Suppression options
rather than flame detection, but all of these types work well with bespoke watermist and clean agent fire suppression systems.
Trommels Trommels are often the most expensive individual items of equipment at an MRF. These are large metal containers that have a rotating separation screen inside, typically 5m to 15m in length and inclined downwards to allow waste to move through the rotating screen from the in feed end to the out feed end. They are most often positioned as the first step in the mechanical separation process, straight out of the shredder/breaker. Some MRFs will have secondary trommels, but the primary trommels are where the ‘raw’ waste can cause problems. If hot materials such as hot barbecue coals or incompatible materials such as batteries and wire wool make it through the loading and shredding stages, they can create ignition in the trommel. Fires in trommels have been known to cause significant damage to the interior of the equipment, as well as spilling onto the outfeed conveyor and travelling onwards to other equipment. Loss of a trommel can lead to downtime of six or more months and multi million pound replacement costs, in addition to multi million pound business costs whilst diverting the waste to landfill or other processing facilities.
Fire protection for trommels is not commonly
installed on the basis that the vast majority of ignition sources are found in the loading and shredding/breaking stages. However, if the incoming waste stream is such that there is a clear risk of ignition sources reaching the primary trommel, then fire protection is worth considering. The most practical option is to install flame
detectors and deluge waterspray/watermist nozzles over the exit of the trommel where it releases material onto the exit conveyor. Nozzles can be positioned to direct water into the trommel itself, as well as onto the conveyor. As soon as the presence of hot material
is detected, all upstream equipment and conveyors should be halted via a machine interlock. However, the trommel and the first exit conveyor should continue operating so that any burning material inside the trommel body can be delivered to the mouth. The downside of this solution is that if hot burning material gets trapped inside the trommel body, it will not be extinguished unless by chance. Manual firefighting efforts would be required once the trommel is stopped. An alternative is to drill the casing of the
trommel and install deluge waterspray/ watermist nozzles through the body wall to discharge directly into the rotating screen. This has been done at several waste handling facilities, usually when a previous fire has been experienced in a trommel and the operator does not want a repeat incident.
FOCUS
www.frmjournal.com NOVEMBER 2018
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