January 2014 Recycling & Waste World
An article explaining the importance of this kind of belting.
If a conveyor belt that is specified as being fire retardant catches fire but does not resist the fire the way that it should do then it will literally ‘convey’ the fire throughout the site. The consequences can be catastrophic.
The importance of fire-retardant conveyors
If a conveyor belt that is specified as being fire retardant catches fire but does not resist the fire the way that it should do then it will literally ‘convey’ the fire throughout the site. The consequences can be catastrophic, warns Sytze Brouwers.
Fire retardancy standards and test methods applied to conveyor belts are becoming increasingly stringent and can be very confusing. The first and most important thing to bear in mind is that conveyor belts cannot be totally fire proof. Using special additives and chemicals, the rubber used in the top and bottom covers that protect the carcass of the belt and the rubber skim between the fabric plies of the carcass can be engineered to resist fire, but the complete structure of the belt cannot be made fire proof. The fabrics used in the carcass of the belt most commonly contain polyester and nylon. These materials have little or no resistance to fire. In other words, every belt will burn when it is exposed to a naked flame that is sufficient to ignite the belt. When choosing a fire retardant conveyor belt, deciding on the actual level of fire-retardancy needed for a specific application or environment is of crucial importance.
Recycling and general service applications
Rubber belts reinforced by layers of textile fabrics (multi-ply) or steel cord reinforcement are the most commonly used type in the recycling industry. The basis of most tests for belting used in normal industrial applications is EN /ISO 340. This standard makes the distinction between fire resistance with covers (K) and fire resistance with or without covers (S). The relevance of ‘with or without covers’ is that as belt covers wear during their operational life the amount of fire resistant rubber protecting the flammable carcass reduces. The best way to decide between ‘K’ and ‘S‘ grades is to consider the material being carried. For moderately abrasive materials such as general household waste for example then ‘K’ grade is usually perfectly adequate. However, if the material is abrasive and tends to wear the top cover quite rapidly then the safest option is to choose the ‘S‘ (Class 2B) grade. In both ‘K’ and ‘S’ grades, the rubber skim that bonds the fabric layers of the carcass together should also be fire resistant. In the case of ‘S’ grade (fire resistant without covers), the rubber skim should be thicker than the skim used for ‘K’ grade.
The easiest way to tell if a ‘K’ grade belt has the required thicker rubber skims is to obtain technical data sheets from the manufacturer for both ‘S’ grade and ‘K’ grades and compare the carcass thickness figures. Another important reason why buyers should always request technical data sheets before placing an order is that they include information on the level of abrasion (wear) resistance. The ingredients used to create a fire resistant rubber compound generally have an adverse effect on its wear resistant properties. Consequently, fire resistant belts tend to wear faster and as the thickness of the rubber reduces so does the level of protection given to the inflammable carcass. To avoid premature wear, in the case of purely fire resistance belting, buyers should always demand an average abrasion resistance level of no more than 150mm³. Thankfully, at Dunlop our rubber compound technicians have proved that it is possible to have the best of both worlds by developing a fire resistant rubber that also has good resistance to abrasion. In fact, our technicians have created a compound that has 50% better wear resistance than the DIN Y standard for abrasion resistant rubber. This means that the belt retains its resistance to fire for much longer and at the same time extends the operational lifetime by the same proportion.
EN/ISO 340 testing
EN /ISO 340 tests involve exposing six individual samples of belt to a naked flame causing them to burn. The source 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 selfextinguish 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. Even if a manufacturer states that their fire resistant belt has passed the ISO 340 test, the buyer should still exercise caution. A typical conveyor belt can easily travel more than 40 meters within the 15 seconds sufficient for a belt sample to pass the test but which would still allow the belt to carry flames over a potentially dangerous distance. For this reason our required time limit standard in Dunlop is no more than one second, ideally 0 seconds. Buyers of fire resistant belt are therefore recommended to ask to see copies of the test results and to check that the laboratory that has carried out the tests has EN ISO 17025 certification.
What standard of fire resistance do I need?
One of the most difficult challenges for users of conveyor belts is establishing the correct level or standard of fire resistance needed. For the vast majority of belts being used in the open air, Class 2A or 2B would be perfectly adequate. Class 2A demands that the belt is able to pass the ISO 340 test described earlier with the covers intact on the belt samples (K grade). Class 2B requires that the belt that can also pass the ISO 340 test with the top and bottom cover rubber removed (S grade). If you are still unsure of the fire resistant grade of belting needed then it is best to carry out an internal risk assessment. If the expertise for this does not exist within your company then there are a number of external organisations (and almost certainly your insurers) who can perform this function for you. For conveyors carrying materials that contain oil, particularly household waste and engine parts, rubber compounds that have a combined resistance to fire, abrasion and oil are available. There are, of course, two types of oil resistance - mineral and vegetable. This is yet another important consideration when deciding on the correct type of fire resistant belt.
CEN fire test standards
Most fire test methods were established many years ago specifically for underground mining belts and have to be certificated by independent testing institutions. Such tests can easily cost up to €20,000 or more. These large-scale tests present a huge problem to manufacturers of rubber belting for above ground use because there is a much wider range of belt tyres compared to those used underground. Such complex test methods have made it extremely difficult to develop improved levels of fire safety because if a belt sample fails the tests then the technicians have to go back to the drawing board to make further changes to the rubber compound and then embark on another round of expensive tests.
New CEN fire test standards to be introduced
Following recent meetings attended by technical experts from all over Europe, the Committee European de Normalization will be introducing several positive changes in 2014. Agreement has been reached on using and adapting tests already in existence for quality standards such as DIN and BS that will involve much smaller scale tests using much smaller equipment. This will mean that major manufacturers will be able to experiment and carry out testing in their own laboratories. Ironically, these new test methods will be even more demanding than the old, large-scale tests. Major insurance companies are already showing interest and are becoming involved in discussions.
Don’t play with fire
Although manufacturers and suppliers may be able to provide test certificates, in some cases that certificate may only relate to the belting that the manufacturer produced for test certification purposes. The actual belt delivered to site may well not be up to the required standard. For greater peace of mind we would suggest ordering an extra meter of belt and then have that piece of belt tested by an accredited testing authority or laboratory. It may well be that some insurance companies would consider reducing annual premiums if fire resistant conveyor belts were introduced. The price of not exercising caution simply cannot be calculated.