No abstract

To gain an understanding of the risks associated with a hydrogen pipeline failure, Air Products commissioned GL Industrial Services UK to perform two experiments where a buried 6″ diameter pipeline at an initial pressure of 60 bar was intentionally failed using an explosive charge to generate a full bore release of hydrogen gas from two open ends, simulating a pipeline rupture event in which a ground crater is formed naturally in the surrounding soil by the released gas. The first experiment was performed with the pipe buried 1m deep in a typical soil and the second experiment was performed with a 1m deep backfill of a mixture of sand and soil. The hydrogen released was ignited immediately following the pipeline failure. Following initiation of each experiment, the properties of the hydrogen gas release and resulting fire were measured. The two experiments were conducted under similar conditions, with the main differences being the nature of the soil used for the pipeline backfill and the wind speed (which was significantly higher in the first experiment). The initial pipeline pressure was very similar in the two experiments, with complete depressurisation of the gas pipeline and reservoir taking place over a period of approximately 80 seconds. Maximum flame lengths of up to approximately 100m were measured in each experiment. A number of previous experimental programmes have been carried out by GL in order to investigate the fire characteristics of natural gas releases from ruptured pipelines, conducted under nominally similar conditions. Recently, experiments of this type were also conducted to investigate releases of mixtures of hydrogen and natural gas. The paper will present a high level overview of the results including a discussion of the observed differences between the release and fire behaviour of the different gases.

Analytic methods used to establish thermal radiation hazard safety boundaries from ignited hydrogen plumes are based on models previously developed for hydrocarbon jet fires. Radiative heat flux measurements of small- and medium-scale hydrogen jet flames (i.e., visible flame lengths < 10 m) compare favorably to theoretical calculations provided corrections are applied to correct for the product species thermal emittance and the optical flame thickness. Recently, Air Products and Chemicals Inc. commissioned flame radiation measurements from two larger-scale hydrogen jet flames to determine the applicability of current modeling approaches to these larger flames. The horizontally orientated releases were from 20.9 and 50.8 mm ID pipes with a nominal 60 barg source pressure and respective mass flow rates of 1.0 and 7.4 kg/s. Care was taken to ensure no particles were entrained into the flame, either from the internal piping or from the ground below. Radiometers were used to measure radiative heat fluxes at discrete points along the jet flame radial axis. The estimated radiant fraction, defined as the radiative energy escaping relative to chemical energy released, exceeded correlation predictions for both flames. To determine why the deviation existed, an analysis of the data and experimental conditions was performed by Sandia National Laboratories’ Hydrogen Safety, Codes and Standards program. Since the releases were choked at the exit, a pseudo source nozzle model was needed to compute flame lengths and residence times, and the results were found to be sensitive to the formulation used. Furthermore, it was thought that ground surface reflection from the concrete pad and steel plates may have contributed to the increased recorded heat flux values. To quantify this impact, a weighted multi source flame radiation model was modified to include the influence of planar surface radiation. Model results were compared to lab-scale flames with a steel plate located close to and parallel with the release path. Relative to the flame without a plate, recorded heat flux values were found to increase by up to 50% for certain configurations, and the modified radiation model predicted these heat fluxes to within 10% provided a realistic steel reflectance value (0.8) was used. When the plate was heavily and uniformly oxidized, however, the reflectance was sharply attenuated. Model results that used the surface reflectance correction for the larger-scale flames produced good agreement with the heat flux data from the smaller of the two flames if an estimated reflectance of 0.5 was used, but was unable to fully explain the under predicted heat flux values for the larger flame.

Hydrogen is a critical component in the production of cleaner fuels. Underground pipelines provide a safe, reliable supply of hydrogen to refineries and the petroleum industry. Proper assessment of the risks associated with underground hydrogen pipelines requires an accurate model of the jet fire consequence. This article will describe experimental and modeling work undertaken in order to define the appropriate methodology for utilizing DNV's PHAST software tool to represent the hydrogen jet fire from the rupture of underground hydrogen pipelines. Two experiments were conducted to measure the flow and radiation from an intentionally ignited rupture of a 6 in. diameter, 60 barg hydrogen pipeline buried 1 m underground. Adjustments to PHAST modeling parameters were required in order to obtain agreement between the measured and predicted radiation and flame length values. The modeling assumptions and parameter adjustments include:Velocity modification to account for interaction of the flow out of the two ends of the ruptured pipe and to model the subsequent discharge from the crater. Specification of the fraction of heat radiated. Specification of the angle of the release.