Zhang Jie and Gavin Ding, Nanjing Zhongcai Cement Spare Parts Co., China, explain how the sealing device is one of the most important components of a rotary kiln.
Kiln outlet sealing technology
The sealing device is one of the most important components of a rotary kiln and plays a critical role in its normal process operation. As the kiln is operated under negative pressure, an unsatisfactory seal between the kiln outlet and the kiln outlet hood will cause external air to enter the kiln and spoil the proper percentage of kiln material volume, burning volume, and air volume. As a result, secondary air volume from the cooler to the kiln is reduced, secondary air temperature is decreased and, subsequently, heat consumption increases. Under positive pressure, dust from the kiln outlet hood will pass through the poorly conditioned sealing and be released from the hood. Therefore, the site environment around Kiln Tyre I will be heavily polluted. Meanwhile, the heavy dust that falls on the supporting rollers accelerates the abnormal wearing of kiln tyres and supporting rollers, as well as shortening the items’ working life.
The sealing is required to provide excellent sealing performance, minimum air leakage, and to be adaptable to the movements and deformations of the shell.
As the kiln hood is stationary, while the kiln moves, the shell is elongated when the temperature increases. During daily operation, the shell can thus deform along its hanging extension section and at the end surface.
Considering the contact and wear between the movable parts and stationary parts in the sealing device, as well as the heavy dust, high temperature, and bad lubrication conditions, the material used for the sealing parts must be of high wear resistance and heat resistance.
Operating principal and structure of a new type of forward and reverse combination sealing
The kiln outlet sealing consists of the following:
- Dust collecting cover I.
- Dust collecting cover II.
- Sealing ring.
- Forward sealing.
- Reverse sealing.
- Cooling sleeve.
- Fixing device.
The air that contains dust passes through the kiln outlet guard plate and is released along the outer circle of the air cooling sleeve under positive pressure at the kiln outlet. The air is blocked by the castable protected sealing ring; 90% of the particles lose their kinetic energy and are collected by the dust cover; particles go through the dust hopper and enter into the grate cooler. The remaining 10% will move forward against the reserve sealing and be induced by the angle of the installed plate. The air flow is then diverted by the reserve sealing plate and forms a swirl in the space between the reverse sealing, dust cover, and sealing ring, where materials continuously clash against each other, lose kinetic energy, and fall into the dust collecting cover. The materials also enter the grate cooler. The dust can only just pass through the two sealings to enter the forward sealing inside. The forward sealing consists of two layers of sealing discs with a layer of carbon silicon aluminum fabric in the middle. This is a last and safe sealing defense and solves the air leakage problem.
The components of the sealing include the following:
- Labyrinth sealing.
- Reverse sealing.
- Air cooling sleeve.
- Steel wire rope counterweight.
- Forward combination sealing.
- Forward sealing cone.
- Reverse sealing cone.
The forward combination sealing contains upper and lower sealing discs with a layer of carbon silicon aluminum fabric in the middle.
The sealing angle between the reverse sealing disc and the air-cooling sleeve is provided by hydromechanicsal analysis and calculation and fluid analysis software. Such an angle enables the sealing disc to change the direction of the internal fluid and dust at the kiln outlet and provides a better sealing effect. This prevents the internal dust flow from washing the sealing disc away and increases the working life of the sealing disc. By analysis based on engineering thermodynamics, the materials selected are of strong wear resistance, heat resistance, and high elasticity, which ensures a longer working life and better sealing performance for the sealing discs.
The main function of the labyrinth sealing is to block the high pressure draught. This means it blocks the direct impact of the high pressure draught on the reverse sealing, decreasing the pressure on the disc and enhancing the sealing effect and working life.
The forward combination sealing contains upper and lower sealing discs with a layer of carbon silicon aluminum fabric in the middle. The lower sealing disc handles wear and heat resistance. while the middle layer of carbon silicon aluminum fabric seals and stops the air from entering the kiln outlet. The upper sealing disc protects the carbon silicon aluminum fabric and the whole sealing device.
The air-cooling sleeve and cone body seals the casing. The structure is also manufactured with wear resistance and heat resistance in mind. The inner circle surface of the air-cooling sleeve is welded with Z-type radiation fins. Cool air is blown from the outer fan to reduce the temperature at the the kiln outlet and protect it from high temperatures.
Steel wire rope counter-weights provide joint force to the forward combination sealing, which sticks completely to the air-cooling sleeve. By adjusting the number of counterweights after installation, the forward sealing and air cooling sleeve stick to each other at a critical state to ensure a good seal. Meanwhile, the friction force is almost nil and the working life of the forward sealing is increased.
Kiln inlet sealing technology
As the kiln is operated under negative pressure, and the inlet negative pressure is higher than outlet negative pressure, the kiln inlet is liable to take in external air that is difficult to expel from the kiln. This leads to the incomplete combustion of the fuel, and increases fuel consumption and load on the kiln inlet fan. In the case of the kiln downdraft, the powdery dust will overflow at the join between the flue chamber and kiln inlet, and cause pollution in the plant.
As the kiln inlet flue chamber is stationary, while the kiln moves, the shell is elongated as temperature increases. As the kiln moves, the shell can expand and move up and down, resulting in deformation to its hanging extension section and at the end surface. In cases of serious deformation, the flue chamber can be damaged. Therefore, the sealing device must be adaptable to movement and deformation.
Considering the contact and wearing between the movable parts and stationary parts in the sealing device, as well as the heavy dust, high temperature, and poor lubrication condition, the required material for the sealing parts must be of high wear resistance and heat resistance.
Operating principal of heavy hammer pressing end surface contact sealing
Heavy hammer pressing end surface contact sealing includes axial sealing and radial sealing. The basic principal is as follows. A radial seal ring hangs on the flue chamber using the boom: it moves but does not rotate as the kiln moves up and down. The middle of the radial seal ring and flue chamber sleeve is sealed by asbestos packing that winds around the sleeve and is pressed tightly by press plate. The movable friction ring of the axial sealing that connects to the hopper and fixed friction ring on the radial support plate forms an end surface sealing. The two friction rings are kept in contact by the pulling force produced by the heavy hammer that is evenly set on the radial sealing. The lubrication system between the two friction rings can inject lubrication to weaken the abrasion and strengthen its sealing effect.
Structural features of heavy hammer pressing end surface contact sealing
The sealing has an extensive contact area, adaptable to the movement of the kiln inlet end.
Eight evenly distributed heavy hammers are placed in a circle where the stress is equal, to avoid partial cracking.
Both sides of the boom include a separated-type structure with spiral buckles added in the middle to greatly modify the axiality of the fixed seal ring and shell.
Both ends of the boom apply joint bearings that remedy the tilting inaccuracy of the support plate after installation, and make it stress evenly in a circular direction.
The material of the friction rings at both ends is spheroidal graphite cast iron, RQTAL4si4. Performance data is shown in Table 1.
Sufficient clearance between radial seal ring and fixed sleeve is retained; it avoids a collision between the seal ring and the fixed sleeve, and makes it adaptable to the movement of the kiln inlet end.
To prevent excessive abrasion of the axial seal ring, a one-shot lubricating system is adopted in the seal structure. A grease pump is equipped with No.2 complex calcium lubricating grease, SH/T0370-1995. The actual grease supply is adjustable in accordance with the actual application.
Read the article online at: https://www.worldcement.com/special-reports/20072017/seal-of-approval/