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Coal handling belt conveyors can look like relatively simple machines, but they are, in fact, considered complex as most every coal handling system is unique in design and application. And, although many can design a belt conveyor to move material from one point to the next, conveyance systems are only considered successful when they run reliably over many years with limited attention. As with any complex machine, design decisions can and will have significant impact on long term reliability and maintenance requirements. Recent advances in numerical analysis and simulation techniques are leading to changes in system and component designs. The application of time-based, numerical modeling tools that consider belt elasticity and discrete particle-based bulk material flow methods are giving engineers unprecedented looks at expected performance during the design cycle when changes are easy and cheap. Examples of coal conveying applications along with the numerical tools required to insure reliability and availability are reviewed.
Introduction to coal conveyor
Reliability and Availability Belt conveyors play an important role in the transportation of coal. They are used extensively during mining, processing, storage, transportation and, finally, in the generation of power. Because a conveyor delivers a very small amount of material over long periods of time, it is essential they operate efficiently with maximum availability and minimum downtime. Unlike a truck or train that delivers large loads intermittently, a conveyor must deliver a small, steady stream of material continually.
However, reliability and availability are much more difficult things to quantify or even define as there are so many factors to consider when determining the appropriate goal. And because belt conveyors usually operate in a series, with transfers from one to another, downtime on one conveyor will mean total conveyance system downtime. For example, in a plant with 5 conveyors in series, availability of 98% on each conveyor (that might be considered acceptable) yields a system availability of 90% (that would probably be considered unacceptable). Therefore, belt conveyors need to be designed for very high availability. Although classical design methods have historically been considered adequate, it can be argued they are not adequate to meet the availability needs of current market requirements. In order to discuss belt conveyor reliability, at some point we have to evaluate specific problems common to existing applications. What type of problems do we see that creates unscheduled downtime? A recent survey of consulting projects and a review of the literature over the last five years have helped produce the following list of frequent complaints:
coal conveyor features
1. Transfer Plugging
2. Drive Pulley Slip
3. Belt Splice Failure
4. Pulley Failures (Figure 2)
5. Dust and Spillage
6. Belt Tracking
7. Belt Wear and Damage
8. Concave curve liftoff
9. Not enough power
10. Idler Bearing Failures Root Cause
Analysis One reason to take a good look at specific problems encountered in a plant is to determine the root cause of these problems and therefore treat the reasons for the problems rather than the symptoms. Root Cause Failure Analysis (RCFA) is a reliability technique used to identify the causal factors for component, equipment, or system failures.
The key to a successful RCFA program is to identify and implement a set of recommendations that address the cause of each failure that is analyzed. RCFA and pro-active maintenance is one of the least used techniques for improving system reliability. The ten items listed above (and many more) can be segregated into 4 distinct categories (Figure 3). Improper usage of the equipment and inadequate maintenance practices are both significant concerns but are both areas that can usually be addressed and corrected through management focus and direction.
Since design inadequacies are much harder to identify and correct once equipment is operating, this is the focus of the remainder of this paper. Once a machine is operating and a design issue is established, the next step is to decide whether it costs more to remove the root cause or simply continue to treat the symptoms. This is often not an easy determination. Even though it may be relatively easy to estimate the cost to remove the root cause it is generally very difficult to assess the cost of treating the symptom since the cost of the symptom is generally wrapped up in a number of customer and employee satisfaction factors in addition to the resource costs associated with just treating the symptom. Regardless, it is obviously a more difficult task to correct design issues after construction than to get it right to begin with. If we expand the design process a little further (Figure 6), we can identify one of the major obstacles to achieving the best system design in the early stages of the process. A key component of the process is ‘Test the System.’ A conveyor system is made up of many individual components from multiple manufacturers sometimes from many different countries. Every conveyor is unique in design and function. Although individual components can and are tested to make sure they meet performance criteria, the system is not tested until it is commissioned in the field.
The systems engineer who is responsible for ensuring all the components are working together is critical to the final product. A system is a set of interrelated components working together toward some common purpose. The properties and behavior of each component of the system has an effect on the performance of the whole system; the performance of each component of the system depends on the properties and behavior of the system as a whole.
There are many individual discipline specialists in the fields of mechanical, electrical, civil engineering, et cetera. Similarly, there are many components specialists in rubber, bearings, motors, controls, et cetera, that must be knowledgeable is their respective fields, but seldom are these individual specialists knowledgeable of the system in which their expertise in used. The field of systems engineering was established and is growing out the need for the system management function in the design of complex machines. The influence of the systems engineer is critical during the early stages of the design process when the emphasis is on the optimization of the system and not the individual components. Since a system ‘test’ is not possible, the systems engineer must usually rely on mathematical models to ensure the machine will perform as expected. The quality of the mathematical tools used is directly related to long term performance and reliability.