The last decade has seen a whole new technology swing in the fire detection arena.
From the original era of fire detection utilising thermal devices and pneumatic operation, the market has passed through many changes progressing from smoke detectors working on a simple switch principle connected to relay-based alarm panels to highly sophisticated microprocessor-based analog addressable fire detection systems as we know today.
The following is an analysis of fire detector system types currently available.
* Conventional alarm systems - conventional detection systems utilise one pair of wires to fire detection devices that terminate at a resistor providing line monitoring facility.
Installation of 20 detectors per line (zone) is recommended due to 'search' distances to locate the detector in alarm. Any combination of smoke detectors or manual call points is connected per zone. The 'fire' decision is made by the detector.
Panels are configured from one zone through to, commonly, 32 zones allowing the partitioning of the building or complex for zonal identification.
* Addressable alarm systems - in the early '80s, local companies Istron and Remtek were the pioneers in developing 'addressable' PCBs and fitting them into the base of conventional detectors, thus providing the facility to identify which detector was in alarm.
Many problems such as excess power needs requiring large cable conductors were encountered and although not well received, some of these systems are still operational today.
* Analog addressable system - world leading detector manufactures Apollo and Cerberous moved on to introduce the analog addressable detectors commonly in use today by manufacturing detectors encompassing all communication electronics. The ability to now not only know which detector is in alarm, but which type of detector are just two of the many added advantages. Aritech and Ziton, South Africa's local companies are among the pioneers of analog addressable systems.
Networking of panels, fibre-optic links, computer graphics and modern communications now provides a highly sophisticated system suitable for any environment. Eskom power stations, Telkom and numerous other organisations commonly use this high-tech option.
Since its inception, analog addressable systems have appeared in the upper echelons of fire detection systems. Large and complex installations have been the domain of analog systems and have generally been associated with high prices. This has kept the marketplace relatively small with specifiers and buyers tuned to premium priced, superior specification products with complex electronics embedded within them. This has led to the introduction of the one loop panel providing high-tech solutions for smaller buildings and bringing costs more in line with conventional systems.
Analog addressable systems now represent over 50% (by value) of new systems installed worldwide. The analog addressable detection system relies on the fire panel to make the 'fire' decision.
* Distributed intelligence systems - with the introduction of smaller and more powerful microprocessors fire detection technology has gone full circle, with intelligent detectors now being able to analyse room environments and make the 'fire' decision.
An intelligent addressable system can be defined as a system of devices designed to interface with a control panel that has built-in intelligence to communicate with sensors at their individual addresses and to interrogate them at a remote distance.
The intelligent system can operate on two fundamentally different principles: either the sensor transmits the fire sensitivity information to the panel, where the alarm, fault and maintenance decisions are made, or the sensors determine whether a fire condition is present and transmit the decision to the panel. In both cases, within the devices, complex algorithms can be configured to process the incoming environmental data; thus the choice of technique is dependent on the core system design.
When system intelligence is spread over the complete fire system, its operation is more efficient as the signals are processed at the point of generation.
Increased reliability, speedier response times, reduced false alarms and reduced maintenance costs have significant benefits of an intelligent fire detection system which can be fully achieved only when 'on-sensor' and 'on-panel' intelligence is distributed.
* High sensitivity smoke aspiration systems - as a totally different concept of sensing fire conditions, the air aspiration systems are making a big impact on the South African market.
An aspirating smoke detection system samples air from the protected area for the presence of smoke. The air-sampling network provides the means for transporting air from the protected zones to the detector.
Air is continually drawn through a simple pipe network to a central detector by a high efficiency aspirator. Air entering the unit passes through a dual-stage dust filter (the majority of air is exhausted from the detector and, where required, back vented to the protected area). The first stage removes dust and dirt from the air sample before it enters the chamber for smoke detection. The second ultra fine stage provides a clean air supply to be used inside the detection chamber to form clean air barriers which protect the optical surfaces from contamination.
The original detector chambers consisted of a xenon tube light source and receiver but today's systems use a detection chamber with a stable, highly efficient laser light source and unique sensor configuration to achieve the optimum response to a wide range of smoke types. When smoke passes through the detection chamber it creates light scatter which is detected by the very sensitive sensor circuitry.
Benefits of these units allow for three or four levels of alarm configurable by the user. Some units have a sensitivity as low as 0,005% obscuration, making them up to 100 times more sensitive than some point type smoke sensors.
Manufacturers of these units claim their benefits to be:
¤ Sensitivity both fine at 0,005% and advantageous in dirty environments with adjustable sensitivity ranges up to 20% obscuration.
¤ Unobtrusive pipe work is concealed in voids and only small sample pipe holes are required.
¤ Adjustable alarm levels.
¤ Air flow monitoring.
¤ Ability to distinguish dust from smoke.
* Video smoke detection system - although not commonly used in South Africa, smoke detection via closed circuit television cameras (CCTV) has now been introduced to the worldwide market providing fire detection of outside areas and large open areas indoors.
The principle of operation of video smoke detection is based on sophisticated computer analysis of the video image, ie the area covered by the CCTV camera (sensor) field of view. With a video smoke detection system, detection zones can be placed anywhere within the camera view on or around the items or areas to be protected. Add to this the ability to visually verify the alarm condition from the front end processor screen or CCTV surveillance monitor and the system represents a powerful new tool in the fight against fire.
The system continuously monitors standard CCTV video images frame by frame and immediately alerts the system operator to the presence of even small amounts of smoke anywhere in the picture, sometimes before it is even visible to the naked eye. This provides a very wide angle of view for a large area of coverage from a standard CCTV system.
The video smoke detection system uses standard CCTV equipment linked to a self-contained processing system which is capable of recognising small amounts of smoke within the video image and alerting the system operator both at the processor and by a variety of remote outputs.
The system employs highly complex algorithms to process video information from up to eight cameras simultaneously. Under normal conditions with all eight cameras connected the system achieves a 5 Hz frame rate for each channel.
The video hardware is designed to allow simultaneous realtime digitising of all eight images, which means that the system does not multiplex images and therefore no information is lost or delayed. Alarm conditions are stored within the system's log that has the capacity to store in excess of 5000 time and date stamped images.
The video system detects smoke rapidly by looking for small areas of change within the image at the digitisation stage and only passing these pixel changes to the main processor for further filtering.
The video information is passed through a series of filters, which seek particular characteristics, which can be associated with smoke. Further analysis is then carried out on the relationship between the filtered characteristics to determine whether all the conditions have been met for the system to confidently predict the presence of smoke.
The system installer has the ability to vary the quantity of smoke and the length of time that the smoke exists before an alarm condition is raised to cater for situations where there may be background smoke present. The installer may also divide the video image into zones and program the system to alarm only if smoke is present in two or more zones.
For even greater system performance, two camera images can be associated together such that smoke in one image is to be treated as a pre-alarm and smoke in two associated camera images is treated as a full alarm.
To provide compensation for areas of the images which could prove troublesome such as windows, mirrors or smoke-producing processes, the installer or system user has the ability to eliminate or mask off parts of the image from detection on an individual pixel-by-pixel basis. These systems are particularly suited for large areas or outdoor applications where line of sight viewing is available, eg:
¤ Power stations.
¤ Forests.
¤ Industrial mills.
¤ Aircraft hangars.
¤ Storage facilities, etc.
* Fibre-optic fire detection system - one of the newest technologies on the market is the utilisation of fibre-optic networks to provide fire monitoring by linear temperature measurement alarms, ideally suited to long areas, tunnels or conveyors, etc.
Quartz glass for fibre optics is not only suitable for the transmission of information, but is also used for distributed sensory analysis. Physical measured values, such as temperature, pressure and tensile force can have an effect on the fibre-optic cable and change the characteristics of the light carried by the fibres. Depending on the measuring principle, the physical measured values can be registered and evaluated.
Another feature of fibre-optic measurement is the location of the event. This provides distributed sensory analysis, which is used in many application fields.
For fire detection, the temperature measurement using Raman scattering is of major importance to the fire industry.
Lattice vibration of the molecular bonds in the quartz glass occurs as a result of a temperature change. When light impinges on the thermally excited molecules, interaction occurs between light particles (phonons) and electrons. This results in scattering of the light in the glass fibre known as Raman scattering. The light back-scattered from the glass fibre contains three different spectral components, viz:
* Rayleigh scattering with the wavelength of the laser source used.
* The Stokes components with the higher wavelength with which the phonons are generated.
* The Antistokes components with a lower wavelength than Rayleigh scattering with which the phonons are destroyed.
The intensity of the Antistokes line is dependent on temperature while the Stokes line is almost independent of temperature. The local temperature of the glass fibre can be determined from the ratio between the intensity of the Antistokes line and the Stokes line.
The fibre-optic measuring system comprises of an analysis unit with frequency generator, laser light source, optical module, high frequency (HF) mixer, receiver, microprocessor unit and a quartz glass fibre cable as a linear temperature sensor. The system is equipped with three channels, one reference channel and two measurement channels (Antistokes and Stokes).
According to optical frequency domain reflectometry (OFDR) technique, the output of the laser is sinusoidally swept through the frequency domain from a start frequency (kHz range) to an end frequency (100 MHz) within the measurement time period using the frequency generator. The resulting frequency deviation is a direct measurement of the spatial resolution on the sensor cable. The frequency modulated laser light is coupled through the optical module in the sensor cable. The interface connects the sensor cable to the control unit of the fire detection system. The continuously backscattered Raman light from the fibre is spectrally filtered in the optical module and converted into an electrical signal using photo receivers.
Finally, the signal is amplified and converted down to the low frequency spectral range. The transformed average signal contains the two Raman backscattering curves. The amplitudes of the backscattering curves are proportional to the intensity of Raman scattering at the location in question. The fibre temperature along the sensor cable is then given by the amplitude ratio of the two measurement channels. This new technology represents a major improvement and allows new fire protection concepts in difficult ambient conditions such as tunnels or other underground installations, steelworks or petrochemical plants.
This technology has undergone extensive field-testing and has been installed in recently opened highway tunnels in several European countries. The fire detection system can determine the exact location of the fire by continuous temperature measurement over the whole length of the tunnel. In addition, the system provides information on fire development, size of fire and the direction in which the fire is moving. As the system evaluates both maximum temperature level and temperature difference, stronger air currents do not affect the detection capability of the sensor cable.
With this information the safety authorities can react early, and direct effective interventions by the fire brigade respectively monitor the traffic flow or rescue operations.
For further details contact the FDIA, tel: (011) 496 1701.
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