(47a) A Methodology to Evaluate Flameless Venting Devices for Dust Explosions

Venting is a common technique used to reduce the potentially-damaging overpressures that can develop inside an enclosure during an accidental explosion. As a result of the venting process, however, several hazards are generated outside of the enclosure such as an external explosion (of the vented-unburned explosive mixture), a high-velocity flame jet, and blast wave effects. For dust explosions, it is possible to minimize these external hazards through the use of a flameless explosion venting device. This device is essentially a cylinder or a box that is made of layers of wire mesh and is installed over an existing vent. As a dust explosion is vented through the device, the layers of wire mesh retain a large portion of the burned and unburned dust and act as a heat sink, arresting the vented flame jet by reducing the reaction-zone temperature.

A flameless venting device also acts as an obstruction that increases the reduced overpressure inside the enclosure (compared to venting without the device) and thereby decreases the effective area of the vent. Because the resulting increase in the reduced overpressure can potentially exceed the design strength of the protected enclosure, it is important that the performance of a flameless venting device is evaluated in order to ensure its proper use.

Limitations and safety considerations for these devices are outlined in NFPA 68 (2007) but require the end user to collaborate with the manufacturer to address these concerns. More detailed guidance is described in the European Standard EN 16009 (2011), which includes test requirements where the reduced overpressure using a flameless venting device is compared to a case of an open vent. The effectiveness of a flameless venting device is then determined by its efficiency factor, which is defined as the ratio between the effective vent area of the device to the geometric vent area.

Although there is a clear quantitative methodology that is described in the EN Standard, a flameless venting device is generally mounted on top of a burst disc with a finite specific mass and burst pressure, which is not taken into consideration in the EN Standard. Furthermore, the effective vent area cannot be directly determined from experimental test results and require the use of an empirical vent sizing methodology. Depending on which methodology is used, the efficiency factor can vary.

Therefore, in the present investigation, the reduced overpressure using a flameless venting device (mounted with a burst disc) is investigated experimentally and compared to that using a burst disc only. The results using cornstarch as a representative fuel in an 8 m³vessel are presented. An overall efficiency of the device is determined, which is defined as the ratio between its effective vent area and the nominal vent area. Because a flameless venting device also includes the use of a burst disc with its own efficiency, the functional form of the overall efficiency is taken as the product of the area efficiency of a flameless venting device (i.e., the ratio between the effective vent area of the device to that of the burst disc) and the burst efficiency of the burst disc itself (i.e., the ratio of the effective vent area of the burst disc to the nominal vent area). The effective vent areas are calculated from measured overpressures using three different empirical correlations: FM Global (see Tamanini 2001), NFPA 68, and VDI 3673 (2002). Furthermore, due to significant variations in the effective reactivity from test to test, a correction factor proportional to the initial flame speed is applied when determining the area efficiency. In general, it was found that the FM Global and NFPA methodologies yield consistent results with less scatter than VDI 3673.

References:

EN 16009 (2011) “Flameless explosion venting devices.” European Committee for Standardization: Brussels, Belgium.

NFPA 68 (2007) “Standard on explosion protection by deflagration venting.” National Fire Protection Association: Quincy, MA, 2007.

Tamanini, F (2001) “Scaling parameters for vented gas and dust explosions.” J. Loss Prev. Process 14:455-461.

VDI 3673 (2002) “Pressure venting of dust explosions.” Verein Deutscher Ingenieure: Dusseldorf, Germany.