Water mist in tunnels - First hand experience

What follows are some personal reflections on my attendance at the SOLIT2 project workshop in Gijon, Spain, on 22nd & 23rd June 2011. But first, some context.

As might be expected for somebody who has worked in Fire Safety research for over a decade, I have seen quite a few experimental fires. We regularly fire test things in our lab and I've seen plenty of fires with heat release rates in the range of 300-500 kW. I have also seen fires in our lab as big as 700-800 kW and have seen the reactions of people to fires on this scale. They generally start backing away slowly and start feeling uncomfortable, both with the level of radiant heat they start experiencing and the 'what would happen if...' thoughts that start going through their minds.

Those fires are the limit of what our lab can handle (we can go transiently to 1000 kW - that is, 1 MW - but we rarely go that big). For various reasons, I've also been witness to some larger fire tests in the burn hall at BRE, for example, I've seen a pool fire that was a little over 2 MW and a solid plastic fire that was also about 2 MW. I've stood a few metres away from such fires and know what the radiant heat feels like. 2 MW is a big fire. When we did the Dalmarnock Fire Tests a few years ago, the peak heat release rate (for an entire living room / home office on fire) was around about 5 MW. 5 MW is a big fire.

But when we start talking about design fires for tunnels, we start hearing numbers like 30 MW or 100 MW and its hard to grasp just how big a fire that means.

So when I got the opportunity to witness an alleged '100 MW pool fire' in the San Pedro de Anes test tunnel, well, how could I refuse?

The SOLIT2 workshop was held in a nice hotel in Gijon, Spain, and featured not one but two visits to the nearby TST test tunnel to witness fire tests with water mist. These were allegedly not demonstration tests, but were part of the SOLIT2 test programme, investigating the abilities of a water mist suppression system to mitigate the effects of a fire in a tunnel.

The workshop started at the hotel on Wednesday 22nd June 2011. The first presentation, by Stefan Kratzmeir of IFAB, gave the context of the SOLIT2 project - the aim is to develop and test water mist technology to either:

1. Achieve the same same level of safety in a tunnel with a water mist system at a reduced cost compared to other common tunnel safety systems (i.e. by installing water mist, you can 'trade-off' and reduce the specifications of other safety systems, such as structural fire protection, or ventilation systems, etc.), or
2. Achieve a greater level of safety in a tunnel with a water mist system at the same cost as would be spent on other systems (i.e. still trading off systems).

So it was clear from the outset, the objective of this project was to reduce costs without increasing risk. But enough on the presentations, we were rapidly shipped off to the test site to witness a large pool fire test.

The fire was not 100 MW as advertised. It was probably about 50-60 MW, which is still [insert adjective or expletive of your choice] big! The 700 l of diesel fuel was distributed across 7 large rectangular fuel pans (each at least 2 m.sq). Once lit, the fire grew rapidly in severity (as pool fires do) and soon we (the observers, standing about 45m upstream of the fire) found ourselves looking up at a layer of smoke billowing across the ceiling above us. This is the dreaded phenomenon of 'backlayering' - even though there was a longitudinal flow of about 2 m/s, it wasn't sufficient to drive the smoke away from us.

It became clear that this hadn't been an intentional part of the demonstration. Some of the Fogtec and IFAB people started looking worried. But the water mist system (spanning a stretch of 50m of tunnel straddling the fire location) was started about 90 s after ignition (I think the intention had been to start it after 60 s) and soon the backlayering began to shrink back and ultimately vanish into the mist.

What rapidly became clear was that the water mist was not extinguishing the fire. It was hard to tell from an observer's point of view, but it appeared that the water mist was also not suppressing the fire, at least, not using the dictionary definition of suppressing (which involves concepts like halting growth and reducing size). The fire appeared to burn at about the same level of severity while the water mist operated. However, what the mist achieved was a reduction in backlayering, possibly due to a reduction in smoke production, or possibly due to a reduction in the buoyancy of the smoke. The mist also provided thermal protection for the tunnel structure and (had there been anyone there) from the people in the vicinity of the fire.

After the pool fire test we were returned to the hotel for an odd lunch of nibbles and finger food and an afternoon of fairly uninteresting presentations (in which the same point was made over and over again - you can trade off other systems against water mist. OK. I get it). Apologies to the speakers, if they're reading this, it wasn't that they were all boring, it was just that I'd heard most of it before, and there was a lot of repetition.

Day 2 of the workshop featured another presentation on the findings of the SOLIT2 project and then another coach trip to the test tunnel. This time, the fire test was to involve a 'simulated truckload' - that is, lots and lots of wooden pallets, arranged in the basic shape of a HGV trailer, covered by a tarpaulin and held in place by a steel frame. The fire was lit, the fire was allowed to grow for about 4 minutes, and the water mist system was activated.

Once again, the mist did not extinguish the fire. Indeed, the fire grew from about 3-5 MW at the point of activation of the mist to about 20-25 MW while the mist was active. But in this test, we (the observers) got the chance to put on waterproofs and approach the fire location.

Here's where the context above comes in. I know what it feels like to stand about 2 m away from a 1-2 MW fire. I now know what it feels like to stand about 2 m away from a 15 MW fire (the size when I approached the fire) in water mist. It feels more or less the same. That is, the radiant heat from a 15 MW fire is attenuated by the mist to such a degree that its similar to the heat flux from a fire a tenth of the size without mist.

And I also know just how wet you get in water mist.

And how good the visibility was. That surprised me. Standing beside the wall on one side of the tunnel, I could clearly see the way-finding lights on the opposite wall. In other words, visibility was still of the order of 10 m.

After about half an hour of burning, the fire brigade were sent in to extinguish the fire. This they could do easily. After this we saw another demonstration of a portable mist system on a burning car (in the open air) and were returned to the hotel for another odd lunch of nibbles and a final summing up session.

So I now have first hand experience of what water mist systems can do for fires in tunnels. They can block heat and reduce smoke production / backlayering.

I have published quite a lot on the subject of the limitations of water mist in the past. Has this experience changed my mind? Well, yes and no.

Yes, in that I now consider heat release rate to be a largely irrelevant parameter when assessing the life safety situation in a tunnel with an active water mist system.

No, in that the workshop still has not addressed some of my other questions, the primary one being 'are water mists systems better than conventional sprinkler systems for fires in tunnels?'

But I've got more to think about and more research to do. This won't be the last thing I publish on the subject of water mists.