DGUV Information 209-027 - Machine Tool Fire and Explosion Prevention and Protec...

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Abschnitt 3.3 , 3.3 Technical and engineering design measure...
Abschnitt 3.3
Machine Tool Fire and Explosion Prevention and Protection (bisher: BGI/GUV-I 719 E)
Titel: Machine Tool Fire and Explosion Prevention and Protection (bisher: BGI/GUV-I 719 E)
Normgeber: Bund
Amtliche Abkürzung: DGUV Information 209-027
Gliederungs-Nr.: [keine Angabe]
Normtyp: Satzung

Abschnitt 3.3 – 3.3 Technical and engineering design measures

3.3.1 Machine tool

Fire hazard

In order to avoid a fire or to limit its consequences, it is important to keep the "fire hazard" as low as possible. This is especially important in areas like the drive unit or on top of the machine, as these areas are generally not protected by extinguishing systems. Horizontal surfaces or areas with possible MWF pool formation or chip accumulation should be avoided. Slightly sloped machine tops facilitate the drainoff of cutting fluids.

Figure 21 Oil pool formation in the drive room

Figure 22 Machine tool in "oil bath"

Flame ejection from the machine

In case of an ignition of the MWF-air mixture and during fires, flames and hot gases may escape from the machine tool. The pressure increase in the interior of the machine causes flame ejections from door gaps, housing doors forced open, loading and unloading openings and pressure relief flaps. Flame ejections can also occur during flooding with extinguishing gas.

Figure 23 Flame ejection from a door labyrinth

Figure 24 Temperatures during flame ejection from the door in the area of the machine operator

A hazard to the operator and the surrounding area by the ejection of flames and hot gases must be avoided. The following requirements are therefore specified for labyrinths in the area around the machine tool doors (see research reports VDW 3001/1 and VDW 3001/2):

  • Flame propagation-inhibiting design: overlapping on both sides with several switch-backs and a maximum gap width of 2 mm. As to the fire hazard, the labyrinths should be so designed that no oil can accumulate,

  • Reduction of the gap widths when the internal pressure increases (ignition of MWF mist) or when the door seals age. Construction of the doors so that they are forced into the machine housing and held in place along with a decrease in gap widths when the MWF ignites (see Figure 30),

  • Design in a way that any ejection of flames or hot gases is not directed towards the area around the operator.

Figure 25 Detail of s sliding door designed for machining with emulsion

Figure 26 Detail of a sliding door with additional labyrinth for machining with neat oil

Furthermore the following should be taken into account:

  • The safety distance at the doors in front of the labyrinth sealing is 30 cm (ejection area of hot gases with temperatures > 60C).

  • If parallel loading and machining is possible at the setting point of the pallet exchanger, the loading area should be separated from the working area with a flame retarding design

  • Rubber or brush sealing is not recommended for technical fire protection reasons (Figure 28).

  • Unavoidable openings such as workpiece openings should be carefully sealed, e. g. by flaps or sliders, which only release during a workpiece change.

Figure 27 Door labyrinth resistant to the outbreak of flames (acc. to VDW 3001/1)

Figure 28 Door sealing not suitable for technical fire prevention and protection

Working area and enclosure

When an MWF-air mixture ignites, there is a danger of insufficiently pressure-resistant housing parts being ejected into the surrounding area, thus allowing the fire to propagate to adjacent machining areas.

The working area and encapsulation should have the following properties

  • Pressure strength of the machining area encapsulation (doors, windows, telescopic covers and other covers) at least 0.1 bar,

  • Design without large gaps, through which the combustion products may escape into the machine surroundings,

  • Sealing-off of adjacent machine areas like handling area or drive unit,

  • Doors should be secured against bursting open or off, for example, with circumferential wrap-arounds and/or locking bolts,

  • Doors equipped with locking devices or interlocking controls,

  • Transparent screens made of framed polycarbonate.

The mechanical containability and, if necessary, the exchange intervals of transparent screens (e. g. polycarbonate) should be considered. A sufficient overlap of the cladding panels on both sides should be provided for the framing (e. g. not enchased in rubber). For more details see research reports VDW 0209 and VDW 0209/1, DIN EN ISO 23 125, DIN EN 12 417 and DIN EN 13 218.

Figure 29 Mounting of transparent screens

Figure 30 Door with wrap-around

Integration of controls into the overall concept

The optimum information exchange between the machine tool controls, the extraction unit and the automatic extinguishing system is the basis for the safe operation of the overall system. An example of the individual switch/control commands are shown in the flow chart.

Figure 31 Switch/control commands between the machine tool, extraction system and automatic extinguishing system

It should only be possible to start the machine if

  • The extraction and chip removal systems are ON,

  • Door are locked and interlocked,

  • The extinguishing system is ready to operate.

Indicated failures should be automatically notified and cleared without delay. Only then, may the system be started. When a fire is detected, the following machine control functions must be initiated:

  • Stopping axes and drives as quickly as possible,

  • Immediate halting of any metalworking fluid air purging,

  • All covers must remain locked and interlocked shut,

  • Shutting-down of the extraction system and closing of the exhaust air shut-off valve (this should stop the fresh air supply and prevent extraction of the extinguishing gas),

  • Initiation of the extinguishing process (for extinguishing gas, e. g. CO2, the possible time lag should be taken into account),

  • Activation (optical and acoustic) of the alarm system,

  • If applicable, isolation of the machining area (extinguishing area) and loading areas (for example, closing doors, blinds, etc.).

3.3.2 Extraction systems

During chip-forming operations in machine tools with defined and undefined cutting edge geometries using non water-miscible metalworking fluids (MWF), MWF mists and vapours are generated. In order to reduce enrichment of the flammable and possibly explosive MWF emissions inside the machine tool and in the immediate surroundings, they are captured, extracted and separated by extraction systems.

Separation systems for metalworking fluids

There are several filtration methods for the separation of metalworking fluids.

The minimum requirements for the choice of the separator are the following:

  • Observation of the pure gas values over the whole maintenance cycle,

  • Low volume variation and observation of the minimum volume flow over the whole maintenance cycle,

  • Safe observation of the pure gas values even during raw gas peaks,

  • The safe discharge of the separated fluids/oils should be guaranteed.

Filtering separators

In filtering separators, the extracted air to be cleaned is passed through a porous medium, where the dispersed solid or aerosol particles are retained by various mechanisms.

There is a distinction between surface filters (see VDI 3677 Sheet 1) and depth filters (see VDI 3677 Part 2).

Electrostatic separators

The function of electrostatic separators is based on the physical principle of the deflection of electrostatically charged particles in an electrical field.

The solid and/or liquid particles contained in the extracted carrier gas (air) are unipolarly charged in the ionization zone. Separation takes place in the electrostatic field between charged plates in the downstream separation zone.

For further information on electrostatic separators see VDI Directive 3678 Sheet 2.

Inertia separators

In all inertia separators, the particles and aerosols are separated by means of inertia, gravitational or centrifugal force by aimed deflection from the gas flow. There is therefore a distinction between gravitational separators, deflection separators and centrifugal separators.

The most commonly used inertia separators for metalworking fluids are generally metal filters acting as pre-filters. They are especially used as pre-separator systems for non water-miscible metalworking fluids. For inertia separators, see also VDI 3676.

Design criteria of extraction systems with regard to fire and explosion prevention and protection

In general, systems for the extraction of flammable air impurities and explosive mixtures should be made of conductive or dischargeable electrostatic materials and should be earthed.

The precondition for the start of the machine is an operating 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, the machine must be stopped. Delayed shutting-down may be incorporated into the control unit to allow machining cycles to end.

Figure 32 Pressure control

Figure 33 Flow monitors

Separators

  • The separator should be designed so that no moving parts or electric equipment with surface temperatures above the ignition temperature of the extracted oil mists are on the intake gas side (ignition source free type).

  • The extraction fan is on the air intake side.

  • The extracted and cleaned air from the machine should be directed, if possible, into the open to minimize fire hazards inside the hall caused by residual emissions.

  • As regards the machine tool, it should be checked if an ignition source can travel to the oil mist separator via the extraction ducts (e. g. hot chips). If this cannot be excluded, the oil mist separator must be integrated into the fire extinguishing concept of the machine tool.

Extraction capacity

In order to achieve the best possible degree of efficiency, the air should be extracted from a totally encapsulated machine tool. The optimum exchange of air inside the enclosure by appropriate supply air openings should also be ensured.

To avoid MWF aerosols and vapours escaping, low pressure must be maintained inside the enclosure. The air motion should always be directed towards the machining room and not vice versa. An air flow directed towards the machining room can, for example, be verified by misting tests.

  • The reference value for the air flow velocity at enclosure openings into the machining room is 0.1 m/s [VDW 3001, Page 42].

  • The extraction capacity should be individually adjustable via a throttle valve or a speed control at every extraction point.

  • Depending on the machining process and the design of the machine tool, 100 to 300 air exchanges per hour in the machining area (m3) of the machine tool are recommended.

Example: Air exchanges machining area= extraction capacity
 300 h-1 air exchange 1,5 m3 work room= 450 m3/h extraction capacity

Extraction point

The extraction point (connector) in the machine interior should be designed so that no coarser particles, metalworking fluid and chips can get into the extraction system and accumulate in the pipes. The optimum extraction of metalworking fluid emissions is achieved if the following criteria are taken into account or observed:

  • Extraction point as far away as possible from the machining zone,

  • Avoid lateral flows at the extraction point,

  • Consideration of the arrangement of MWF nozzles, nozzle placing, main atomization direction and chip flight when selecting the extraction point,

  • Installation of baffle plates or mechanical pre-separators. This avoids the introduction of MWF aerosols and chips into the extraction circuit,

  • The air velocity at the extraction point should be as low as possible (< 8 m/s).

The desired flow rate (16-18 m/s) in the ducting can be achieved by reducing the duct cross-section after about the first meter.

Figure 34 Baffle plate above extraction point

Figure 35 Clogged chip grating

Ducting

  • Ducting should be non-inflammable and should not be electrostatically chargeable (ensure that ducting is earthed),

  • No use of folded spiral-seam ducts,

  • Ducting should be routed so that no introduced or condensed MWF can accumulate inside (avoid cavities and uneven ducting),

  • Avoid flexible (corrugated) plastic pipes/hoses or the like if they are not compulsory for vibration isolation (if possible, fit vertically and keep as short as possible, only use electrostatically conductive materials; Figure 37).

Figure 36 Accumulation of oil in the extraction ducting

Figure 37 Connecting piece for vibration isolation: vertical, earthed

  • The flow velocity should be < 8 m/s at machine connections and between 16-18 m/s in the supply lines.

  • For the interior control of the ducting (oil deposits and chip accumulations) control/inspection hatches should be installed at required intervals.

  • For the drainage of condensed oil along the extraction piping/ducting, drainage pipes with suitable siphons capable of being adapting to the pressure level should be fitted. The drainage pipes should be easy to check and to clean.

  • Rapid-action shut-off valves or flame arresters should be installed in the piping/ducting at interfaces outside the machine.

Figure 38 Inspection holes

Figure 39 Inspection holes 2

Rapid-action shut-off valves and flame arresters

Rapid-action shut-off valves are intended to reduce the risk of flames entering the piping and ducting and propagating to other areas. In case of fire, the rapid-action shut-off valve seals-off the machine tool from the extraction system and vice versa. In addition, the rapid-action shut-off valve also serves to disconnect the machine tool in case of a malfunction or shut-off of the extraction system. The shut-off valve actuation signal may either be generated by the machine tool or directly from the central fire alarm system.

If a fire occurs in the machine tool, the shut-off valve immediately closes in order to protect the piping, ducting and extraction system. In addition, the quantity of extinguishing agent, e. g. CO2, necessary for the machine tool is thus significantly reduced.

Ideally, the activation is via the machine tool, as it is then also possible to disconnect the extraction for the machine tool in question in case of a malfunction without the necessity of disabling the whole central extraction system.

The rapid-action shut-off valves should be appropriate to the actual set up. The valve should have the following characteristics:

  • Non-flammable material,

  • Closing time: < 1,5 sec,

  • Final position monitoring,

  • Closed without electrical power or pressure.

The piping/ducting should be protected against flame propagation to other areas by, for example, the installation of reliable flame arresters.

Figure 40 Shut-off valve for extracted air

Local extraction

Electrostatic or mechanical filters:

  • Start-up of the machine should only be possible with the extraction system working.

  • In case of fire, interruption of extraction within 10-30 s after detection (immediate activation), for example, by means of a motor brake or an automatic shut-off valve (the time delay until interruption of the air flow is determined by the programmed quantity of extinguishing agent released in the case of automatic extinguishing systems).

Central extraction

  • Connection to a central extraction system is only permitted if no explosive substances or mixtures, e. g. from other processes, are present (information in the operating instructions!). This measure, among others, is necessary in order to avoid the propagation of fires into surrounding piping/ducting.

  • Every effort should be made (baffle plates, mechanical pre-separators etc.) to ensure that as little MWF as possible enters the extraction system from each machine.

  • Start-up of the machine should only be possible with the extraction system working.

  • In order to avoid fire propagation, an automatic shut-off device (e. g. shut-off valve for extracted air) should be installed. Activation is generally by the extinguishing system control unit in the case of centrally operated systems.

  • Any malfunction in the extraction system must be signalled. In case of a breakdown, the relevant machine tools must be shut-down at the end of the machining cycle if extraction cannot be ensured. This is necessary, if in case of breakdown of the system, explosive mixtures form, which could ignite after escaping from the machine enclosure.

  • A malfunction inside a machine tool must be signalled. The machine tool is then isolated from the extraction system by means of a shut-off valve. The rest of the extraction system can remain in operation.

  • In the flow monitor of an individual machine indicates malfunction, the machine should be shut-down at the end of the machining cycle.

  • The extracted air shut-off valve should be closed if machines are not running or if in emergency-stop mode.

3.3.3 Pressure relief devices

In case the machine's encapsulation is insufficiently pressure-resistant, a potential injury hazard exists to persons if housing parts blow-off or flames eject when a MWF mixture is ignited. In such cases, a pressure relief device should be fitted for such pressure peaks.

The pressure relief valve has the purpose of releasing excess pressure generated by the ignition of a mixture to the machine's surroundings. Flames and hot combustion gases resulting from ignition should be directed to safe areas.

The pressure relief valve is usually installed in the cover of the machine tool. It is intended to relieve pressure as quickly and directly as possible and thus reduce risk to machine operators.

As machine enclosures often only have low pressure resistances ( 100 mbar), the response pressure of relief devices fitted to machine tools should be less than 5 mbar. The device only opens briefly and shuts back closed. This should prevent the rekindling of flames by the introduction of air as well as avoiding flame propagation.

Figure 41 Pressure relief valve open

Figure 42 Pressure relief valve with wire cage, outside

When a MWF/air mixture ignites, long jets of flame may escape from the pressure relief device which pose a hazard to the surroundings of the machine. As a result, no flammable materials (wooden crates, insulation, etc) should be located above the pressure relief valve. A minimum distance to the hall ceiling should be maintained (details from the manufacturer of the pressure relief valve) in order to avoid the reflection of flames. Personnel should be instructed to keep the danger zones around and above the pressure relief device free.

Many pressure relief devices are also equipped with a wire mesh or fine wire gauze in order to hinder flames shooting out. However, such wire mesh guards affect the pressure relief process. In practice, such guards can represent an additional fire hazard if they are heavily wetted/polluted with oil and this also applies to the pressure relief device itself.

The possible danger of workpiece or tool parts being ejected through the pressure relief valve and the hazard of the pressure relief valve catching fire should also be taken into account during planning.

Figure 43 Flame ejection from a pressure relief valve

Figure 44 After-Fire subsequent to extinguishing

As a rule of thumb for dimensioning, the companies Total Walther GmbH, Cologne, and Deutsche Montan Technologie (DMT), Dortmund have, based on tests, determined that a minimum pressure relief area of 0.1 m2 per m3 of encapsulated volume is satisfactory. For higher strength enclosures, it may be reduced (e. g. 0,05 m2 pro m3 enclosed volume) or replaced by alternative concepts (e. g. opening of the chip conveyor into safe areas).

A more detailed design including transfer to common pressure relief devices may be carried out according to research report VDW 3002.

3.3.4 Fire prevention and protection

3.3.4.1 Extinguishing agent

Extinguishing agents for fires of flammable metalworking fluids can be:

  • Extinguishing gases, e. g. oxygen displacing gases like CO2, N2, inert gases and their mixtures,

  • Water (using water atomizing technology, water misting technology),

  • Foam,

  • Powders of fire classes ABC or BC (oil fires correspond to fire class B).

Attention:

If carbon dioxide is used as the extinguishing agent, health hazards have to be anticipated at concentrations of 5 per cent by volume or more. Concentrations of more than 8 per cent by volume can pose a danger to life (see "Regeln für Sicherheit und Gesundheitsschutz bei der Arbeit - Einsatz von Feuerlöschanlagen mit sauerstoffverdrängenden Gasen" BGR 134).

Metal fires (e. g. Mg, Al, Ti) cannot be extinguished with extinguishing agents of fire classes A, B and C! At present, inert gases (e. g. argon) and powder extinguishing agents of fire class 4 exist for the fighting of metal fires. Suitability for the extinguishing of metal fires must be proven for all other extinguishing agents.

Some extinguishing agents (e. g. extinguishing powders) do not extinguish without leaving residues and may pollute machines and the surroundings.

Extinguishing of machine fires

If the operation of a machine tool involves a high risk of fire, integrated fire alarm and extinguishing systems must be installed (DIN EN 13 478). Here, the order should be as follows:

  • Manual extinguishing system,

  • Fire alarm system in combination with a manually operated extinguishing system,

  • Fire alarm system in combination with an automatic extinguishing system.

Figure 45 Choice of suitable extinguishing systems

In practice, the implementation ranges from a fixed fire extinguisher with corresponding piping to a fire alarm system coupled to an automatic extinguishing system.

The choice of the extinguishing method and the integrated fire alarm and extinguishing systems used for machine tools depends on the degree of potential hazard to persons, property and the environment.

Figure 46 Warning notice for CO2 extinguishing gas

Figure 47 MWF ignition during grinding with subsequent fire

The fast detection and the fast extinguishing of a fire by automatic fire extinguishing systems is essential, depending on the degree of risk of:

  • Personal injury,

  • Heavy damage to assets and the environment,

  • Hazards of subsequent metal fire.

Figure 48 Extinguishing after fire detection

3.3.4.2 Manual extinguishing

For manual extinguishing, portable or mobile fire extinguishing devices are generally used. Suitable fire extinguishing devices must be available in sufficient number and size and in the vicinity of the machine tool (see "Ausrüstung von Arbeitsstätten mit Feuerlöschern" BGR/GUV-R 133).

Prior to the start of manual extinguishing it should be ensured that

  • The local extraction system is disabled,

  • The metalworking fluid supply and the purging air supply is interrupted

and

  • The machine is in a safe state.

This can, for example, be realized by activating the machine's central emergency stop.

If a fire is to be extinguished manually, machine doors must only be opened by specially instructed personnel or by the fire service.

By opening machine doors, atmospheric oxygen can enter the machining area resulting in the hazard of

  • Fanning the fire,

  • Backfiring

and

  • Flame ejections.

Another possibility is extinguishing through an extinguishing hole, which is easily opened (e. g. pushed open) in the case of fire. In practice, special holes through which fire-fighting lances can be held, have been proven of value. After the extinguishing holes or openings in the machining room door have been opened, the source of the fire can be extinguished by introducing an extinguishing lance or a fire extinguisher hose by the company fire service or by specially trained personnel.

Figure 49 Extinguishing hole with marking and information

Figure 50 Machine tool extinguishing hole

The extinguishing hole should be positioned so that the entire machining room can be flooded. Open extinguishing holes should not cause other hazards (e. g. ejection of workpieces, crushing or trapping hazards for personnel).

3.3.4.3 Fixed fire extinguishing systems

For the protection of machine tools, automatic extinguishing systems with gaseous extinguishing agents or water atomization technology are generally used. The aim is the extinguishing of the any burning metalworking fluids (oil fires). When a central extraction system is used, the use of an automatic fire extinguishing system is generally recommended because of the higher hazard potential.

In individual cases, when risks are low and where no unattended operation is possible, machine tools may also be equipped with manually activated fire extinguishing systems. It has to be ensured that a fire is detected as early as possible (e. g. by means of automatic fire detection) and that the fire extinguishing system is activated without delay.

An automatic extinguishing system for machine tools consists of, among others, the following components:

  • Fire detection elements (in the machining room of the machine tool and at other places, where fire hazards exist, e. g. extraction system, chip conveyors),

  • Fire alarm centre and/or control device (fire detection, alarming, monitoring and control of the extinguishing system, if applicable, control of equipment like machine shut-down, shut-off or closing of the extraction system - see Figure 51),

  • Manual activation (at the control panel or in the vicinity of the machine)

  • Extinguishing agent tank (including loss monitoring device) and distributor pipe circuit into the interior of the machine,

  • Extinguishing nozzles (appropriate arrangement inside the machine, in order to uniformly distribute the extinguishing agent over the entire extinguishing area),

  • Alarming devices, optical and acoustic (acoustic alarms must be at least 5 dB louder than the background noise),

  • If applicable, interlocking option for the extinguishing system,

  • If applicable, time switch (electrical, non-electrical),

  • If applicable pressure relief device.

Extinguishing systems should conform to the state-of-the-art. Relevant information is e. g. contained in

  • The standard series DIN EN 12 094 for gas extinguishing system components,

  • The regulations (e. g. Regulation "Einsatz von Feuerlöschanlagen mit sauerstoffverdrängenden Gasen" (BGR 134), Information "Sicherheitseinrichtungen beim Einsatz von Feuerlöschanlagen mit Löschgasen" (BGI 888), Principle "Grundsätze für die Prüfung von Feuerlöschanlagen mit sauerstoffverdrängenden Gasen" (BGG 920),

  • VdS-Richtlinien für Planung und Einbau von Löschanlagen (z. B. VdS 2093).

Water mist systems must be specified for the relevant application. In addition, the efficiency and reliability should be verified by means of fire and extinguishing tests and corresponding component and system tests (see e. g. European Technical Specification DIN/CEN TS 14972 for water mist systems).

Design, planning and installation

When designing the extinguishing system, the parts of the machine tool which should be protected, e. g. the interior of the machine (with or without extraction) are specified with regard to the risk. The planning and project development of the fire detection and extinguishing system is carried out on the basis of these specifications.

For orientation and estimation of the quantity of extinguishing agent needed for automatic CO2 extinguishing systems, a reference value of 5 kg CO2 per m3 protection volume applies (extinguishing area).

The exact value should be determined for each machine/system in accordance with the Rules of Technology (e. g. VdS 2093).

Extinguishing agent losses, which can e. g. be reduced by rapid machine shutdowns, short extraction overruns and small openings in the machine enclosure should also be considered.

When calculating extinguishing agent quantities, the extraction system and its overrun should also be considered.

When gas extinguishing systems are used, there should be sufficient pressure release possibilities fitted to the machine to allow the pressure build-up after release of the extinguishing agent to be vented. Existing openings or pressure relief valves often suffice for explosion protection. However, this must be checked.

Design and construction criteria are e. g. published by "VdS Schadenverhütung GmbH" (testing and certification institution for the prevention of damage), (see www.vds.de).

The planning and installation of the fire extinguishing system should be done by a specialist company, if possible, in collaboration with the machine tool manufacturer.

Preconditions for operation

For a safe operation of machine tool extinguishing systems, the following should be taken into consideration:

  • Electrical supply and control of the extinguishing system (including emergency electrical supply) should be independent from the machine tool,

  • Interlocking option (blocking) of the extinguishing gas supply for personnel protection, e. g. during setting and maintenance work (non-electrical blocking device or electrical if an equivalent level of safety is ensured),

  • Regular testing of the amount of extinguishing agent in the tank, e. g. by monitoring the extinguishing agent pressure and/or by an automatic weighing devices. Due to its physical properties, CO2 cylinders cannot be monitored by testing the pressure.

Figure 51 Weighing device

Figure 52 Fire alarm centre with manual activation

Activation of extinguishing process

When evaluating the personnel hazards caused by extinguishing gases and the extinguishing gas concentration inside the machine and its surroundings, the specific ambient conditions always have to be taken into account (e. g. size of the machine, openings, size of the surrounding area, propagation of the extinguishing gas into the environment, if applicable, rooms below).

The above evaluation results in requirements for alarming and delay. The relevant regulations contain advice and specifications (see Regulation BGR 134 and the Information BGI 888.

For the safe activation of the machine tool extinguishing process, the following should be taken into consideration:

  • When the extinguishing system activates, extraction flows and metalworking fluid supplies must be interrupted and the machine drive shut-down.

  • The extinguishing system can be activated manually or automatically.

    Caution: Effusion of the extinguishing agent may lead to flames escaping through openings and enclosure gaps.

  • In the case of gas extinguishing systems, the time delay should be considered (personnel protection measures, interruption of extraction flows). For water mist systems, there is generally no delay.

  • After every activation, the extinguishing agent tanks must be refilled (no multiple use possible as the containers are always completely emptied).

3.3.4.4 Fire detection elements

Fire detection elements are a key criterion for fire protection. They must guarantee the safe detection of fires in a fast and reliable way and activate the extinguishing process via the control system.

For automatic activation of the extinguishing system

  • Thermal fire detection elements (e. g. thermo elements, bimetals)

and

  • Optical fire detection elements (IR, UV) are available. Suitability should be tested for each individual case.

Thermal fire detection elements react slower than optical systems and are thus sometimes used in combination with optical sensors.

MWF mist is only partly permeable by UV radiation (depending on oil mist density). The use of UV sensors is preferable for dry machining and in areas free of MWF mists. The suitability of UV sensors should therefore be tested in each individual case.

Optical sensors must be kept clean. This is done, for example, by air purging. The functions "cable breakage" and "window malfunction" should also be monitored (the optical detector controls itself for vision).

Fire detection equipment must correspond to the state-of-the-art (e. g. DIN EN 12 094-9). For planning and installation, the manufacturer's specifications and the rules of technology should be taken into account besides risk-specific aspects.

Special solutions must be specified for each application and their efficiency and reliability must be verified by fire tests and tests of the relevant components and systems.

Figure 53 Fire detection elements (optical sensor) with purging air

3.3.4.5 Extinguishing nozzles

The extinguishing nozzles must be compatible with corresponding extinguishing agent and application and be suitably arranged e. g. they should not be directed towards door labyrinths). Consultation with the manufacturer is recommended!

When using CO2 extinguishing systems, the extinguishing agent is stored as a liquid in storage tanks (extinguishing gas cylinders). Gasification only occurs at the extinguishing nozzles and must be complete.

The other extinguishing gases mentioned are generally stored in a gaseous state.

Figure 54 Extinguishing nozzle in operation

3.3.4.6 Organisational measures for fire prevention and protection in the machine tool surroundings

In order to avoid propagation of a machine fire to its surroundings and personal injury during a fire or extinguishing, general rules of behaviour in case of fire and general rules of preventive fire protection must be observed (see also Information "Arbeitssicherheit durch vorbeugenden Brandschutz" BGI/GUV-I 560).

This includes:

  • Reduction of combustible substances near to the machine (flammable materials, cardboard, oil),

  • Provision of a sufficient number of manual fire extinguishers (BGR/GUV-R 133),

  • Enforcing smoking prohibition,

  • Keeping emergency exits, escape and rescue routes free,

  • Behaviour in the case of fire: rescue chain, emergency calls, fire service.

Whether a machine fire can propagate and flash over to other areas is strongly dependent on the "conditions" surrounding the machine. The most frequent causes for the fast propagation of a subsequent fire are oil pans filled to the rim and gratings with large surfaces, large-area MWF pools and other flammable materials (paper, cardboard, cleaning rags etc.).

Figure 55 Oil-filled gratings with large surface areas

To reduce fire hazards, there should be as few combustible materials in the immediate vicinity of a machine tool as possible. Packing materials or oil-soaked cleaning rags should, under no circumstances, be stored in the immediate vicinity. Regular emptying and cleaning of oil pans and gratings (provide drains, use oil extractors) and the disposal of cardboard boxes and oil-soaked rags significantly reduces fire hazards.

Attention:

Oil and grease on used cleaning materials wick and have large surface areas.

Under certain circumstances (temperature, pressure), MWF-soaked rags can self-ignite. Such "ignition sources" have repeatedly caused fires in chip containers, machine interiors and open waste containers. As a result, used and soiled cleaning materials should be kept in non-flammable, closed containers.

Figure 56 Source of ignition in the chip container

Figure 57 Fire hazard near to a machine

Furthermore, the chip containers should be emptied regularly in order to reduce the fire hazard and prevent possible self-ignition. If the chips are stored for several days and the volume of chips generates high internal loads, exothermic reactions may cause a heating process which could possibly lead to self-ignition.

Also cigarette ends and combustible materials (cleaning rags, cardboard boxes, paper cups) should not be thrown into chip containers. Furthermore, observance of a general ban on smoking is indispensable in these areas.