Fire is one of the most destructive and dangerous forces known to man, destroying everything in its path. Insurance companies are becoming increasingly alarmed about conveyor fires. Anecdotal, as well as factual evidence gained from numerous laboratory tests, certainly indicates that even some of Europe’s biggest users of conveyors, are using belts that do not meet the claims of the supplier. Here, Rob van Oijen, one of Europe’s top application engineers, provides an essential guide to choosing fire resistant conveyor belts.
Combustible environments
In environments where potentially combustible materials such as coal dust, fertilizer, grain or biomass are present, it is essential that the conveyor belt cannot create static electricity that could ignite the atmosphere. Belts need to be able to allow static electricity to pass through the metal frame of the conveyor structure down to earth rather than allow static to build up. The safest approach is for all belts to meet EN 12882 Category 1, which demands that the belt is anti-static and conforms to EN ISO 284 international standards. This means that they meet the primary requirement for use in ATEX 114 (Directive 2014/34/EU) classified zones if necessary.
Basic grades of fire-resistance
The basis of most tests for belting fire resistance is EN ISO 340* and is included within the classifications of EN 12882 on electrical and flammability safety requirements for general purpose conveyors used above ground. These standards make the distinction between fire resistance with covers, which is Class 2A (or often referred to as K grade) and fire resistance with and without covers, which is Class 2B (or often referred to as S grade). The relevance of “with and without covers” is primarily because the rubber skim layers between the synthetic plies protect what is essentially the most flammable part in the construction and which can actually be the path for flames to propagate. The rubber skim material therefore needs to be as equally fire resistant as the outer covers. Worryingly, manufacturers who want to minimise costs to achieve a more competitive price often use rubber skims that have insufficient resistance to fire.
For the majority of ‘open air’ applications, EN 12882 Class 2A (K grade) or Class 2B (S grade) levels of fire resistance should be perfectly adequate. The best way to decide between Class 2A and Class 2B is to consider the material being carried. For moderately abrasive materials, then Class 2A is usually suitable. However, if the material is more abrasive then the safest option is to choose Class 2B.
*EN/ISO 340 tests involve exposing six individual samples of belt to a naked flame causing them to burn. The source of the flame is then removed and the combustion time (duration of flame) of the test piece is recorded. A current of air is then applied to the test piece for a specified time after the removal of the flame. The flame should not re-ignite.
The time it takes for the belt sample to self-extinguish after the flame has been removed is then measured. The duration of continued burning (visible flame) should be less than 15 seconds for each sample with a maximum cumulative duration of 45 seconds for each group of six tests. This means that the average allowable time per sample is 7.5 seconds. This factor is of paramount importance because it determines the distance that the fire can be effectively carried by a moving belt.
Even if a manufacturer states that their belt has passed the ISO 340 test, caution should still be exercised. A typical conveyor belt can easily travel more than 40 meters within the 15 seconds that is allowable for a belt sample to pass the test, which is a potentially very dangerous distance. For this reason, Fenner Dunlop in The Netherlands strive for an average maximum time limit of less than one second, which is more than six times faster than the required standard.
Conveying biomass
A higher level of fire resistance for conveyors handling biomass is necessary. One of the biggest dangers is dust emission. In the production process of biomass wood pellets, wood chip and similar renewable resources, the materials are continually broken down. This results in high levels of combustible dust that can be easily ignited by static electricity. Biomass dust can also be highly prone to self-ignition, especially if it becomes damp. A chemical reaction can take place that causes self-heating and what is referred to as “off-gassing” (carbon dioxide, carbon monoxide and methane emissions).
Because of the increased risk of self-ignition, the use of covered conveyors is becoming increasingly commonplace. In enclosed environments the risk of human life is heightened because burning rubber and other synthetic materials such as polyester and nylon release thick toxic smoke that contains cyanide, carbon monoxide, sulfur dioxide, and products of butadiene and styrene. For this reason, EN 12882 Class 4A is usually the best choice for conveyors operating in closed or covered conditions because it involves a more severe fire test according to EN12881-1 method A, C or D in addition to EN/ISO 340 testing.
Rubber compounds that have a combined resistance to both fire and oil are available for materials that contain oil. This is an important consideration, so it is necessary to be very specific when requesting quotations from manufacturers and suppliers. If you are still unsure of what grade you need then do not hesitate to seek expert advice.
For the majority of manufacturers, the chemical ingredients used to create fire-resistant rubber have an adverse effect on its wear resistant properties so fire resistant belts tend to wear faster. As the thickness of the rubber reduces so does the level of protection given to the inflammable carcass so always look at the manufacturer’s technical datasheet to check that the abrasion figure is no higher than 160 mm³.
Choosing the right supplier
Deciding on the correct grade of fire-resistant belt is much easier than deciding who should supply that belt. Always select a supplier that has a reputation for quality, who will be easy to reach and communicate with and who will provide a high standard of professional technical support. Most importantly of all, always select belts based on their safety and longevity (whole life cost) rather than for short-term ‘economic’ or budgetary motives. Experience categorically shows that the price of the belt will invariably be reflected in its overall quality and its ability to self-extinguish fire in good time.
The biggest source of low-price, low-grade belting is Southeast Asia, primarily China. Their low prices are usually attributed to much lower labour costs, but this is a fallacy because labour accounts for as little as 5% of the total production cost. The real reason for the difference in price is that raw materials, including the expensive chemicals needed to create rubber that has a good standard of resistance to fire, ozone & ultraviolet, oil and wear, represent up to 70% of the cost of producing a conveyor belt. Consequently, the only way to manufacture a low-price belt is to use low-price (low grade), raw materials and/or minimise or omit the use of key ingredients. This is confirmed by extensive laboratory testing that shows that some so-called ‘fire-resistant’ belts actually burn like paper and up to 90% of all belts sold in Europe are not even resistant to the damaging effects of ground-level ozone and ultraviolet light.
Of equal concern is that manufacturers located outside of the Europe are not subject to REACH (Registration, Evaluation and Authorisation of Chemical substances) and POP’s (Persistent Organic Pollutants) regulations. This provides them with an open door because they are free to use unregulated chemicals, which cost much less compared to their regulated counterparts, even though they may be entirely prohibited or at least have strict usage limitations within Europe.
Always check the provenance
Even if the manufacturer is based in Europe, always ensure that you are totally confident of the true provenance (origin) of what you are being quoted for by asking for certified confirmation of the actual place of manufacture rather than where the belt will be shipped from. This is important because, with the exception of Fenner Dunlop in The Netherlands and North America, virtually all manufacturers supplement their production with imported belting.
About the author:
Rob van Oijen has specialised in conveyors for over 18 years, supporting businesses throughout Europe, Africa, the Middle East and South America. He is one of the leading application engineers in the conveyor belt industry.
