Preventing a problem is always easier then dealing with the problem after it has had time to develop. Combustible dust in an air conveying dust pipe can develop a problem very fast and fire prevention is the reason for using spark detection and extinguishment.
Most state governments have adopted into law National Fire Protection Association (NFPA) standards for fire prevention. NFPA standards are very specific about providing spark detection and extinguishment system for ducts that convey combustible dust. These regulations are also very specific about how the spark detection system must perform.
The enforcement of these standards is generally left up to the local fire authority and wide variations of enforcement exist. However, the national trend is definitely towards meeting the NFPA standards.
Detecting, identifying and hitting a target (spark) moving at over 55 miles/ hour is a rather complicated physics problem. Some of the topics that must be addressed are detector sensitivity, detector cone of vision, response time, temperature range, optimal obscuration, noise immunity and the degree of spectral matching between the detector and the ember.
Past ex[werience has shown the requirements for intermittent, repetitive application systems. The first is that the detector must not immediately latch into the alarm state with condtinuos water spray and shutdown. Instead the detector must provide a distinct alarm signal for each ember that passes in view. Second, the input circuitry which receives the alarm signal must be able to process the alarm signal such that the extinguishment system solenoid commencing with the alarm signal and ending after the ember has been quenched. Thirdly, the water delivery equipment, that is the solenoid valve and nozzle, much be suitable for intermittent, high speed operation. At a conveying speed of 5,000 feet/ minute, all of the above must be accomplished in less that one second.
Sprinkler systems, which can be less expensive, are not at all suited to the high speed dynamics of a dust conveying system. Typically a sprinkler system waits for a fire to develop, waits for the temperature to reach 190 degrees F and then responds by flooding the area with hundreds of gallons of water. If the dust filter is located outside and the fire occurs during freezing weather, the clean up can be very time consuming.
The benefits of a high speed spark detection and extinguishment system are significant:
When cleaned air is returned from a bag house back into the plant, spark detection takes on a new level of urgency. A fire or explosion in the bag house could send a lame front back into the plant putting both the plant and personnel at risk. A spark detection system equipped with an abort gate in the clean air return pipe, can sense the spark and dump the flame front outside before it can enter the plant.
A spark detection system is composed of three basic components. First is the detector, 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: 1. Central Control, 2. Detector, and 3. Extinguisher are all integral parts of a successful spark detection and extinguishing 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, cellulosic fibers, sawdust, chemical dust, combustible vapors and any other combustible material.
The detector 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 purpose 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 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 Spark Detection controller 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 passed through the curtain of water. 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 be too long.