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

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Abschnitt 3.1 , 3.1 Selection of suitable metalworking fluid...
Abschnitt 3.1
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 3.1 – 3.1 Selection of suitable metalworking fluids

Properties

During chip-forming machining, different emissions result from the metalworking fluid and machining process used. On one hand mechanical stresses on the metalworking fluid generate aerosols (particles of 0,5 to 50 µm of diameter), and on the other, thermal stresses on the metalworking fluid generate vapours or ultrafine aerosols [3].

Oil mistsVapours
Oil particles between 0.1 and 50 µm
Caused mechanically
Gaseous hydrocarbons
Caused thermally
  • High MWF pressures and cutting speeds

  • Impact of MWF on machine bed, tool and workpiece

  • Nozzle shape and arrangement

  • High feeds and cutting speeds

  • Tool wear

  • High MWF temperature

  • High vaporization of MWF

Figure 15 Emissions during chip-forming machining

High mechanical and thermal loads at the cutting edge cause the metalworking fluids to atomize and vaporize. In the interior of the machine, a mixture of oil mist/vapour and air is formed. The higher the thermal and mechanical stresses during the machining process, the higherthe proportion of MWF vapours. The highest emissions have to be expected for machine tools with high cutting speeds and low-viscosity MWFs (e. g. during grinding).

By selecting low-emission metalworking fluids, MWF aerosols and vapours at the workplace can be reduced. Low-emission metalworking fluids are characterized by the following properties:

  • Formulated with low-evaporation mineral oils or synthetic esters or special liquids, e. g. polyalphaolefins,

  • Addition of anti-mist additives.

Table 2 Characteristics of non-water-miscible metalworking fluids

It is principally recommended to select the MWF with the lowest vaporization losses and the highest flash point at the viscosity required by the machining process, which should be as high as possible.

Example: Grinding with flammable MWF based on oil (non-water-miscible) MWF
Characteristics:- Flash point> 140C
- Viscosity> 6 mm2/s at 40C
- Vaporization losses at250C < 80 %

Note:

With increasing viscosity, the filterability and the feed rate (flushing rate) of the oils in a given system decrease.

The formation of respiratory aerosols (particle size: 0,5 pm to 5 µm) can be reduced by using metalworking fluids with anti-mist additives. Anti-mist additives are especially effective in low-viscosity metalworking fluids such as grinding and honing oils.

The effect of the anti-mist additives may, however, "fade" with time due to mechanical and physical stresses (shearing at the machining point). Furthermore, certain anti-mist additives may lead to problems with the micro-filtration (10 µm filters, matt filters) of the MWF (e. g. obstruction, blocking of the filters). The suitability of the metalworking fluid for the process should therefore be agreed upon with the manufacturer.

If the MWF temperature is successfully monitored and the MWF is kept at room temperature by suitable measures, misting behavior can be significantly improved.

This can be achieved by:

  • Sufficient quantities of MWF,

  • Sufficient flooding of the cutting zone,

  • Baffle plates for improved cooling,

  • General cooling.

Investigations show that increasing the temperature of an MWF by 10Ce results in the doubling of aerosol formations.

Figure 16 Temperature dependency of aerosol formation

Figure 17 MWF - Lower explosion threshold; Influence of viscosity [6]

Besides the vaporization and misting behavior of the MWF, the following technical safety characteristics are relevant for the evaluation of the explosion risk:

  • Lower explosion threshold in g/m3,

  • Maximum explosion pressure in bar (g),

  • Maximum pressure increase, expressed by the KP value in barx m/s.

For MWF aerosols, the following values for the above characteristics are given in technical literature [4, 5, 6, 7]:

Lower explosion threshold25 g/m3 ... 60 g/m3
Maximum explosion pressure7,2 bar (g) ... 7,7 bar (g)
KP value75 bar m/s ... 103 bar m/s

The explosion pressures and KP values are determined experimentally and are maximum values.

Use of multi-functional oils

In practice, mixtures of the metalworking fluid with other working substances may occur. As far as quantity is concerned, the most important tramp oil contamination of MWF circuits is caused by hydraulic oils. This may lead to a deterioration in the tribological properties of the MWF. The consequences are problems during machining, high tool wear through to tool breakage and machine damage.

The introduction of tramp oils and residues such as

  • Machine cleaning and care products,

  • Cleaning agents and solvents on workpieces,

  • Tramp oils etc.,

into the metalworking fluid circuit of the machine tool should therefore be avoided as far as possible (information on MWF care see VDI 3397 Sheet 2, "Tätigkeiten mit Kühlschmierstoffen" BGR/GUV-R 143).

A possibility of reducing the above hazards is the use of compatible multi-functional oils (see VDI 3035). All lubricants used in the machine tool such as hydraulic oils, slideway oils, gear oils and spindle oils are matched to each other and fully compatible. Leakages then only have a very small influence on the metalworking fluid. Other advantages are increased of process reliability and longer life of the metalworking fluid together with a reduction in servicing and maintenance expenditure.

Figure 18 Multi-functional oils