Simulating hydrogen VCE

Hydrogen leaks and ignition

Hydrogen leaks can arise from a variety of sources such as damaged pipes, faulty valves/dispensers or failure of a shutdown mechanism. Hydrogen is typically stored as a gas at high pressures (up to 950 bar). When a leak occurs, the gas escapes rapidly. Escaping hydrogen quickly disperses into the air, forming a flammable mixture. Hydrogen is highly flammable in a wide concentration range (4-75% by volume in air) and has a relatively low auto-ignition temperature (520°C). Hydrogen can generate electrostatic charges when released, which can potentially cause sparks that can be responsible for the ignition of hydrogen.

Scenario

Modelling approach

To simulate the case, we start at the top of the tree and continue downward, filling in information node by node:

• Add background. Define the context and environment for the simulation.

• Add receivers (optional). Identify vulnerable areas or congested regions where flammable cloud dispersion might occur.

• Add equipment. Set the location of the hydrogen storage tank on the map.

• Select models. Start from the point of release and add subsequent models in the order of events to create a model tree.

Model selection

One of the approaches to simulate a Vapour Cloud Explosion event is using an explosion model the ‘‘Vapour Cloud Explosion’’. A more precise approach is to specify a confined space within which the flammable cloud may overlap and calculate the actual confinement with in this 3D area using ‘‘Receivers’’. To determine the source condition for these models, the release model  ‘‘Gas Release’’ may be applied. Following the release model, a dispersion model ‘‘Dispersion - Flammable cloud’’ needs to be followed to predict the dispersion behaviour of a flammable cloud.

Alternatively, the whole chain of events can be simulated using the combined model ‘‘Gas LOC Scenario Continous Leak’’.

Source definition

Using an approach of modelling the order of events by linking models, the process starts with the ‘‘Gas Release’’ model to calculate the outflow conditions for the subsequent ‘‘Dispersion - Flammable Cloud’’ model. It is highly recommended to use a release model and a dispersion model to define the source terms that will be used as input into explosion models. This approach helps avoid defining unrealistic initial conditions.

To counteract the consequences of the negative Joule-Thomson effect, isothermal expansion should be applied for all hydrogen scenarios.

The model inputs for the Gas Release model are as follows:

Dispersion

Subsequently, the Dispersion model is linked to the release model. The exit condition will be automatically filled in as input parameters into the Dispersion model. Additionally, the result of this model can be used to report Flash Fires (see results section). The additional parameters that need to be defined are presented in the table below. These include a definition of release type, meteorological and environmental conditions and reporting rules.

• The type of release depends on the release scenario and needs to be selected by the user. For continuous release from process equipment above the ground can only be applied a vertical or horizontal jet.

• The meteorological and environmental conditions will influence and determine turbulence in the atmosphere.

• Reporting parameters define the moment in which the results are reported.

  • The ignition time specifies the moment at which the flammable cloud results will be reported (when the flammable cloud reaches its maximum area, the max. flammable mass or at a time defined by the user).

  • Concentration averaging time (expert parameter) is the duration over which the concentration is averaged to take into account the meandering behaviour of the plume.​ For a flammable cloud value of 20 seconds is set as default because we are most interested in the peaks of maximum concentration as this will determine whether the vapour cloud will ignite.

VCE

The EFFECTS Vapour Cloud Explosion model can be used to simulate the explosion phenomenon, which is based on the Multi-Energy method. Defining input parameters for this method might be challenging. Therefore EFFECTS includes the GAME correlations (Guidance on Application of Multi-Energy), which assists with the definition of the scenario. There are two ways to approach the explosion scenario:

Receivers - Congested area

The ‘‘Receivers’' node is situated above the Stationary equipment node. For confined area definition, the congested area type can be applied. The resulting degree of confinement of the cloud and its corresponding curve number are based on the actual overlap of the drifting cloud, by calculating the the fraction of the cloud captured in the congested area according to the real geometry of the congested area.​

This scenario includes 2 congested areas where the flammable cloud may be trapped and explode.

• Under the rooftop of the hydrogen fuelling point

• Under the rooftop of the regular refuelling station

Both areas are defined by area on a map and  by input parameters:

Connect the receiver to the dispersion model to calculate the overlap of the dispersion cloud with the congested area. This option is available in the pop-up menu after right-clicking at the dispersion model (see instructions below).

VCE - explosion model

This model estimates the overpressure impact of a Vapour Cloud Explosion (VCE) from a drifting cloud, triggered at a specific ignition point. The VCE model can be connected to a dispersion model, allowing several input parameters to be automatically populated. The key parameters for this model are outlined in the table below. The VCE model supports two calculation methods: the Multi-Energy method and the GAME overpressure method.

• If  '‘Use GAME overpressure method” = No, then the model uses the Multi-Energy method. In this case, we use the Multienergy method without GAME correlation.

• The Multi-Energy method requires the definition of the “fraction cloud involved in the explosion” and the “curve number” (can be calculated by the receiver). Due to hydrogen’s high laminar burning velocity, we expect curve number 10. The fraction is set to default.

• The vulnerability settings determine the translation of overpressure into damage effects.

Results

The results typically include map contours, grids, graphs and comprehensive reports. The map visualisation shows the flammable cloud area, overpressure contours and lethality contours in model and receiver nodes.

These data can be used to evaluate:

Flash Fire

The heat radiation from Flash Fire outside the flame area can be neglected. Therefore a Flash Fire is defined by the flammable area outlined by the Lower Flammable Limit (LFL) contours in the dispersion model results. Within these contours, the flame will cover the area, where a 100% lethality may be assumed.

Overlap with congested areas and overpressure

These results are available in Receivers. Additionally, the report and graph provide information about the amount of confined mass and damages due to the explosion. The overpressure contours based on a location of explosion based on ignition time are available in Vapour Cloud Explosion model as well.

Dynamic dispersion

Enabling dynamic dispersion provides a clear time and location-dependent overview of the Flammable cloud behaviour.

Combined model vs Linking models

Using individual models and linking them usually provides more visibility in the selection of calculation methods. However, linking the model vs using a combined model does not result in any differences in the results. While the combined model can run all possible scenarios, you can select to include only a VCE or Flash Fire and include receivers, making both variants identical.

The results can be customized to meet specific requirements. You can adjust parameters such as contour levels, contour and grid colours, combine graphs, etc. This customization allows for a more tailored and informative analysis, depending on the particular aspects of the flammable dispersion you need to emphasize.

For detailed guidance on how to adjust and interpret the presentation of results, explore the results chapter. You are also able to export your contours to Google Earth, the steps are detailed in the article, How to export consequence results into Google Earth.

Conclusion

Hydrogen leaks pose significant risks due to its high flammability, wide flammable range (4-75% in air), and rapid dispersion, forming explosive clouds. Advanced modelling techniques, implemented as the Multi-Energy method and overlaps with congested areas, are essential for simulating scenarios like Vapor Cloud Explosions (VCE) and Flash Fires, enabling precise evaluation of overpressure effects, confinement levels, and flammable mass distribution to apply safety measures and mitigation strategies.

Download the project file

Explore the project file simulating the VCE at the refuelling station. Compare the results from the combined model and the individual model. Use the dynamic concentration grid to track the cloud between the Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL) over distance and time. Assess the potential explosion impact and compare the approach with overlap with the congested area with a VCE model calculation.

Click this link to download the project file

To view the project file, please open it using the EFFECTS software. If you don’t have the software, you can download and use the free viewing demo version of EFFECTS via the link below.

Download EFFECTS free viewing demo

References:

1. RIVM. (2023). Calculation method for calculating the risks associated with the transport of compressed gases and cryogenic gases. Available at:https://rivm.openrepository.com/handle/10029/626793

2. NIPV. (2023.) Hydrogen filling station – Leak in buffer storage gas cloud explosion. Available at:https://scenarioboeken.nipv.nl/waterstoftankstation-lek-in-bufferopslag-gaswolkexplosie/