Serge Jorget and David Kotrys, Fives FCB, France, discuss rotary kiln maintenance offerings.

Cement kilns: evolutions and induced problemsThe rotary kiln has been used for cement production for about a century and has become the only burning technology, with the less efficient and polluting shaft kilns largely disappeared. The kiln can be considered the heart of the cement plant. Its reliability directly influences the plant’s production capacity. Therefore it has to be kept in optimal condition and deserves the highest attention.

Mechanical and process stresses have increased over the years, thus raising the risk of incidents and wear.

l Specific production capacities have risen within the last decades by approximately 25% (from 160/kg/h/m3 up to 200).

l The production process has evolved as concerns the clinker quality; the LSF has also been raised to improve the cement resistance and often to compensate for the use of additives. This induces a higher burning temperature.

l Alternative fuels bring impurities, which are damaging to the refractory lining.

l As a consequence the length of chemically resistant but more conductive refractory has been extended from the burning zone up to the middle of the kiln.

All these evolutions increase the shell temperature; even the use of fans cannot prevent incidents when the refractory lining is worn out.

While operating conditions are becoming more severe, much progress has been achieved in kiln optimisation systems. The composition of the processed meal is more stable; the flame shape is better controlled owing to modern burners such as the Pillard Novaflam®; the cooler operation is less sensitive to clinker quality; the refractory lining offers a better resistance. This reduces the occurrence of incidents and increases kiln availability.

Nevertheless, in the medium and long-term, any kiln will suffer brick falls, flame deviations, overheating, unexpected shutdowns and other events that will affect

the condition of the kiln body, as well as the wear of its elements. In particular:

Deformation of the kiln axisThe consequences of the deformation, permanent or not, can be:

l A bad contact between the kiln and its supports, with cyclic, deviated and excessive loads. Roller bearings suffer accelerated wear and may break. So may the roller itself.

l A moving contact between the teeth of the pinion and of the girth gear. This will result in abnormal wear and possible breakage of teeth.

l Cyclic load also for the motor, which suffers and obliges a reduction of the production capacity.

Deformation of the shell

l When refractory lining is worn out and not replaced on time, bricks fall, and each time parts of the shell are submitted to excessive temperatures and the steel loses its properties.

Wrong adjustment of the tyre clearance

l With so-called floating tyres, the clearance between kiln and tyres in operation ensures that the ovality of the kiln is kept very low and that the bricks do not move during the rotation.

Deformation of the nose-ring

l The end part of the kiln is subjected to the high temperatures of the flame.

These examples demonstrate the need for a correct evaluation of the kiln geometry and condition, which should be carried out every 2 to 4 years. With adjustment and repairs carried out in time, the possible damage and the potential for reduced kiln capacity are minimised. The cost of such expertise is easily covered by the savings realised.

Kiln alignment methodologyThere are usually two types of kiln alignment procedure: cold kiln alignment and hot kiln alignment.

Fives prefers to perform the kiln alignment in hot conditions. The kiln does not have to be stopped and is observed under the normal operating conditions. Hot kiln alignment (HKA) gives a better assessment of the kiln situation in its real dynamic and thermal conditions.

Because the HKA is done in operation, it does not disturb the production and so can be planned more easily with the plant management.

Referential systemThe first phase of the HKA, as for cold kiln alignment, is the building of a fixed referential system in which the position of the equipment can be defined for the three directions.

Figure 1. Position of kiln supports on the piers.

Figure 2. Definition of kiln centre.

Reprinted from February 2015 \ World Cement

The next phases consist of the kiln measurement programme itself. First, a global positioning is established, showing the kiln’s ‘morphology’. The longitudinal positions of the rollers and tyres are also represented, to show any abnormalities, wear or errors.

Foundations and frame The kiln is supported by several concrete blocks. Although massive and anchored in the ground, their foundations can move; checking their position enables the detection of anomalies, which could be due to subsidence, cracks or other phenomenon. Periodic monitoring is therefore necessary to ensure the proper kiln operation and avoid unseen moves leading to heavy mechanical damages.

Measurements made on fixed references positioned on the frames indicate the behaviour of the foundation, allowing the calculation of a local axial inclination, slope or possible twist.

These measurements give the possibility to have a global reference of the installation, allowing better tracking over time.

Geometry rollers/tyres According to Fives defined procedures, each tyre and roller is precisely measured for its dimensions and position (diameter spacing, slope, etc.)

It is therefore possible to evaluate the conicity, the skewing* and the deviation from the parallel line. Several parameters can also be deduced, as follows:

l The behaviour at the contact surfaces (e.g. Figure 1, support 1 contact point).

l The mechanical environment (e.g. Figure 1, support 3, roller skewed in opposition).

l Generalised constraints (in case two supports are skewed in opposition).

l The wear.

*Skewing is a mechanical effect coming from the position of the rollers relative to that of the tyre.

Diameter and centre definition Using the measurements previously performed, it is possible to define the relative positions of all elements, and to calculate the position of the kiln centre.

All measurements taken by Fives are defined at a given shell temperature.

Thrust position(s) In many cases, the hydraulic thrust is positioned on the wrong side of the axis, which creates an upward force to the thrust roller. Consequently, the roller can be lifted from its body and damaged (marks on tyres, folding of the thrust shaft, deformation of the base frame).

Fives takes into account the position of the hydraulic thrust roller on the radial axis, relative to the kiln and the rotation direction, in order to assess the risk of mechanical failure (thrust roller, thrust tyre, thrust body, etc).

Definition of axis and slopes The axis line position of the kiln is calculated from the kiln centre position on each supporting station.

This helps define the corrections to apply to make it come back to a straight line.

Shell deformation and prediction of brick fall risk areasThe stability of the refractory lining highly depends on the shell shape stability. Refractory falls can therefore be induced in particular by the shell deformation.

Such brick falls entail overheating of the concerned areas and can result in further irreversible deformations. These damages can also cause a permanent deformation of the kiln axis over some length, known as ‘banana shape’, causing mechanical disorders both on the gear girth/pinion as well as the upstream and downstream seal. The kiln reliability and the production stability are inevitably decreased. It is therefore of great interest to control the shell

Figure 3. Real axis position given from measurements on a kiln.

Reprinted from February 2015 / World Cement

geometry with its deformations in order to identify areas at risk as concerns the refractory stability.

Once again and for each section, the position of the real kiln centre is calculated.

A 3D representation of the kiln shell deformation, based on the shape determined for each perimeter, gives a better understanding of areas where the level of deformation is predominant.

From this geometric analysis, and thanks to its long experience and its knowhow of equipment, Fives developed a model to evaluate the risks of brick fall related to deformation.

Various parameters are taken into account, such as the diameter variation, the radius curvature variation and the presence of hollows and bumps.

Fives cannot only define the areas with a risk of brick fall, but also quantify this risk as shown in Figure 5.

According to the level of risk, recommendations can be given such as the use of an adapted type of bricks or kiln shell replacement.

Understanding kiln shell deformation is the perfect complement to a hot kiln alignment and

provides useful information to understand kiln behaviour and provide tailored recommendations.

Results, analysis and onsite expertise Completing the geometrical analysis, a mechanical audit is performed on the kiln. The goal is to check the whole mechanical parameters of the equipment (tyre sliding, contact surfaces, girth gear/pinion and shaft line, upstream/downstream sealing, supporting bearing, lubrication, hydraulic thrust, etc).

These mechanical checking and visual inspections complement the geometrical measurements to help gain an understanding of the kiln situation. With the global mechanical audit and the geometric data in hand, Fives is able to give OEM recommendations on the complete equipment.

Prioritised action plan These recommendations are provided as a full report from Fives with the support of its kiln engineering department. They are prioritised in order of importance to facilitate the work of the teams onsite and anticipate needs in terms of budget.

Beyond making recommendations, Fives is able to solve the problems identified during the HKA directly, such as roller adjustment, girth gear/pinion adjustment, etc.

Fives is also able to support the plants in areas such as roller and tyre machining, adjusting shims under tyres, or by replacing major parts of the kiln, such as shell, tyres, girth gear and/or pinion, upstream/downstream sealing, etc.

ConclusionThe kiln can be considered the most important piece of equipment in a cement plant since its reliability influences the ouput of the plant. Many parameters can influence its mechanical condition and wear. One small defect that is not corrected can have major consequence after a few months or years. Fives helps its customers to reach the maximum availability of their equipment, offering hot kiln alignment, shell deformation and brick fall analysis, mechanical supervision or mechanical revamping. Fives is convinced that proper kiln maintenance is the primary step to ensure a stable production (excluding the operation) and reduced maintenance costs in the long-term.

Figure 4. 3D exaggerated view of the kiln shell deformation.

Figure 5. Identification of zones at risk for the lining stability.

Reprinted from February 2015 \ World Cement