Industrial Fire Journal - Fire & Rescue - Hemming Group Ltd
Gas-powered buses – hazards and evacuation
Published:  24 February, 2014

New fuels are emerging to replace fossil fuels. The overall benefits are great, but with new technical developments new risks emerge. As a consequence of an accident that involved natural gas buses in Helsingborg, Sweden, SP Fire Technology, an international testing institute based in Sweden, performed evacuation tests where the driver was found to be the key factor, write Glenn Appel, Johan Lindström and Fredrik Rosén (SP).

One safety measure on today's biogas-powered buses is situated on the gas tank. The gas tank is equipped with a thermal fuse to ensure that it does not explode because of the increased pressure resulting from heating of the vessels. The thermal fuses activate when the temperature passes about 110 Celsius. When this temperature is reached, the fuse goes off and the gas is released at a high pressure out in to the surroundings. In most of the known cases the gas ignited after escaping from the vessel causing a jet flame to form. This high-pressure jet flame poses a massive risk to the people surrounding the bus since it appears suddenly and without warning.

In several documented cases, the temperature fuse failed to work because of an overheated fuel tank. If the tank heats up at one single spot – away from the fuse – it can explode due to expansion of the gas inside it. Even when all safety measures on the vessel work as they should, it may still explode when the temperature fuse is not affected by heating. Lionel Perrette and Helmut K. Wiedemann describe three cases where explosion has occurred in their article: Safe Storage of Natural Gas on Urban Buses: Case Early Investigation and Learnings. During these three incidents, the tanks exploded within 10-20 minutes after the start of the fire.

The start of the fire

Following a collision between two natural gas fuelled buses, the Swedish Accident Investigation Authority (Svenska Haveri kommisionen, SHK) commissioned SP Fire Technology to perform fire tests on a bus to identify when critical conditions occur and what parameters affect the outcome of the fire. Fires in buses often start in the engine compartment due to hot surfaces, leakage of fuel, electrical failures etc. When a fire occurs inside the engine compartment it is quite hard to notice at first, since the engine often is situated in the rear part of the bus. This gives the fire time to grow before being discovered. There are several parameters that affect how the fire evolves after ignition. Some of them are related to whether or not the bus is moving; if the engine is running; and how the wall between the engine and the bus compartment is constructed.

The tests that SP performed involved a conventional Swedish city bus powered by natural gas standing still with the engine turned off. This resulted in high carbon monoxide values inside the bus after 3:13 (min: sec). Worth mentioning is that the fire growth was even faster during the accident that occurred in Helsingborg.

If a detection system is installed in an engine compartment, the fire will be discovered earlier and extinguishing media will suppress the fire to some degree. The placement of the detection tube and the nozzles are the most important parameters if the suppression is going to be successful. During the two tests done by SP, a water mist system was used in one of the tests as a suppression system. Even though the nozzles were placed too low to extinguish the fire, the critical conditions inside the bus were delayed by 10 minutes longer than in the free-burning test. The beneficial effect was due to good placing of the detection tube, which caught the fire in nine seconds from ignition.

Risk analysis

The conclusion is that a detailed risk analysis needs to be carried out when installing the nozzles and detection tube and protect the areas that are most prone to ignition.

Fire risks in enclosed spaces

A fire incident in an enclosed space, such as a tunnel or a garage, can in many ways have greater consequences than the corresponding accident above ground level. When a fire occurs in an enclosed space, the hazardous gases follow the ceiling and quickly create a bad environment for the people within the confined space. Also, the fire will spread much more easily in these types of environments.

If the temperature fuse works the way it should during a fire, a jet flame is a likely consequence. The jet flame poses a higher risk for evacuators inside a confined space than out in the open since there is a lot less room to escape from the burning bus.

Evacuation and driver behaviour

As a part of SHK’s project, several evacuation drills were performed at SP Fire Technology. The parameter in focus was mainly how the evacuation times varied with each available escape route. One of the tests simulated blocked doors, either by an obstacle or by the fact that the driver couldn’t get the doors open. The other test simulated that everything worked as it should and all the doors were open. The volunteers trying to evacuate the bus comprised 74 men and women in the age range between 59-93. Several of the volunteers had a walking aid inside the bus, including two people who relied on a walking frame.

In the evacuation exercise where everything worked smoothly, the evacuation time was measured as 1 minute and 25 seconds, and in the scenario with only one free passage way (the front door), it took 2 minutes and 46 seconds. In the free-burning scenario where the limit value for carbon monoxide was measured, the evacuation took 3 minutes and 13 seconds. The driver has to be aware that when a fire occurs there is only a very small time frame for the passengers to evacuate the bus before their health is in danger.

SHK’s conclusion was that the driver is the essential factor in whether or not the evacuation will succeed. The driver needs to stop the bus in time, park it in a position so there is no accelerated fire spread – such as being tilted in an unfavourable direction – and make sure to disengage the doors so they are able to open.

During driver training and during operation, it is important to perform training and exercises related to evacuation, such as a schedule of actions, learning to cope with panic-like feelings and to handle passengers in case of an emergency. In a real evacuation situation, the driver will be under a lot of pressure to make the evacuation as safe as possible, and training will significantly enhance the chance of a successful outcome.

Testing of the bus engine compartment

As most bus fires originate in the engine compartment, SP Fire Technology has developed SP Method 4912 for testing fire suppression systems for use in bus engine compartments. Moreover, if the fire suppression system and the manufacturer meet additional requirements, the system can be certified in accordance with SPCR 183 (Certification rules regarding fire suppression systems in buses and coaches) and entitled to display SP’s own quality label, the P-mark.

Certification obligates the manufacturer to sell the certified system under the same condition, namely, that a real life engine compartment of the same size as the test apparatus is protected by the same amount of extinguishing agent and nozzles as during fire testing. The system can be up- and down-scaled in accordance with rules provided in SPCR 183. Moreover, prior to an installation of the system in new layout of the engine compartment, a risk analysis must be performed and documented, showing how the particular fire risks are handled by system design, in particular nozzle placement. This risk analysis has to be performed by qualified staff appointed by the manufacturer with documented experience for this task.

To ensure that the products displaying the P-symbol fulfil the requirements of the certification rules, the manufacturer has to operate FPC (Factory Production Control) that will be scrutinised by SP during an initial visit and subsequent annual inspection visits. See for more information.

Numerous common reasons for the ignition of buses or coaches have been identified, including (but not limited to): heat, vibration, material fatigue or malfunction, inadequate maintenance, and faulty electrical system design. As written above, most bus and coach fires start in the engine compartment or surrounding areas. In addition, fires can originate in the electrical system of the bus and the wheel houses. In order to fully understand the risks associated with any given vehicle design it is important to consider fire hazards, electrical hazards and EMC/EMI performance.