Many plants require a material handling system to transport bulk solids and powders. These solids may come from stockpiles of material transferred from ship, rail, truck, etc., or directly from process operations (e.g., intermediates or finished products). A variety of conveyors can move such solids. However, plants most commonly select a belt conveyor because of its reliability. The overall material handling system also includes transfer points (or towers), trippers and chutes to discharge materials from one belt conveyer to another or to equipment.
A belt conveyor contains many rotating and vulnerable elements such as a drive system, gear reducer unit, pulleys, idlers, etc. All these components rely on rolling element bearings, manufacturer’s standard seal, lubrication system, etc. Even when using the best technologies and most durable components, the overall reliability and availability of a single belt conveyor line won’t match the availability required for many critical processing units. Therefore, redundancy usually is essential.
A single belt conveyor line might suffice for some routes, for instance, from the unloading system to stockpiles. However, for most services, installing two separate conveyors is prudent. For example, recommended practice is to transport material from a major stockyard into the processing area via a dual conveyor system. Likewise, critical lines, especially those where a conveyor trip can result in a costly shutdown of a crucial chemical processing unit, demand two independent conveyor systems to ensure that a breakdown doesn’t affect flow to the unit.
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The Basics
A belt conveyor system consists of two or more pulleys and a belt that rotates around them. One or two powered (drive) pulleys move the belt and the material on it forward; the unpowered (idler) pulley maintains tension on the belt. The belt consists of multiple layers of specially selected materials. Underlayer(s), called the carcass, provide linear strength and shape, and often are made of a woven fabric. The overlayer(s), called the cover, typically consist of various rubber or plastic compounds specified to suit the material being handled. Unusual applications can call for covers composed of more exotic materials. Conveyor belts should be made continuous by hot vulcanizing. Mechanical fastening traditionally was used in some applications but has caused many operational problems; it is not recommended for modern conveyors.
As an indication, the maximum inclination of a belt conveyor should not exceed 15° to the horizontal. A greater angle might be possible but requires special design. As a rough guide, limit the speed of a conveyor belt to 3.2 m/s; there are successful high-speed conveyors but many of them use special designs. The distance of the belt line including pulleys from the supporting floor should allow easy maintenance; I generally recommend a minimum clearance of 800 mm below the return side of the belt.
Consider all operating cases and scenarios when designing and sizing the conveyor. A conveyor should be capable of accepting a 10–15% surge; therefore, power calculations usually incorporate 10–20% surge capacity for the full length of the conveyor. Carefully check calculations related to all starting/start-up cases, including restart of a fully loaded conveyor. The method of restart and adequacy of power for the restart of a fully loaded conveyor are important, but sometimes overlooked, factors. Each conveyor should be capable of being started under all load conditions without any slip occurring between the drive pulley and belt.
A common requirement is to limit the maximum belt tension at normal operating condition to around 10–14% of the tensile strength of belt to ensure sufficient margin for belt mechanical strength. Some specifications set the limit at 8% for special cases (for instance, for a nylon carcass belt); lower limits might be prudent sometimes. On the other hand, conveyors have operated successfully for many years with tension exceeding 14%. So, higher limits might make sense in some situations. Carefully evaluate and verify each case. The starting and braking tensions imposed on the belt may cause problems over time.
Starting characteristics of the drive unit and the braking effects during deceleration should be such that the maximum tension in the belt is limited to somewhere around 130–150% of the belt tension at normal operating condition. In other words, limit maximum transient tension to 14–20% of the tensile strength of belt. Again, these are rough figures; detailed evaluation may show that deviations are acceptable.
Key Components
Conveyors commonly use pulley shells, end discs and hub assemblies of all welded construction and manufactured from suitable grades of carbon or low-alloy steel. Do not employ pulley shells made from pipe or tubing. The pulley face width usually is 50–100 mm wider than the belt. To be on the safe side, the maximum combined stress in the shell and end discs should not exceed 20–30 MPa. Reported shaft failures often stem from fatigue failure; a common culprit has been a far greater loading on the shaft than theoretical expectations. Shaft deflection usually is limited to a maximum of 0.05% (1/2,000) of the end disc span and an angular deflection of five minutes at the shaft/hub connection.
Idlers bear the load of material on conveyor. Idler assemblies should have heavy duty construction with roller shells made from precision-finished steel tube with end discs securely welded to the shells. Limits usually are specified for the idler diameter — e.g., 100, 115, 125 or 150 mm depending on application. Transition idlers at both the head and tail end feature an adjustable trough angle. Buffer idlers are installed at the material receiving point. Idler spacing on the carry strand should limit the belt sag to the lowest of either 1% of the idler spacing or 1.2 m, and on the return strand to the lowest of either 2% or 3 m. As a general rule, feed points to conveyors need great attention and robust design because these points must withstand great dynamic loads and harsh conditions. Impact idlers are installed at the skirted feed points of all conveyors; they are spaced at a nominal pitch of 0.3 mm and should be designed to allow retraction of the idler supporting base to assist maintenance without the need to remove skirt plates or structures in the load area.
Gear reducer units lower the speed of electric motors driving conveyors. Gears are designed for infinite life, with power rating equal to the full motor rated power multiplied by the relevant factors, often above 1.25 or 1.35.
A backstop device (also known as a holdback) prevents back moving due to material weight in case of any malfunction or accident such as sudden power failure. This device usually is fitted to the drive pulley shaft extension opposite the drive end; it should have the capacity to hold as a static load 100% of the stalled torque of the electric motor (including a suitable service factor, often 1.4 or 1.5).
Flat belts sometimes are used in material handling; they are simple to engineer and probably the most widely used type of conveyor belt for low capacity applications or for short distances. However, they have limited capacity to transport materials and pose some disadvantages. Obviously a completely flat belt would not work well for handling granules or powders on any angle of incline for a distance; materials would spill right off the edges. Trough belts commonly are chosen; they are very suitable for relatively long distance paths where materials have some time to settle down into the trough. These belts are concaved to create a suitable groove or trough for material to ride along the conveyor path. This allows for high capacity. Self-aligning mechanisms installed at suitable intervals correct any belt wander. Belts are required to bend and stretch lengthwise as well as laterally at the end wheels. Carrying idlers come in different designs and various trough (or groove) angles such as 20°, 30°, 35° and 45° from the horizontal. The most common option has been carrying idlers of three equal rolls with trough angle of 35°.
Bearings cause the most reliability issues and failures. So, all aspects of bearing selection and sizing demand great care to ensure adequate bearing life and reliability. Some specifications stipulate a service life of 100,000 hours (say, 10–11 years). Simulating worst possible loadings on each bearing is important. Make allowances for shock loading when calculating the design load on bearings; an impact factor of 1.25 (or more) usually is needed for calculating bearing life. Bearing selection depends on many factors such as application, overall loadings, etc. Good options for pullies are self-aligning spherical roller bearings, and for idlers, deep groove bearings; evaluations may suggest choosing other appropriate options. Another important component is the bearing seal. A poor seal can’t prevent dust and material from entering the bearing, and can’t properly manage the grease and lubrication in a bearing. A multi-cavity labyrinth seal in conjunction with a rubber lip seal fitted between the labyrinth and bearing chamber often is a good choice.
Case Study
A chemical plant needed a 3,250-t/h belt conveyor to transfer solids (density of around 852 kg/m3) a distance of 385 m while lifting the material around 10 m. The selected speed is 3.15 m/s and belt width is 1.8 m. The conveyor design features four segments: a horizontal length; a length with 4.5° inclination; a third length with 7° inclination; and a final horizontal length from which material discharges through a chute. The design incorporates carrying idlers of three equal rolls with trough angle of 35°. Calculated drive force is around 100 kN; estimated shaft power of the drive pulley is about 310 kW and the selected electric motor is 450 kW. Drive pulley diameter and idler diameters are 1,300 mm and 160 mm, respectively.
AMIN ALMASI is a rotating equipment consultant based in Sydney, Australia. Email him at [email protected].
