Operating Principles of VCM

The following points are concerning a correct operation, product quality, safety as well as economical aspects:

1. Material Feed

• feed mass flow is measured by belt conveyor weighing cell 
• must be continuously, but variable (speed of the feeding belt is controlled)
• grain size must be according to layout (not to big/fine material)
• temperature of feeding material should be constant 
• fast variation / fluctuation cause a disturbance of the operation (vibration)
• pieces of metal must be separated before mill, otherwise they cause a damage of parts inside the mill

2. Temperature mill outlet

• is measured directly behind classifier, is controlled (must be constant)
• is the indication for the drying process in the mill
• to low temperature: insufficient drying - clogging in the cement transport system
• to high temperature: damages of roller & classifier bearings and sealing
• HH, LL limit, fixed during commisioning cause the stop of the feed automatically

3. Temperature mill inlet

• is measured on mill inlet ducts
• should not exceed a certain value (especially during heating up)

4. Mill gas flow

• is measured in the clean duct behind mill filter / mill fan and controller
• should be kept constant independent of fluctuation in pressure or temperature of the system
• quantity will be adjusted during commisioning (depends on the kind, quantity and quality of production)
• gas flow is combination of : 
• circulation air
• fresh air
• hot gas
• evaporated water
• false air
• too low gas flow: 
• accumulation of the material in the mill (product finer)
• increasing of reject amount
• operation disturbance
• decreasing of the feeding is necessary
• too high gas flow 
• more material cas pass the classifier (product coarser)
• high wear
• no economical operation

5. Mill inlet pressure

• static pressure is measured in both mill inlet gas duct
• need to be kept constant (controller by position damper "R" or "S"
• negative pressure should be maintained in all operation conditions
• too high pressure (0):
• dangerous (getting overpressure)
• disturbance of the gas flow + grinding pressure
• too high negative pressure: increasing the power consumption of the mill fan 

6. Mill differential pressure 

• static pressure before and behind mill is measured separately, but 1 pipe each goes to another trasmitter for dp (or dp will calculated)
• indicates the present charging of the mill
• depends on:
• feed mass flow
• gas flow
• grinding pressure
• classifier speed 
• dp should kept constant between min and max
• too low dp:
• part load condition of the mill
• uneconomical operation condition
• increasing of  feed rate possible
• too high dp:
• check feed rate, mill motor power consumption
• grinding pressure , height of grinding bed
• reducing of feed rate necessary
• HH - stop feeding, start feed again after archieve normal value

7. Grinding pressure

• is generated by the oil pump in the hydraulic system (HSLM)
• will be transferred into the grinding bed via hydraulic cylinder /rocker arm/ roller
• grinding pressure and nitrogen pressure in the accumulator must be monitored (even changing the nitrogen pressure)
• is decisive for the efficiency of the process 
• too low pressure:
• lower mill output
• coarser product
• increase of mill dp
• decrease of feeding rate
• too high pressure:
• higher wear
• rough mill operation 
• uneconomical operation
• pressure drop in the hydraulic system below a certain limit causes a mill stop

8. Product fineness

• is measured in blaine (cm2/gr), range about 3500....6000 blaine
• is influenced by:
• classifier speed (higher speed - finer product)
• gas flow
• grinding pressure
• hourly analysis is necessary to kept product quality (adjustment classifier speed if necessary)

9. Mill vibration

• is measured constanly as velocity mm/s in horizontal direection of mill body
• HH limit is set during commisioning , in case exceeding HH limit the mill motor will be stopped to avoid mechanical damages
• reason for high vibration level is an imperfect operation:
• Grinding bed to low with to high grinding pressure 
• no stable feed rate  to the mill (e.g. 1 component missing)
• feeding material is to fine (grinding pressure need to be reduced)