Modelling fires

Published:  18 July, 2013

By 2014, commercial and industrial property insurer FM Global aims to start carrying out risk service testing and modelling of fires for clients – Hemming Fire speaks with Chief Engineering Technical Specialist (UK) Allan Macpherson about the benefits that the open source fire modelling project will bring to the warehousing industry.

How difficult is it to model a fire?

It’s insanely complicated and the number of variables is absolutely enormous. Just to give you an example of the level of detail, if you have a fire that burns with a bright luminous flame it will tend to radiate more heat - as radiant heat absorbs by fuel surfaces - it makes it easier for fire to spread. A clean burning flame will change the way fire spreads. And then you have the business of the configuration of the burning surfaces, the pallets, cartons on the shelf… and to make it more interesting, what we are trying to do at FM Global is model how sprinkler protection deals with the fire, so you have to start thinking about water droplet sizes, to what extent they can penetrate the flames, and whether the updraft is going to carry the droplets away. And then you also look at the ability of a surface to get wet – cardboard absorbs water making it harder for the fire to spread but the exact way and the degree to which it does this needs to be known. So you are modelling the fire down to the flame colour,  burning surfaces of all the materials involved, and water from sprinklers cascading down through the storage array.

At what stage are you now?

We’ve got to the point where we can model fire spread in cartoned commodities and paper. We also have models of a storage rack four tiers high with sprinklers interacting with the fire. The model is  physics-based and validated against testing we’ve done, so it gives us the same results over a specific range of test conditions. But that is not to say the model is complete because we still want to increase the range of storage configurations and heights, as well as include plastics and complex industrial fuels into the model.

The calculations needed to do the simulation – which in a real fire test would last five minutes – take a day in processing time. You need very powerful computers because you are splitting the whole thing into a mesh – an array of cubes if you like – for the mathematical equations. And something like a 3-D storage rack requires many different meshes for gas, solid fuel and water, all interacting with each other, so we have computer systems processing ten trillion operations per second. Fortunately, computers are getting faster, and the ways we solve the equations are getting more efficient.

We are collaborating with academia such as the Imperial College, University of Edinburgh, Kingston University, University of Ulster and University of Maryland in the US on the development of the FireFOAM fire model, which is based on OpenFOAM, an open source computational fluid dynamics (CFD) toolbox.

What is the ultimate aim?

We may never replace real-life testing with computer simulations – or at least it’s a long way off. What we are doing, however, could lead to fewer and more beneficial fire tests. So, in the past, to design a specific fire protection solution you might have needed to conduct eight full-scale fire tests, soon you only might need to run three such tests, covering the two extremes and one in the middle, and have simulation software to cover all eight. If you get the same answers as the extremes and the one in the middle, you can be reasonably confident that you are getting good answers to the ones in between the middle and the extremes. We are getting close to that.

What will be the benefits to clients?

Full-scale testing is costly and can takes months to set up. There are many cases to address, so we are operating our FM Global Research Campus in two working shifts at the moment.

In 2012 we carried out around 600 tests. A typical situation might be someone who has existing sprinkler protection but the storage commodity changes, and we are not sure whether the fire protection will do the job. If we get to the point where we can run computer simulations in a couple of weeks, we may be able to give a more timely answer – for example the existing protection is ok, or you need to do some adjustments but not rip the whole thing out. We might model it and say it doesn’t work, or perhaps model it with a different sprinkler head and then say “yes, that makes the difference.”

The fire modelling will also inform the standards we publish on protecting different hazards and make them more finely detailed. We tend to give protection criteria in ceiling height increments of 1.5 metres. With modelling we could perhaps reduce it to one-metre increments, resulting in more finely tailored protection schemes.

We will also be able to do risk-service testing, where a client has something that doesn’t quite fit our published standards, so we then could build a model and use it, along with any necessary tests, to choose a solution for them.

We are optimistic this is going to work!

  • Operation Florian

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