Operation Of Vertical Roller Mill
For a stable grindingprocedure the following paraments are inportant:
1. Thevibrations of the grinding tabe..
2. The ∆P of thegrinding system.
3. Theheight of the grinding bed.
4. Thepressure of the grinding rollers.
5. Thespeed of the grinding table.
What is grinding bed?
Grinding bed is the material layer between the rollerand table.
Size in feed:5-8% roller’s diameter
Process operation parameters
High grinding pressure
1. Highmill motor current.
2. Thrustbearing pressure increases.
3. Lowmill ∆P.
4. Lowerinternal circulating load.
Low grinding pressure
1. Lowmill motor current.
2. Lowcapacity for given air flow & DP.
3. Increasedrejects level.
4. HigherDP a cross nozzle ring and classifier.
Bed height too high
1. Highmill motor current.
2. Lowmill DP.
Bed height too low
1. Lowmill motor current.
2. Highmill DP.
3. Highrejects level.
4. Lowmill output.
5. Highvibration than normal.
Feed material too dry
1. Highexit gas temperature.
2. Lowermaterial friction on grinding table.
3. Lowdrive power.
4. Low DPacross mill.
Feed material too net
1. Lowexit gas temperature.
2. Materialagglomeration on the grinding bed.
3. Highdrive power.
4. HighDP across mill.
5. Highvibration than normal.
High internal circulatingload
1. HighD.P.
2. Productfineness too high.
3. Classifierchocked.
4. Lowgrinding pressure.
5. Lowair flow.
Parameters used for processcontrol
Process measured Varibles:
1. Finenessor residue.
2. Millmotor speed.
3. Millair flow.
4. Milldifferential pressure.
5. Temeratureafter the mill.
6. Producttransport volume.
7. Mill vibrations.
Manipulated Varibles
1. Classifierspeed.
2. Millfan speed.
3. Waterinjection.
4. Hotgas.
5. Freshair.
Process Problems
High vibration
1. Inconsistentfeed size.
2. Grindingpressure too low.
3. Damring height not uniform.
4. Lowmaterial feed with high grinding pressure.
5. HighDP across mill.
Low output
1. Feedsize too coarse.
2. Grindingpressure too low.
3. Damring height too low.
4. Worn-outliners.
5. Producttoo fine.
6. Highsystem pressure drop.
High mill DP
1. Lowgrinding pressure.
2. Feedrate too high.
3. Damring hight too low.
4. Highinternal circulation load.
5. Producttoo fine.
Product too coarse
1. Lowclassifer speed.
2. Highair flow through the mill.
3. Airvelocity at nozzle ring is too high.
How to control mill vibration
1. Knowthe vibration limit and optimum grinding bed thickness for your mill.
2. Havereliable, calibrated bed depth monitors.
3. Adjustthe mill dam ring height to compensate for table wear.
1. Applystable water spray to control bed, normally fed to the feed size of eachroller, check flow consistency on each spray.
2. Knowthe critical level of gas flow and pressure drop for you mill.
3. Maintainstable mill gas flow to keep internal circulation consistent apply automaticcontrol loop.
4. Avoidmagnetic and non-magentic.