A wire mesh fabric

R. Stahl's 'Expressure' technology is based on a multi-layer stainless steel mesh with flow channels in which explosion energy is safely dissipated.

© R. Stahl switchgear

The metal mesh woven from fine stainless steel wires is built up in several superimposed layers. Each of these layers has been optimized in the course of numerous tests with regard to its geometric and technical parameters such as wire thickness, mesh size and weave type. The different layers are sintered to form a stable composite. As a result, the finished wire mesh has high gas permeability, mechanical strength and heat capacity with relatively low thermal conductivity in addition to its resistance to ignition breakdown, making it suitable for integration into flameproof enclosures. Using special processes, it can be cast into cast aluminum housings as well as welded into stainless steel housings. A ratio between gas-permeable surfaces and closed side walls of 10 to 15 % has proven to be ideal.

The side walls with permeable pressure relief elements are sealed on the outside by bursting discs and protected to IP66.

© R. Stahl switchgear

If an explosive atmosphere ignites inside an enclosure prepared in this way, the energy released is dissipated quickly and efficiently. Various physical effects take effect (shown in very simplified form below). The gas-permeable fabric enables rapid pressure reduction to the outside. Depending on the position of the ignition source in the pressure chamber, a certain amount of the unburned gas-air mixture is also forced out and cannot become explosive. A not inconsiderable part of the resulting reaction heat is absorbed by the grid-air structure of the wire mesh and is therefore not available for internal pressure build-up. The result: while peak pressures of around 10 bar are measured in a conventional Ex d enclosure of the same size, the peak pressure measured in the new enclosures is well below 1 bar. A suitable arrangement also reliably prevented the outer surface of the grid layers from being raised to temperatures above the permissible value for temperature class T4.

However, good protection of the external grille surfaces is necessary to ensure that the solution functions reliably under a wide range of harsh environmental conditions such as contamination or icing. This is made possible by the installation of conventional bursting discs, which guarantee protection class IP66 in normal operation, but open at a set pressure value of 0.1 bar in the event of an explosion, thus paving the way for the escaping gas to escape.

3 mm wall thickness

The pressure reduction described above makes new designs for flameproof enclosures possible. The enclosures developed by R. Stahl under the brand name 'Expressure' have wall thicknesses of around 3 mm and are therefore not far removed from the corresponding dimensions of industrial enclosures. This has a significant effect on the weight and compactness of the design. Comparative sample calculations showed savings of 30 to 50 % in weight and 25 % in external dimensions compared to conventional switchgear combinations. This is particularly relevant when it comes to tightly packed installations in the offshore sector.

A large-format housing is all you need

It is also possible to build flameproof enclosures with very large dimensions. The largest expressure enclosure currently available has a height of 1400 mm with a width of 100 mm and a depth of 700 mm; even larger enclosure dimensions are planned. This makes project planning much easier. Whereas conventional technology required a combination of several enclosures to accommodate complex control units and distribution boards, a single Expressure enclosure is now generally sufficient. Large equipment such as transformers or frequency converters, for example, which previously either could not be made suitable for hazardous areas at all or only with a great deal of effort, can now be safely installed in the large-volume enclosures.

The ATEX and IECEx certifications were carried out in parallel at the two German test centers, and the necessary certificates have already been obtained for the first four enclosure sizes. For the ATEX area, pure Ex d certificates were issued using Annex II of Directive 2014/34/EU (this annex deliberately enables innovative solutions that are not yet 100% included in the relevant harmonized standards). Both the IEC 60079-1 standard and the IEC 60079-33: Special protection standard were used for the IECEx certificates. Both test bodies mentioned acted as the two 'independent verifiers' required by 60079-33.

Author:
Prof. Dr. Thorsten Arnhold is Vice President Strategy & Technology at R. Stahl Schaltgeräte and Chairman of the IECEx system in Waldenburg.