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What To Do When A Kiln Tire Stops Migrating

One of our kiln tires has stopped migrating on the shell. What should we do?

An answer to this frequently asked question invites further questions and discussion on why tires should migrate (creep) in the first place.

There are two different designs of roller-supported rotating equipment that must be considered. First there are Type I units, those that are refractory lined, typified by kilns, calciners and reactors, etc. Secondly there are Type II units, those that are not refractory lined typified by dryers, coolers and granulators etc. Although migrating tires are very typical of Type I vessels, they are less common for Type II vessels, as we shall see.

Since the issue is usually much more critical with type I units, we'll discuss them first.

Type I units (Kilns, calciners, reactors etc.)

To fit the rotating shell with a migrating tire is to accommodate differential thermal expansion between the shell and the tire. The heat of the process will heat up the shell more than the tire. Consequently the shell will expand its diameter more than the tire. For this reason some clearance, when cold, is desirable between them so that the tire does not pinch the shell when the unit reaches operating temperature. Otherwise a pinching tire will frequently and often permanently distort the shell and cause refractories to be crushed. Costly damage, if it needs to be repaired. Since the temperature difference between the shell and tire vary due to natural variations of process conditions, purposely having a slightly larger gap is desirable. This allows some temperature excursions without risking damage. As a result this difference in diameters makes one revolution of the shell occur slightly ahead of the time it takes for the tire to make one revolution. This difference is called tire migration or creep. Its magnitude is usually relative to the difference in diameters.

When the tire stops turning, the safety margin is gone. The alarm now sounded suggests the need for understanding the reason for this condition. Quantifying the magnitude of the problem will give us the options of what to do.

Two possible reasons.

The tire may have stopped rotating for two reasons. First and most usually the shell has expanded faster than the tire, the tire-to-shell gap has disappeared and the tire locks onto the shell. Often this is caused by too rapid a kiln start-up or a rapid temperature rise of the shell associated with partial coating or refractory loss.

A second reason is that the tire is simply jammed on the shell. It may not be free to rotate due to the introduction of a foreign object or a broken piece of one of its support elements into the gap. A wedged piece of steel like this will slowly rip its way along between the bore of the tire and the shell, stopping normal tire migration for the time being. This then is not temperature related but entirely mechanical in nature.

Careful visual inspection may give evidence of which situation is at hand. But nothing is better than knowing what the usual range of tire-to-shell temperature difference is. Most kilns now have shell temperature monitors, like the "T-Scanner", from which a history of temperature differences can provide immediate assessment of the present condition. Foresight suggests that the preparation of a table of temperature differences vs. creep values will give a warning of incipient problems.

The mechanical condition is of less immediate concern. Ample and appropriate lubrication is the immediate "fix". When a shut down permits, the mechanical deficiencies can be addressed.

Loss of clearance, on the other hand, may need immediate intervention. If due to a rapid warm-up schedule, slow it down. If due to partial loss of coating, changing the location burning zone by moving it back or forward, may be possible. Loss of coating, along with partial refractory loss, may be creating a "hot spot" underneath the tire which is reason enough for an immediate kiln shut-down.

A possible third and most difficult problem.

Sometimes insufficient clearance for normal operation exists. This often happens on an older kiln with a section replacement or filler bar (chair pads, or shell pads) repair, when the work has resulted in a fit that's too tight. Such a repair is expensive to reverse. It may be possible to cope with this for awhile, maybe even long enough for natural wear to create a passable situation.

These thermally-caused tire lock-ups may be addressed by the following:

  • pointing shell cooling fans to the area immediately to each side of the shell adjacent to the tire
  • placing "rosebud" torches to heat up the tire
  • building an enclosure, similar to a gear guard, to contain heat and reduce the normal tire-to-shell temperature difference
  • any other methods that reduce the shell-to-tire temperature difference

A snug tire, as long as it does not pinch the shell, has its benefits. A migrating tire unavoidably causes a lot of wear. A tire which doesnÂ’t migrate would eliminate this wear, extending the service life of the parts considerably. Doing this by simple thermal expansion is risky, as explained, but if temperatures are monitored and controlled, it is no reason for immediate panic. New tire mounting designs using splined tires are available. They may be costly, but they certainly eliminate the problems surrounding this entire issue. They should be considered when shell section replacements involving tires are required.

Type II units. (Coolers, dryers and granulators etc.)

Unless the process vessel is a fired unit, a migrating tire design should be questioned. A fixed tire has no wear between it and the shell; a god send for any maintenance department. Unless differential thermal expansion is a real problem (without refractory it becomes a much reduced concern) there is little reason for using a migrating tire. It is not unusual however, to find designs like this.

Without the need to consider refractory, there seems little to worry about if a migrating tire stops moving. Excessive temperature, so much so that it permanently distorts the shell, is difficult to conceive. If such process temperatures were attainable, wouldn't refractory be part of the design? Tire-locking at a lower temperature may still slightly distort the shell. But since the shape of the shell isn't critical in terms of maintaining a refractory arch for example, it becomes a moot point.

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