Major tunnel fires can be catastrophic with the scale of lives and property lost. The particular nature of fires in such confined spaces ensures special challenges to fire safety professionals and fire fighters.
Ali Aleali is business development manager for FireVu, which has designed and installed fire detection solutions for Dubai’s Palm Jumeirah and Sydney Harbour tunnels. With 30 years in the fire safety industry, Ali discusses the on-going issue of major fire tunnels.
The Mont Blanc Tunnel was no stranger to fire.
It had registered 16 truck fires over 35 years prior to the disaster, but on each occasion the situation was recovered early, the fire put out.
The morning of the 24th March 1999 was different. The results were particularly horrific even by the disastrous standards of major tunnel fires.
A Belgian truck carrying flour and margarine was spotted with white smoke coming from under the cab. The smoke had been streaming for some time as tunnel users tried to alert the truck driver to danger before he stopped. He got out and tried to manage the problem. However, flames took hold and he was forced back.
Two minutes after the driver had attempted to put out the fire, tunnel employees raised the alarm. At this point there were at least 10 cars and vans and 18 trucks in the tunnel that had entered from the French side, which were to be trapped because of the airflow’s direction.
In the ensuing conflagration, 39 people died as temperatures reached more than 1,000 °C (1,830 °F). The fire burned for 53 hours. The combustible loads of other trucks prolonged the disaster: the flour and margarine load alone was equivalent to a 23,000-litre oil tanker.
Repercussions included €13.5 million (US $17.5 million) in compensation paid by the Italian tunnel operator alone and a custodial sentence for the head of tunnel security. Renovations and fire safety upgrades took three years to complete while a vital artery of transport and trade was closed.
The Alps region was to suffer more fatal tunnel disasters such as Tauern and Gotthard within a couple of years of Mont Blanc, bringing even more attention to the issue.
|Fire detection systems need to recognise danger early|
The fact that years of incidents and danger signals did not translate into satisfactory fire detection and prevention measures being pursued before disaster struck was certainly a tragic opportunity lost.
The specific danger of tunnel fires
Fires in tunnels present particular dangers, more so than many open air environments.
Firstly,there are the High Heat Release (HHR) rates. Vehicles burn far more vigorously in confined spaces than open areas. The HHR is four times as powerful in tunnels as open areas. While a car fire produces 3-5 MW, tankers produce as much as 200 MW. The Mont Blanc tunnel had a number of tankers and trucks with combustible loads that strengthened the force of the fire.
Then there is the dense smoke, which can be highly poisonous. Mont Blanc’s fumes contained cyanide and carbon monoxide. The speed of the smoke outpaced those who decided to run; those that attempted to escape using their vehicles found that the smoke choked off the oxygen to their engines, which ground to a halt.
Fighting the fires: the challenge
When a fire takes hold in a tunnel it presents a number of specific challenges and dangers to fire fighters:
- Incredible heat – in the case of Mont Blanc, it took 5 days for the tunnel to cool
- Thick toxic fumes
- Poor or no visibility
- Confined operating space for emergency services
- Airflow that generates aerodynamic disturbance affecting smoke behaviour
We can also add the problems of communication and with international tunnels, different authorities from different jurisdictions and countries, perhaps languages, working together under pressure.
The need for fire detection measures and the response
Tunnel designers have often looked at causes of accidents, which although naturally important are perhaps a distraction from concentrating on fire detection and prevention that can help avoid lethal situations altogether.
It is essential that fire detection and prevention systems reduce the risk and enable fires to be contained and extinguished before developing into unstoppable infernos.
Indeed, a new EU directive came into force at the end of April 2014 that should help reduce major fires immensely. In terms of incident and fire detection monitoring, systems need to be implemented for tunnels over 500m long. For those over 3000m, video systems are mandatory.
Good progress has been made in implementing the directive. Many countries have exceeded the mandatory requirements. However, many jurisdictions are lagging behind and might need an extension to meet their obligations.
Fire detection and suppression in practice
Many fires start with a smouldering phase. At this stage fires can be dealt with, even by non-emergency staff, contained and extinguished before disaster unfolds.
Fire detection systems need to recognise danger early. Suggested solutions that rely on heat activation alerts generally take too long. For example heat sensing cable that short circuits when the thermoplastic melts and raises the alarm. Moreover, such systems provide no visual information to help tackle the blaze.
There are other systems that can be considered such as infra red, air sampling and other linear detection systems in addition to the above that include optic sensors.
Visual Smoke Detection (VSD) solutions are possibilities that can be considered to provide early warnings. VSD uses changes in its wide field of view to alert operators, remotely or on site to potential fire danger. Possible interference to fire detection such as car fumes and the movement of vehicles do not affect its effectiveness.
Our controlled tests at Sydney Habour Tunnel, using a vehicle fire generating temperatures in excess of 500°C generated an alert after just 14 seconds with a further 30 alerts during the test using FireVu’s VSD solution.
Let’s not forget about fire suppression solutions, which can be used effectively in conjunction with fire detection solutions:
- Sprinklers, standpipes and hydrants
- Deluge, possibly with foam
- Water curtains – efficient at blocking flames spreading and good for compartmentalising fires, protecting tunnel structures and facilities and cooling temperatures
- Water mist
Other steps such as those introduced by the Mont Blanc authorities, for instance checking trucks before they enter the tunnel are to be commended. Educating tunnel users on how to deal with fire situations is highly valuable in minimising risks and consequences.
With all systems there needs to be co-operation towards finding the best solution for each tunnel from fire engineers, emergency services, government agencies, tunnel workforces, owners and other parties.