In one of our previous articles, we discussed conventional explosion venting using explosion vent panels. It is one of the fundamental methods of protecting process equipment against the effects of dust and gas explosions, widely used in the food, wood, chemical, and pharmaceutical industries, among others.
The operating principle is relatively simple – when a specified pressure is reached, the explosion vent panel opens, allowing explosion products to be discharged into the surroundings in a controlled manner. As a result, the reduced pressure (Pred) inside the protected vessel is limited to a level that is safe for its structure.
This solution is highly effective, provided that flames, hot gases, and combustion products can be safely discharged outside the protected vessel. However, in many modern industrial plants, process equipment is installed inside production halls, often close to workstations or other elements of technological infrastructure. In such cases, conventional explosion venting may pose a threat to personnel and the surrounding environment.
In response to these limitations, flameless explosion venting systems were developed. They allow the reduced pressure (Pred) to be limited while simultaneously retaining the flame and cooling the combustion products before they are released into the surroundings.
Limitations of conventional explosion venting
Explosion venting relies on controlled pressure reduction inside the protected vessel by opening an explosion vent panel and discharging explosion products into the surroundings. The purpose of this solution is not to stop the explosion, but to limit the reduced pressure (Pred) to a level that will not damage the protected equipment.
In practice, this means that the pressure wave, flame front, hot combustion products, and – depending on the course of the explosion – unburned or burning process material are directed outside the process vessel. The extent of their impact zone depends on the substance properties, explosion characteristics, protected vessel volume, venting arrangement geometry, and installation conditions.
Therefore, when designing an explosion venting system, it is necessary to consider not only the structural strength of the protected vessel, but also the impact of venting on its surroundings. It is essential to designate a safety zone free from workstations, walkways, or equipment that could be damaged by the effects of an explosion.
It should be emphasized that explosion venting does not eliminate the hazard, but merely transfers its effects outside the protected vessel in a controlled manner. For this reason, both the EN 14491 standard and the American NFPA 68 standard require that design considerations include not only the selection of the correct vent area, but also the safe discharge of explosion products into the surroundings.
This problem is particularly common in modern production plants, where filters, dust collectors, cyclones, or silos are installed inside production halls. In such cases, designating the required safety zone may be impossible, making the use of conventional explosion vent panels not always feasible or sufficient. It is precisely in these situations that flameless explosion venting systems are employed.
Operating principle of flameless explosion venting systems
The primary function of a flameless explosion venting system is to limit the reduced pressure (Pred) while simultaneously preventing flames from escaping into the surroundings. Just as in conventional explosion venting, when the opening pressure is exceeded, the explosion vent panel opens and explosion products are discharged from the protected vessel. The difference is that before leaving the vessel, they flow through a special flame quenching element.
In most available solutions, the element consists of multi-layer stainless steel mesh packs. Depending on the manufacturer, meshes of various geometries and aperture sizes are used, arranged in multiple layers or offset from one another. Other structural designs utilizing specially shaped metal profiles or strips are also encountered. Regardless of the design used, all solutions form a system of very narrow flow channels with a large heat transfer area. It is these channels that determine the effectiveness of the device.
During an explosion, hot combustion products along with the flame front flow through the metal element at high velocity. Contact with the metal surface results in intense heat absorption from the flame front. The flame is not stopped by a mechanical barrier, but extinguishes due to the absorption of energy required to sustain the combustion process. As a result, flame propagation ceases and cannot continue outside the device.
This phenomenon utilizes the quenching gap effect – the same physical principle applied in Ex d flameproof enclosures, where appropriately sized gaps prevent the transmission of an explosion from inside the enclosure to the surrounding explosive atmosphere.
However, the device does not stop the flow of explosion products. On the contrary – it must ensure their rapid discharge to limit pressure buildup inside the protected vessel. Only the flame front is retained, while cooled combustion products and explosion pressure continue to be vented to the surroundings.
The combination of explosion venting and flame quenching functions allows flameless venting systems to be used in installations where conventional explosion vent panels do not provide the required level of safety.
Limitations of using flameless explosion venting systems
Flameless explosion venting is not a universal solution. Its scope of application is derived from testing a specific device type and the conditions specified in the certificate, manual, and manufacturer’s documentation. Therefore, selection is not limited to determining the required vent area – it is also necessary to verify whether the device has been tested for the properties of the protected dust, expected reduced pressure, vessel volume, and process conditions.
The first limitation is venting efficiency. The quenching element offers resistance to the flow of explosion products; therefore, a flameless device does not provide the same effective vent area as a conventional explosion vent panel of identical dimensions. In practice, this may require using a larger number of devices or a larger geometric vent area.
Dust reactivity, understood as its ignition and explosion characteristics, is of particular importance when selecting a device. Parameters to be considered include minimum ignition energy (MIE), minimum ignition temperature of a dust cloud, minimum ignition temperature of a dust layer, maximum explosion pressure Pmax, and deflagration index KSt. These parameters determine both the course of the explosion and the feasibility of safely using a specific device, particularly when combustion gases are discharged into an explosive hazardous area.
The range of allowable parameters can vary significantly between manufacturers. Therefore, the technical documentation for a specific model must always be verified, including allowable values for MIE, Pmax, KSt, and Pred, type of dust, maximum protected vessel volume, and other limitations resulting from testing and certification. It should not be assumed that devices from different manufacturers with similar designs have an identical scope of application.
Metal dusts represent a special case. Their combustion may be accompanied by significantly higher temperatures and the presence of highly heated particles, which increase the thermal load on the quenching element. An excessive temperature rise impairs heat absorption from the flame front and thus the effectiveness of flame quenching. For this reason, the possibility of using flameless venting devices for metal dusts requires verification on a case-by-case basis according to the scope of testing and certification of the specific solution.
Another limitation is the potential partial blockage of the quenching element. This applies primarily to fibrous or sticky materials, or those prone to agglomeration or build-up. A reduction in the cross-sectional area of the flow channels can also occur during the explosion itself due to the ejection of a significant amount of unburned product, increasing flow resistance and potentially reducing venting efficiency.
The absence of flame emission does not mean complete elimination of the explosion’s impact on the surroundings. Hot gases and combustion products are still discharged from the device, so their potential impact on adjacent installations must always be evaluated. Particular attention must be paid to processes involving dusts with low minimum ignition energy or low cloud or layer ignition temperatures, as even cooled explosion products can act as an ignition source under certain conditions.
Limitations also apply to the size of the protected vessel. The maximum volume assigned to a single device derives from the scope of tests conducted. If the protected volume is larger, multiple devices of the same type may be used, provided that the total protected vessel volume does not exceed four times the maximum volume for which a single device was tested.
Selection of flameless explosion venting systems
The selection of a flameless explosion venting system should be carried out in accordance with the requirements of applicable standards and manufacturer documentation. In addition to the explosion characteristics of the dust, factors to consider include the volume of the protected vessel, its pressure resistance, required reduced pressure, installation conditions, and limitations arising from the scope of device certification.
Although the operating principle of flameless explosion venting systems is relatively simple, their correct selection requires considering many interrelated factors. In practice, the safety of the solution often hinges on seemingly minor differences in dust characteristics, process conditions, or limitations derived from the device’s technical documentation.
For this reason, system selection should be entrusted to specialists with experience in designing explosion protection systems. Experience allows for faster identification of potential limitations, proper interpretation of standard and technical documentation requirements, and selection of the solution best suited to a specific installation.
Summary
Flameless explosion venting is an effective method of protecting process vessels from the effects of a dust explosion, particularly in indoor installations where conventional explosion vent panels are difficult or impossible to use. This solution allows the reduced pressure (Pred) to be limited while preventing flame emission into the surrounding area.
However, the effectiveness of this method depends on correct device selection, taking into account dust properties, process parameters, and limitations resulting from testing and certification. A properly designed flameless explosion venting system serves as an effective element of explosion protection, enhancing the safety of personnel, equipment, and technological process continuity.
Feel free to contact usif you would like to arrange training for your team or discuss a program tailored to the specific needs of your organization.
Author: Sebastian Słaboszewski
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