PREPARATION AND START OF LOESCHE MILL
I.
PREPARATION BEFORE START:
Before the mill is started it is of course a precondition that all measuring instruments are correctly calibrated and all interlocking is functioning through signal and/or sequence tests. In addition all the alarm limits for temperature, pressure, gas, oil and cooling water flows must be adjusted to the prescribed values. Following points must be checked before start of the mill:
1) Oil level in the mill main gear box and separator gearbox. 2) Oil level in the hydraulic skids for the roller system and gear oil supply system.
3) The filters for oil , gear boxes and hydraulics , are all to be checked and cleaned , if necessary.
4) All stop valves in gear box lubricating station , roller tensioning system and roller bearings lubricating station are in their correct position.
5) Hot Gas Furnace, diesel tank, fresh air flaps, flexible hoses & control panel indications.
6) All feed hoppers contain a suitable quantity of raw materials. 7) All manholes and inspection doors are closed properly. 8) Compressed air for bag filter purging & for reject valve is present.
9) Check if bypass dampers 275.07 and 275.05 are closed. 10) The composition of the mill feed is chosen in accordance with the instructions given by the laboratory. Before the preheating of the mill begins the following auxiliary machines are started: •
Pump for hydraulic Roller system
•
Gear oil pumps for mill main gear box lubrication
II.
PREHEATING THE MILL:
When the cold mill has to be started it is necessary to preheat the mill for one ~ two hours to prevent condensation in the system. In order to protect the roller bearings and sealing it is necessary that the preheating of the mill circuit is done at a suitably low temperature. Heating up with Hot Gas Furnace: 1. The raw meal transport to the storage silo and the bag filter for dedusting the silo are started. 2. Start of the dust transport and electrostatic precipitator (ESP) except the ESP high voltage (High Tension). 3. ESP Fan is started. 4. The rotor of the separator is turning in minimum speed, in order to avoid unilateral deformation of these components. 5. ESP Fan damper is opened slightly. The mill outlet damper 275.11 is opened so much that the mill inlet pressure is : - 30 mm H2O. 6. Adjust the fresh air dampers and the system of firing for the Hot Gas Furnace and start it. 7. By the adjustments of mill outlet damper 275.11 and ESP fan damper as well as the oil amount to the Hot Gas Furnace, hot air is drawn through the mill a span of 2 hours. 8. The temperature after the mill must be in the range of 100 ~120°C and the mill inlet pressure should be kept at - 30 mm H2O.
9. If necessary the mill outlet temperature may be lowered through opening of the cold air damper.
III.
START OF THE MILL with Hot Gas Furnace:
Just before the start of the mill the gas dampers are opened in such a way that the differential pressure is at least 900 mm H2O. The fuel amount to the Hot Gas Furnace is slightly increased in order to maintain the mill outlet temperature in the range between 100~ 120 ºC.
The rollers are lifted up from the grinding table. Pre-charge the mill feeding system. The starting up of mill main drive rotating with lifted rollers.
and the grinding table is now
Next the mill feed is started with approximately 50 -60% of the nominal production; 0.5 * 130 ~ 65 or 0,6 * 130 ~ 75 ton per hour and rollers will be lowered. Set point for hydraulic system ~ 1500 psi. The amount of air passing through the mill is adjusted by means of gas dampers and the mill differential pressure (~550 mm H2O) to avoid fall through of material in the nozzle ring. After working pressure adjustment the draught in the mill and the amount of fuel to the Hot Gas Furnace are readjusted (∆ p = 70 mm H2O, Tout = 90 ~ 100 ºC), as the quantity of fresh feed increases the working pressure increases in a range between 1800 – 2000 psi. To prevent damage to the roller mill, the working pressure must not exceed 2300 psi . When this pressure is reached, an alarm message is sent and when the pressure reaches 2300 psi, the mill is shut down automatically. The power consumption of the mill motor will go up as the grinding rollers begin to compact the material on the grinding table. At the same time the vibrations in the mill becomes more pronounced. The mill outlet temperature will decrease. If necessary, the fuel amount to the Hot Gas Furnace is readjusted or the manual cold air dampers closed. The desired operating temperature after the mill is kept constant by:
• Stepwise increase of mill fresh feed • The working pressure adjustment • Quantity of air passing through the mill The speed of the separator is adjusted within the range between 30 ~ 40 % from the maximum rotor speed (according to the fineness of the product and the differential pressure within the mill system).
IV.
STOP OF THE MILL:
There are two types of mill stops: 1. The Provoked stops ; equipment faults , power failure, instruments malfunctioning & production related interruptions. 2. Planned stops; Full silos, preventive maintenance, Lack of raw materials. Only the planned stops will be treated in the following. Before a planned stop of the mill, the fresh feed to the mill is reduced to 60 % in order to reduce the amount of material over the grinding table (inside the mill). The grinding layer thickness will go down and the mill vibration will increase. The Hot Gas Furnace should be stopped. The fresh feed to the mill, rollers lifted up, and main drive will stop. Finally the raw meal transport to the homogenizing silo is stopped. The fresh air damper and the fresh air fan for the Hot Gas Furnace must continue to run for at least 1 hour in order to cool down the Hot Gas Furnace and gas ducts. In case of long lasting stop of the mill department the following machines must be stopped as well: 1. The oil pumps for the hydraulic system for the grinding rollers 2. The gear oil pumps for mill gear box lubrication 3. Cooling water for gear box lubrication
V.
NORMAL MILL OPERATION:
In order to achieve a stable mill operation the mill fresh feed must be as homogeneous as possible. That means it is very important that all weighing feeders are running continuously and the stop of a weigh-feeder for a main component will mean stop of the mill.
The fineness of the product is normally determined by the sieve residue on the 90 microns sieve; 12 ~ 15 %. If the fineness deviates from the desired fineness the setting of the speed of separator rotor; • When the speed of the separator rotor increased the mill will produce a finer product • When the speed of the separator decreased the mill will produce a more coarse product Finally the residual moisture content in the product is determined. VI.
MILL VIBRATION:
During the operation the mill vibrations are measured continuously by means of vibrometer which is placed on the mill gear box. Whenever the vibrations exceed a preset upper limit, the mill motor trips automatically in order not to cause mechanical damage of the grinding parts, the hydraulic system and the main gear box.
The reason for the increased vibration level is normally caused by too little material in the mill. Vibrations may, however, also be caused by excessive quantities of Fine materials in the mill or in the fresh mill feed. When there is TOO little material in the mill resulting in a thin layer of material between the rollers and the grinding table and a LOWER differential pressure over the nozzle ring and the grinding table, the mill fresh feed must be increased until the mill operation becomes normal again. If the vibration level increases very rapid the grinding rollers may be lifted up away from the table for a short time.
When there is too much material circulating in the mill, the fresh feed to the mill must be reduced. By decreasing the separator rotor speed for a short time the level of material in the mill will be reduced.
VII.
OPERATIONAL PARAMETERS AND THEIR CONTROL:
To ensure sufficient and reliable operational parameters for the mill operator, it is necessary to equip the control system with adequate instruments. A. INSTRUMENTATION: By instrumentation is meant both the equipment necessary to measure and register different operational parameters and the equipment used to change those parameters; Valves (regulating, solenoids, proportional), dampers. This means that there are three different processes: • Measuring • Registration • Action Measuring should be made at the most adequate point to give a reliable , stable and correct signal. The signal passes to the control panel for registration in some form or another, often after having been transformed. It may be registered in an instrument giving the actual value, in continuous graphical form and in accumulative form, or in case of a system, the three forms are computerized.
The following automation:
action
may
have
three
different
degrees
of
• Manual action in the field or site • Manual action from the CCR
• Automatic action from CCR The following figure overleaf shows the instruments considered necessary for good process control of mill. The list also indicates the type of registration and the type of control considered necessary.
The list do not include instruments and equipment necessary for machine safety, as for example temperatures in grinding rollers bearings and main gear unit , separator gear unit , oil levels -----etc.
List of Instruments Necessary for Mill
Description parameters
of
Registration
Control Manual
Act. value Raw mix Feeder
Cont. graph
Accumulated value
Field
Auto.
CCR
Weighing
tph
ton
Yes
Yes
Yes
High grate Weighing Feeder
tph
ton
Yes
Yes
Yes
Sand
tph
ton
Yes
Yes
Yes
Total Feed
tph
tph
ton
Yes
Yes
Yes
Power consumption mill motor
KW
KW
KWh
Power ESP fan
KW
Yes, mechanicall y preadjusted
Yes
No
consumption
Power consumption separator drive
amps
Power consumption mill auxiliaries
amps
Grinding pressure
Mill Vibration
psi
mm/s
KWh
Temperature mill inlet
ْ
C
ْ
C
Yes
Temperature outlet
ْ
C
ْ
C
Yes
Pressure mill inlet
mm H2 O
mm H2 O
Yes
Differential Pressure
mm H2 O
mm H2 O
Yes
Pressure mill outlet
mm H2 O
mm H2 O
Yes
Separator speed
rpm
No
Yes
Gas
%
Yes
Yes
No
outlet
%
Yes
Yes
No
Position damper Position damper
Kiln
mill
mill
B. OPERATIONAL PARAMETERS: The operational parameters are divided into: 1. Variables or parameters to be controlled 2. Control parameters The variables are the parameters that must be kept as constant as possible at an optimum values. Control parameters are the parameters which are adjusted or altered to keep the variables at their optimum values. Auxiliary parameters are the parameters that does not influence the operation directly but which help in the form of safety or statistics. VARIABLES 1) Differential Pressure; ∆ p The differential pressure is a measure of the conditions inside the mill. It is showing the relation between the air volume passing inside the mill and the amount of grinding material.
Dropping and rising values of differential pressure indicates that the relationship is changing. The differential pressure is measured at a point in the mill inlet gas duct and at a point above the grinding rollers in the outlet gas duct before the ESP. It is registered as actual value in mm H2O. The control parameters for ∆ p are: o The fresh feed to the mill o The air volume passing inside the mill o The speed of the separator (rpm)
o The grinding pressure (psi) 2) Mill Vibration:
The mill vibrations are measured by means of a vibrometer placed on the mill gear box and is registered as actual value in mm/s, The control parameters for the mill vibration are o The fresh feed to the mill o The speed of the separator in rpm o The grinding pressure in psi o The gas flow through the mill 5) Power consumption of the mill main drive:
The power consumption of the mill motor is registered as actual value in KW. The control parameters affecting the mill motor KW are: o
The fresh feed to the mill
o The speed of the separator in rpm o The grinding pressure in psi o The gas flow through the mill
6) Mill Inlet Temperature:
The mill inlet temperature is registered as actual value in ºC . The mill inlet temperature depends on the construction of the gas ducts; and whether the gases derive from the Hot Gas Furnace or from the kiln exit gases; Tinlet = ~ 250 °C. Generally, the mill inlet temperature is adjusted by the temperature of the Hot Gas Furnace or in the case of kiln exhaust gases by changing the position of the gate between the kiln and mill. In case there is false air, this will have much influence on the inlet temperature. 7) Mill Outlet Temperature:
The mill outlet temperature is registered as actual value in ºC. The mill outlet temperature depends on both the moisture content of the mill fresh feed and volume of the hot gases passing through the mill. It is also influenced by the actual quantity of false air. The control parameters for mill outlet gas temperature: o Adjustments of the quantity and temperature of the hot gases , i.e. fuel valves for Hot Gas Furnace & kiln exhaust gas damper o The ESP damper o The cold air dampers for Hot Gas Furnace 8) Pressure Mill Inlet:
The pressure in the mill inlet is registered as actual value in mm H2O. The pressure must be kept slightly negative between -30 ~ 50, mm H2O in order to reduce the amount of false air. The control parameter for the mill inlet pressure is the adjustment of the ESP fan damper.
CONTROL PARAMETERS: 1) Mill Fresh Feed:
The total mill feed is registered as actual value in tph Adjustments of the mill feed have direct influence on the o Mill differential pressure o Mill motor power consumption o Mill vibrations o Mill outlet gas temperature Identically the total fresh feed influences parameters in the grinding and drying process,
practically
all
2) Grinding pressure: The grinding pressure is registered as actual value in psi, Adjustments of the grinding pressure have major influence on the mill motor power consumption, the mill Vibrations, the grinding layer thickness, the mill differential pressure and the mill outlet pressure. 3) Separator Speed: The speed of the separator is registered as actual value in rpm. Changes in the separator speed first of all imply a change in the fineness of the product and secondly it will influence the mill differential pressure, the mill vibrations , the grinding layer thickness , the mill motor power consumption and the mill outlet pressure. 4) Position of the Kiln Gas Damper
Adjustments of the kiln gas damper will result in changes of the amount of hot gases to the mill. At the same time it will change the mill inlet and outlet temperatures, the mill inlet pressure and indirectly the mill differential pressure.
5) Position of the ESP fan damper:
Adjustments of the damper will change the volume of hot gases passing through the mill. Changes of damper position have direct influence on inlet and outlet pressure and also inlet and outlet temperatures as well as the mill differential pressure. 6) Position of the By-pass damper:
Adjustment of the by-pass damper will change the gas flow through the mill and have a major influence on the mill differential pressure, grinding layer thickness , mill vibrations and minor influence on the mill inlet and outlet temperatures.
AUXILIARY PARAMETERS: 1) Feed of mix bed , limestone , sand:
Said feed parameters are registered in the actual value in tph. The parameters are important for quality control, and for mill operation. 2) Power consumption of separator and mill auxiliaries:
The power consumption of the separator and mill auxiliaries; bucket elevator, rotary blowers, screw pumps, rotary air lock feeders ------etc, are registered as actual value in amp. Said parameters are mostly used as an indication of the load of the mentioned machines. 3) Gas Temperature in ESP Outlet:
Said temperature is registered as actual value in ºC. Heating up of the electrostatic precipitator is used during starting of the mill circuit to avoid the risk of condensation in the system, particularly in the ESP
CHANGES OF PARAMETERS:
The mill operator can change or adjust one of the control parameters:
following
1) The amount of total fresh feed to the mill 2) The grinding pressure 3) The speed of the separator 4) The position of the ESP fan damper 5) The position of the by-pass damper
Direct Action Control parameters INCREASE
Indirect Reaction Controlled Parameters or the VARIABLES Bed Thickness
Differential pressure ,
Mill Vibrations
Mill Power consumption
Mill Outlet Temp.
Product Fineness
Prod. Capacity
Total Mill Feed Rate
+
+
_
+
_
+
+
Grinding Pressure
_
_
+
+
+
+
Separator Speed
+
+
-
+
+
-
ESP fan Damper
_
+
+
_
+
+
+
By-pass Damper
_
+
+
_
_
+
+
↔
SYMBOLS: + : Increase
↔ : Unchanged
- : Decrease
( ) : Small change
The effects of such changes / adjustments, provided that only one parameter is changed or adjusted and that the remaining parameters are kept constant.
Presuming the mill is in normal steady operation the consequences to the mill characteristics resulting from changes of the directly accessible control parameters can be described in a simplified way. The principle is that adjustment of the directly accessible action control parameters lead to a change of the indirect reaction parameters or the variables. The operator can only control the variables by regulating the control or action parameters.
