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from the growing fire moves the stratification level higher and higher. This is the reason that International standards will state the limitation on ceiling heights for certain types of fire detectors. This can be as low as 20 feet for point type heat detectors, 25 feet for point type smoke detectors, 30 feet for beam type smoke detectors, and flame detectors or a series of beam type detectors for anything above this height detectors in standard temperature rooms. Should the room temperature be lower than standard e.g. coldstorage facilities, the stratification height is significantly lower. 3. Thermal Barriers. As previously stated, heat rises, therefore the hottest air will be found at the highest point in the room. This will create a barrier of hot air in which smoke will only penetrate if it is hotter. This is very evident in glass ceiling areas such as atria found in shopping malls. 4. Diffusion. As smoke rises it will tend to drift sideways as well as upwards. Therefore the concentration of smoke will be lower as the smoke rises. It is the concentration of smoke that will eventually take a detector into alarm. 5. Air Movement. Any air movement will take smoke away from the source of the fire. This is not a problem if you can predict the exact route of the air movement. In computer rooms, control rooms and clean rooms, this is usually known and therefore the detection can be placed across air conditioning vents etc. There will however be the need for additional ceiling detection for when the airflow has stopped (e.g. air conditioning switched off) and the smoke will take the traditional path of straight up. Should the protected areas be external (or an area with similar environment such as an aircraft hanger that has large doors open), then there would be no predictable airflow route, therefore impossible to detect efficiently with any other form of detection other than “at source” detection techniques. The solution to these problems is by “looking” at the risk; therefore detecting any impending fire “at source”, by doing exactly this, the VSD system does not encounter any of the above problems. Other “At Source” Detection Methods Other than VSD there are other “at source” type detectors. They are: - 1. Thermal Imaging Cameras. These systems can detect changes in temperature by monitoring infrared emissions. They can be useful in a controlled environment where there are no other forms of unpredictable heat sources. They are impractical areas where heat sources such as cars, trains etc are moving through the “scene”. Line of sight must be achieved; and heat emissions may come after initial smoke emissions. 2. Image Processing of Heat Changes. The way these systems work is by fitting a heat sensitive pad to known risk areas. These heat pads will change colour or contrast with the increase in temperature. This change can then be monitored and processed via the camera. 3. The most popular fire detection systems by far in this category BİLDİRİLER KİTABI TÜYAK PROCEEDINGS BOOK 2009 and have been previously employed for these types of detection scenarios were flame detectors. The problem is that by the time flames appear to trigger the device into alarm, serious damage has occurred and will rapidly increase; also significant amounts of smoke may have been emitted causing serious risk to personal health. How Does VSD Work VSD is based on sophisticated computer analysis of the video image seen by the CCTV camera (sensor). Using advanced imageprocessing technology and extensive detection (and known false alarm phenomena) algorithms, VSD can automatically identify the distinct characteristics of smoke and flame patterns. The fire detection industry has an abundance of known smoke and flame phenomena such as frequency of flicker etc, and all of these factors are built into the systems to give an accurate decision on whether smoke and/or flames are present. VSD systems use standard CCTV equipment (back & white, colour, infrared, existing or new) linked to a self contained processing system which is capable of recognising small amounts of smoke and flame patterns within the video image and alerting the system operator both at the processor and by a variety of remote outputs. VSD systems employ 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 5Hz-frame rate for each channel. The video hardware is designed to allow simultaneous real time digitising of all eight images, which means that the system does not multiplex images and therefore no information is lost or delayed. All alarm condition images are log time & date stamped, and stored within the system’s memory. VSD systems detect smoke and flame 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 that can be associated with smoke and flame behaviour. Further analysis is then carried out on the relationships between the filtered characteristics to determine whether all the conditions have been met for the system to confidently predict the presence of smoke and/or flame. The system installer has the ability to vary the amount of smoke signal, 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 programme the system to alarm only if smoke and/or flame is present in 2 or more zones. For even greater system performance 2 camera images can be associated together such that smoke and/or flame in one image is to be treated as a pre-alarm and smoke and/or flame in two associated camera images is treated as a full alarm. To provide compensation for areas of the image that could prove troublesome such as windows, mirrors or smoke producing processes, the installer or system user has the ability to eliminate TÜYAK 2009 3
TÜYAK BİLDİRİLER KİTABI 2009 PROCEEDINGS BOOK or mask off parts of the image from detection on an individual pixelby-pixel basis. Typical example of VSD system can be seen at picture1 (AlarmLog) and Schematic1 Picture 1. Alarm Log Schematic 1. Conclusion VSD systems are now common place around the World. They are used to solve various fire detection problems and are therefore suitable for a number of applications where no other smoke detection solution is viable. Some of these applications are: - w Power Station Turbine Halls w Aircraft Hangars w Road Tunnels w External Areas (forests, petro-chemical plants etc) Some of the key reasons why VSD is useful weapon in the fight against fire are: - w Detects at source. - Detects smoke very fast by taking the fire detection to the source and not waiting for the smoke to travel to the detector. - Height is irrelevant therefore stratification and diffusion of smoke are not an issue 4 TÜYAK 2009 w Visual Information given. - Accurate information on how to react (not just a red light on a panel). This will increase speed of response, accurate information to fire-fighters and safety to response teams. - You can clearly identify who or what stated the fire by use of rewind function on digital video recorder. w Commercial Viability. - You can use existing cameras so no installation works or materials are required. - Large areas can be covered with few cameras. - Maintenance is simple as the cameras can be positioned anywhere that is convenient and does not require the target/ risk area to be inconvenienced. References Related part of NFPA72 Standard , DTEC Manuel , DTEC Test reports Brief Curriculum Vitae Malcolm Gatenby Export Manager Middle East & Asia Detector Technologies (D-Tec) Malcolm Gatenby has over 30 years of in-depth knowledge of high technology electronic systems, specialising over the last 25 years in integrated fire detection and security systems. Working with Zettler (now Tyco) for 5 years designing Fire Detection systems then moving to, Siemens Cerberus (UK, Abu Dhabi & Dubai for 10 years), then to D-Tec in 2002. Malcolm is an active member of the export council of the UK, and has been the Export Manager of D-Tec since its inception based in Dubai and spends much time speaking at seminars around the World on the subject of innovation in fire detection systems.
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