Tunnels Fire Safety - US expert

TUNNELS

Fire Safety - US Expert

Sergiu Luchian is in charge of fire safety systems for the huge Central Artery and Williams tunnel project in Boston MA at the Mass DOT. He says what still amazes him after seeing dozens of fires in tunnels – most of them staged for drills or safety studies – is how quickly they can fill a huge section of the tunnel with thick black smoke.

“You’d be amazed how quickly it gets pitch black.”

He was invited by the NY/NJ Port Auth to a Holland Tunnel fire drill three years ago and he timed and filmed it as usual. He said in less than 20 second the space around the ignited car was filling with smoke. And as soon as the smoke rises to the ceiling the tunnel lighting is lost, so the tunnel is darkened very quickly and before the smoke accumulates downwards to the level of the motorist.

Fuel and the body paint of vehicles are the source of the smoke to begin with, he says. Upholstery and tires, and then loads, burn too. He remembers in the last Holland Tunnel test fire that by the time a fire truck came he could hear it as if it were about to run him down it was so loud. But until the truck was within 50 feet he couldn’t see its headlights or even the spinning white and red strobes.

CCTV & Cell Phones Key

Luchian says the key to preventing or minimizing fire casualties once a fire has started is speed of response. In the Ted Williams Tunnel in Boston there was a car fire in September last. Motorists with cell phones were calling in at about the same time the operations center saw it on their closed circuit TV monitors within 10 or 20 seconds of the fire producing smoke. Variable message signs were quickly switched and stopped traffic entering and opened the way for emergency services. Fire trucks were at the car within 3 minutes and were able to contain the fire and put it out within a quarter of an hour. Noone was hurt and there was no damage to the tunnel or to other vehicles. That’s the good news.

The bad news is that in the Williams tunnel fire the tunnel’s fire detection systems didn’t trigger an alarm until 14 minutes after the start of the fire, not until the fire fighters were almost done. So if the operations room had had to wait for the alarm system there could have been a much more serious incident. And that’s the best fire sensing system available, Luchian says. A problem is that if every truck exhaust stack that passes is not to trigger a fire alarm it has to be set at a temperature threshold so high that a quite considerable fire can go undetected down at the level of a vehicle engine before the ceiling based detectors give the alarm. (There would seem to be a need for detectors based on measurement of particulates and smoke, though perhaps dirty diesels would trigger those?)

A major mitigation measure at the outbreak of a tunnel fire is to rev up the ventilation fans to maximum power to extract as much smoke and hot air as possible, as fast as possible, to preserve some visibility, reduce smoke inhalation and get some of the fire’s heat away. The hotter the fire the faster it grows. Presumably the extra air sucked in by the ventilation on full power has a less than offsetting effect in promoting combustion.

Slow Old Centrfugal Fans

A problem is that the large centrifugal fans common in tunnels ventilated with ducts can take two or three minutes to rev up to full power. Axial fans are generally faster and jet fans fastest. The sheer speed of jet fans (see TRnl#12 Feb 97 p3) in accelerating to full power seemed to be a major factor in their demonstrated success in fire mitigation in a run of tests in an abandoned tunnel in West VA in 1995, which led to a major rewriting of the fire code for tunnels in the US. Jet fans are hung below the ceiling of the tunnel and simply push the bad air out one end of the tunnel, pulling fresh air in the other.

Luchian says that ducted tunnels like the Mont Blanc with exhaust duct above and fresh air below are best for fine tuning air quality.

“They are the Rolls Royces of tunnels. You can play them like a piano. The jet fan tunnels are the Toyotas. They are affordable tunnels, not quite as beautiful for day to day operations, but they do very well in emergency situations.”

The underground roads being built in Boston in Luchian’s Central Artery/Tunnel project have a mix of ventilation systems. Earlier sections with the full ducting were in design before the fire code was changed in the US to allow jetfans, which have been incorporated in the last designed parts - at a saving of $45m. (National Fire Protection Assoc, Rule 502)

He thinks that the extraordinary duration of the Mont Blanc fire – it burned for over 50 hours – was probably the truck’s load of margarine and flour, both potent fuels for fire.

Luchian says he doesn’t have more than sketchy news reports on the Mont Blanc fire to go on yet, but he wonders what systems they had in place to detect vehicular fire before the tunnel entrances, what plans they had to close the tunnel, and how quickly they reacted. The first line of defense is to stop any afire-vehicle from entering. The Chunnel fire of a couple of years ago involved a failure to stop a train carrying a truck that was already afire from entering the tunnel.

“It is quite criminal not to have systems in place to stop an afire-vehicle from entering the tunnel, because the fire inside the tunnel is so much more dangerous than a fire outside. But if the fire is inside the tunnel, then you need very quickly reacting systems to warn motorists of the danger and stop them entering, and you need to get help into there to contain the fire very fast, before it escalates into something very big.”

Lowtech & Low Cunning Needed

Luchian says the two most effective means of learning about a tunnel fire are cameras, and motorist pull-boxes. Also you have to install an antenna for cell phones. He says noone is going to spend money on second tunnel tubes and cross-passages where the traffic doesn’t warrant it. But in that case good refuges are needed with multiple doors. A fire will jam a nearby fire door, so there has to be another door a distance away so that one door to a refuge is operable. Refuges need insulation, positive air pressure, ventilation, and communications too. Plenty of fire drills are needed and constant reassessment of weak points in fire protection.

America’s worst tunnel fire, he says, was in the early 80s in the Caldicott tunnel on CA-4 just west of Oakland CA. An irresponsible trucker with a tanker truck and trailer drove recklessly and jacknifed, and a bus and car careened into the burning wreckage. About 6 people died.

“That was a yahoo fire, caused by an irresponsible slob crashing with a highly flammable volatile load inside the tunnel. That’s the most difficult kind of fire to avoid.” (Contact Sergiu Luchian 617 342 1174 sergiu@aol.com)