Using flameless venting to protect your plant
Jef Snoeys, Fike Corporation
Flameless venting is a relatively new dust explosion protection method for indoor equipment that combines conventional explosion venting with flame-quenching techniques. Flameless venting devices meet increasingly strict explosion safety regulations for bulk solids plants and overcome the limitations of conventional ducted explosion venting in some applications. Flameless venting devices can also be retrofitted to existing equipment installations.
The need for flameless venting
Conventional explosion venting has long been the most common method of protecting indoor process and storage vessels, enclosed conveyors, and other equipment from the effects of a dust explosion. An explosion vent is an opening in a vessel's top or side that provides pressure relief during an explosion. Ducting directs the vent discharge outside the building, protecting people indoors and preventing dust deposits outside the vessel from becoming involved in a secondary dust explosion. This ducting has a substantial effect on the pressure experienced inside the vessel during the venting process, and the vent area calculation must allow for this effect.
However, in some cases, the pressure increase can be so great that an alternative protection method is required. The flameless venting device provides a ready solution for these cases by combining explosion venting and flame quenching techniques. The device can be added to existing equipment, allowing bulk solids processors to retrofit their equipment to meet today's more stringent explosion safety regulations.
In the US, flameless venting devices must comply with the National Fire Protection Association (NFPA) standard NFPA68: Standard on Explosion Protection by Deflagration Venting (2007), which OSHA has the authority to enforce. In Europe, the devices must comply with European Standard prEN16009 "Flameless explosion venting devices." This article concentrates on US requirements.
While the flameless venting device is used primarily on indoor equipment, it's also well-suited to outdoor equipment that’s adjacent to buildings or roads, where a vented explosion’s flame and pressure effects can’t be safely controlled. The flameless venting device can be used in most applications handling nontoxic dry bulk materials.
How a flameless venting device works
Components. A flameless venting device is a passive device that typically includes an explosion vent panel and a flame-quenching element that's inside a flanged metal frame; the frame attaches the element to the process equipment, such as a dust collector. The element is closed at one end and open at the other, and it's bolted onto the explosion vent opening on the equipment so that its open end overlaps the opening. The flame-quenching element's frame encloses layers of particle-retaining, high-temperature stainless steel mesh or carbon steel mesh or ceramic material. The flame-quenching element may be cylindrical, rectangular, or square to fit the vent opening's shape.
Operation. During the early stages of an explosion inside the process equipment, the explosion vent panel opens. As the explosion expands, flame and burnt and unburnt dust discharge through the open vent into the flame-quenching element. Most of the dust is retained inside the element, and the energy (heat) dissipates as it travels through and is absorbed by the steel mesh or ceramic material inside the element. This reduces the burning fuel’s temperature below the fuel's ignition temperature, extinguishing the flame and preventing flame propagation beyond the device. Only a small amount of dust passes through the flame-quenching element, and post-combustion gases from the explosion are safely vented through the device into the external atmosphere around it. The flameless venting device's operation not only extinguishes the flame and retains dust, but eliminates the need for explosion vent ducting and minimizes the vent relief area required for indoor explosion venting.
Available types. Flameless venting devices are available in various shapes. Many are designed to suit particular storage and process equipment such as silos, bins, hoppers, dryers, mixers, dust collectors, and cyclones, and handling equipment such as enclosed belt conveyors, screw conveyors, and bucket elevators.
To learn more about flameless venting devices and how to choose one to comply with NFPA 68, you can read the article "Meeting explosion safety requirements with flameless venting" by Jef Snoeys, Fike Corporation. You can also find articles listed under "Safety" in PBE's Article Index.