevent the fire from spreading over to the magnesium chips. The inert gas argon has proved to be a suitable extinguishing agent.

If CO2 is intended to be used as fire extinguishing agent for the oil fire, it is essential to ensure within the frame of a case-by-case analysis that a fire within the machine can in no way spread over to the magnesium chips.

It has to be ensured that magnesium chips cannot accumulate in the interior of the machine, e. g. by means of sufficient flushing devices.

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Figure 9
Schematic diagram; protection concept for neat oil machining (non water-miscible MWF), machining centre for automatic loading and unloading of workpieces

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Figure 10
Schematic diagram; protection concept for neat oil machining (non water-miscible MWF), machining centre for automatic loading and unloading of workpieces; top view

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Figure 11
Extinguishing nozzle and heat detector/temperature sensor in the interior of the machine

To reduce the fire hazard, as few combustible materials as possible should be present inside the machine tool and in the vicinity of the workplace. Chips have to be removed from the interior of the machine, e. g. by chip conveyor and flush cleaning. Chip containers have to be emptied regularly and on demand (preferably at least once per day).

The work area has to be kept free from further ignition sources and marked with the prohibition sign P003 (no open flame; fire, open ignition source and smoking prohibited - source: ASR A1.3) (see Clause 8 Marking). The prohibition of open flames, fire, open ignition sources and smoking must be implemented (safety marking according to ASR A1.3).

2.1.4.2 Minimum quantity lubrication (MQL)

Minimum quantity lubrication (MQL) is a total-loss lubrication method. The lubricant is either sprayed onto the tool via nozzles from outside (external feed) or from inside through the tool (internal feed). In industrial series production, the internal feed is mainly applied.

In contrast to conventional "wet processing", minimum quantity lubrication requires an average of not more than 50 ml lubricant per process hour and tool for the machining process. In exceptional cases, however, the process may be supplied with more than 150 ml/h lubricant for a short period of time (see DGUV Information 209-024 "Minimalmengenschmierung in der spanenden Fertigung", (English version: DGUV Information 209-025 "Minimum Quantity Lubrication for Machining Operations")).

Malfunctions (e. g. incorrect movements, tool crash, blunt tool) can cause sparks. As a result, the oil in the oil-soaked chip accumulations/chip residues may ignite and might then cause a fire in the interior of the machine or the immediate vicinity.

In the further course of the fire, a secondary metal fire can be caused by ignition of the magnesium chips/residues.

Protective measures:

For fault-free, low-emission metal machining when using minimum quantity lubrication, lubricants with very good lubricity and a high thermal rating are best.

In industrial manufacturing, synthetic ester oils and fatty alcohols with favourable vaporisation behaviour and a high flash point are used (DGUV Information 209-024 "Minimalmengenschmierung in der spanenden Fertigung", (English version: DGUV Information 209-025 "Minimum Quantity Lubrication for Machining Operations").

The following guide values have proven to be useful for selecting a low-emission lubricant.

Viscosity at 40 °C DIN 51562 Part 1Flashpoint CoC DIN EN ISO 2592Evaporation losses at 250 °C DIN 51581 Part 1
> 10 mm2/s > 150 °C< 65%

Table 2 Guide values for selecting a low-emission lubricant (source: IFA workbook "Measuring hazardous substances", category 6)

To reduce the risk of fire, the use of lubricants with a flash point of at least 150 °C and a high viscosity (> 10 mm2/min at 40 °C) is recommended.

Within the framework of a research project, the effects of ignition incidents during minimum quantity lubrication on the risk of fire and explosion were investigated under practical conditions (see DGUV Leaflet Fachbereich AKTUELL FBHM-088 "Minimalmengenschmierung - Risiko von Bränden und Explosionen").

If the following two key points are observed, MQL machining with lubricant quantities up to

1000 ml per processing hour and per m3 workspace/machine interior (~ approx. 15 ml per minute per m3 workspace/machine interior)

is possible without the hazard of an aerosol-air-mixture ignition.

1. Use of low-emission MQL lubricants

For application in machine tools, the use of low-emission MQL lubricants with the properties mentioned in table 2 according to DGUV Information 209-024 "Minimalmengenschmierung in der spanenden Fertigung" (English version: DGUV Information 209-025 "Minimum Quantity Lubrication for Machining Operations") is recommended.

It is strongly advised against using low boiling liquids such as ethanol.

2. Extraction with continuous volume flow and monitoring

The extraction volume flow should be set high enough so that there is a slight negative pressure as well as an inward flow inside the machine to prevent emissions escaping from the enclosure.

An extraction volume flow of 400-600 m3/h per m3 machine room has proven its value in industry. Information on the optimal setting is included in VDI 3802 sheet 2 and VDI 2262 sheet 4.

Generally, an extraction system has to be provided for the efficient disposal of the oil mists produced inside the machine.

The chips in the machine tool should be removed, e. g. by continuous discharge via chip conveyors.

Chip accumulations can be reduced by a particular design inside the work space of the machine (e. g. by workroom panelling that is steeply inclined, smooth surfaces (e. g. made of stainless steel)).

Depending on the risk assessment, in practice a fire detection system and an additional manual fire alarm system outside the machine (manual release) are installed, for example, if a large amount of chips has accumulated inside the machine. The signal is usually forwarded to the works fire brigade or a central control centre. To enable access for manual suppression or removal of the source of fire, access doors are provided with interlocking guards with guard locking, e. g. with sealed emergency unlocking.

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Figure 12
Interlocking guard with guard locking/position switch with emergency unlocking at the protection door magazine maintenance

g_bu_948_as_7.jpgNote
If manual fire suppression is required in the event of a fire, the machine door may only be opened by specially instructed personnel or by the fire brigade. Any whirling up due to the use of the extinguishing agent must be prevented (hazard of dust explosion).

For useful information on instruction, see DGUV Leaflet Fachbereich AKTUELL FBHM-043 "Brand an Werkzeugmaschinen - Was ist zu beachten?" (English version: "Fire on machine tools - What has to be considered?") Furthermore, regular maintenance and cleaning of the system has to be ensured. The intervals have to be specified in a cleaning schedule (e. g. Sample cleaning schedule, see Annex 2 of DGUV Information 209-024

"Minimalmengenschmierung in der spanenden Fertigung", (English version: DGUV Information 209-025 "Minimum Quantity Lubrication for Machining Operations").

2.1.5
Machining with hand-held grinding machines

When grinding, brushing and deburring magnesium components, ignitable magnesium dusts and fine chips are produced. Magnesium dusts are highly flammable and explosive in air in a whirled-up state.

Deposited dusts in the work area represent a fire hazard. Maximum caution is required if there is a risk of dusts and chips adhering to the work clothes. Adhering dusts are very easy to ignite and, in the event of a clothing fire, lead to an extremely rapid and violent fire spreading with extremely high temperatures.

Flammable sparks are often the cause of clothing fires. The sparks can be created by unintentional contact of hand-held machining tools (e.g putting down the overrunning hand grinder) with spark-generating metallic parts (e.g tool storage, linings, frame parts, extraction hoods, etc.).

Protective measures

The dry grinding dusts produced must be extracted as close as possible to the machining point. In addition, an extraction system especially suitable for magnesium dust must be used at the workplace (see description of suitability for magnesium dust in the Chapter "Intended use" in the Operating Instructions for the products concerned. Requirements for extraction systems for dry magnesium dust, see Clause 2.1.2).

In practice, workplaces for grinding magnesium (e. g. consisting of work table, work surface and edge extraction as well as hand-held grinding machines) are usually extracted by means of wet scrubbers. The extraction system runs permanently during processing and the extracted air volume flow is continuously monitored (flow velocity: v > 20 m/s). The wet scrubber has vents, so that inevitably generated hydrogen escapes and cannot accumulate. A continuous equipotential equalization (earthing from the grinding booth to the wet scrubber incl. lightning protection) is also a prerequisite.

Furthermore, it is required to wear additional suitable protective clothing with a smooth surface without pockets (e. g. flame-resistant rubber or leather apron with a smooth surface). Thus, the adhesion of magnesium dust to the work clothes is minimized. Clothing made of knitted fiber and fleece materials is absolutely unsuitable.

Ignition sources, e. g. grinding sparks from sparking tools, must be excluded from the work area. In the direct vicinity of the work area, metal parts (frame parts, extraction hoods etc.) should therefore be made of "non-sparking" material (e. g. aluminium or copper) if possible. Only the use of compressed-air operated tools is permitted; electrical grinding devices, such as angle grinders, are excluded.

Dusts which are not collected by the extraction system must be removed by regular cleaning of the work area. When doing so, avoid whirling up the dust with compressed air. For this purpose, suitable industrial vacuum cleaners without internal ignition sources (e. g. identification plate type 22, EPL Dc) and, if necessary, with hydrogen venting should be used; the dust collection containers should be emptied every working day. Cleaning intervals and methods must be documented in a cleaning schedule. For information on industrial vacuum cleaners and dust extractors, see DGUV Information 209-084 "Industriestaubsauger und Entstauber".

The work area must be marked with the prohibition sign "Open flames, fire, open ignition sources and smoking prohibited".

Extinguishing agents of fire class D for metal fires shall be kept available. Under no circumstances may water be used to extinguish the fire!

See DGUV Leaflet Fachbereich AKTUELL FBHM-051 "Trockenschleifen von Magnesium".

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Figure 13
Fire of molten magnesium

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Figure 14
Ejection of molten metal on contact with water, approx. 50 ml