This paper presents results from a test program carried out to examine the impact of congestion on the peak deflagration pressure achieved within a vented enclosure. The industry standard for predicting the peak pressure developed in a vented deflagration is the National Fire Protection Association's Guide for Venting Deflagrations (i.e., NFPA 68). However, the NFPA 68 vent area correlations were developed using data from tests with enclosures that contained little or no congestion. Based on the results of external vapor cloud explosion (VCE) tests, it is reasonable to expect that the presence of congestion in a vented deflagration will increase both the rate of pressurization and the peak pressure achieved. The NFPA 68 guide explicitly cautions that congestion will create turbulence and increase the needed vent area.

These tests were conducted in a rig with a 24-foot by 24-foot footprint and a height of 6 feet. Steel plates were attached to the roof and all four sides served as vents. The vent panels weighed 2 lbm/ft2 and were configured to release at 0.3 psig; panel restraint devices were not utilized. The congestion was provided by a regular vertical cylindrical tube array (2-inch schedule 40 pipes). This array utilized provides a congestion pitch-to-diameter ratio of 7.6 with area and volume blockage ratios of 13% and 1.5%, respectively. This corresponds to a “low” congestion level in the Baker-Strehlow-Tang (BST) VCE blast load prediction methodology. Tests were conducted using near-stoichiometric methane and propane mixtures.

The average peak pressures achieved with methane and propane mixtures were 4.0 psig and 5.0 psig, respectively. These pressures are 20 to 30 times the values predicted by the NFPA 68 weak and strong enclosure correlations, respectively. On the basis of these tests, it is concluded that NFPA 68 correlations can significantly underpredict the peak pressure achieved in a vented deflagration for an enclosure with even a low level of congestion.