Vehicles and mobile plant are used in various industrial settings, such as aviation, mining and forestry. Neil Crowther looks at their hazards and the design of effective suppression.
WHEN DESIGNING a fixed fire suppression system for industrial vehicles and mobile plant, it is important to establish what fire protection is required and where to place system components. Knowledge of fire hazards in the vehicle or equipment and the fire suppression system capabilities is essential, and should be fully evaluated. A good understanding of national and local fire protection codes and standards is also necessary.
Every hazard must be identified while there is still design flexibility, since adding protection for another hazard becomes more difficult once the system is installed. The areas of protection are fixed at installation and limited in number.
In addition, a fixed fire suppression system does not remove the need for a hand-held portable fire extinguisher in the vehicle or equipment. Fuel spills, welding heat, or other unforeseeable hazards may cause fires that cannot be tackled effectively by the installed suppression system.
The fire suppression system protects those areas with a strong likelihood of fire and potential for signi-ficant damage – seldom would a system be designed to protect every square inch of the equipment.
An effective system design is based on a thorough hazard analysis. A fire hazard is found wherever the three elements that create a fire – heat, fuel and oxygen – can be brought together. As oxygen is always present, identifying the fuel and heat sources is most critical. A potential fire hazard exists whenever a fuel comes into contact with any heat source, and both these elements are easy to find in vehicles.
Common fuel sources in vehicles and mobile plant include flammable liquids, greases, rubber, plastics, upholstery, and environmental debris such as wood chips or coal dust. Typical heat sources are engines, exhaust systems, pumps, turbochargers, bearings, gears, brakes and electrical equipment.
Where there is dripping or leaking fuel, the hazard can become even more dangerous than first antici-pated. Consulting experienced operators and owners of similar equipment can help to identify the source locations of previous fires and special hazards not normally considered (see box).
After completing the hazard analysis, it is important to determine the best fire suppression agent, or combination of agents, as well as nozzle coverage.
Choosing the correct agent(s) for use in fixed systems takes skill and knowledge of the particular hazard and the vehicle or equipment’s working environment. The most commonly used agents include:
Multi-purpose dry chemical
Currently one of the most widely used fire suppression agents, this has been utilised in most of the United States Bureau of Mines trials and tests. It has been proven to perform on all classes of fires, and currently has the highest level of documented evidence for its effectiveness.
Dry chemical systems are relatively compact in size, requiring little if any modification to the vehicle during installation. On a weight basis, they are probably the agent most effective in extinguishing fires. These systems also have a very wide operating temperature range – from -53ºC to 98.9ºC – without any form of modification.
Being water based, aqueous film-forming foam (AFFF) systems have the advantage of cooling down the area, thus lessening the chance of re-ignition. An incidental benefit of these systems is that the run-off agent may suppress secondary fires which can occur under the vehicle if fuel, etc, has collected and subsequently ignited. This has been the preferred agent for vehicle fire protection systems in Australia and some parts of the Pacific Rim region, where temperature constraints are not severe.
AFFF systems are relatively large in size compared to dry chemical systems, with a typical canister containing up to 80 litres of agent, which can make their installation difficult on smaller items of plant. Discharge times are usually in excess of 1 minute to give good cooling effect.
These systems have similar properties to AFFF, but provide a good measure of freeze protection (-40ºC). As with AFFF, wet chemical systems are relatively large when compared to dry chemical systems, with a typical canister containing up to 115 litres of agent, which may make installing them difficult on smaller items of plant. Discharge times are up to 2 minutes to give good cooling effect, and they place a blanket over the fuel to reduce oxygen.
Twin agent systems use a combination of dry chemical and AFFF/wet chemical to give fire suppression and surface cooling in the hazard area. Multi-purpose dry chemical systems discharge first to give rapid fire knockdown and then an AFFF/wet chemical system will discharge to give effective cooling.
Inergen, argonite, argotec, FM-200 and carbon dioxide are the gases most commonly used for vehicle systems. These agents are suitable for use in enclosed spaces and preferably fitted with fast responding detectors. The systems must be designed and, where necessary, the vehicles modified to retain the agents for as long as possible in order to use their inerting properties.
This can cause a number of practical problems in most heavy plant applications, but they are not insurmountable. For example, if belly pans are not fitted to the machine, then panels will need to be fitted to retain the gas. This is essential because most of the gases in use are heavier than air and will therefore disperse rapidly, while there may be overheating due to the closure of these openings.
As any breeze will carry the gas away and render it ineffective, all access panels and doors should be kept closed under normal operating conditions.
It is debatable how effective gases are on off-road and on-road vehicles, and it is recommended that they be avoided wherever possible in over and off-road vehicle and plant applications. The only exceptions would be where electrical equipment is present and compartments are well sealed.
Having selected the correct extinguishing agent or agents, it is important that the right coverage is given. In a rising number of cases, incorrect nozzle coverage has led to fire hazards being inadequately protected, placing a machine at risk.
The first step is to determine which nozzles are needed and where they should be placed to best protect the hazard.
Nozzles are selected by determining the size of the hazard and comparing it to a nozzle’s effective discharge pattern. When using any of the nozzle types, ensure that no obstructions interfere with the discharge pattern, since it is directed to the hazard.
The nozzles and their locations can be selected according to the following rules:
- the entire hazard area must be within the nozzle’s pattern and maximum effective discharge range
- some areas may exceed the area coverage of a single nozzle, and may require one or more additional nozzles for protection
- in some cases, a single nozzle can cover more than one area of a common hazard, such as a transmission and torque converter – both areas must be within the nozzle’s discharge pattern
- when planning nozzle locations, check that the effective flow of extinguishing agent to all recognised hazard areas will not be obstructed
- if obstructions cannot be avoided, one or more additional nozzles may be required to provide appropriate coverage
- in areas where the environment may cause an extreme build-up of Class A materials – such as wood debris, coal dust, garbage or oil – use the largest system tank available and keep the number of nozzles per tank to a minimum, to allow the maximum amount of chemical per nozzle to be discharged and the longest discharge time
- never settle for less than full coverage of each hazard
By ensuring that both extinguishing agent and coverage are considered and correctly implemented in these ways – and, if possible, consulting a vehicle protection specialist – the risk of fire to industrial vehicles and mobile plant will be minimised.
The engine compartment contains an assortment of fluids, fuels, oils and greases, as well as congested wires, hoses, and accumulated debris – all of which are extremely close to high heat sources.
When combustible materials build up on the top of the battery in the presence of moisture, they can cause a short circuit and become a fire hazard.
Transmissions, torque converters, parking brakes
These components are a possible high heat source that could cause ignition to combustible material.
Hot fluid spraying from a ruptured high-pressure hose, or leaking from a loose flange or fitting, could find its way to a source of ignition.
The belly pan can accumulate not only leaking fuel from the vehicle, but also external debris. A fire starting here could quickly engulf the entire vehicle.
Hydraulic fuel pumps
Due to the high pressures involved with these pumps, fluid spraying from a leaking pump could find its way to a heat source and cause ignition.
Neil Crowther is managing director of Ardent
More information is available at www.ardent-uk.com