Release the pressure!
Flameproof enclosures receive a cutting-edge update courtesy of R. STAHL

R. STAHL1In the event of an explosion inside the enclosure, the blowout panels are blasted open; the mesh elements underneath then allow the pressure to safely dissipate towards the outside

Waldenburg, Germany – Flameproof enclosures have undergone an impressive facelift. Instead of heavy, bulky cast and steel constructions, innovative material combinations and technologies can now be employed to create innovative control cabinets that, despite being virtually indistinguishable from their counterparts in nonhazardous
industrial environments, can still be used in Zone 1 without restriction.

Flameproof enclosures are a type of protection that came into use over 100 years ago. In their original state, electrical and electronic components and apparatus exhibit a number of potential ignition sources – either as an inherent aspect of normal operation or as a result of a fault – which preclude their use in hazardous areas. Installing this equipment in an enclosure that is sufficiently robust to withstand the outward force of an explosion within it without sustaining serious damage, however, is a solution that allows it to be used in the safety-critical industry sectors mentioned. Appropriate technical measures must be taken to ensure that the outer surfaces of the enclosure remain at a temperature below the ignition point of hazardous mixtures of this nature. The technical requirements pertaining to this type of protection are defined in detail in part 1 of international standard IEC 60079; the code used in this standard for this type of protection is “Ex d”. The widespread use of flameproof enclosures in the field of explosion protection can be attributed to the fact that they make it relatively easy to upgrade virtually any electrical equipment for use in hazardous areas. This type of protection is incredibly robust and reliable. These benefits are offset by a number of considerable drawbacks, however: Above a certain enclosure size, equipment installed in a flameproof enclosure becomes extremely heavy and bulky. Depending on the size and geometry of the flameproof enclosure’s inside capacit, the composition of the ignited explosive atmosphere and the location of the ignition source, peak blast pressures of 8-15 bar may be experienced. This often results in exceedingly high installation and operating costs because the load-bearing system and parts of a building must be sufficiently robust and adequately proportioned to accommodate this. Conventional technology encounters another critical stumbling block: in order to obtain the requisite flameproof properties, the enclosure must have thick walls in the region of 10-20 mm. If the enclosure size is to be at all cost-effective or practical, this effectively limits the enclosure capacity to no more than 500 litres. Large, complex electrical control panels and distribution boards must therefore be divided between a number of smaller flameproof enclosures, which are then combined. With these enclosure combinations, the project engineering and production stages are considerably more complicated and time-consuming than they are for conventional industrial control cabinets. Flameproof bushings are used for the electrical connections that must be established between the different Ex d enclosures. As a result, control panels and distribution boards of this kind that
are suitable for hazardous areas are incredibly time- and cost-intensive not only at the project engineering and production stages – operation, maintenance and repair work all increase this burden too.

New pressure-relief solution
For many decades now, R. STAHL has been an explosion protection technology specialist of international renown. Back in the 1960s, R. STAHL was already breaking new ground as the first manufacturer to develop and produce flameproof enclosures using plastic. A few years ago, R. STAHL began working with advanced light weight construction principles with the aim of eliminating the technological drawbacks and limitations described above. Developing, and subsequently patenting, a pressurerelief element which obtained its ATEX certification back in 2015, was able to reduce pressure by up to 30% more than conventional flameproof enclosures. Spurred on by this success, R. STAHL joined forces with several well respected German universities of applied science and Germany’s two leading test bodies, PTB and DEKRA EXAM, working closely with these institutions. The end result was a special, woven wire mesh developed specifically for the purpose of providing effective, flameproof pressure relief. The metal mesh is woven from fine stainless steel wires and consists of several layers, stacked one above the other. Numerous tests were conducted to optimise the geometric and technological parameters of each of these layers, such as wire gauge, mesh size and weave. Using special processes, it can either be cast into cast aluminium enclosures or welded into stainless steel enclosures. In the event that an explosive atmosphere ignites within an enclosure that has been prepared as described, the chemical energy that is released is very quickly and efficiently dissipated. Various physical effects come into play at this stage, that cannot be touched briefly here. The result is remarkable: While a conventional Ex d enclosure of equivalent size will give peak pressure readings in the region of 10 bar, the new enclosures produce staggeringly low peak pressure far below 1 bar. By means of a carefully considered configuration, it has also been possible to reliably prevent the outer surface of the mesh layers from reaching temperatures above the permissible value for temperature class T4. And in order to ensure that the new solution works under a huge range of harsh ambient conditions, such as environments where it is exposed to dirt or ice, it is important to ensure that the exposed surfaces of the outer mesh layers are properly protected. This is achieved by fitting conventional blowout panels, which provide IP66 protection in normal operation but are blasted open at a target pressure of 0.1 bar in the event of an explosion, allowing the gas to escape to the outside.

Slimmer, lighter, more versatile
The pressure reduction system described opens up whole new dimensions in flameproof enclosure design. The enclosures developed under R. STAHL’s “EXpressure” brand have walls around 3 mm thick. This brings them almost into line with the dimensions of the equivalent industrial control cabinets, and has a significant impact on the weight and compactness of the structure. Comparative calculations performed by way of example have indicated weight savings of approximately 30-50% when compared to conventional switchgear and controlgear assemblies, with external dimensions being reduced by around 25%. This is particularly useful in instances in which installations are to be packed tightly together for offshore applications. On the other hand, it is now also possible to build extra large flameproof control cabinets. The largest “EXpressure” cabinet currently available is 1400 mm high, 1000 mm wide and 700 mm deep. Even larger cabinets are in the pipeline. This makes project engineering significantly easier for
the reasons explained above. Sizable equipment, such as transformers and frequency converters, can now safely be installed in the spacious enclosures. ATEX and IECEx certification was conducted in parallel by the two well known German notified bodies. The necessary certificates are already available for the first four enclosure sizes. With the ATEX system, certificates have been issued solely for Ex d protection based on the provisions set out in Annex II of Directive 2014/34/EU, IEC 60079-1 and IEC 60079-33. Special protection have been used for the IECEx certificates. The two test bodies mentioned acted as the two independent verifiers required by standard 60079-33.

In short
With the advent of R. STAHL’s “EXpressure” brand, the market is witnessing a trailblazing new technology for switchgears, control cabinets and distribution boards, as well as other electrical equipment such as luminaires and process analysis technology. R. STAHL has been able to overcome many of the disadvantages of conventional flameproof enclosures and replace these with considerable customer benefits.