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Figure 1: Flowsheet of a pellet production with spark extinguishing system

Spark extinguishing systems in pellet production

René Schwertfeger, T&B electronic GmbH

Due to the positive market development for renewable energies, wood pellet production throughout Europe has recorded high growth rates in the last few years. In recent months, however, the attention of operators (and insurers) has been focused on the necessary fire protection in these plants as a result of major incidents: fires and explosions repeatedly occured as a result of flying sparks or concealed embers in processing equipment as well as pneumatic and mechanical conveying systems,. The consequences are hazards to human life, loss of production and major property damage. This risk can be eliminated by monitoring production plants and securing them with spark extinguishing systems. T&B’s latest generation of control panels complies with the guidelines of VdS Schadenverhütung GmbH and offers maximum safety when protecting machines, conveying equipment, filters and silos. Pellet production can be divided into the fields of wood preparation, wood drying and actual pellet production.

Figure 2: Operating principle of a spark extinguishing system

Whereas the fire risk in wood processing is rather low due to the material’s high residual moisture, there is a regular fire hazard in the adjacent areas.

Essential danger spots are:

a) dryers

b) pneum. conveying lines to silos and filters

c) Elevators

d) Hammer mills

e) pellet mills

f) coolers

Figure 1 illustrates the minimum level of protection required for a pellet plant:
In the dryer area, the inlet and outlet must be monitored with spark detectors for the detection of glowing residues. The detectors detect the slightest infrared radiation and report it to the spark detection centre, which triggers extinction. The middle sections of the dryer can be monitored with flame or thermal detectors. These control an extinguishing system installed in the dryer, which, depending on the dryer construction, will be activated section by section similar to the functionality of a spray flooding system according to VdS.  In addition, it will be useful to monitor the dryer exhaust air using spark detectors, which also act on the extinguishing system in the dryer.

Spark detection and spark extinguishing devices should be installed behind the fans in all conveying lines to filters and silos as shown above:

The spark detection (A) comprises at least two spark detectors which monitor the entire cross-section of the pipeline. The spark extinguishing device (B) consists of a fast-opening solenoid valve and at least one extinguisher nozzle. Thanks to its newly developed automatic extinguishing system, T&B can realise extremely small distances between spark detection and extinguishing units which are far below the previous standard values of the VdS-directive. The extinguisher nozzles are provided with a shutter to  prevent contamination. Extinguishing spray will be maintained until the last detected spark (C) has passed the fire extinguishing section, then the automatic system closes autonomously. The spark extinguishing system remains ready for operation, so that newly emerging sparks can be fought immediately again. The minimum extinguishing time is five seconds. It extends automatically when the flight of sparks continues. By means of an adjustable spark threshold the machine will be switched off (D). In order to ensure a faultless a spray pattern, a flow pressure of at least 5 bar must be available. If this is not abvailable, a pressure booster (E) is used. It is possible to connect to an existing Sprinkler system for fire-fighting water.

Fire extinguishers located in frost-prone areas must be equipped with electrical trace heating. The trace heating (G) is controlled via an outdoor thermostat with associated heating tape monitoring. All heated extinguishing water pipes as well as the extinguishing device must be insulated. The spark detectors feature built-in test equipment. Each spark detector can thus be checked from the spark detection control panel for its functionality.

The new generation of BM 6 (H) spark detection control panels features an automatic detector testing loop, manual detector testing is not not available here. An extinguishing water monitoring system constantly controls, whether extinguishing processes are running smoothly, and displays these processes on the spark detection control panel. The spark detection centres can store up to 2500 fire alarms or malfunctions and display it in plain text.

Elevators are equipped with a spark detection system. to allow for an elevator shutdown in case of a revision,  e.g. via a cross conveyor. The downstream hammer mills are monitored at the outlet by means of spark detectors, behind the hammer mill an extinguishing system is mandatory which must also include the downstream filter. Thus a spreading of the eventual fire will be prevented. At the outlet of the pellet press, the elimination of so-called embers is an important component for safe production. In contrast to a spark, an ember is larger, but often encapsulated, and therefore emits significantly less energy than a spark. By installing special detectors for detecting embers, downstream areas are reliably protected. An extinguishing system downstream of the pellet press is recommended in order to prevent the spread of fire to further parts of the plant. During the start-up process, high product temperatures will occur at the outlets of both hammer mills and the pellet press. T&B detectors are able to distinguish between these production-related short high  temperature-events from hazardous embers. A spark detection system after the cooler ensures that sparks are not transported into the loading area.

Figure 3: FST It – Indicator for the detection of embers

The VdS guideline 2106 is decisive for the installation of a spark extinguishing system. It states, among other things, that a spark extinguishing system may only be installed by a VdS-approved installation company. In addition, structural fire protection measures must be observed. For example, pneumatic conveying lines that pass through fire or complex partitions must be fitted with fire protection sliders. In the event of a continuous flight of sparks, these can be actuated by either the spark extinguishing system or, in the event of fires in the building, by conventional smoke detectors. A combination with central spark extinguishing systems is possible.

For each spark extinguishing system, an installation certificate, a schematic drawing showing the function and protection range and a hydraulic calculation must be prepared by the recognised installation company. The documents must be submitted to the VdS in case of an inspection. Spark extinguishing systems must be surveyed and maintained at regular intervals by a recognised installation company. As a rule, bi-annual maintenance is sufficient. In addition to maintenance of the system, T&B offers additional services such as a ‘winter check’ (inspection of the system before the frost period for insulation defects, trace heating, etc.) or the development of protection schemes for the turnkey installation of spark extinguishing systems.

T&B is a recognised installing company for spark extinguishing systems and one of the market leaders in Europe in the field of plant-engineering related fire protection systems using spark extinguishing systems. Since its foundation in 1984, more than 30,000 T&B systems have been sold worldwide. Concept, development and production of extinguishing systems for customised fire protection solutions are currently carried out by some 30 employees. All components are manufactured in Germany. The customer portfolio ranges from asphalt preparation to the sugar industry. In addition to the protection of classic spark extinguishing areas, T&B electronic specializes in the protection of processing machines and filters by gas extinguishing systems. Managing partner of T&B electronic is Thomas Warnecke, graduate physicist.