Understanding Flameless Venting Application and Design Considerations

Jason Krbec, PE, director of business development, CV Technology

Flameless venting is a now well-known explosion protection technology that is used to provide passive explosion mitigation to shield people and processes from the known hazards associated with handling combustible dry bulk materials. Many of these materials are key ingredients used in processes that produce the myriad of items we require for daily life. For example, these dry materials could be spray dried, mixed, packaged, transported, stored, or otherwise processed in a plant, potentially creating combustible dust hazards in one or more common industrial process steps as items for consumers, businesses, and other entities are produced. Flameless venting, along with other safety equipment can be used to improve outcomes if a devastating deflagration event related to these combustible materials were to occur.

Flameless Vent Basics

The typical flameless vent used in a processing facility incorporates a deflagration vent, or explosion vent, which is attached to a process vessel as a means to mitigate the effects of a potential explosion event. An explosion vent is designed to relieve the explosion event’s pressure wave generated during an event while also directing the by-products like heat, flame, dust, and gases to a safe location as well. An explosion vent--which is integrated into the flameless vent--opens when the pressure exceeds a certain level during a deflagration. The by-products created from the incident--such as dust and flame--are then directed into the flame arresting mesh element of the flameless vent. The mesh is the main element that quenches and stops propagation by forcing the by-products into a tortuous escape path through layers of mesh, which are often stainless steel or sometimes ceramic. These mesh layers retain the flame while relieving the pressure during an incident.

In North America, NFPA 68, Standard on Explosion Protection by Deflagration Venting provides the basic criteria for design and application of flameless vents. All vents must be listed or approved according to specific design limitations to verify their function and performance. Flameless venting can often be used in any area where traditional venting is possible. In cases where equipment cannot be located outdoors or adjacent to an exterior wall, then flameless venting may be the most practical explosion protection option, but there may be other advantages to using flameless venting.

Flameless Venting Benefits

One benefit to using a flameless vent to mitigate explosions is that the design eliminates the need for an explosion vent to be ducted outside the plant while also minimizing the vent relief area required indoors. This clear benefit of the flameless vent makes it appealing for various applications, including for use with bucket elevators, storage vessels, dryers, mills, mixers, pneumatic conveyors, and dust collectors.

There are three main types of flameless vents available on the market to protect vessels and process equipment, the tube-style, box-style, and elevator-style, as shown in Figure 1. The tube-style flameless vent was the original explosion protection flameless vent created more than 30 years ago, followed by the rectangular variations with either a curved or flat flame arresting mesh surfaces that we see available in the market today. Each flameless vent design has its benefits and drawbacks, and many suppliers offer variations to suit applications in a broad array of processing plants, including those in the biomass, chemical, pharmaceutical, grain, food, metal, and pet food industries among others.

Tube-style flameless vents are sometimes referred to as “quench tubes” and feature a cylindrical design that pairs with a round explosion vent. This flameless vent’s highly efficient design makes it suitable for almost any application, including high-KSt, organic, melting, fibrous, and some lighter metal dusts.

Box-style flameless vents have a slim profile for smaller-footprint and retrofit applications. Rectangular stainless steel mesh flame arrestor pairs with a rectangular explosion vent and is suited to dust collectors, storage bins, conveyors, and bucket elevators.

Elevator-style flameless vents feature a rectangular explosion vent paired with a curved, stainless steel mesh flame arrestor in a slimmer, lightweight profile that maximizes the provided vent area for use on elongated vessels such as bucket elevators and mechanical conveyors.

It is important to understand that all three styles of flameless vents provide varying levels of dust retention and thermal radiation reduction during a deflagration event and all flameless vents offer passive protection, which means they do not require an outside sensor or other device to continuously protect a process. The key is the mesh structure of the flame arrestor that dissipates the energy (heat) as the flame spreads out from a deflagration point. The mesh reduces the fuel’s burning temperature and extinguishing flame to prevent further propagation from spreading beyond the initial vessel. Note that flameless explosion venting should be paired with explosion isolation equipment to also prevent upstream and downstream spread of a deflagration to other equipment and plant areas.

Accounting for Real-World Conditions in Flameless Venting Efficiency

The efficiency of the various flameless vent styles depends on effectiveness of the flame arrestor (mesh) and the performance of the explosion vent. A flameless vent’s sizing and selection should be based on using data from the worst-case scenario for a given application as the vent’s efficiency can range from 45 to 95%, depending on certain variables.

Flameless vent efficiency during real-world conditions is a factor that may be overlooked during discussions about flameless vents because the vent’s flame-arresting medium has limitations on how much dust it can handle during a deflagration event and the dust level can fluctuate in a vessel or pipeline during operation. For example, a flameless vent with a larger vessel volume certification that contained a small dust concentration during testing may actually indicate the vent’s limitations in real-world applications. In the most commonly protected equipment — a dust collector — the dust suspension fluctuates regularly, with an internal dust concentration from 500 to 750 g/cu m during normal operation (or even higher during pulsing). Because of the frequency of high dust concentrations in this piece of equipment, the flameless vent certification process should include testing protocols that use real-world dust concentration levels or levels that match the application that the flameless vent will be used for in the field to ensure adequate protection for vessels, workers, and the overall plant.

To achieve these real-world comparable test results that will better suit a high dust loading application is possible by using either the FM 7730 or the EN 16009 (ATEX) testing protocol methods. While either test can be used, it is important to note one key difference in these tests: The newer FM 7730 certification requires a minimum 750 g/cu m dust concentration while the EN 16009 does not require a certain dust concentration during the testing process and allows a manufacturer to instead choose the dust concentration. Knowing which testing parameters were used by an equipment supplier in regard to dust concentration and vessel volume can help you to select the best flameless vent for your process equipment.

The newer certification, FM 7730, was released in 2015 from FM Global, which is a leader in industrial-grade product testing. The company’s testing protocols require a baseline dust concentration for flameless vents to meet FM Approval, so this testing method is more suitable in assessing the flameless vent’s capabilities in a real-world situation, including in the event of a process upset where an inordinate amount of dust (fuel) is present to generate a potentially stronger explosion if ignition were to occur.

Effective Vent Area

Beyond considering what testing standard is most useful to consider when selecting a suitable flameless vent, you should also consider the vent’s effective vent area. Essentially, the effective vent area is the efficiency of the flameless vent (which is a measure of how efficient the flameless vent is compared to if the process was protected by only an explosion vent of the same size). A general rule of thumb is that tube-style flameless vents will have a better efficiency than alternatives since the geometry of the flameless vent creates a better ratio of flame-arresting material to vent area compared to a box- (rectangular-) style flameless vent. The efficiency of a flameless vent is directly affected by the dust concentration it can handle. A flameless vent that proves to be only highly efficient with a low dust concentration will perform with a reduced efficiency at higher dust concentrations.

Manufacturers have some data about how their flameless vents perform by combining past experience in real plants and the test results from performing either the FM 7730 or the EN 16009 testing protocols. However, when considering which flameless vent is best for a particular application, it’s important to consider your dust and process vessel along with some other factors when calculating which flameless vent will offer enough vent area to provide adequate safety.

Flameless Vent Critical Design Points and Other Key Considerations

Understanding how important test data and certification is compared to real-world parameters is essential for proper flameless venting design. In addition to considering how a flameless vent has been tested to determine efficiency, keep in mind other design limitations that are part of good practice and the NFPA 68 standard:

K_st: The deflagration index of a dust cloud, which is the normalized rate of pressure rise in a 1-cu-m vessel. Higher K_st means that a material will have a faster deflagration rate (explosion strength) versus a material with a lower K_st.

Dust Loading: The amount of dust present in a volume (sometimes called dust concentration). As noted, this can have an outsized impact on efficiency.

Dust Type: The material being handled, which is often broken down into the following types: standard organic, melting, fibrous, or metal. Different dust types can affect the efficiency of a flameless vent.

Vessel Volume: The volume of the given vessel or area, for example a dust collector or perhaps a storage vessel

Room Volume: The volume of the room where the vessel is located must also be accounted for as some overpressure outside the vessel will occur during a deflagration.

P_red: The resultant or reduced pressure when a deflagration in a vessel is vented needs to be within the strength limits for the flameless vent to handle

Pairing the limiting criteria outlined in NFPA 68 with the following application-specific design considerations will help to determine how a flameless vent will impact a processing operation.

Safe Operating Distances: An effective safe distance for personnel to protect them from elevated temperature and pressure effects external to the unit during a deflagration or explosion

Area Classification: A hazardous area classification around the flameless vent to avoid the potential of a secondary ignition

Environmental Considerations: Emissions such as smoke, combustion gases, and in some cases unburnt particles, depending on the type of flame arresting material used in the vent’s design

Process Interlocking: Interlocking/monitoring the flameless vent with a burst indicator to allow for the interlocking of process systems during an activation

Process Compatibility: Weighing overall process compatibility with flameless venting. Some high-temperature, sanitary, and elevated-pressure applications may require different flameless vents to meet safety standards.

Flameless venting is a passive, effective method of explosion mitigation, allowing for safe processing in indoor and occupied areas. Understanding how different variables impact flameless vent performance and application is critical for process safety. If proper consideration is given to appropriate data-driven testing, then any safety-conscious company can adequately protect even the most challenging applications. Selecting a supplier that designs and manufacturers their own flameless vents, understands the regulatory standards, and has the engineering expertise of different processes will help you select a flameless vent for the best outcome: continuous process safety.

Jason Krbec, PE, is director of business development, CV Technology (Jupiter, FL). For more information, visit cvtechnology.com.