CFD simulation of gas dispersion

Ensuring safety in environments where dangerous gases are present is of utmost importance. Production environments may deal with gases that are toxic or asphyxiating, flammable or explosive, or corrosive. That means not just the installations, but also the workers’ health and even lives are at stake if special procedures or unforeseen gas releases are not properly dealt with.

Using CFD simulations, our engineers model leak scenarios with precision, predicting the behaviour of gases under different wind, leak rate, or ventilation conditions. This allows the process engineers or the environment, health and safety professionals to come up with improved workspace layouts, ventilation systems, and emergency response strategies. Depending on the location of the risk, two typical models are:

• Indoor gas leak: The dispersion of a gas inside a building depends, apart from the gas properties and release parameters, on the ventilation of the building. Typically, the ventilation system is designed for a number of air changes per hour. As a first step, CFD can show whether the ventilation is well distributed, or whether there are dead zones. Then, release scenarios can show the impact of safety measures on the expected concentrations of the dangerous gas.
• Outdoor gas leak: The dispersion is determined by the wind direction and speed, interacting with the local relief and buildings or other constructions. In this case, CFD studies are used for risk assessments, or to investigate the possibility of safeguarding the most impactful locations by quick detection and intervention.

In both cases, the gas density is important to consider. Some toxic gases have high densities by nature, and densities can especially be high for compressed gases due to the low temperatures of the escaping gas. High densities can lead to higher concentrations of close to the floor, as can be seen in the simulation below.

Example In the simulation below, a leak of a toxic gas (heavier than air) is simulated. The room is ventilated by suction from the top, with air inlets through transport gates. The gas concentrations are higher on the bottom, but due to the flow pattern of the ventilation, the gas spreads around the room and across its height relatively quickly.

Detecting leaks Using transient simulations, CFD can predict the time it will take before the dispersing gas reaches the location of sensors in a sufficient concentration to be detected. This is helpful to design a detection strategy based on catch-all sensors and/or sensors at high risk locations. The time until detection, together with the time needed to intervene (shutting down installations, closing valves, evacuating…) determines the exposure risk and expected dosage.

CFD is useful to model scenarios with several types of gases:

• Toxic gases such as CO, H₂S, Cl₂ or NH₃ can be harmful from low concentrations. When odourless like CO, the risks are particularly high as the poisoning gas may be breathed in without being noticed.
• Flammable or explosive gases such as CH₄, C₃H₈ or H₂ can cause fires or explosions when mixed with air and ignited.
• Asphyxiating gases such as N₂, CO₂, or Ar aren’t toxic in small concentrations, but by displacing air, can lead to suffocation especially in confined spaces or vessels.

The versatility of our CFD tools and high-performance computing power allows us to run detailed cases at building, site, or neighbourhood level. Feel free to contact us to discuss possibilities.