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Explosion Protection |
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Spark
Detection and Spray Systems |
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Spark Detection System. A spark detection system is composed of three basic components. First is the detector head which is highly sensitive to optical radiation emitted by hot or burning particles. The detector receives an optical signal and converts it into an electronic signal. The electronic signal is received and processed by the central control unit which in turn triggers the extinguishing system. The three components; detector head, central control, and extinguisher, are all integral parts of a successful spark detection system. A spark detection system is typically used in pneumatic particle transport systems where there is potential for combustion or explosions. These include wood chips, cellulose fiber's, sawdust, chemical dusts, combustible vapors, and any other combustible material. Other applications can be in mechanical transport systems such as conveyors, or in the drop chutes. The detector head reacts to rapid changes in the optical signal so it is especially suited for viewing a cross section of a duct where the particles are in the field of view for only a short period of time. The idea of the system is to provide a zone in the path of transport where no potentially hazardous spark can pass without being seen by the detector or detectors. Usually, two or more detectors must be used in order to ensure that the entire cross section of the transport zone is within the field of view of at least one detector. The extinguishing system commonly used is a solenoid operated water spray located downstream from the detection zone. The water spray is set up to create a curtain through which the hazardous particle must pass. The central control unit provides the essential timing and the impulse to open the solenoid valve for a fixed period of time during which we are ensured that the spark has been extinguished. The timing is important as the activation of the spray must not be too late as it will miss the spark. Also, to avoid unnecessary shut down and cleanup, the spray must not begin too early. Other extinguishing systems could be Dry Powder, C02, other clean agent gases where water is not suitable; complete flooding where combustion could be catastrophic; system shutdown; or a combination of methods. All methods can be operated from the central control panel. The basic ASSASSIN system includes two detectors, a control panel and an extinguishing system. This is the basic requirement for a successful spark detection system. |
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Detection System Features Before describing the specific models we will describe the novel features which are part of the standard detection system. Each detector is equipped with an infrared LED located behind the protective lens. This LED is used to test the opposing detector when more than one detector is used in a duct. In this way each detector can test whether or not the opposing detector is operational. In operation the user would use the diagnostic functions to call up a program that will allow the user to test any detector on any zone. After the system has been installed this program will be run and it will store a number that represents the amount of energy the LED had to emit to cause the opposing detector to respond to the pulse. This pulse is sent in increasing steps until the detector responds. The panel will store this as an initial value which the user can compare against in the future. Typically this number will go up as dirt accumulates on the lens. When the test is run again the panel will show the initial value and the current one. A difference of greater than 50% would indicate that maintenance is required for the detector site.
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Detection System
Specifications: The definition of sensitivity, in
accordance with the current NFPA standards, is: The sensitivity of a
detector is the wattage per square centimeter incident on the
photosensitive surface of the detector from a radiating source, whose
optical range lies within that of the sensitivity range of the detector,
which will trigger the alarm after the given response time. |
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Detection System Components MODEL 100-1:STANDARD 1.0uW DETECTOR: This is the basic model detector which will be suitable for most applications. The detector head is contained in an indoor applications aluminum enclosure. It is equipped with a quick-release mounting bracket for mounting onto the surface of a duct. The quick-release mount bracket allows for easy periodic cleaning of the lens surface. The specifications of the # 100 -l detector are:
MODEL 100-2: 1.0µW DETECTOR WITH AMBIENT LIGHT FILTER: Each detector is equipped with an ambient light sensor. If it is found that the application requires the detector to be in conditions where there is some ambient light present, the detector equipped with an optical filter should be used. The sensitivity of the detector is the same but the spectral range of the detector is limited to a narrower bandwidth. The filter used is a black glass filter which only passes light wave lengths above 780nm. In other words, all visible light is absorbed by the filter (hence the name black glass). The physical characteristics of the detector are the same as model 100-1 except that the clear Pyrex lens is replaced by a black glass filter. All the specifications are identical to the model 100-1 except for the spectral range which is from 0.780 to 1.2 microns. MODEL 100-3: 1.0 µW DETECTOR WITH AIR PURGE: In some conditions, buildup of dust and debris on the surface of the lens may be a problem. This can be overcome by using the air purge option with the standard detector. The lens is kept clear and dry by supplying a dry air purge on the outer surface of the lens. The air is blown into a cowling which surrounds the outside of the lens but does not restrict the viewing angle of the lens. The dry air creates a high pressure zone at the surface of the lens which prevents the dust from touching ft. The physical dimensions of the MODEL 100-3 are the same as the basic model with an extra connection for air purge and the cowling which surrounds the lens. The air purge requires air at a pressure of 1 0 psi for most applications. MODEL 100-4: 1.0 µW DETECTOR WITH AIR PURGE AND AMBIENT LIGHT FILTER :This is simply the standard detector with both the air purge and the optical filter as described above. The physical dimensions are the same as MODEL 100-3. MODEL 100-5:4 HIGH SENSITIVITY DETECTOR :In some applications, more sensitivity is required. With some adjustments, the standard detector can be adapted to a higher sensitivity. An increase of a factor of 1 0 is possible. The high sensitivity detector will be used for large ducts or in applications where there is high obscuration. As there is a greater potential for unnecessary false alarms, the high sensitivity detector is only used in special applications. All of its specifications are identical to the standard detector except for its sensitivity which is increased to 0. 1 µw/cm2. MODEL 110-1: HIGH TEMPERATURE DETECTOR: For high temperature applications, the detector must be kept remote from the surface of the duct in order to protect the electronics. This is accomplished by using a large diameter light pipe which transmits the light from the surface of the duct to the detector. The angular acceptance of the high temperature system is slightly lower than the basic model (Half Power Cone of Vision = 80 degrees) but is kept high due to total internal reflection within the light pipe. The high temperature system requires a different mounting bracket. There is a loss in sensitivity of a factor of 5.0 (Sensitivity = 5.0 µw/crn2) with the high temperature system. This loss in sensitivity is due to the extra distance of the detector from the surface of the duct and some absorption in the Pyrex rod. |
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System Application Conveyor Applications: If a detector is placed at a distance h from a conveyor, the width of the conveyor which is viewed is 3.4h. In order not to exceed d when using one detector, the distance h must not exceed 0.5 d. For increased sensitivity, more detectors can be used to overlap the fields of view. Spray Extinguishment Application:
The water spray system is comprised of the 900-1 Nozzle and Spray
Assembly. This unit is always supplied as an assembly and is specifically
designed to operate and be installed as an assembly. The solenoid valve
portion of the assembly partly relies upon differential water pressure
across a diaphragm to open and close the valve. This flow of water takes
place in small diameter orifices which should be kept free from debris as
much as possible. Therefore, the second requirement of the Nozzle and
Spray Assembly installation is to use a strainer immediately upstream of
the assembly installation. It is recommended that all newly installed
piping upstream of the assembly be thoroughly cleaned out prior to
start-up. This will help reduce the chances of foreign matter lodged in
the pipes from entering the assembly. An option available with the water
spray system is a set of pressure transducers. These transducers are to be
located upstream and downstream of the Nozzle and Spray Assembly. The
upstream location allows verification of the presence of adequate supply
pressure of the water source. The downstream location verifies that flow
took place during a particular event. The maximum water pressure
recommended is 100 psi - achievement of which may require a pressure
reducer.
micro-processor sampling cycle:
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System Components The Detectors are solid state electronic units that have been designed to detect the presence of relatively low energy embers such as might be found in ' a dust collection system. A minimum of two detectors are mounted on the side of an air duct opposite each other. When an ember of sufficient energy (typically 1 µW) passes through the detection area. The unit will detect the event and pass the information to the main panel. The unit is able to detect high ambient light levels and inform the panel of this condition, it will continue to detect embers until the light level saturates the sensor. Each detector has been designed to 'test' it's partner. This is accomplished during an operator initiated "Sensitivity Check". The main panel will instruct the Detector to send a pulsed I/R signal of increasing output until the Detector opposite senses this. At this time the main panel will note what level of I/R was being sent and compares it to a stored value that was done when the Detectors were installed. An increase of 50% over the initial value will cause a trouble light to come on and the display will indicate this fact. This would normally indicate a dirty lens and it would be appropriate for maintenance to be conducted at this time. Main Panel is composed of several sub systems: Display/Keypad(220-4): Is the main processor board responsible for communicating with and supervising all the I/O control boards (301). All troubles and alarms are shown on the front of the panel box (200-2). Control Board (301): Is the board that monitors one zone with up to four detectors and four auxiliary inputs as well as controlling the output board (303) and all its outputs: water solenoid (900-1) horn (910-1), alarm and trouble relays. Module (303): Is the board responsible for the switching of all outputs when instructed to do so by the I/O control board (301). These outputs are: water (900-1), dust collector shutdown, abort damper horn (910-1), alarm and trouble relays. Solid State Relay (950-1): Is used to switch inductive loads like dust collector controllers ( Normally Closed applications ) Mechanical Relay (960-1): Is used to switch inductive loads like the Abort Damper (Normally Open applications) |
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