Tyre Creep 

 Purpose of tyre creep control:

 From a tyre creep point of view, the minimum heating-up period is determined by the time required to stabilize the temperature difference (and hence the clearance) between the kiln shell and the tyre, since the shell will heat up faster than the tyre.

 Tyre  # 2 is particularly critical.  Too rapid heating can result in a constriction of the kiln shell, which will cause permanent deformation if the yield strength of the kiln shell material is exceeded.  This in turn will cause excessive play in the tyre (tyre slip) after the normal working temperature is regained, as well as increased ovality of the kiln shell, a factor that may contribute to excessive refractory lining wear.

 As a general rule, the tyre creep (the relative motion between the tyre and the kiln shell) should be monitored at regular intervals or continuously with a kiln shell scanner if available, manually if not.   If the tyre creep is too low or non-existent, the heating-up process should be slowed down or interrupted until a measurable amount of relative movement is established.  For this reason, the tyre creep may become the limiting factor in determining the heating-up schedule.

 Every tyre has its specific designed cold clearance that allows a maximum temperature difference between the kiln shell and tyre during heating.  If the temperature difference exceeds the maximum value during the heating-up schedule, the shell below can be plastically deformed due to restricted expansion.  To avoid such a deformation, some plants have established artificial heating or exterior heating elements on the tyre themselves, or define a start-up sequence for the shell cooling fans.  This allows for controlled heating of the tyre in line with the heating-up schedule for the kiln.  The thermal expansion for each unit can be controlled so that deformation of the kiln or the tyre can be avoided.

Proper clearances for tyres during heat up will range from a minimum of 5 mm  to a high of 25 mm.

Procedure for tyre creep monitoring       

Purpose and Scope:        

 To monitor and react to Kiln Tyre Slippage levels in order to prevent damage to the Kiln Shell, Tyre, and Refractory. It is important to note that all support tyres must be monitored during a preheat, however, the tyres in the area of the burning zone are the most critical and susceptible to temperature changes. Due to the potential for damage, this monitoring operation should be considered critical in a kiln start up with dedicated manpower provided.

 Specific Points

 a.)  Slippage and temperature measurements during a kiln startup should be taken in the field only. Do not rely on thermal imaging scanners for temperature or slippage indication during a preheating. Use thermostatic pencils when necessary.  Slippage and temperature measurements must be taken at each tyre every ½ hour during the preheating

 b.)  The temperature difference between the tyre and the shell must be less than 125 °C at all times during the preheating. The shell temperature must be taken on both sides of the tyre. The tyre temperature must be measured on the side of the tyre and not the rolling face. Measure both the uphill and downhill side of the tyre.

 c.)   If the temperature difference rises above 125 °C, hold the kiln heat input until the

Temperature falls below the 125 °C. When the temperature is below 125 °C, resume the preheating schedule.

 d.)  The minimum tyre slippage per kiln revolution is 8mm.

 e.)  If the kiln tyre slippage falls below 8mm/rev, hold the kiln heat input until the tyre slippage reaches 8mm / rev. At 8mm / rev, resume the preheating schedule.

 f.)    If the kiln tyre slippage falls below 5mm / rev decrease kiln heat input until the tyre slippage reaches 5mm / rev. Ensure that the shell and the tyre temperature are rising at the same rate. At 5mm +, hold the kiln heat input until the slippage reaches 8mm +, at this point, resume preheating schedule.

 g.)  If there is no measurable slippage, follow the instructions for slippage below 5mm/rev with aggressive moves on heat input. If slippage has not returned in ½ hour, shut down.

 h.)   If tyre slippage is in excess of 25mm plant personnel should investigate the shimming of the tyre.  Excess slippage of the tyre can cause problems with kiln ovality which may result in shell, refractory and nose ring damage.

Notes:

• From a mechanical point of view, the slower the preheating, the less chance there is to encounter a slippage problem.

 Note:  Tyre temperature is determined by taking the measurement on each side of the tyre.  The recorded temperature should be the average of the two.

• The combination of tyre/shell temperature differences and slippage is extremely valuable information. Temperature OR slippage alone is meaningless. Both parameters must be recorded for proper assessment. Modify record sheets accordingly to record this data every ½ hour.
•  Consider external means of heating or cooling to assist with reduced or excessive slippage.