Fire Suppression Basics: Detection Systems
In case you missed any of the previous posts in the Vehicle Fire Suppression Basics series…
Part 2 of 8: Detection Systems
In this post, we continue our series looking at the detection methods used in vehicle fire suppression. Even the most effective extinguishing agents depend upon a rapid and reliable detection system. Here, we look at the most common types of detectors and the advantages and disadvantages of each.
There are three types of detectors commonly used to sense the presence of fire: smoke, flame and heat detectors.
Smoke detectors sense the presence of visible or invisible particles released in combustion. In a hostile/off-road environment, the debris generated by exhausts or the dust and debris in the atmosphere would generate frequent false alarms and this makes them unsuitable for use on vehicles.
Modern flame detectors use infra-red, visible and ultra violet radiation to detect fires. These are used widely in the vehicle and mobile heavy plant industry. Some models also have the capability to detect a wide range of electro-magnetic radiation (emitted from combustion or other sources), meaning that the detectors can eliminate false alarms from sources such as sunlight, welding torches and other heat sources.
- Pros: Ultra-fast detection in less than 5 milliseconds.
- Cons: All electromagnet flame detectors require line of sight. Maintenance of the detectors is crucial and it is important that they are tested frequently. Expensive.
Heat detectors are widely used in vehicle fire suppression. Several groups of detectors fall into this category: point or spot heat detectors, linear heat detectors, linear pressure heat detectors (loss of pressure) and pneumatic heat detectors (rise in pressure).
Point or spot heat detectors
These are set temperature detectors or rate of rise sensitive detectors. Spot detectors are used in heavy mobile equipment because of their rugged design and simple actuation. They come in different formats and with varying actuation temperatures to suit different applications.
- Pros: Relatively low cost and effective means of detecting fire.
- Cons: By their very nature, spot heat detectors only monitor a local area for temperature. Unlike flame detectors, they have fewer checks in order to determine if a fire or heat source is a false alarm.
Linear heat detectors
The most common type of linear detector used in vehicle fire protection is a linear detection cable that uses two spring steel conductors, separated by a heat sensitive insulator. At a predetermined temperature, the insulation melts. This allows the conductors to come into contact, resulting in a change in signal relayed back to the control module. To overcome the problems of different temperature sensing requirements, spot heat detectors or thermal switches can be placed in areas requiring specific point detection. These can then be interlinked with linear detection cable.
Electronically monitored systems that use linear heat detection cable significantly reduce the chance of false discharge in comparison with pneumatic loss of pressure systems, as they are able to differentiate the electric signal resulting from an increase in temperature form the one caused by damage to the cable. They also communicate any damage to the system, providing increased reliability.
- Pros: Reliable, relative, low cost. Reduced chance of false discharges due to damage to the cable.
- Cons: Linear detector is a ‘one shot’ operation. Therefore, following any fire or overheat the cable needs to be replaced. Detection time (small flame) can be 20 seconds.
Linear pressure heat detectors / Loss of pressure (LOP)
Pneumatic and pressure detectors have been used for many years and operate by using simple mechanical/pneumatic principles. A pressurised detection tube runs throughout the protected area, and when ruptured by heat or flame, the pressure is released, activating the system.
These systems can be used without electronics, and are appropriate for hazardous areas where the expense of using intrinsically safe or flameproof electrical systems may be prohibitive.
- Pros: No need for electronic control, it can be used in hazardous locations e.g. ATEX. Low cost.
- Cons: Relatively low detection temperature – 120°C – which can lead to false alarms. Reliant on reservoir of gas to keep the tubing pressurised. Damage to the tubing can result in false discharges.
Pneumatic heat detectors / Rise in pressure
Pressurised fire and overheat detectors are used on many aircraft and railway locomotives. The detector sensor tube contains Helium and a metal hydride core material. The application of heat to the sensor tube causes an increase in internal gas pressure, which in turn operates a pressure diaphragm and closes an electrical contact, actuating the alarm circuit. Mechanical damage to the tubing cannot result in false alarm; it will instead give a ‘fault’ indication.
The detector has two sensing functions. It responds to an overall average temperature threshold or to a highly localised ‘discrete’ temperature caused by flame or hot gases. Both the ‘average’ and ‘discrete’ temperatures are factory pre-set. They also have a fire resistance of 1100°C for 5 minutes and resets all function after use. However, if the detector is physically damaged then the whole unit will need to be replaced.
- Pros: They require little, if any, scheduled maintenance. Rugged and reliable, often used in aerospace.
- Cons: Very expensive.
Next blog post in the Vehicle Fire Suppression Basics series: Control Systems
We start this blog series with the essential component of any fire suppression system, the extinguishing agent.
Extinguishing agents and detectors are key components of any system, but it is the control module that dictates the response to fire.
No fire suppression system is complete without the correct suppression agent tanks. In this blog post, we look at the two main types of tanks.