CNG propelled buss exploded in central Stockholm - reason unknown

A city bus propelled by natural gas has exploded and started burning in central Stockholm. Photos and videos show a massive fire ball with flames reaching 40 to 50 meters tall.

Luckily the bus was empty when the explosion occurred. The bus driver was injured and taken to hospital, according to Swedish media.

It also appear very lucky that the explosion didn´t occur further into the tunnel, where the rapid fire development could have caused much more damage, both to human lives and to vital infra structure in the city.

According to Swedish media, 20 % of all public buses in the country now run on some form av natural or bio gas.

by CTIF Communications Coordinator Björn Ulfsson

"The incident involves a (natural) gas propelled bus that has hit the top of the Klara Tunnel and subsequently exploded", says Björn Westerdahl, communicator at the Greater Stockholm Regional Fire Department.

Swedish media mention "gas", not referring to gasoline, but to some form of natural gas. 

One of CTIF´s experts on the subject of alternative fuel propelled vehicles, Kurt Vollmacher in Belgium, believes the bus may have been running on CNG, since Keolis Sweden, the company operating the buses, recently equipped their fleet with MAN Lion’s City Articulated Buses fuelled by CNG. (compressed bio methane)

According to Vollmacher, a Belgian firefighter and an active member of the CTIF Commission for Extrication & New Technology, the Stockholm incident is very similar to a recent incident in the Netherlands, where a bus of identical type and make was involved in a near explosion in December. (Read more about the Dutch incident later on this article.)

A spectacular eye witness video in Swedish media - filmed by a dash cam from a vehicle driving right behind the bus -  shows a rapid fire development, almost like a detonation, when the bus hits something in the ceiling of the tunnel.

Within a fraction of a second the bus bursts into a gigantic fire ball.

The cover photo (above) is a screen shot of the moment of explosion from the video on Expressen.se.

The gas bottles hit an obstruction and were damaged

"It is a massive intervention with several ambulances and many other emergency vehicles", said a reporter at Swedish newspaper  Expressen.se, Fredrik Sjöshult, who was on location in central Stockholm.

According to the police, the bus has probably hit the pontoons which hang down as height indication warnings in the opening of the tunnel.

"Probably the bus has been damaged. It has a propane tube with fuel that may have hit the tunnel opening and then it has started to burn and explode," says Carina Skagerlind, communications officer at the Stockholm Police, to Expressen TV.

No passengers were on the bus, but the driver was injured. The bus company Keolis announced that the driver was taken to hospital in an ambulance.

Only light facial burns

The call came in at 11.25. The explosion occurred at Klaratunneln in the middle of Stockholm city.

Considering the size of the explosion, the driver was lucky: Representatives of the employer Keolis announced that the driver has light burns on his face and possibly on one hand. No other injuries have been reported.
 

Over 100 feet tall flames

Klas Mellbourn lives in one of the houses just in the vicinity of where the bus exploded, and spoke to Expressen as an eye witness:

"We were at home and heard a very heavy explosion. The house shook a little. From the balcony we saw heavy smoke development and flames up against the house. The flames reached above the roof of our house - and the house is 40 to 50 meters high so there were really tall flames", he says to Expressen.

What went through your mind when you saw this?

"We thought about what it could be and were afraid that it might be terrorism or something like that. Shortly after 1 pm local time police confirmed that the driver was seriously injured".

There is no information on injured persons in vehicles behind and next to the bus during the explosion. However, at least one vehicle was allegedly hit by flying rubbish from the explosion.

"Went the wrong way"

So how could this happen?

Patrik Åberg at Keolis says that the buses are not allowed to be driven through the tunnels:

"They must never go that way because the buses are too tall. The driver went the wrong way and entered the barrier before the tunnel", said Patrik Åberg at Keolis to Aftonbladet.se.

Do you know why he chose this road?

"No, we must investigate this", says Patrik Åberg.

The Stockholm Transit Company, SL, states that this type of accident should not be occur on regular bus lines.

"The busses normally drive on roads that are free of obstacles", says Henrik Palmér at SL's press service.

No suspicion of terrorism - and not a BLEVE

Säpo (The Swedish National Security Forces) confirms to Expressen that they have been informed of the incident. At present there is no increased level of preparedness, says Säpo's press officer Sofia Hellqvist:

"We are informed about the event, but it is the police and emergency services that are responsible for handling this", she says to Expressen.

The CTIF Commission for Extrication & New Technology will continue to investigate what happened in Stockholm. Kurt Vollmacher who has worked heavily for the Commission with ISO Standardization Procedures for new fuel type vehicles, says that it likely wasn´t a Boiled Liquid Expanded Vapour Explosion, since a BLEVE is only possible with gases while they are in their liquid phase:

"With CNG you can instead have a rupture of the tank due to over pressure. I think there was a ideal mixture between gas and air in combination with a spark that set off the explosion", Kurt Vollmacher said on Sunday to CTIF News. 

Similar incident in Holland in December - only the vents were damaged:

Kurt Vollmacher also compares the Stockholm explosion to an incident in the Franeker region of the Netherlands on December 6, 2018, involving the same type of bus, where the bio gas tanks caught the top of a road tunnel and were torn off the vehicle, landing on the street. The gas bottles started leaking heavily, but there was no explosion or fire:

"A bus from Arriva drove on Thursday under a viaduct on Hottinga street, where the gas bottles caught on something in the tunnel. On the roof of the bus were gas bottles. Six of them came off and landed on the street and leaked gas.

As a precaution, the police closed a few streets. The accident occurred around half past eight. The fire brigade has taken care of the gas cylinders and made stabilized them. Around ten o'clock, gas was still leaking from one bottle."

There were no injuries in the incident in the Netherlands incident - however; had circumstances been different, the Dutch incident could also have costed many lives.

"The big difference between these two accidents is that in Stockholm, one of the tanks appears to have been severely damaged due to getting in contact with the structure. Then, there was an ideal mixture of gas and air, and a spark:  Such an "ideal" fuel/air mixture in which both the fuel and the oxygen in the air are completely consumed is called a "stoichiometric" mixture. In the Netherlands there was no explosion, because the tanks were not damaged", Kurt Vollmacher explains.

In the case of the Dutch incident, only the vents of the fuel bottles were damaged, and by the time the fire services had arrived,  almost all of the gas had escaped. Luckily, there was no source of ignition nearby, and the gas was diluted with fresh air and reached inflammable mixture before any ignition could occur.

In both cases, according to Vollmacher, there is a pressure between 200 and 250 bars in the CNG bottles. 

"I think in the case of the Netherlands accident, it was extremely lucky there wasn´t an explosion or fire, says Kurt Vollmacher.

by CTIF Communications Coordinator Björn Ulfsson

The International Association of Fire Services, CTIF.org, has worked for many years towards standardization of symbols for vehicle marking and Intervention Procedures for new and alternative fuel type vehicles.

Many of the new vehicles are constructed in ways that make the propulsion type difficult to recognize during a fire.

Firefighters can often put themselves at severe risk trying to extricate and extinguish on vehicles which may carry batteries with very powerful electrical currents, or compressed, liquified or hybrid fuels that may explode easier and more powerfully, or at least behave very different, compared to more conventional fule types like gasoline or diesel.

Therefor, the CTIF is continually working towards creating standardized intervention cards for all known vehicles on the market, and standardized vehicle markings for fuel type and placement of dangerous parts such as power cables, fuel lines and batteries or hydrogen containers.

In November 2018, CTIF unveiled some of the work the organization has been doing around ISO standards. Read more about this work here.

During the CTIF Seminar, "Fire, Rescue & New Challenges", Kurt Vollmacher received the CTIF Special Achievement Reward for his work with New Fuel Type Vehicle Standardization.

If you want more information about ISO Standards for new vehicle types, operating procedures for interventions, or general information about the CTIF´s work for safer citizens through better educated firefighters, please contact the CTIF Office or the CTIF Commission for Extrication and New Technology.