SAFETY/FIRE | TECHNICAL
and plan against them. Ventsim has a Dynamic Fire Simulator that gives access to some of the emergency tools and other tools for gas explosions and emergency escape as well. Preset fire events can be selected using the library and
entered into the model within any part of the tunnel. This can be moved around to understand variances, acting as a tool for training or for planning. It could also be a tool for actual emergencies if the model was set up with these correct presets in advance. An incident of fire is not defined with a simple start
and finish. Fire will progress from its initial small state, growing and burning for some time, before decaying and ceasing. So it can be described as a series of three stages within a single event. This is matched in the Ventsim software model through the event wizard, allowing the fire to have the stages mapped out based on data entered. Simulations can be adjusted based on changes to the
fire itself as well as for other things in the ventilation system, for example, turning a fan on or off. This flexibility enhances the range of insights across the model. When a ventilation system is modelled, the normal
flow of air and gases can be noted and seen. The expectation might be that smoke from a fire will follow a similar path. However, as previously noted, fires can behave in different ways, so the value of the fire simulator is in providing that further tested insight. One thing, in particular, to monitor is the level of
carbon monoxide. The Ventsim model will produce a carbon monoxide generation estimate automatically for a fire simulation to help establish the potential risk of danger to people. As the simulation proceeds, the fire stages can be
observed graphically in colour along with the charting of carbon monoxide levels. Airflow direction can also be observed and that may indicate changed states due to heat and buoyancy or other effects taking place, requiring a response around evacuation. Fire requires oxygen and the model will automatically
alter the burn rate of the fire to match the oxygen available – and to the amount of fuel it can affect, even if there is a huge amount of fuel that could potentially be added to the fire being modelled. This results in more realistic fire behaviour. In some cases there may be an oxidising agent within the fuel source. The Ventsim software feature that automatically accounts for changes in oxygen can be turned off, therefore, to reflect that the chemical would also contribute oxygen. Rollback occurs where you have a hot fire causing
the smoke to rise to the roof of the tunnel and then to move backwards along the roof against the ventilation flow. This is effectively a bi-directional movement of air and an extremely complex scenario to model even with computational fluid dynamics (CFD); however, this effect can be simulated to an extent by applying the rollback feature to areas either side of the fire that could potentially be affected.
Useable learnings from a fire simulation Looking to the objective of planning and design for the best outcomes, a lot of information can be learned from a simulation. Monitors can be placed throughout the tunnel to
understand air flow and gas levels at each point. This can be done along the course of the fire as it goes through the stages so we can understand the full dynamic and impact: ● How does the fire behave? ● Are the planned escape routes workable over the event?
Answers can be tested in simulations with fires of different sources placed at different locations along the tunnel model. The modelling can help to examine if the ventilation design might be adapted to negate issues identified, or as an active agent against fire impact, to increase safety and enable escape routes to be maintained for longer. It is important for the simulation tools to integrate
well into the actual system design as placement of fans, fire doors, and escape routes can all be tested out to optimise from a safety perspective. Actively altering the design means that each scenario output can be improved. When developing escape route plans, in particular,
the Ventsim software, for example, has both an Escape Route Analyzer and a Calculator to help establish the safest and most efficient pathways for exit, and the time to travel those routes. Through this, the fire safety modelling can help to
ensure that the nominated safe locations in a tunnel (e.g., refuge bays) can be reached during an incident, and how long it might take for people passing through dangerous conditions. In the case of an actual event, the Escape Route
Calculator could also help rescue teams to assess the best routes to get to a particular location, and to find routes to best avoid, where possible, dangerous levels of smoke and gas.
SUMMARY Fire simulation can play a vital part in the development of safe ventilation systems for tunnels. By combining all the information provided by the simulation, designers and operators can gain more insight and predictable views of the dangers that need to be addressed in establishing the infrastructure and for emergency planners during operations. Originating in the underground mining industry and
extending into infrastructure tunnels, Howden’s Ventsim software has been used for many decades for ventilation design, providing users with a graphical and intuitive way of developing and testing fire safety systems. The software’s Fire Simulator, in particular, has a depth of features to fully illustrate and understand the complex impact of fire on a design leading to optimised final designs and informed emergency planning.
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