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Crying 'Zwolf'

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World Cement,

Albrecht Schall, VDZ, discusses the ‘Zwolf’ and the role of maintenance in ensuring that cement kilns operate effectively.

There is an old piece of German cement industry shorthand sometimes still used – plant personnel will occasionally report that ‘The Zwolf’ (short for: ‘Zwangsläufige Ofenlängsführung’ or ‘forced longitudinal kiln guiding/hydraulic thrust device’) of the kiln has broken down.

Of course, everyone knows about the kiln hydraulics that push the kiln up and down (Figure 1), but the use of the term ‘Zwolf’ is less common. When reviewing old cement industry literature, it becomes apparent that plant personnel from 60 – 70 years ago were battling with similar issues.

One engineer, working in the maintenance department of a plant 57 years ago wrote the following: “The Zwolf (forced longitudinal kiln guiding) has already proven itself at two rotary kilns. Of these two, the first, which was built in 1912, quite small in diameter, was considered to be ready for scrap before the installation of its ‘Zwolf’ in March 1953, with the result that the management decided that new tyres and rollers would be purchased, because the ‘Zwolf’ had fixed the quite serious situation at the two tyre stations.”

It is, historically speaking, remarkable that this process occurred exactly 100 years after the invention of the rotary kiln in 1853 by two English engineers, G. Elliot and W. Russell.

Today, it is no longer surprising when children operate their cell phones or when we hear about the ever-increasing wonders that electronic controls can produce. Everything in human life and in nature works better when the controls are working properly.

Among the thousands of rotary cement kilns, which are 50 – 200 m long and have to withstand internal temperatures of up to 2000°C, tyre durability seems to benefit from a slow walk back and forth on the support rollers. The number of tyre stations of rotary kilns can vary between 2 – 12.

From an engineering point of view it is important to understand that where the tyre rests on the rollers, undefined forces and stresses cross the mechanical systems. It is also necessary to accept that there will never be exact static calculations for rotary kilns for reasons of static indeterminacy, and even the best computer programme cannot resolve the issue completely. A rotary kiln with only three tyres tends to behave similarly to a wooden table where the fourth leg is supported by a piece of cardboard. The reason for this is that a heated rotary kiln causes a constant change of forces and pressure on the support roller, for which no engineer is able to come up with valid rules.

A rotary kiln drum filled with hot, rolling material is dimensionally unstable. As was written years ago: “It is not plastically formable like a big sausage; The real operational behaviour is changeable over time and partly plastic, somewhere between a rigid barrel and a big sausage.”

Today’s requirements are that the tyre is dimensionally stable. Tyres and rollers can be calculated quite well, and with preventive maintenance methods it is possible to successfully avoid even tyres with diameters up to 6.2 m and larger having low ovalities. This stability helps to improve the life of the brick lining in the kilns considerably.

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