Max kilnshell temp.pdf

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MAXIMUM KILN SHELL TEMPERATURE Ricardo Mosci INTRODUCTION The maximum recommended kiln shell temperature varies by plant, by country and by kiln manufacturer, despite the fact that most kiln shells are made of low alloy carbon steel (v.g. ASTM C27). Kiln control room alarms are set in a wide range, between 400 C and 550 C. Three are the most frequent questions on the subject: 1. What is the maximum continuous shell temperature a kiln stands without permanent damage to the shell? 2. What is the maximum spot temperature on the shell to force a kiln shutdown? 3. Is it advisable to cool a hot spot with a water mist? To properly answer to questions 1 and 2, the following additional information is absolutely necessary:  Age and condition of the kiln shell.  Age of the refractory lining.  Type of refractory lining.  Distance between tires.  Proximity of the hot spot to the tires or gear.  Extension of the hot spot.  Kiln alignment conditions.  Whether the hot spot is exposed or under roof.  If exposed, is it under rain?  Presence and stability of coating on the lining.  Shell temperature on the hot spot.  The presence of shell cracks in the vicinity of the spot.

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In a snapshot, here are the reasons for so many questions. Old kilns shells have been exposed to creep for a long time and are more prone to develop fatigue cracks than newer shells. Old refractory linings are usually infiltrated with salts and less prone to develop a new coating. Dolomite products have higher tendency to form a new coating than magnesia spinel products, and pure magnesia spinel products have fewer tendencies to form coating than impure magnesia spinel products. Magnesia chrome products exhibit the same coatability as dolomite products. The longer the shell span, the less it will resist high temperatures without sagging. Therefore, longer spans have more tendency to develop permanent deformation than shorter spans. Hot spots near tires and bull gears require immediate action. These hot spots almost invariably force the kiln down. The longer the circumferential extension of the hot spot, the greater the risk of shell permanent deformation or collapse. Misaligned kilns induce localized stresses along the kiln length. If the hot spot coincides with an area of stress concentration, the shell sometimes elongates or twists beyond recovery. If the kiln shell is directly exposed to the elements and a heavy rainstorm hits the hot spot, the shell may develop cracks under sudden quenching. Sometimes the brick results severely crushed in the hot spot area. The presence of cracks in the vicinity of the hot spot calls for a immediate kiln shutdown to avoid shell splitting.

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THE PHYSICS OF KILN SHELLS Kiln shells are made with structural rolled steel plate, such as A.S.T.M. A 36. The properties for this type of steel are: Carbon – 0.25% Manganese – 0.80% to 1.20% Phosphorus – 0.04% Max. Sulphur – 0.05% Max. Silicon – 0.40% Copper – 0.20% Min. The mechanical properties of this type of steel at room temperature, are: Tensile Strength – 50,000 to 80,000 p.s.i. Yield Strength – 36,000 p.s.i. Min. Elongation – 20% Min. Linear Thermal Expansion Coefficient – 11.7 x 10 –6 / ºC Elastic Modulus – 207 GPa Poisson Ratio – 0.3 in the elastic range, 0.5 in the plastic range. These properties, as stated before, are measured at room temperature. What happens to the shell strength as its temperature is raised? It drops considerably, as shown in Fig. 1. It is interesting to notice that there is a gain in strength between room temperature and 200 C, followed by a sharp loss in strength as the temperature goes up. At 430 C the ultimate strength of the steel drops from 75,000 p.s.i. to 50,000 p.s.i., a hefty 33% loss. Some investigators report a 50% strength loss for the same temperature range.

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TENSILE STRENGTH

80 70 60

73

68

60 50

50 40

25

30 20 10 0 100

200

300

400

550

TEMPERATURE CELSIUS

KILN SHELL DESIGN CHARACTERISTICS From a purely structural approach, the kiln shell may be compared to a continuous “O” beam, support in several points along its axis, and subject to a uniform load comprised of its own weight, the load weight and the refractory weight. Through finite elemental analysis the bending momentum and stress on the shell can be calculated at any point between tires, at any desired temperature. Mathematical modeling has proven that sagging is not the main source of stress in a rotary kiln. In modern two-pier kilns, the shell is built purposely flexible to avoid excessive stress concentration at the rollers and tires. In these kilns brick crushing in the proximity of the shell became quite common. It is known to the industry that the kiln shell flattens under load, thus deviating from its quasi-circular shape. This type of deviation is called ovality. Even at room temperature, without any load, the cross section of the kiln is not circular. The greater the ovality, the greater the pinching stress on the steel and on the refractory lining. In order to keep the shell format under tires, the steel plate is made progressively thicker towards the centerline of the tire. The point where the thicker shell meets the normal shell is a point of great stress concentration as evidenced by frequent brick shifting at these areas.

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If excessive ovality and stiffening are bad for the kiln shell and the refractory lining, why not make thicker, more rigid shells? Because the alignment of the kiln shell is far from perfect. The imaginary axis of the kiln is not a straight line. During rainstorms, power failures, heat up and cooling, some parts of the shell develop into a crankshaft. As the kiln turns, tremendous Hertz pressure develops between rollers and tires. By resorting to relatively thin and elastic shells, kiln designers are able to divert the stress away from the tire stations. Other sources of stress concentration on the shell are misaligned rollers in the horizontal and vertical directions. The forces thus generated force the brick lining into diagonal and triangular patterns, followed by partial or total crushing. Hot spots in these areas are usually catastrophic for the kiln shell, as the lining collapses instantly. Table 1 contains some real situations encountered in U.S. kilns. Shell Diameter (mm) 3,950 5,182 3,658 5,639

Thickness Under Tire (mm) 50 75 25 100

Thickness Elsewhere (mm) 25 31 20 31

Span

Temperature

(mm) 27,700 34,440 26,000 22,631

ºC 320 450 480 360

This table indicates that the impact of a hot spot will be different for each kiln. The reader is encouraged to identify and justify the worse case scenario on the table. HOT SPOT OR RED SPOT? A hot spot is the one that gets the production manager’s attention. A red spot is the one that gets the corporate office’s attention. Hot spots are isolated areas on the kiln shell with abnormally high temperature. Hot spots are quickly detected by a shell scanner or with a portable infra-red pyrometer. They cannot be seen during the day, and they can hardly be seen at night. Therefore, based on the visible

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radiation spectrum for hot surfaces, their maximum temperature must be below 600 C. Red spots differ from hot spots in that they are visible at night. While a hot spot is just a warning, a red spot always demands some kind of action from the kiln operator. Red spots can be temporary, if caused by sudden coating detachment. If the brick is thick enough and not deeply densified with low melting salts, coating may develop again and remedy the situation. Red spots caused by lining failure are not temporary and require a kiln shutdown. Only experienced operators, with good knowledge of the residual lining thickness, can tell the difference. Unfortunately, brick drillings are not made available to kiln operators, despite the fact that it is a critical decision tool during emergencies. Red spots create a relatively small area on the shell that expands faster than the adjacent areas. Since the shell expansion is confined to a small region, the hindered expansion develops a tremendous amount of potential energy. Using the elastic modulus and the thermal expansion coefficient of carbon steel, the amount of stress developed can be calculated and compared to the ultimate strength of the steel. A red spot generates 25 kgf/cm2 for every degree of temperature difference. Assuming that the steel surrounding the red spot can absorb half of that stress, the residual stress will be 12.5 kgf/cm2. For a thermal gradient of just 200 ºC, the creep limit of the steel will be exceeded and its ultimate strength will be almost reached. From the previous analysis it becomes evident that not only the value of the temperature is important, but mostly its distribution along the kiln length and circumference. If the stress caused by kiln misalignment, ovality and distortion is added to the temperature stress, it is easy to understand how bubbles and large cracks develop on the kiln shell. It is just a matter of time, load and temperature before permanent damage occurs.

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QUESTIONS AND ANSWERS Q. What is the maximum continuous temperature a kiln shell stands without permanent damage to the shell? A. 450 ºC or 870 ºF for a structural carbon steel shell. Q. What is the maximum spot temperature on the shell to force a kiln shutdown? A. 550 ºC or 1022 ºF if the spot is permanent and persistent. If the red spot is near or under a tire or bull gear, the shutdown procedure must start immediately. Any persistent red spot covering more than 10% of the kiln circumference should follow the same previous procedure. Q. Is it acceptable practice to cool down a red spot with a water mist? A. Provided the mist is a mist, and just a mist, yes, it can be tried without serious consequences to the integrity of the shell. If properly done, the procedure can avoid a costly permanent deformation to the shell. If improperly done, the consequences to the shell can be serious. The goal of this procedure is to cool down the hot air layer that permanently envelops the kiln shell.

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Kiln shell badly damaged by heat.

Hot spot along the kiln circumference.