The differences between natural gas and hydrogen, such as density and combustion characteristics, mean the dynamics of hydrogen in high-volume gas escapes needs to be analysed to ensure safe network interventions.
Nowadays, Gas Operatives utilise a tool named “High Volume Gas Escapes Tool” (HVGET) to predict indicative ignition distances associated with high-volume natural gas escapes. This project seeks to upgrade the methodology of the HVGET to make it suitable for hydrogen gas streams.
This project will provide Gas Operatives with an efficient tool to predict and identify distances of explosive atmospheres ensuring customer supply continuity and lessening the risk of explosion/injury to the public and operatives.
The upgraded methodology will be ready by Q2 2023.
Benefits
This project aims to update the methodology used in the High Volume Gas Escapes Tool (HVGET) to make it suitable for Hydrogen. The HVGET is an Excel-based tool that predicts indicative ignition distances associated with high-volume gas escapes from gas distribution pipelines. This project will set the baseline to update the spreadsheet and provide Gas Operatives with an efficient tool to predict and identify distances of explosive atmospheres by improving the understanding of below-ground releases as well as the dynamics of the hydrogen escape. As a result, the project might bring on updates on existing piping repair procedures to be adapted for future hydrogen networks, reduction on emergency response time and lessen the risk of explosion/injury to the public and operatives.
Learnings
Outcomes
Prior to this project, there was no clear understanding of the ignition distances associated with gas escapes from intermediate to low pressure mains carrying hydrogen.
The methodology utilised to generate the models for hydrogen was based on experiments carried out as part of H21 – Phase 1 and data created with DNV’s FROST and CONIFER software. The tool has also been updated to use the latest version of the model for outflow from covered releases. The models used are described below:
- Covered Outflows. This model is composed of two sub-models. The first sub-model is for the pressure drop through the soil, which for low pressures gives a volumetric flow rate approximately inversely proportional to the viscosity of the gas. Hence, the ratio of the volumetric flow rate of hydrogen to the volumetric flow rate of natural gas is approximately equal to the ratio of the viscosity of natural gas to the viscosity of hydrogen, or 1.22. The second sub-model is for the velocity-driven pressure loss as the gas accelerates from being stationary upstream of the release into the orifice, which has a volumetric flow rate that is inversely proportional to the density of the gas. Hence, the ratio of the volumetric flow rate of hydrogen to the volumetric flow rate of natural gas is approximately equal to the square root of the ratio of the molecular weight of natural gas to the molecular weight of hydrogen, or 3.02 for the relatively rich natural gas used in the tool. Combining these two sub-models, the ratio of the volumetric flow rate of hydrogen to the volumetric flow rate of natural gas is expected to lie between 1.22 and 3.02, and the ratio of the mass flow rates is expected to be between 0.13 and 0.33.
- Exposed Outflows. Two sub-models were used to predict the outflow from exposed leaks. The first one was for exposed leaks that are small compared to the diameter of the pipeline, where the pressure in the pipeline remains close to the initial pressure. The second sub-model was used for pipeline ruptures, where the pressure in the pipeline adjacent to the rupture drops rapidly after the start of the release. It was also used for leaks that are smaller than a full-bore rupture, but which are a significant fraction of the cross-sectional area of the pipeline. Both models were appropriate to predict the outflow of hydrogen and natural gas, and both models give mass flow rates which are approximately proportional to the square root of the molecular weight, so the ratio of the mass flow rate of hydrogen to the mass flow rate of natural gas is expected to be about 0.33 for the relatively rich natural gas used in the tool.
- Covered Fires. There is considerable uncertainty in the way that gas emerges from the ground following a covered release. The release is likely to have low momentum, so jet fire models are unlikely to be suitable. For natural gas, the fire has been modelled as a simple point source at ground level, assuming that 20% of the heat is radiated. A similar model has been used for covered hydrogen releases, and it is assumed that the fraction of heat radiated for hydrogen is the same as for natural gas. There is little experimental data for this type of fire, but experimental data for jet fires suggests that the fraction of heat radiated is similar for hydrogen and natural gas. The ratio of the mass flow rates of hydrogen to natural gas is expected to be between 0.13 and 0.33, hence the ratio of the heat radiated by a hydrogen fire to a natural gas fire is expected to be between 0.32 and 0.82. The distance to a given thermal radiation is approximately proportional to the square root of the power, hence, the ratio of the distance to a given thermal radiation level for a hydrogen fire to the distance for a natural gas fire is expected to be between 0.57 and 0.90.
- Exposed Fires. The jet fire model used a flamelet to predict the fraction of heat radiated from the jet fire. If the predicted fraction of heat radiated was the same for both gases then the ratio of the distance to a given thermal radiation level for a hydrogen fire to the distance for a natural gas fire is expected to be at the upper end of the range given for ignited covered releases, that is, around 0.90.
- Covered Dispersion. Considering that the LFL of hydrogen is 4%, compared to 4.6% for natural gas, and the ratio of the volumetric flow rates of hydrogen to natural gas is expected to be between 1.22 and 3.02, hence the ratio of the distances to half the LFL for a hydrogen release to a natural gas release is expected to be between 0.32 and 0.82. A model for passive releases illustrated that the ratio of the distance to half the LFL for a hydrogen release compared to the distance for a natural gas fire is expected to be between 1.20 and 2.00.
- Exposed Dispersion. Unignited releases from exposed leaks result in dispersing jets, possibly with some momentum loss and dilution for releases which impact on a trench or crater. If the release has a lower momentum, the ratio of the distances to half the LFL for hydrogen and natural gas is around 1.4. Otherwise, the ratio of the distance is expected to be approximately 3.40.
To summarise, the ignition hazard distances for hydrogen tend to be slightly shorter than those for natural gas, but the dispersion distances for hydrogen tend to be significantly longer than those for natural gas. The additional overpressure hazards for hydrogen are often smaller than the distance to half the Lower Flammability Limit (LFL), hence, including the overpressure hazards often does not affect the size of the zones. However, for exposed ruptures of medium pressure (MP) and intermediate pressure (IP) pipelines with diameters larger than about 8” or 250 mm, the distance to 40 mbar can be greater than the distance to half the LFL and including the overpressure hazards increases the size of the zones. The zone sizes predicted for hydrogen are generally larger than those predicted for natural gas, typically by a factor of between 1.5 and 2.0
Lessons Learnt
The project achieved the desired outputs, therefore there are no particular takeaways to be drawn from this project.
