Playing with fire!? - Belt conveyors are sophisticated equipment widely used in the transportation business. Most often, nevertheless, their function is of paramount importance for the plants they serve: If the belt conveyors fail, the plant (or port) might be paralysed, so their weaknesses should always be kept in mind.
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Burning belts do not only mean production losses, but often also a thread for personnel and equipment. (Pictures: © Dunlop Conveyor Belting)
If a belt does not perform according to the manufacturer&#;s claims by wearing prematurely, ripping too easily or disintegrating due to excessive heat or because of oil penetration, the risk to life and limb is relatively small.
Yes, it can be very expensive but hardly life-threatening. But 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.
Due to the financial pressures on the cargo industry caused by the uncertain economic climate, more and more organisations are being forced to examine their day-to-day running costs and, almost inevitably, seek cost savings. The pressure to cut costs now seems to be influencing buying decisions concerning fire retardant conveyor belts.
Is price being put before safety? Or to put it another way, is the operator being lulled into a false sense of security by conveyor belt manufacturers and suppliers?
Poor quality fire resistant belt can be expensive!
The discovery that a belt is not sufficiently fire retardant is only likely to be made when it is too late.Anecdotal as well as factual evidence gained from laboratory testing certainly indicates that even some of Europe&#;s biggest users of conveyor belts, including some major ports, may be using belts that are not as resistant to fire as they are supposed to be. In a growing number of instances, many sites that should be using fire resistant belting are operating with non-fire resistant belts simply because of &#;economy&#;.
At the same time, insurance companies are becoming increasingly concerned. According to at least one major insurer, claims for fires directly involving convey-or belts are costing an average of nearly USD 8 million per claim.
Fire retardency standards on specifications 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, which have little or no resistance to fire. In other words, every belt will burn when it is exposed to a naked flame that has sufficient energy to ignite the belt. When choosing a fire retardant conveyor belt, deciding on the actual level of fire retardency needed for a specific application or environment is of crucial importance.
Dunlop BV XS conveyor on the left stopped the spread of fire.
EN is the standard for safety requirements for conveyor belts for general-purpose use (not underground). The most basic electrical and flammability safety requirement is EN Category 1. For environments where coal dust, gas, fertilizer, grain or other potentially combustible materials are involved, 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. At Dunlop we decided some time ago that the safest approach was for all of our belts to be anti-static and conform to EN/ISO 284 international standards.
This means that they can all be used in ATEX 95 (94/9/EC Directive) classified zones. Some people mistakenly believe that all belts used in ATEX classified zones must be flame retardant but actually this is not the case.
ATEX regulations apply to industrial environments where there is a risk of explosion because dust or gas is present in the atmosphere. For those organizations that are buying conveyor belts for use in ATEX regulated areas it is very important to ask potential suppliers for a copy of a certificate provided by an appropriate independent testing authority such as Dekra Exam in Germany.
Interestingly, a belt that has good anti-static properties is also a good indication of the quality of the rubber used on the belt. All (black) rubber belts contain carbon black, which is an ingredient in the rubber compound needed to achieve good mechanical properties. The higher the quality of carbon black used to produce the rubber compound then the better anti-static properties it will have.
Because fire safety is such an important issue there are numerous safety classifications and international standards for which there are many different tests used to measure the self-extinguishing properties of conveyor belts. Rubber belts reinforced by layers of textile fabrics (multi-ply) or steelcord reinforcement are the most commonly used type for transhipment and in general service applications. 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 and without covers (S).
DIN testing at Dunlop.
The relevance of &#;with and 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 grain, biomass for example then &#;K&#; grade is usually perfectly adequate. This also applies to elevator belts. However, if the material is abrasive and tends to wear the top cover quite rapidly then the safest option is be 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 datasheets 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 datasheets 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 than150mm³.
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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. However, laboratory tests have revealed that this is very much an exception to the rule within the conveyor belt industry.
Conveyor belt specialist Jeremy Clark shares his essential dos and don&#;ts when buying industrial conveyor belts.
Conveyors are indispensable but expensive pieces of equipment. With operating budgets under continuous scrutiny from senior management, it is all too easy to make a costly mistake when buying conveyor belts. Here, conveyor belt specialist Jeremy Clark shares what he considers to be the most important dos and don&#;ts when buying industrial conveyor belts.
Never assume that the specifications of your conveyor belts are correct. Most conveyors have been in operation for a number of years. During their lifetime they are likely to have been operated by several different managers and maintenance personnel. It never fails to surprise me to find that mistakes have been made by the original builders of the conveyor when calculating the belt specification best suited to the conveyor. Likewise, subsequent attempts to solve problems such as rapid wear and ripping and tearing, may have resulted in the wrong specification of belt being fitted. The same applies if the material mix or volumes have changed significantly. It is imperative that the belt specification always matches the conveyor design and the materials being conveyed.
For example, belts that are too thick for the design of the application can cause problems such as excessive rigidity (lack of troughability) and steering and handling difficulties. The same applies to tensile strengths that are too high. It is important to remember that for every step increase in tensile strength, the pulley and drum diameters need to be increased by 25%. The belt carcass may fail due to dynamic stress if this action is not taken. Quality belt manufacturers and their service partners who supply and fit their belts, will have experienced engineers. When provided with sufficient information, they can verify the belt selection.
Always select belts based on their durability, suitability and longevity (whole life cost) rather than for short-term &#;economic&#; or budgetary motives. Experience shows, without doubt, that the price of the belt will invariably be reflected in both its quality of performance and the length of its working life. I regularly see cases where a good quality belt can produce a working life of more than five years compared to low-grade belts that have been purchased on the basis of their low price but need regular repairs and then have to be replaced after only a year or two, often less.
The biggest source of low-price, low-grade belting is, of course, the Far East, primarily China. Their prices may be very tempting, but it is important to understand how those prices are achieved. The &#;lower labour costs&#; argument is a fallacy because the labour element accounts for as little as 5% of the production cost. The true reason for the difference in price is that raw materials make 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), unregulated raw materials. There simply is no other way. Cost-cutting practices include using cheap, low-grade carbon black made by burning old car tyres, the use of &#;bulking fillers&#; such as clay instead of good quality polymers and using low-grade inner synthetic plies. The two best indicators of poor quality rubber are its low price and its pungent smell. Good quality rubber rarely has any smell at all. As the old saying goes, price is what you pay but cost is what you spend.
Always (unless significant quantities are involved), choose belts from within the manufacturers standard stock range whenever possible Any deviation from the standard specification of a belt held in stock means that the belt would have to be made to special order, invoking higher production costs and potential minimum length order quantities. It also increases the delivery lead time, which can be especially bad newsif a belt needs to be replaced inan emergency.
Only buy belts when you are totally confident of their provenance (manufacturer&#;s origin) and where the manufacturer clearly states the level of anticipated performance rather than simply claiming to have been tested to a certain standard. Unfortunately, in today&#;s market, with the exception of Fenner Dunlop in The Netherlands and North America, nearly all manufacturers now supplement their production with imported belting, mostly from the Far East. My advice is to always ask for certified confirmation of the actual place of manufacture.
Avoid over-specifying the type of belt edge. This may surprise you, but it can have a very significant impact on belt costs, repairs, maintenance, lost production and lead times. Many years ago, moulded edges (fully covered in rubber) were the standard because cotton was used as the reinforcing fabric in multi-ply belts. A moulded edge was therefore essential to prevent moisture penetrating the cotton fabric and causing it to rot. Since the introduction of synthetic ply fabrics using polyester and polyamide, this problem effectively no longer exists. Despite this, many conveyor operators and their buying departments continue to specify moulded edges, even though they do not provide any structural or performance advantage and can actually be susceptible to damage if the belt wanders off-track.
Nowadays, the most commonly used type of belt edge is the &#;cut & sealed edge&#; or simply &#;sealed edge&#;, which is sufficient to prevent moisture being drawn into the carcass from the edge by capillary forces. Although the synthetic fibre plies are barely affected, moisture can ultimately cause vulcanising problems when making splice joints. A sealed edge also enables a belt to be used in very wet conditions and makes it better suited to long-term storage outdoors.
The reason why it is important not to over-specify the type of belt edge is that manufacturers need to manufacture belts at the optimum width and length for cost and production efficiency. Consequently, standard width stock belts are usually made as wide as the manufacturing machinery will allow, creating huge rolls known as &#;slabs&#;. These are then cut and sealed to create a range of narrower widths. For example, using metric measurements in this case, 400 meters of 2,200mm wide belt &#;slab&#; would be manufactured and then be slit (cut) to produce 400m of 1,000mm wide belt and 400m of 1,200mm wide. This is why the vast majority of rubber multi-ply belting held in stock by manufacturers, distributors and traders has sealed edges. Moulded edges can only be created when a belt is manufactured in isolation and to an exact width. Consequently, the manufacturer is unable to achieve optimum production line efficiency, the cost of which has to be reflected in the selling price.
Never order a conveyor belt without first obtaining written confirmation from the supplier/manufacturer that the belt being supplied is fully resistant to ozone and UV. As every manufacturer of rubber products will know only too well, at high altitude ozone (O3) acts as a protective shield by absorbing harmful ultraviolet rays but at low altitude, it becomes a pollutant that attacks the molecular structure of rubber. Ground level ozone increases the acidity of carbon black surfaces with natural rubber, polybutadiene, styrene-butadiene rubber and nitrile rubber being the most sensitive to degradation. The reaction that occurs is known as ozonolysis.
The first visible sign is when cracks start to appear in the surface of the rubber. Further attacks then occur inside the freshly exposed cracks, which continue to grow steadily until they complete a &#;circuit&#; and the product separates or fails.
Ultraviolet light from sunlight and fluorescent lighting also has a seriously detrimental effect on rubber because it accelerates rubber deterioration by producing photochemical reactions that promote the oxidation of the rubber surface resulting in a loss in mechanical strength and wear resistance. This is known as &#;UV degradation&#;. The combination of ozone and UV has a seriously limiting effect on the operational lifetime of a rubber belt, regardless of geography or type of climate.
Fortunately, ozone and ultraviolet damage is easy to prevent. Unfortunately, laboratory testing consistently reveals that some 90% of belts tested according to EN ISO /1 procedure B static ozone resistance test are not resistant. In fact, the vast majority typically start to crack within the first 6 to 8 hours of the 96-hour test duration. This is because the anti-ozonants needed to protect the rubber have been omitted from the rubber compound mix because of cost so always insist on ozone & UV resistance when selecting any rubber conveyor belt.
Choose belts that are specifically engineered for the conditions they are required to cope with. This is especially important on sites where rip, tear and impact damage problems occur. Attempting to solve the problem by fitting belts with thicker covers and heavier carcasses is invariably a mistake. Simply using more of the same material will not provide the solution. As mentioned earlier, thicker, heavier belts can also cause other problems including lack of troughability, steering and handling problems and dynamic stress due to the pulleys and drums now being
too small.
The most economical solution is to fit a conveyor belt that has been specially designed for the purpose, such as Fenner Dunlop&#;s UsFlex, Nova X and Ultra X range belts that are proven to last up to four or five times longer than conventional multi-ply conveyor belts. The initial buying price might be appreciably higher, but the cost will be substantially less over the working life of the belt, with the added benefits of far less repairs, change-out costs and disruption to output.
Always choose a belt supplier who you can rely on to be accountable in the event of a problem. It is important to have a supplier that will readily provide a high standard of professional technical support and who has a good reputation for quality. This, I feel, should always be an essential part of the criteria when choosing a supplier. A tall order perhaps, but they do exist.
More information at Fenner Dunlop.
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