Formation of potentially explosive atmosphere in solvent washing machines

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Ignition tests to determine the probability of a formation of potentially explosive atmosphere during the spraying of AI, AII, and AIII solvents

Definition of the hazard zones for explosion protection
Explosion protection requirements according to directive 94/9/EG (ATEX 95)

Prof. Dr. rer. nat. Tammo Redeker, IBExU
Dipl. Ing. M. Dickopf, D.W: Renzmann Apparatebau GmbH

D.W. Renzmann Apparatebau GmbH manufactures solvent cleaning and distillation systems for the printing, dyeing and varnish, and chemical industry. To evaluate the consequences for explosion protection which result from the directive 94/9/EG for these products, a meeting was held at Renzmann in July of 2002 between experts from the PTB1), the IBExU2), the technical committee for metal and surface treatment3) and the VDMA4).

In the Renzmann test center, these experts were able to obtain a close-up view of sprayer and brush washing machines and observe spraying processes. In view of the large quantities of flammable solvent sprayed during the washing process, it was considered probable that the aerosol/vapor/air mixtures (hybrid mixtures) are not ignitable. This would be the case if the high density of droplets in the washing chamber or an oversaturation of the atmosphere beyond the upper explosibility limit were virtually "smothering" possible ignitions.

This theory represented a chance for Renzmann to avoid defining the interior of the washing machines as hazard zone 0. It was decided to build a test machine and conduct ignition tests at IBExU. The aims of these tests were:

A clear definition of the hazard zone in solvent washing machines.
Creating spraying conditions in which the hybrid mixture would not be ignitable (nozzle design, homogeneity of the nozzle distribution, rate of flow, pressure etc.).

Design of the test machine

Washing chamber made of 4 mm steel plate, volume 0.8 m³, one side open and covered with transparent foil as pressure relief surface
Under the washing chamber, a container for the circulated washing solvent with a supply hose from the washing chamber
In the washing chamber, various flat fan and jet spray nozzles of the design, number, and arrangement typical for solvent sprayer and brush washing machines
Rotatory pump with a max. flow rate of 45 m³/h and a max. pressure of 8 bar to supply the flat fan nozzles with solvent from the washing solvent container
Reciprocating pump with a flow rate of 3 m³/h and a max. pressure of 50 bar to supply the jet nozzles with solvent from the washing solvent container

Testing method

The test machine was set up in a concrete bunker at IBExU. The interior of the washing machine was clearly visible from a safe distance due to the transparent foil. During each ignition test, the flame propagation was filmed with a video camera. For each ignition test, a pyrotechnic igniter with 0.170 kJ ignition energy was installed in the washing chamber.

Then, different scenarios were simulated by activating and deactivating nozzles, using the rotatory pump (sometimes throttled) and the reciprocating pump, using different solvents etc. The following variation range was tested: Between 2 and 40 nozzles of different designs and diameters, widely differing distribution of the nozzles in the chamber, spraying pressure from 2 to 50 bar, flow rate from 1.5 to 45 m³/h, AI, AII, and AII solvents.

Test results

With AI solvents, the atmosphere in the washing chamber could not be ignited in individual cases. This seemed to confirm the theory of the oversaturated hybrid mixture. However, when the test was repeated under the same conditions, ignitions with explosions and subsequent combustion occurred in parts of the washing chamber. This also happened when solvent was sprayed into the washing chamber from all nozzles at a flow rate of 45 m³/h, a very high flow rate for the size of the washing chamber. Consequently, it was impossible to find reproducible spraying conditions in which hazard zone 0 could have been reliably avoided.

With AII and AIII solvents, explosions and subsequent combustion occurred in all tests, even with the minimum flow rate and number of nozzles. Evidently, the aerosol-air mixtures containing vapor below the lower explosibility limit offered more favorable conditions for explosion than the partially oversaturated AI-aerosol-air mixtures with higher vapor content in the explosibility range.

Upshot: The spraying of AI, AII and AIII solvents and the creation of aerosols always creates a hazard zone 0 in solvent washing machines. The flammability of AII and AIII-aerosol/air/vapor mixtures is at least as high as that of AI mixtures.

Consequences resulting from the application of directive 94/9/EG

In sprayer and brush washing machines in which AI, AII or AIII solvents are sprayed and aerosols are formed, a flammable atmosphere of hazard zone 0 is generated. The movable and driven cleaning devices installed in the washing chamber, such as spray nozzle systems, supports for parts or rotating brushes must conform to category 1 according to directive 94/9/EG.

This calls for a type examination of all devices inside the machine which may be potential ignition sources, to be performed by a notified body in accordance with directive 94/9/EG. In addition, the manufacturer must use a quality assurance system in accordance with directive 94/9/EG, appendix IV. As an alternative to the type examination, the notified body may also conduct an examination on an individual basis.

An inerting of the machine interior, e. g. using nitrogen, can be conducted to prevent hazard zone 0.

Physikalisch-Technische Bundesanstalt, Fachgruppe 3.3 - Braunschweig
(German national institute of natural and engineering sciences, division 3.3 - Braunschweig)
IBExU Institut für Sicherheitstechnik GmbH, An-Institut der TU Bergakademie Freiberg
(IBExU institute for safety engineering)
Fachausschuss Metall- und Oberflächenbehandlung, Norddeutsche Metallberufsgenossenschaft - Hannover
(Technical committee for metal and surface treatment of the employer's liability insurance association for the metal producing and working industries in Northern Germany)
VDMA - Verband Deutscher Maschinen- und Anlagenbau e.V. - Frankfurt/M
(German federation of the engineering industries)