Are unconfined hydrogen vapor cloud explosions credible?

Abstract

Owner/operators of chemical processing and petroleum refining sites often ask whether unconfined hydrogen vapor cloud explosions (VCEs) can actually occur. This question normally arises during the course of a consequence‐based facility siting study (FSS) or a quantitative risk assessment (QRA). While it is generally recognized that a hydrogen release within a process enclosure could lead to an explosion, the potential for an external hydrogen release to cause a VCE is not as widely recognized and is often questioned. This uncertainty appears to stem from the impression that a hydrogen release always ignites quickly and near the point of release such that a flammable cloud does not have time to develop prior to ignition and/or that a hydrogen release never produces a flammable cloud of any significant volume due to its positive buoyancy. Unfortunately, neither impression is correct. Hydrogen releases are actually susceptible to delayed ignition, and hydrogen releases can form significant flammable gas clouds near grade level. Unconfined hydrogen VCEs can and do occur. Furthermore, given the potential for rapid flame acceleration associated with hydrogen, the consequences of a hydrogen VCE can be severe. Consideration of such events in FSS and QRAs is, therefore, warranted. Prior accidental hydrogen VCEs are reviewed to establish that such events do occur. Selected hydrogen VCE tests are also discussed to establish the potential severity of such events. Moosemiller and Galindo [10th Global Congress on Process Safety, 2014 Annual AIChE Meeting, New Orleans, LA, March 30–April 2, 2014] reviewed the ignition characteristics of hydrogen relative to the potential for a delayed ignition, and only the conclusions from that article are presented here. Example dispersions, using both simplified dispersion and computational fluid dynamics methods, are presented to illustrate the flammable gas volumes that can be created by hydrogen release scenarios. Blast load predictions are presented to illustrate the range of loads that could result from a hydrogen VCE due to such a release. © 2014 American Institute of Chemical Engineers Process Saf Prog 34: 36–43, 2015

Elongated VCE blast waves and structural response

“…Figure illustrates deflagration versus DDT regimes as a function of normalized distance and flame speed based on these observations. DDTs in BakerRisk unconfined test rig were reported for both ethylene and hydrogen mixtures, as shown in Figure a. A DDT resulted for ethylene within a normalized distance ( L f /L FV ) of 4 for an approximately stoichiometric mixture.…”

“…This cloud is referred to as “20/4/1 – 1.83 m” or “20/4/1 – R 2.92 m” within the context of this paper. The BakerRisk test rig – has dimensions of L = 14.6 m (48 ft), W = 3.66 m (12 ft), and H = 1.83 m (6 ft) and is referred to as “8/2/1 – 1.83 m” or “8/2/1 – R 2.06 m.” The Buncefield JIP Test 3.2 #4 configuration has dimensions of L = 30 m, W = 4.5 m, and H = 3 m is referred to as “10/1.5/1 – 2.25 m” or “15/1.5/1 – R 2.93 m.” The SRI RPSEA (GexCon) test #5 configuration with dimensions of L = 48 m, W = 8 m, and H = 4 m is referred to as “12/2/1 – 4 m” or “12/2/1 – R 4.5 m.”…”

Very lean hydrogen vapor cloud explosion testing

“…The lean hydrogen mixture was ignited at grade near the rig center. 9 The next tests, described in Section 3.2, were performed in 2022 in a test rig that was twice as long as the original test rig: 96 feet long, 12 feet wide, and 6 feet tall, with a high level of congestion. The mixture was ignited at grade, 24 ft from the west edge of the test rig.…”

“…2,3 Furthermore, assertions that hydrogen cannot pose a VCE hazard are contradicted by the existence of accidental unconfined hydrogen VCEs that have produced damaging blast loads. 4 Given the expected push for the expansion of hydrogen-related infrastructure, it is critical that industry leaders, regulators, and safety professionals understand the potential for hydrogen-air clouds to produce damaging blast loads. The specific purpose of this research effort was to explore the lower concentration limit at which hydrogen-air mixtures can be expected to produce damaging blast loads in an unconfined environment.…”

Hydrogen jet vapor cloud explosion: A model for predicting blast size and application to risk assessment

“…In the case of the delayed ignition of jets, the turbulence induced by these releases can lead to flame speeds sufficient to produce damaging VCE blast loads, even in the absence of confinement or congestion. The ignition of hydrogen released as a jet has resulted in accidental VCEs . Hydrogen jet release VCE scenarios may therefore be relevant to consequence assessment studies, particularly in the absence of alternative release scenarios involving large congested volumes.…”