January 2005

Web alarm system prevents fire in tissue paper plants

The Firefly WebScan alarm system from Orthos has been shown in tests to deal reliably with a common fire risk in plants manufacturing tissue-paper based hygiene products that not only frequently shuts down production but can cause extensive danger and damage as recently proven in a £17.5 m fire in the storage area of a tissue plant in Mannheim, German.

All tissue mills suffer from an unavoidable accumulation of highly flammable cellulose dust and fibre on flat surfaces throughout the plant. The smallest outbreak of fire in such circumstances is extremely dangerous and can rapidly spread throughout the entire plant.

One specific area of increased risk now causing concern is the dryer. The tissue web typically travels through the dryer at speeds of over 20 m/sec and is separated from the drum by a knife or doctor blade as it exits the dryer. It is then wound onto the mother reel, which is often about 5 m wide and is commonly split into two or three smaller reels by an air knife.

Hot spots of glowing paper can arise at random within the fast-moving web and are rapidly enclosed and insulated within the reel. These can be generated by friction where the knife impinges on the dryer drum or by a drum´s aging non-stick coating becoming exothermic, or may arise within the dryer itself.

The high web speed means it is not possible to detect and cool or quench the hot spots before they are wound into the reel, so it is essential that they are detected and plant operators warned of their existence. Infected reels are at risk of spontaneous combustion and can cause severe damage to the plant.

Developed in Scandinavia and test-proven in the tissue products industry, the WebScan system is installed just prior to the reel-up and covers the whole width of the web. It comprises up to eight specially designed low-temperature flame/radiation detectors linked to a central controller.

A hot spot passing through the scanning zone of a given unit is immediately detected and its location indicated by the controller. An alarm is triggered to enable the operators to stop the plant and remove the infected reel to a safe quarantine area for inspection and disposal or re-pulping.

The detectors are highly sensitive and can detect problems within thicker, very absorbent tissues such as those used as workshop wipes. Particular attention has also been paid to the design of the detectors to ensure that false alarms are minimised.

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