A revolution in explosion protection

Engelmann: I have known Dr Arnhold for more than 15 years now. We first got to know each other during joint research work at the Physikalisch-Technische Bundesanstalt (PTB, the National Metrology Institute of Germany). At the time, I was still Head of Department at the Otto von Guericke University in Magdeburg and invited Dr Arnhold to lectures about safety technology. When I later moved to the Ernst Abbe University of Applied Sciences in Jena, we set up a research partnership together.

Arnhold: 43 percent of IECEx-certified devices are based on the flameproof enclosure (Ex d) type of protection. This type of protection is extremely robust and reliable, but also has significant disadvantages. These include the fact that Ex d solutions become bulky and heavy once the enclosure size passes a certain threshold. The reason for this is the need for flameproof enclosures to contain high blast pressures in the event that an explosive atmosphere finds its way in and ignites. Depending on the size and geometry of the flameproof enclosure's inner cavity, the composition of the ignited explosive atmosphere, and the location of the ignition source, peak blast pressures of 8 to 15 bar may be experienced. Another disadvantage of conventional technology is that the walls are thicker – between 10 and 20 mm. This means that cost-effective, technically practical design sizes are limited to an enclosure volume of around 500 litres at most. For this reason, large, complex electrical control panels and distribution boards have, until now, been divided between a number of smaller flameproof enclosures, which are then combined. This results in increased expenditure during the project engineering and manufacturing phases when producing enclosure combinations of this kind. Furthermore, any subsequent changes to the internal electrical wiring lead to modifications, which must normally be accepted by an explosion protection expert before the equipment can be put back into service.

Engelmann: To eliminate these drawbacks, we spent the last few years focusing on state-of-the-art lightweight construction principles. In this context, numerous tests involving different porous materials were performed. The idea behind these tests was to discover reliable, effective ways of multiplying the effect of flameproof joints that would allow for a significantly greater reduction in pressure, in significantly less time, following an explosion within the enclosure. We achieved initial success when we developed and patented a pressure relief element made of porous, sintered material. This element, which obtained ATEX certification in 2015, was able to reduce pressure by up to 30% more than conventional flameproof enclosures.

Limbacher: In fact, this was just the beginning. To end up with a fundamentally new design, we worked with the Ernst Abbe University of Applied Sciences in Jena to research exactly what happens when an explosion occurs in a pressure relief Ex d enclosure. We came to an important conclusion – the pressure generated by an explosion is dissipated very quickly. This is helped by two different physical processes. One of these is the volume work – the gaseous combustion products are dissipated towards the outside via the ignition transmission-protected joint. The other is the heat dissipation via the enclosure wall. We discovered that cooling is the most important factor for pressure reduction. To accelerate and intensify heat dissipation, we jointly developed a metal mesh, which is mechanically stable and features high gas permeability and high heat capacity with low thermal conductivity. Our aim was to find a material that reduces the pressure the most when under load, while at the same time preventing ignition transmission. The ignition transmission resistance is achieved because the reaction heat that is released is absorbed quickly by the mesh/air structure; additionally, some of the unconsumed gas/air mixture is pushed out of the woven mesh. The woven wire mesh, which is made up of layers placed on top of one another, is either cast into an aluminium enclosure or welded into a stainless steel enclosure.

Engelmann: In the event that an explosive atmosphere ignites within an enclosure that has been designed as described, the chemical energy that is released is dissipated very quickly and efficiently. This causes a variety of physical effects. 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.

Arnhold: The result is striking – while you can find pressures of 10 bar in a conventional Ex d enclosure of the same size, the peak pressure in the new enclosures is far lower than 1 bar. As a result, EXpressure cabinets can be designed with much thinner walls and are significantly lighter. Additionally, they can prevent the external surfaces of the mesh layers from exceeding the permissible values for temperature class T4. To prevent the metal mesh from becoming contaminated or icing over in harsh everyday operating conditions, rupture discs are installed on the relief surfaces; in the event of an explosion, these open at a target pressure of 0.1 bar and achieve IP 66 degree of protection in normal operation.

Poidl: For offshore applications, the installation area and overall weight is an important cost-related factor when designing system technology. This is equally applicable to process engineering for creating MTP (Module Type Package) systems. In this context, the modularity of the systems enables modifications to be performed quickly during production. The required production area and the weight of the entire production modules are also important characteristics when establishing the feasibility of this concept. EXpressure's new construction possibilities and unique design, featuring large, lightweight cabinets, offer the solution for explosion protection applications.

Poidl: The weight reduction makes transport, handling and mounting easier. The space-saving enclosure concept and smaller setup area make it possible to design machines and systems to be more compact. The large, undivided space inside the enclosure provides easy access for installing and wiring switching components and for performing maintenance and inspection work.

Schmitt: Additionally, the volume available within the enclosure for installing equipment is comparable to that of many conventional industrial cabinets and control boxes, and allows the switchgear layout used for non-hazardous areas to be used unchanged for hazardous areas without any additional engineering work. Thanks to EXpressure, even last-minute changes to the order can be accommodated with little difficulty at the commissioning and SAT stages. Cable glands can easily be retrofitted at any point in time, and new components can be added to pre-installed equipment.

Schmitt: Thanks to the small dimensions of the EXpressure cabinet, installation in narrow spaces is now possible. Furthermore, equipment such as transformers can now be safely installed inside the cabinet.

Poidl: Among others, the crane manufacturer Liebherr uses EXpressure for significantly more compact explosion-protected machine control for offshore cranes. After all, EXpressure technology is – as mentioned earlier – ideal for use on offshore installations with limited available space, thanks to its lightweight, compact construction design. Our customers also include a number of other users in the machine building industry.

Poidl: The large single enclosures mean that the enclosures have high power density. To produce especially compact and lightweight complete solutions, transient thermal calculation by our engineering department is an important part of the design process. This simulation guarantees a long service life for the machine control system.

Walch: The EXpressure cabinets, which are certified by the PTB and DEKRA, are certified as "Ex d flameproof enclosures." As such, they are suitable for applications in Zone 1 and Zone 2 hazardous areas.

Walch: This new development is not portrayed in the currently valid standard version of DIN EN 60079-1 Explosive atmospheres – Part 1: Equipment protection by flameproof enclosures "d" (IEC 60079-1:2014). To obtain certification in accordance with this standard, either all the requirements set out in the standard must be met, or it must be ensured that this development is safe regardless. A means of performing the tests in a way that is as close to the standard as possible has been developed; this guarantees the safety of the enclosure using additional design measures (e.g. rupture discs).

Since EXpressure is a completely new approach for Ex d enclosures, we decided to carry out this certification project together with two independent test bodies. A joint solution was developed and implemented. Finally, both the ATEX certification required for Europe and IECEx certification were obtained.  By selecting the two expert test bodies, DTC (Dekra Testing and Certification, also known as DEKRA EXAM or Bergbau Versuchsstrecke BVS) and the PTB (Physikalisch Technischen Bundesanstalt, the National Metrology Institute of Germany), we were able to accelerate the market acceptance of this new development.

Limbacher: Pressure reduction in the flameproof enclosure sector is a fully new approach. This means that the test bodies involved in the process focused strongly on each design detail and espacially the pressure relief material used.  We investigated the porous material used, in particular. Coordinating with our suppliers and defining the required inspections and quality criteria were challenges that we had to overcome as a team.

Schmitt: We brought in experts from each specialist department within the company. Each brought their own expertise and methods. One major advantage of this collaboration was the fact that we used a joint project space, which enabled us to achieve a high level of interdepartmental knowledge during the development phase. At the same time, the EXpressure project team kept their aim clearly in mind. Additionally, we were able to drastically shorten the development cycle by performing a number of interconnected activities in parallel or in quick succession.

Limbacher: We were able to perform explosion tests in accordance with the Ex d standard in our laboratory. As far as I know, we are the only manufacturer with this testing capability. This meant that we were able to test porous materials for enclosure sizes of approximately 200 litres to inspect their characteristics and their behaviour in the event of a gas explosion. On the basis of these tests and our years of experience in developing flameproof enclosures, we were able to obtain important findings and derive design rules. Our close, continuous exchange of information with the Ernst Abbe University of Applied Sciences in Jena that occurred during our joint research was always important, interesting and inspiring.

Schmitt: Of course – there are always setbacks. However, we can learn a little from sport in this regard – winners come to the fore, while losers fall by the wayside. Gold medals aren't just made of gold; they are made of a rare combination of sweat, determination and guts. In line with this philosophy, we always got back to work after a setback in order to find new solutions. Our motivated, goal-oriented approach meant that we continuously redesigned and challenged estimations, analyses, simulations and model calculations. In the end, the fact that we created or registered more than 20 new patent concepts during the EXpressure development process is proof of our success.