Make the Right Moves With Belt Conveyors

Understand important factors in their selection, design and operation

By Amin Almasi, rotating equipment consultant

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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.

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