REINER FURTHMANN, AUMUND FÖRDERTECHNIK …...REINER FURTHMANN, AUMUND FÖRDERTECHNIK GMBH, GERMANY,...

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AN ANCIENT IDEA FOR THE MODERN DAY AN ANCIENT IDEA FOR THE MODERN DAY REINER FURTHMANN, AUMUND FÖRDERTECHNIK GMBH, GERMANY, OUTLINES THE HISTORY AND MODERN DEVELOPMENT OF BUCKET WHEEL ELEVATORS. Introduction The history of the bucket elevator goes back to antiquity, when buckets on a rope were used to raise water for ancient irrigation systems in early Middle Eastern civilisations. In these early designs, the rope runs over two wheels, one above the other, raising water from a pool (boot) at the base to a reservoir (silo) at the head. No doubt there were improvements as time progressed, but the major impetus to the design and execution came in around 1785, with the introduction of continuous milling systems for flour production that were driven both by wind and, latterly, by steam. These machines were constructed in wood, with a belt and bucket system directly comparable to modern designs – but with much smaller handling rates. The bucket elevator raises the grain to silo storage, from which it is allowed to fall by gravity through the process stages. This concept was expanded for the grain elevators found on US waterways from 1840 onwards. Here, belt-bucket elevators would raise large volumes of cereals for storage in silo. The stored grain would then be raised again by elevator and allowed to fall by gravity through an inclined spout to barges or deep-sea vessels. Developments in the cement industry The cement industry offers similar challenges, albeit under far more arduous conditions. With the introduction of the rotary kiln in around 1900, traditional batch handling, for example by skip-hoist, could no longer be tolerated. The resulting continuous process demands placed extreme pressure on the associated handling systems to maintain uninterrupted raw material availability and clinker discharge. These pressures have significantly increased over the years and, with the introduction of the precalciner concept

Transcript of REINER FURTHMANN, AUMUND FÖRDERTECHNIK …...REINER FURTHMANN, AUMUND FÖRDERTECHNIK GMBH, GERMANY,...

Page 1: REINER FURTHMANN, AUMUND FÖRDERTECHNIK …...REINER FURTHMANN, AUMUND FÖRDERTECHNIK GMBH, GERMANY, OUTLINES THE HISTORY AND MODERN DEVELOPMENT OF BUCKET WHEEL ELEVATORS. Introduction

AN ANCIENT IDEA FOR THE MODERN DAY

AN ANCIENT IDEA FOR THE MODERN DAY

REINER FURTHMANN, AUMUND FÖRDERTECHNIK GMBH, GERMANY, OUTLINES THE HISTORY AND MODERN DEVELOPMENT OF BUCKET WHEEL ELEVATORS.

IntroductionThe history of the bucket elevator goes back to antiquity, when buckets on a rope were used to raise water for ancient irrigation systems in early Middle Eastern civilisations. In these early designs, the rope runs over two wheels, one above the other, raising water from a pool (boot) at the base to a reservoir (silo) at the head. No doubt there were improvements as time progressed, but the major impetus to the design and execution came in around 1785, with the introduction of continuous milling systems for flour production that were driven both by wind and, latterly, by steam.

These machines were constructed in wood, with a belt and bucket system directly comparable to modern designs – but with much smaller handling rates. The bucket elevator raises the grain to silo storage, from which it is allowed to fall by gravity through the process stages. This

concept was expanded for the grain elevators found on US waterways from 1840 onwards. Here, belt-bucket elevators would raise large volumes of cereals for storage in silo. The stored grain would then be raised again by elevator and allowed to fall by gravity through an inclined spout to barges or deep-sea vessels.

Developments in the cement industryThe cement industry offers similar challenges, albeit under far more arduous conditions. With the introduction of the rotary kiln in around 1900, traditional batch handling, for example by skip-hoist, could no longer be tolerated. The resulting continuous process demands placed extreme pressure on the associated handling systems to maintain uninterrupted raw material availability and clinker discharge. These pressures have significantly increased over the years and, with the introduction of the precalciner concept

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Reprinted from May 2017World Cement

in around 1966, allied to five-stage twin string preheater systems that require a vertical lift of up to 170 m, handling some 600 tph (or more) of raw meal, the pressure is on the elevator supplier to deliver reliability and economy in operation in a single lift belt-bucket elevator.

The tallest belt-bucket system thus far delivered by AUMUND stands at 173 m and is located at Ambuja Cement’s Himachal Pradesh plant, one of the largest and most modern cement plants in India. At present, this is one of the highest belt bucket elevators installed in the world and, as such, only a very few companies possess the expertise to deliver a competent design at this performance point.

In principle there are two types of bucket elevator found in the cement plant: the chain-bucket system, which is generally used for coarse raw material or hot clinker on route from the kiln to the silo; and the belt-bucket elevator, which is generally used for handling smaller granular material, such as the raw meal before calcination and the finished cement to storage silos before dispatch. Generally belt-bucket and standard central chain elevators run faster and use centrifugal discharge (effectively the material is thrown out of the bucket over the drive pulley), whereas double chain elevators run more slowly and use gravity discharge, where the material falls free of the bucket and is discharged into the head chute.

Belt-bucket elevatorsConsidering first the belt-bucket elevator, the external differences between the major suppliers may not seem important. But, as always, the devil is in the detail. For the belt itself, while outwardly similar and both based on steel cord construction (certainly for the larger machines), there are two basic systems of bucket interface. In some designs, the belt is manufactured with ‘cord-free spaces’ through which the bucket is fixed, making the belt unique and non-standard.

The AUMUND belt concept differs in that continuous close pitch longitudinal ropes are employed, without rope-free bucket mounting spaces, with additional wire ropes running laterally across the width of the belt, adding considerable lateral stiffness, without significantly changing the belt’s longitudinal flexibility. This cross-stabilised construction gives improved stability to the belt with improved bucket fixing, the Achilles’ Heel of every bucket elevator and probably the most common cause of elevator failure, as well as improved directional stability (better tracking).

Using continuous wire ropes at close pitch, both longitudinally and horizontally, without rope free spaces, gives maximum strength and stability but does require the bucket fixing is punched through the belt, steel ropes and all. Of course this is taken into account with the tension calculation and the ruptured ropes are ignored in the full belt width tension rating.

To make the necessary bucket fixing, AUMUND has developed a special punching machine design, which gives clean fixing holes that are accurately aligned relative to the belt edge and at precisely the correct pitch. The belt is held in a cassette and advanced automatically under the punching frame using a precision encoder to measure the pitch. The belts are prepared at the company’s works in Germany and shipped to site at the exact required dimensions, including a prepared joint and the required clamping strips.

Belt-bucket elevators, raising the raw meal to the blending silo and preheater tower.

Cross-stabilised elevator belt.

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Reprinted from May 2017World Cement

With these huge belt elevators, including widths to 2000 mm, theoretical lift height to 200 m, and handling rates to 2600 m3/hr, the belt joint is highly stressed. Here AUMUND has developed a special clamping connection that brings the belt ends together and provides for the individual jointing of the steel cords, ensuring the belt strength is maintained. The cord jointing is encapsulated in a steel casing and filled with a resin compound to protect the exposed wires.

New developments Traditionally the belt-bucket elevator has been limited to a 10 mm grain size and is most often used for the raising of the raw meal to the blending silo and from the blending silo to the preheater, as well as at the final stage the lifting of the finished cement from the mill to the dispatch silos. However, with recent developments, AUMUND has blurred some of these traditional application areas, developing new belt-bucket systems to handling coarse material, such as the BWG-GK elevator, which allows for grain sizes up to 80 mm.

This new range of elevators is based generally on the original BWG design, but with special buckets that overlap using a profiled interlock, preventing the ingress of coarse material between belt and bucket. Furthermore, with the close pitch buckets set wider than the belt, the belt edge is protected under

the most difficult conditions, even digging material from the boot.

The BWG-GK design is still limited by temperature to 130˚C or up to 150˚C with a special belt. Nevertheless, the ability of these belt elevators opens new possibilities for the handling of raw materials, where traditionally a chain elevator would be the only viable solution. Wet and sticky materials remain a challenge for any bucket elevator: often, in older designs, the material will accumulate on the inside surfaces of the bucket, effectively reducing the bucket capacity and, therefore, the handling rate capability of the elevator. AUMUND has solved this problem with a new bucket design for the BWG and BWG-GK machines that includes a flexible casing panel that prevents internal build-up and ensures the bucket contents are fully discharged with no residue remaining.

In addition, AUMUND has a raft of detailed developments in progress to tailor the elevator construction and performance in line with the demands of the minerals industry, where cohesive materials, such as coal, coke, ore, and clay, are the norm.

Central chain elevatorsOf course, there are alternatives to the bucket elevator. At the preheater tower in particular, the Air-Lift was a popular solution in the past. This used pneumatic transport to lift the raw meal from the blending silo to the preheater inlet at high level. Pneumatic conveying is notoriously inefficient, with only 23% of the absorbed energy being used to transport the material compared to over 90% with a bucket elevator. Not only is the pneumatic solution expensive to run, it also impacts on the ID fan power demand by injecting high volumes of air into the preheater cyclone, which effectively reduces the ID fan performance. By replacing the Air-Lift with a bucket elevator, not only does the system consume less energy, if all else is suitable, by improving the ID fan performance, the kiln output may also be increased.

However, for grain sizes from 10 – 100 mm, the BWZ central chain elevator and its derivatives are the preferred solution, particularly when handling hot materials to 450˚C, where it is the only solution. This elevator design is based on a single central forged chain developed to operate under the most arduous conditions, typically handling hot clinker or in mill recirculation, where the material temperature may be consistently over 150˚C and, under flush conditions, much hotter. The AUMUND design stands out in the detail of the chain forging, but also the bucket mounting, where an intermediate bracket is bolted to the bucket and carried on extended chain pins, with a clearance to allow some flexibility in the relative chain-bucket alignment. This prevents the transfer of any twisting stress to the bucket

Central chain elevator with flexible bucket mount.

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Reprinted from May 2017World Cement

structure, which causes premature cracking and ultimate failure.

Chains are forged with a labyrinth seal to minimise fine material ingress to the bearing surfaces. The pin-bush assembly is also pre-filled, using a specific grease formulation, during assembly, not for lubrication purposes but to promote the effective lapping of the pin-bush bearing, which extends the chain life long after the lubricant has disappeared. At the head, a single central friction-tyred rim drives the chain directly; the chain and rim are an integrated design, such that the rim engages the chain at three points on each side plate, plus on the central bush, giving maximum driving traction. The AUMUND central chain elevator type BWZ is available with a maximum single flight lift height of 90 m and buckets to 1200 mm width, giving a handling rate of around 1050 m3/ hr at 1.87 m/sec.

In addition to the single strand elevators, the chain design system is available with twin strands, designated as BWD central chain elevators, operating in a common casing, which extends the handling rate pro-rata to 2100 m3/hr. The latest development – the BWT central chain elevator –extends the handling rate to 3000 m3/hr, using three 1100 mm wide bucket strands, operating in separate but integrated casings, using a common drive shaft and shared tandem drives. By integrating the casings with a shared drive system, the gap between the

elevators may be minimised, simplifying the feeding and feed boot design to ensure an even feed to each elevator strand.

Continuing improvements Working hand-in-hand with the University of Magdeburg, Germany, AUMUND has carried out detailed discrete element method (DEM) analysis of the material flow from the single inlet chute to the three discharge chutes (one to each elevator strand) to ensure an equal flow splitting within a compact chute system to minimise the feed boot height. This arrangement would not be practical using three separate elevators. For lifts to 45 m and handling rates up to 4000 tph, the BWT elevator is the most economical solution, ideal for recirculation in the larger roller press systems that are now coming back into favour at modern cement plants.

In recent years, AUMUND has made detailed improvements to virtually every part of the bucket elevator and the associated equipment. These benefits may be realised in almost any situation by retrofitting chain or belt, buckets, and drives into an existing casing to minimise downtime and cost, while realising the benefit of the most modern solutions and often improving performance at the same time.

Of course, it is always nice to talk of the biggest and fastest, but the majority of the machines found in the cement plant are of more modest specification. Here, also, dependability is critical; the plant output is equally reliant on individual machines within the process regardless of size.

While it is not a conventional design, the pendulum bucket elevator has been resurrected by AUMUND based on its design of some 60 years back. One particular example of this concept remains in operation at the port of Duisburg-Schwelgern, Germany, but the general concept went out of fashion in the cement sector. By applying the lessons learnt in pan conveyor chains using external rollers to support the chain and bucket, the company has implemented a new and more compact solution, bringing this concept fully into line with modern plant demands. The advantages of the pendulum design remain in that the material may be carried and discharged on both the upper and lower strands and at any point along the conveyor length, as well as allowing horizontal loading with any combination of angular change in the conveyor routing between.

ConclusionIn conclusion, the elevator remains today a core component of the cement production line, installed in key points, where dependability is essential to maintain the kiln production and plant output. While the principle may remain similar to the design concept of Leonardo Da Vinci, the detail has most surely moved forward.

BWD double strand elevator.

BWT triple strand elevator.