Work Package 2: Safety

Leader: Loughborough University

Partners

Leeds University, CEA, Shell Hydrogen, UK HSE, National Grid.
 

Scope and Objectives

The existing gas pipeline networks are designed, constructed and operated based on the premise that natural gas is the material to be conveyed. However, hydrogen has different chemical and physical properties which may adversely affect (increase) the risk presented to the public. Risk is a combination of likelihood of an untoward event (such as failure frequency of pipelines or ignition probability) and the consequence (hazard) of the event (such as the severity of a fire or explosion). Adding hydrogen to the gas infrastructure may affect both the likelihood and severity of untoward events and hence potentially increase the risk to the public. It is important to quantify this effect in order to establish if the risk remains acceptable and to identify the maximum hydrogen concentration that can be added to the natural gas without this risk becoming unacceptably high.

To achieve these objectives, the Safety Work Package is re-examining the hazards presented by fire and explosion situations pertinent to the gas infrastructure, based on a natural gas/hydrogen mixture being involved. Additionally, information on the likelihood of accidental events will be revised so that it is relevant to the new fuel, for example, pipeline failure frequencies and ignition probabilities.

To assess the change in consequences small and large scale experiments are being undertaken to provide data to aid model development and validation, specifically:

• Laboratory experiments to assess the burning velocities of methane/hydrogen mixtures

• Large scale experiments of gas build up in a domestic environment

• Large scale experiments of gas build up and explosions in an industrial enclosure/building

• Field scale and large scale experiments of explosions in congested regions (Vapour Cloud Explosions)

• Large scale experiments to study jet fires and underground pipeline failures

Simple and CFD models are being developed and validated using this data and then the models used to assess the impact of different levels of hydrogen on the severity of the hazards which may arise from a wide range of accident scenarios.

Data from WP3/4 will aid re-evaluation of the failure frequencies of pipelines and equipment. The ignition probability and minimum ignition energy of gas mixtures is being determined through laboratory experiments. This data will be used to revise the ignition probability used in risk assessments of the gas network.

The above work on likelihood and consequences is being combined to produce a risk assessment tool, which can be applied to gas networks conveying methane/hydrogen mixtures, enabling the risk to the public to be calculated for differing levels of hydrogen introduction and compared with the risk for a system conveying natural gas. The tool will be provided as input to the Decision Support Tool of Work Package 6.

Current Status

To date, all the large scale experimental work quantifying the change in consequences (hazard) have been completed. The gas build up experiments showed that the nature of methane/hydrogen gas accumulation appears to be similar to methane although increased buoyancy of the accumulation results in increased buoyancy induced ventilation which is beneficial in limiting gas concentrations. The confined vented explosions showed that adding 20% hydrogen had little effect on explosion severity, but adding 50% hydrogen resulted in a significant increase. In unconfined congested regions, studies of Vapour Cloud Explosions showed that with 50% hydrogen, the overpressures increase significantly and there is potential for a transition to detonation. The data from the jet fire experiments has shown that the characteristics of a methane/hydrogen (with 25% hydrogen) flame are similar to a methane flame but the flame temperature was slightly higher leading to higher heat loads to engulfed objects. The data from the pipeline fire experiments is still being analysed.

The development of simple and CFD models of gas build up and explosion hazards are well advanced. Model predictions have been compared with the experimental data and the models used to study the effect of changing parameters. These models made use of the results obtained during the laboratory experiments of burning velocities (laminar and turbulent) for methane/hydrogen mixtures.

In terms of assessing the likelihood of events, a series of laboratory based experiments have been completed to assess the minimum ignition energy and ignition probability of methane/hydrogen mixtures containing 0, 25, 50, 75 and 100% hydrogen. The data has been analysed and a correlation developed will enables the prediction of the minimum ignition energy for any methane/hydrogen mixture across the flammable range.

Failure probabilities for natural gas pipelines have also been collated from industry databases and these have been included in the risk assessment tool being formulated for transmission pipelines. This risk assessment package can be run as a stand-alone code or from within the DST. Currently, the risk assessment package can be run to predict the risk to the public from pipelines conveying natural gas. However, the structure of the package also allows natural gas/hydrogen mixtures to be considered, but further work is required in order to be able to provide suitable failure probabilities and ignition probabilities for the mixtures. The results of the fire experiments may also result in changes to the model of the hazard presented.