Release the pressure! Flameproof enclosures receive a cutting-edge update courtesy of R. STAHL
Waldenburg, Germany – Flameproof enclosures, atype of protection already prevalent internationally for explosion-proofing electrical equipment, 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.
New pressure-relief solution
A few years ago, R. STAHL began working with advanced lightweight-construction principles with the aim of eliminating the technological drawbacks and limitations.
Their work also encompassed numerous tests involving different porous materials. The idea behind these tests was to discover reliable, effective ways of multiplying the effect of flameproof gaps that would allow for a significantly greater reduction in pressure, in significantly less time, following an explosion within the enclosure.
Developing, and subsequently patenting, a pressurerelief element made from porous, sintered material was an important first milestone for the company. This element, which obtained its ATEX certification back in 2015, was able to reduce pressure by 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. The different layers are then sintered together to form a robust, integral structure.
Besides its flameproof properties, the finished woven wire mesh boasts good gas permeability, mechanical strength and thermal capacity, while at the same time exhibiting relatively low thermal conductivity, making it the optimal choice for integrating into flameproof enclosures.
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, which we can only touch on briefly here. The gas-permeable woven mesh permits a rapid release of pressure to the outside. In the process, depending on the location of the ignition source in the flameproof chamber, some of the uncombusted gas/ air mixture is forced out, preventing it from fuelling an explosion. However, a significant fraction of the resultant reaction heat is absorbed by the mesh/air structure of the woven wire mesh and is consequently not available for reducing the pressure inside the enclosure.
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 readings, with measured values 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. Whilst it may have been necessary in the past, using conventional technology, to combine more than one enclosure in order to accommodate complex control panels and distribution boards, you now generally only need a single “EXpressure” enclosure.
Sizable equipment, such as transformers and frequency converters, which could previously not be upgraded for use in hazardous areas or could only be upgraded with great difficulty can now safely be installed in the spacious enclosures. The novelty of the solution means that there was no risk to R. STAHL; ATEX and IECEx certification was conducted in parallel by the two well known German notified bodies and the certificates are already available.
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 (this annex deliberately permits innovative solutions that are not yet fully covered by the relevant harmonised standards). 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. The innovative enclosure
design with pressure-relief elements offers a whole host of benefits to mechanical, apparatus and plant engineers and operators of process plants situated in hazardous areas.
R. STAHL has been able to overcome many of the disadvantages of conventional flameproof enclosures and replace these with considerable customer benefits.
