Large-scale vented deflagration tests

Abstract

This paper presents results from a test program carried out to determine the peak deflagration pressure achieved within a congested enclosure vented through one wall of the enclosure. The industry standard in the United States for predicting the peak pressure developed in a vented deflagration is the National Fire Protection Association's Standard on Explosion Protection by Deflagration Venting (NFPA 68). The NFPA 68 (2013 edition) vent area correlation accounts for varying degrees of congestion if the ratio of the obstacle surface area (Aobs) to that of the enclosure (As) is greater than 0.4 (i.e., Ar = Aobs/As > 0.4). The tests described in this paper were performed using an obstacle array with an Ar ratio of less than 0.4. These tests were conducted in a rig with a 48-foot width, 24-foot depth, and 12-foot height. The rig was enclosed with solid walls, roof, and floor, allowing for venting through one of the long walls (i.e., 48-foot by 12-foot). The venting face of the rig was sealed with a 6 mil (0.15 mm) thick plastic vapor barrier to allow for the formation of a near-stoichiometric propane-air mixture. The flammable gas cloud was ignited near the center of the rear wall. Steel vent panels (20-gauge, 2 lbm/ft 2 ) were installed over the plastic vapor barrier using explosion relief fasteners. The vent panels were configured to release at 0.3 psig; vent panel restraint devices were not utilized. The congestion inside the rig was provided by a regular array of vertical cylinders (2-inch schedule 40 pipe and 2-inch outer diameter cylinders) giving area and volume blockage ratios (ABR and VBR) of 4.9% and 2.2%, respectively, within the congestion array. The obstacle to enclosure surface area ratio (Ar) for this obstacle array pattern is 0.3 with the array extended throughout the rig, which is less than the critical value to account for congestion in the NFPA 68 correlation.Four series of tests were conducted with varying vent parameters, flammable gas cloud sizes, and congestion levels. Baseline tests were performed with the congestion array and flammable gas cloud extending throughout the rig without vent panels present (i.e., vapor barrier only). The second test series included the addition of vent panels for the same congestion pattern as that employed for the baseline tests. The third test series utilized a flammable gas cloud which filled only the back half of the rig. For the fourth test series, the congestion array only occupied ¼ of the rig. The peak pressures and impulses for each test series are provided, along with pressure histories internal and external to the rig for selected tests. The steel vent panel throw distance is also provided as a function of internal peak pressure.The test data were compared with the predictions of the vent area correlations provided in NFPA 68. For all but the fourth test series (i.e., congestion array occupying ¼ of the rig), the average internal peak pressures were approximately a factor of 2 larger than those predicted by NFPA 68.

Comparison of large-scale vented deflagration tests to CFD simulations for partially congested enclosures

“…The internal impulse for each test series is shown in Figure 15, along with those predicted by the FLACS simulations and NFPA 68. Previous work comparing internal peak pressure data from the DLG to NFPA 68 predictions [4] discussed the limitations of NFPA 68 for accounting for varying levels of congestion at Ar ratios of less than 0.4. For Test Series B and C, the Ar ratio is less than 0.4 (0.32 and 0.17 for Test Series B and C, respectively), and this causes the NFPA 68 predictions to be identical for these two series.…”