Detecting and suppressing vehicle fires

Published:  27 July, 2010

Passenger-carrying vehicle fires can put lives at risk, cost the vehicle owner a great deal of money, and jeopardise the company’s ability to continue to provide the service to which it may be contractually committed.  Nick Grant of Firetrace International – a Gold Sponsor of the FIVE conference, held in Gothenburg, Sweden from the 29th until the 30th of September – examines the contribution that targeted, risk-specific fire detection and suppression can play in improving vehicle fire safety.

According to the US Department of Homeland Security NFIRS [National Fire Incident Reporting System] and NFPA [National Fire Protection Association], mechanical and electrical failures or malfunctions account for around 75 percent of on-highway vehicles fires.  In something like 60 percent of the cases, the most common site for an outbreak is in or around the engine compartment, the running gear or wheel areas.  These are, therefore, the areas most in need of detection and suppression. 

 However, the real nature of the particular fire hazard has to be taken into account if the solution is to offer maximum protection.  In addition to fuel and the risk of fuel-line ruptures, there are a number of flammable liquids present throughout any vehicle engine compartment.  These include hydraulic, brake, automatic transmission and power steering fluids, as well as combustible accumulated grease on the engine block, for which frayed or damaged electrical wiring can easily provide the ignition source.  

 Regenerative braking systems on hybrid diesel-electric buses and coaches result in significantly improved fuel economy and greatly reduced emissions when compared with standard diesel engine vehicles.  However, they are another potential fire risk that is best eliminated by fitting a dedicated fire detection and suppression system around the rechargeable energy storage system.

 When it comes to rail rolling stock, most fires are the result of either a collision or derailment.  However, locomotive fires do occur.

 Recently, the development of high-power variable-frequency / variable-voltage drives, or "traction inverters," has allowed the use of polyphase AC [Alternating Current] traction motors, resulting in a more efficient and reliable drive that is better able to cope with overload conditions that often destroyed the older types of motors.  Fires though do still occur.

So, many of the fire safety challenges present in road vehicles also apply to rail locomotives, particularly those relating to diesel engines and generator sets.  However, trains also have several other high risk areas that need dedicated fire protection.  These include driver’s control panels, electrical panels and electrical distribution equipment, air conditioning units and heat exchangers, kitchen galley equipment, battery boxes and fuse boxes.  Significantly, particularly when it comes to suppression agent selection, many of these enclosures and cabinets house energised equipment.

 

Targeted solutions

Dealing first with engine compartments, the dynamics of airflow in and around an engine compartment when a vehicle is in motion can seriously limit the performance and reliability of traditional detection and suppression systems when heat and flame – that typically rise from the source of a fire – are likely to be propelled elsewhere.  The inevitable build-up of dirt in and around engines, intense temperature variations and vibration are also factors that are known to cause traditional detection and suppression systems to fail to provide the essential fast and accurate fire detection and suppression.

One resolution to these challenges is Firetrace International’s FIRETRACE, which can be fitted to new vehicles or retrofitted to existing vehicles.  It is already being used to protect over 5,000 vehicles and there has not been a single instance where a FIRETRACE system has either false alarmed or failed to detect and suppress a genuine fire.  It is the only UL Underwriters Laboratories] listed, FM [Factory Mutual] approved and CE [Conformité Européene or European Conformity] marked tube-operated system in the world that is tested as an automatic fire detection and suppression system.  The system is also endorsed by the Danish Institute of Fire & Security Technology and the Swedish Fire Protection Association that has confirmed FIRETRACE’s compliance with the SBF 128 bus fire test. 

 Briefly, FIRETRACE is an automatic, self-seeking, intrinsically safe fire detection and suppression system that does not require manual activation or monitoring, virtually no maintenance and no electricity or external power source.  It comprises an extinguishing agent cylinder attached to a proprietary Firetrace Detection Tubing via a custom-engineered valve.  This polymer tubing is a linear pneumatic heat and flame detector that is immune to the vibration, shocks and temperature extremes found in vehicle engine and generator compartments.  It was specially developed to deliver the desired temperature-sensitive detection and suppression characteristics of harsh environments. 

 

This leak-resistant tubing is routed throughout the engine compartment.  Immediately a fire is detected, the tubing ruptures and automatically releases the suppression agent, extinguishing the fire precisely where it starts and before it can take hold.  The tubing is placed both above and behind the potential source of fire to ensure that the airflow actually helps by directing the heat and flames towards the tubing, providing faster and more reliable detection and suppression.  Depending on the particular FIRETRACE system that is chosen – either Direct Release or Indirect Release – the suppression agent also flows through the delivery tubing to the front of the engine, again working with the airflow to flood the entire compartment.

 

The Direct Release System utilises the FIRETRACE tube as both the detection device and the suppressant delivery system.  The Indirect Release System uses the FIRETRACE tube as a detection and system activation device, but not for the agent discharge.  The rupturing of the tube results in a drop of pressure causing the indirect valve to activate.  This diverts flow from the detection tube and the agent is discharged from the cylinder through diffuser nozzles, flooding the entire enclosure.  

 Suppression agent selection

The extinguishing agent cylinder is usually mounted inside the engine compartment and, for vehicle engine applications, ABC dry chemical powder is the appropriate choice, as it is suitable for the suppression of ordinary combustible, electrical, and flammable liquid fires that involve petrol, diesel, solvents, lubricants and spirits. 

 However, ABC dry chemical powder is not necessarily the most appropriate choice for other vehicle fire risks, in particular enclosures and “micro-environments” that house energised equipment.

 Any energised equipment – both low voltage and high voltage – can catch fire.  Typically, fires in electrical cabinets are caused by loose connections and faulty cables that, when power is running through them, the electricity can arc.  This arced electricity is extremely hot and can cause the cable sheathing to burn and fire to spread to other components. It is therefore essential for the fire detection and suppression to be targeted on the connections and components, such as switches and transformers. In these instances, the most appropriate choice is a clean agent such as DuPont FM-200 or the more recently introduced 3M Novec 1230 Fire Protection Fluid.  Both are non-conductive agents that have been used extensively by Firetrace International in its FIRETRACE systems, of which there are now more than 150,000 installations around the world.

 FM-200 and Novec 1230 are approved by UL [Underwriters Laboratories] and FM [Factory Mutual] and are listed in the appropriate codes and standards, such as NFPA 2001 [Standard on Clean Agent Fire Extinguishing Systems] and BS EN 15004:2008 [Fixed firefighting systems. Gas extinguishing systems]. 

 However, one agent that is not suitable for these particular applications is CO2 [carbon dioxide], as using CO2 to protect electrical cabinets and enclosures risks doing considerable and expensive damage to the equipment it is seeking to protect. 

 With direct-discharge, tube-operated systems in enclosed electrical cabinets, the CO2 discharge may be a matter of only a couple of inches away from delicate circuit boards or microchips.  On discharge, the CO2 is released in liquid form that transmutes instantly into a gas, reducing the temperature to a super-cooled minus 94 degrees F [minus 70 degrees C].  This instantly freezes the humidity throughout the cabinet and effectively transforms the electrical panels into “snow boxes”.  This “snow” melts into water inside the panels and then comes into contact with the energised components.  The “snow” can also collect dust and dirt particles from inside the cabinet, which are deposited on to surfaces inside the enclosure, creating an electrically conductive substrate.  The rapid cooling can also damage sensitive electronic components through a process known as thermal shock. 

 Additionally, when the CO2 transmutes, it expands at a rate of 500:1. This sudden expansion in volume creates a significant over pressurisation that can also seriously damage sealed enclosures and deform metal panels. 

These important shortcomings are what make FM-200 and Novec 1230 appropriate choices.  They discharge at much higher temperatures and have proven to provide fast and reliable suppression without the detrimental side effects of direct-discharged CO2.  They also do not involve the huge temperature change from ambient, or the huge pressure change from ambient that precludes CO2 from being suitable for these applications. 

  • Operation Florian

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