Kiln Mechanical behavior

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Kiln Mechanical behavior

 

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Thermal Expansion

 

 

How to evaluate Expansion?

 

Thermal Expansion effects on the shell

Shrinkage of the shell into the tyre

Under tyre shell temperature is greater than tyre average temp.

  • In normal operation
  • .. and more in start-up phase

 

High risk to shrink the shell at start-up

  • too quick preheating
  • too small cold clearance value

Consequences

  • Shell will be deformed permanently
  • Will possibly lose some bricks
  • Will probably have some problems of fixation
  • Will lead to potential shell cracks in the future

Thermal bending

 

 

Variation of Temperature along kiln

 

Due to the mass of the kiln, there is still a contact on each pier

  • .. but load is lighter at central pier and higher at the other one
  • .. and opposite after half a rotation !

If the thermal bending is very high some tyres could even not touch the rollers!

Thermal bending causes

A defect in coating can lead

  • To different temperatures on the same section
  • Then to differential expansion
  • Then to kiln thermal bending

When the kiln is not properly rotated

  • During start-up
  • During kiln stoppage

When the kiln is not rotated after one incident

  • Importance of barring drive!

Thermal bending corrective actions

Modify process parameters

  • to get proper equal coating

Barring drive always to be ready to operate

If too late, shell replacement: Figuil (Cameroon) Example

  • Production incident, no barring during 10 min.
  • Tropical rain (which cools only top of shell)
  • Kiln bent forever (tyre/roller clearance 25mm)
  • Need for cut or shell replacement

Thermal Expansion effects

Girth-gear root clearance increase

  • Girth-gear fixed on the shell
  • Example of a 3-pier 4.6 m diameter kiln with pre-heater: Root clearance increases about 6 mm

Kiln roller bearing

  • Kilns are generally rotating on roller shaft with liners
  • Few roller bearings because
  • very high loads and low rotating speed
  • expensive to design and use

Are there any limits?

  • Yes, there is a limit of pressure on a liner
  • Special mode of friction, low speed, negative influence

Kiln roller bearing liner Temperature

-Depends on friction -> related to rotation

  • Most of the torque used to lift the material inside the kiln / Very little converted to heat through the shaft/liner contact
  • Torque liner about Torque motor x (0.03 to 0.05)

-Depends on the load -> related to

  • Design. (FLS, POLYSIUS, CLE, FCB using approx. same design limit)
  • Quality of geometry and lubrication
  • Alignment of kiln tires
  • Shell axis deformation (permanent or thermal)

-Shell axis deformation (permanent or thermal)

Expansion effect on Kiln bearing liner

-When the kiln starts, tyre will heat the roller, then the shaft

-After 48 hrs, the shaft diameter will be bigger but the liner support will remain cool (water cooling)

-The expansion of the shaft will reduce the oil film thickness… and the contact will increase

Kiln bearing liner overheating cycle

Kiln bearing liner -> Emergency procedures

Best method = Oil re-circulation with cooling and cleaning

 Small units available on the market with installation on request

 In case of small problem, putting the kiln on barring can save you!

 Time to decrease shaft temperature, get proper viscosity back, come out of the vicious cycle

 Cool by air roller flank to limit kiln heat transmission to shaft

 Oil usage

 The best oil cannot solve contact pattern problems

 Very thick oil or synthetic oil cannot solve too low clearance between shaft and liner

 Check if this synthetic oil can be mixed with mineral

 Check if this synthetic oil does not chemically attack any bearing part

 put the highest viscosity you can, assuming the real viscosity will drop very quickly because of temperature (oil viscosity diagram versus temperature must be known)

Kiln bearing liner -> Musn’t

 Fan cooling: dangerous on bearing

 Housing open = Dust contamination = Be careful!

 Are you sure you are cooling the shaft or the housing? (think of expansion)

 Cool the roller flank with water

 Water on roller is dangerous

 Permanent application or heat shock leads to spalling

Use grease or molycot is forbidden  very dangerous

 can block the oil pocket, preventing any oil from entering to lubricate liner and shaft

Expansion Conclusion

-Heat is a very important parameter for kiln shell

-Attention should be always given to temperatures

  • Maximum temperature
  • But also differential

-Mechanical staff must take extreme care in case of kiln thermal bending

Mechanical crankshaft

Kiln Misalignment

-As an example on 3 piers, a mechanical bend is one tube which is deformed so the 2nd section is rotating when sections 1 and 3 are perfectly set-up in line

Wrong Kiln Alignment

Wrong alignment of shell at erection due to

 Subcontractor’s lack of know-how

 Deformed new or old shells

 Not properly chosen shells to be replaced

 

Wrong Kiln Alignment effects

 On a poorly aligned 3-station kiln, tire 2 too high by about 10 mm  Overload = 8 t/mm x 10 mm = 80 t (TC calculation)

 Permanent load supported by this tire = 80 t higher than normal

 Possible problems (depending on design safety coefficient) on

 Bearing liners overheating

 Tyre (wear, holes, crack, …)

 Rollers (wear, holes, crack, …)

 Shaft crack

 Brick failures

  • Too high load => too high ovality => problem of bricks

 Frame cracks  Shell cracks

Brick Failure

If not followed by stoppage (major cause)

 Too long run with missing bricks / Too high temperature on the shell with bricks

 Some indicators

 If the circumference of hot-spot is > 90° of the total, it becomes dangerous for the kiln shape, otherwise it will create only local deformations (but still bad)

 Axial hot-spots are less dangerous than radial

If not followed by stoppage (major cause)

 Too long run with missing bricks / Too high temperature on the shell with bricks

 Some indicators

 If the circumference of hot-spot is > 90° of the total, it becomes dangerous for the kiln shape, otherwise it will create only local deformations (but still bad)

 Axial hot-spots are less dangerous than radial

Brick Failure effect

 

 

Mechanical crankshaft without visual indications

Possible

 Since kilns are generally very flexible, you may have a crankshaft but as the kiln deforms during rotation, the deformation is not visible

 Sometimes kilns are affected by several crankshafts in opposite phases

How to check there is a crankshaft?

-By looking for

  • Periodical noise at the speed of kiln rotation
  • Periodical motor amperage fluctuation
  • Periodical deflection of roller shaft

-If you see a significant radial run-out on kiln seals or on the shell between two piers

-IMPORTANT : To detect a potential problem you must wait for 5 or 6 rotations!!!

 

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