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Solution for operating and commissioning of vertical mill
2011-07-22
Solution for operating and commissioning of large-scale vertical mill
 
Abstracts [Jiangsu Pengfei Group Co., Ltd supply complete set of equipments and excellent after sell service of the new type dry process cement rotary kiln production line with the daily capacity less than 8000ton, compound fertilizer plant with annual capacity of 300,000ton, electrical plant, metallurgical enterprise. Main product is rotary kiln, vertical mill, tube mill, roller mill, drying equipment, crushing equipment, dust-collecting equipment, conveying & lifting equipment and cooling equipment, etc.]
 
The commissioning & operating of large-scale vertical mill means from trial-commissioning, normal operating, full process of maintenance of normal equipment. It is a very complicated system project.
 
1.      Working principle
 
Vertical mill is one kind of grinding machinery utilizing material bed crushing principle to grinding material; it is one kind of full air swept type mill, inlet material will fall into spray ring via disc, the high speed air near to this location will blow them, metal and heavy iron will fall into the spray ring and then they will be discharged. Fine powder zone to upper part of vertical mill, separating via separator, finished product will enter into dust collector complying with air to collect them, rough powder will circulate back. Rough powder and rough particle will be thrown up, complying with the decreasing of air speed, it loses the support, subsides on the surface of disc and they will enter into the new circle after entering into milling rail. In multi-circle, the heating transmission between particle and gas will make the water evaporation. Thus, MPS vertical mill has the ability of grinding, conveying, separating, drying and iron separating, etc.
 
2. Ventilation inside of mill and inlet & outlet temperature control
 
2.1 The source of inlet air and matching
Inlet hot air utilizes the waste air of rotary kiln system, only the primary utilizes hot air from air heating furnace, in order for adjust air temperature and saving energy, it could also blend cooling air and circulating air.
 
Utilizes hot air furnace supplying to hot air process system, in order saving energy, blend 20%-50% circulating air via moisture condition of material. Utilizes the waste-gas of pre-calcining kiln as the source of hot air system, expect waste gas could fully enter into mill. If there is margin, the waste gas will be exhausted into dust collector via tubes. If all the waste gas are fed into mill which is not enough, it could confirm the blending cooling air or circulating air, via the temperature of inlet mill of waste gas.
2.2 Air volume, air speed and air temperature control
 
a. The principle of air volume choice
Dust concentration of outlet gas should be in the range of 550-750g/m3, normally it should be lower than 700g/m3;
Air speed of outlet pipelines of mill normally should be larger than 20m/s and it should avoid to be set up in horizontal;
The standard air speed of spray ring is 90m/s, max. fluctuation range should be 70%-105%;
When the grind ability of material is not good and the output of mill is low, meanwhile the outlet air volume is suitable and the air speed of spray ring is very low, iron plate should be used to ward the hole of spray ring after roller to reduce the ventilation area and improve air speed.
Allow adjusting air volume inside the range of 75%-105% according to the condition of vertical mill, but the serial system of kiln and mill should not affect the gas exhaustion of kiln.
 
b. The control rule of air temperature
Outlet air temperature of raw mill should not exceed 120 centi-degree, normally it should be controlled inside 90 ± 5 centi-degree, or the soft connection will be damaged and the division of cyclone could be stop for expansion.
In the hot air furnace system supplying hot air, only the moisture of outlet material should meet the requirements, inlet air temperature of dust collector is over 16 centi-degree above the dew point, inlet and outlet air temperature could decrease properly to save energy, normally this reduction should be controlled to be less than 200 centi-degree.
When drying the mill, inlet air temperature couldn’t exceed 200 centi-degree to avoid that lubrication oil goes bad inside of roller.
 
2.3        Avoid air leakage of system
 
Air leakage of system means the air leakage of main body of vertical mill, outlet pipelines of mill and dust collector. Under the same total air volume, air leakage of system will reduce the air speed of spray ring and cause the serious remuage. The reduction of outlet air speed cause the lower output of finished product, higher circular loading and higher pressure difference. As the vicious circle and reduction of total air volume could easy cause the full grinding and vibration and stop. And it also could cause the insufficient conveying capacity inside of mill and the output reduction. Otherwise, it could also reduce the air temperature of dust collector which will easy for dewing.
If in order for keeping the air speed of spray ring, updating the ventilation will add the load of fan and dust collector and this will waste power. Meanwhile it also could be limited by the capacity of fan and dust collector. Thus air leakage of system brings nothing but hard thus this should be worked out. According to the requirements of Germany, the air leakage of MPS vertical mill should be less than 4%, while according to our known, air line should be designed with the air leakage less than 10%, and thus air leakage of system is not allowed to exceed 10%.
 
3. Choice between several kinds of parameters
 
3.1        Choice on tensioning force
 
Grinding ability of vertical mill is mainly comes from the hydraulic tensioning device. Under normal condition, the choice on tensing force is related to the characters of material and material layer thickness on disc, as vertical mill is material bed grinding, the larger the extrusion force is, the higher the crushing degree is, thus the harder material needs higher tensioning force; in the same way, the more thicker material layer needs higher tensioning force. Or the effect will be bad; normally the thickness of material layer should be controlled inside of 70-85mm.
For the material with good grind ability, too large tensioning force is a kind of waste, under thin material layer, it could cause vibration meanwhile for the material with bas grind ability, the tensioning force should be large, thinner material layer could gain better grinding effect. The choice on tensioning force depends on the current of main motor for mill. Under normal working condition, it is not allowed to exceed the rated current (143A), or the tensioning force should be reduced, while if the output is 190t/h, roller pressure should be controlled in the range of 150-175ber.
 
3.2        Choice on rotating speed of separator
 
Main factor affecting the fineness of product is the rotating speed of separator and the on site air speed. Under the same rotating speed of separator, the more the air speed is, the rougher the fineness of product, while under the same air speed, the faster the rotating speed of separator is, the more the gained centrifugal force of the particle is, the less the passed particle is and the finer the fineness of product is. Under normal condition, outlet air volume is stable, the change of the on site air speed is small. Thus the main method for controlling the fineness of product is to control the rotating speed of separator. Normally the grain size of product of vertical mill is even and it should be controlled in reasonable range which is 10% residue with 0.08mm screen and this could meet the requirements on fineness of raw meal for rotary kiln, as too finer could reduce the output, waste the energy meanwhile it add the circular loading inside of mill and this could cause pressure difference which is hard to control.
 
3.3        The choice on the thickness of material layer
 
Vertical mill is material bed grinding equipment, under the same equipment; grinding effect depends on the grind ability of material, tensioning force and the material quantity bearing this extruding force.
The adjusting range of tensioning force is limited, if material is hard to grind, power consumption for each unit of surface area is large, meanwhile the material layer is thicker causing the material quantity absorbing these power is increasing and this will cause the rough powder increasing and the requested fine powder decreasing, thus the output is lower, power consumption is higher, circular loading is larger and the higher pressure difference is hard to control, all of this will worsen the working condition. Thus when the material is hard to grind, the thickness of material layer should be less to add the ratio of qualified particle in extruded material. In contrary, if the material is easy to grind, material layer could be thicker and the qualified particle is also abundant, thus adjust the material layer to be thicker and the output could be higher accordingly. Or it will cause over-grinding and energy wasting meanwhile under normal condition, the thickness of material layer should be controlled in the range of 70-85mm.
 
4.Normal problem during the operation
 
4.1        Vibration of mill
 
Under normal operation, vertical mill is very stable, vibration will be 1-1.25mm/s, but if the adjustment is bad which will cause vibration, the amplitude of vibration exceed 3.5mm/s the system will alarm. Thus, during the commissioning, the main problem will be vibration. While the main reason for vibration is:
If there is metal part entering into disc, it will cause vibration.
If there is no material liner on grinding table, direct contact between roller and grinding table will cause vibration. The reason for no material liner is:
1st, discharging quantity. The discharging quantity of vertical mill should adopt the capacity of vertical mill, when discharging quantity is lower than the output of vertical mill, material layer will become thinner gradually, when the thickness material layer reaches specific value, under the function of pull force and its weight, it will appear the direct contact between roller and grinding table which will cause vibration.
2nd, Hardness of material is low and friability is good. When the material has good friability, low hardness and high pull force, even there is a specific material layer, instant empty pressing could cause vibration.
3rd, low retention ring. When material has good grind ability and friability meanwhile the retention ring is low, it’s hard to guarantee the stable thickness of material layer, thus if material has good grind ability, retention ring should be updated accordingly.
4th, full grinding & vibration. Full grinding means after subsiding of material inside of mill could almost bury the roller.
Reasons for full grinding is: too large discharging quantity causes the circular load inside of mill increasing; too fast rotating speed of separator causes the circular load inside of mill increasing; too large circular load causes too much powder which exceed the conveying capacity of air inside of mill; air flowing inside of mill is not enough which causes big air leakage of system or improper adjustment. 
 
4.2        Concerning remuage
 
    Under normal condition, air speed of jet ring of MPS vertical mill is about 90m/s which could blow material, meanwhile allows the impurity such as metal and stone with larger density fall into scrap plate via jet ring and then they will be discharged from mill, thus few impurity will be discharged which is normal, this process is called remuage. But if the remuage is added obviously, it should be adjusted and control the working condition. Reasons for large remuage is too low air speed of jet ring. Reasons for low air speed of jet ring is:
1st, system ventilation is imbalance. Due to the mistake of air flow meter or other reasons, system ventilation largely decreases. Air speed of jet ring decreasing cause large remuage.
2nd air leakage of system is serous. Though air flow quantity of fan and air flow meter doesn’t reduce, but due to large air leakage of pipelines, cyclone of mill and dust collect, air speed of jet ring decreases which causes serious remuage.
3rd Ventilation area of jet ring is too large. This phenomenon normally appears on the mill grinding material with bad grind ability, due to bas grind ability, if the same capacity should be kept, the specification of chosen vertical mill will be larger, however the output hasn’t been added, ventilation should not enlarge according to specification but  
4th Damage of sealing device inside of mill, there are sealing device between grinding table base and lower bracket body of mill and upper & lower two sealing device among Lycra poles, if these sealing devices are damaged, the air leakage will be serious which will affect the air speed of jet ring and cause the more serious remuage.
5th The gap between grinding table and jet ring is enlarged. Normally the gap is normally 5-8mm, if the iron parts which is used for adjusting the gap is wear or come off, the gap will be enlarged, hot air will pass this gap which will reduce the air speed of jet ring and cause the increasing on remuage.
 
4.3        About the controlling on pressure difference
 
Pressure difference means the static pressure difference between grinding chamber at lower section of separator and the hot air inlet during operation, this pressure difference mainly is composed of two sections, one is the part ventilation resistance caused jet ring for hot air incoming, under normal condition, it is about 2000-3000Pa; and the other is that the space between the upper of jet ring and (lower part of separator) pressure tapping point is full of hydraulic pressure of suspending material, while the sum of these two resistance form the pressure difference of grinder. Under normal operating condition, air volume outlet of mill could keep in a reasonable range of 30-50mber, the outlet air speed of jet ring normally is about 90m/s, thus the changes of local resistance of jet ring is small, the vary of pressure difference of grinder depends on the vary of hydraulic resistance inside of grinding chamber. This vary is mainly caused by the vary of the volume of suspending material, while the volume of suspending material depends on the feeding volume, the other depends on the volume of circular material volume inside grinding chamber, feeding volume is the factor to be controlled, under normal condition, it is stable, thus the vary of pressure difference directly reflects the volume of circular material inside of grinding chamber.
Under normal working condition, the pressure difference of grinder is stable, this means that the inlet material volume and outlet material volume has reached dynamic balance, circulating load is stable. Once this balance is disturbed, circulating load will change, pressure difference will change accordingly. If the vary of pressure difference couldn’t be controlled effectively, this will cause the terrible result, mainly as follows:
1st,dcreasing of pressure difference indicates that inlet material volume is less than outlet material volume, circulating load will decrease, the thickness of material bed will gradually become thinner, when it reaches the limitation, it will cause vibration and stop grinding.
 
2nd, gradually increasing on pressure difference indicates that the inlet material volume is larger than outlet material volume, circulating load gradually adds, finally it will cause the unstable material bed or serious remuage which will cause full grinding and vibration and stopping.
 
Reason of increasing on pressure difference is that the inlet material volume is larger than outlet material volume, normally it is not caused by the immoderate feeding, thus due to variable unreasonable process cause the decreasing on outlet material volume. Outlet material should be the qualified product. If the grinding efficiency of material bed is bas, it will cause the decreasing on outlet material and the circulating volume will be added; if the grinding efficiency is good but the separating efficiency is low, it will also cause the decreasing on outlet material.
 
The factors which could affect the grinding efficiency are as follows,
 
1st tightening force of hydraulic tightening device
 
Under the same condition, if the tightening force of hydraulic tightening device will become larger, positive pressure of material on material bed will be larger and the grinding effect will be better. But too high tightening force could add the probability of vibration, the current of motor will be added accordingly. Thus operator should consider the fixed value of tightening force considering the grind ability, output and fineness of material, the form, thickness and vibration of material bed, meanwhile when the output is 190t/h, roller pressure should be controlled in 150-175ber.
 
2nd thickness of material bed
 
On the premise of the fixed tightening force, different thickness of material bed, the effect of bear pressure could be different. Especially the material has different grind ability, its requested break stress will be different, thus the best value of thickness of material bed should be different, and normally it should be controlled in 70-85mm.
 
3rd, extruding surface of grinding table and roller
 
During production process, complying with the wear of grinding table and roller, the grinding effect will decrease, due to variable reasons could cause the uneven on extruding surface between grinding table and roller which will appear partial over-grinding or lack of extruding force for part, so the grinding effect will be bad.  Thus grinding table and roller liner has better to be exchanged together, or the grinding effect will decrease.
 
   4th friability of material
 
   The friability of material could greatly affect the grinding effect, the design and choice of vertical mill depends on the test parameters of material and capacity request. But please pay attention to that: the same vertical mill is used for different mineral, material with different friability of material; relevant parameters should be adjusted in time to avoid the changes of pressure difference.
   Separating effect is the main factor which affects the circulating load.  This means the situation of separating qualified material and discharging out of mill in time. Separating effect depends on the rotating speed of separator and the fluid flow formed by the air speed inside of mill. Under normal condition, the rotating speed of separator increase, the outlet product becomes finer, while under the fixed rotating speed of separator, air speed inside of mill increase, the outlet product becomes rougher. Normally these two parameters should be stable and balanced. 
 
5th, heating of mill and mill system
 
Only grinding, drying and separating process are all under good operation, the operation of whole vertical mill will be stable. In order to add the moisture content of raw material, the whole system should be preheated (continuous heating, slow preheating to avoid partial overheated) for a specific time before starting vertical mill, or, vertical mill will take more heat energy during raw material drying process under low-temperature, meanwhile finished product will be wet – thus the process of raw meal conveying into silo and raw meal extracting from raw meal silo will meet the same problem; at the same time, raw material block will become more in grinding area. If raw material sticks to grinding table or roller, it will cause too high vibration or overflowing of raw meal. Heating mill is necessary which could avoid the too high hot pressure between grinding parts, roller and grinding table. As the weight and thickness of roller and grinding table is large, the inner temperature will be lower than the outer temperature in a quite long time – heat transaction and thermal capacity. This uneven temperature distribution – outer heater inner cooler – form the heat pressure which could make the parts crack. Thus increasing of inlet of vertical mill should be slow. As the min. heat energy used for drying process is concern with the inlet temperature ( higher than 120 centi-degree), so heating the vertical mill during operation is impossible—first preheat with lower inlet temperature (95-120 centi-degree). During heating process, enough air inside of mill to strengthen the heating of parts. Enough air will cause the inner pressure difference to be larger than 5mbar. Heating should at least keep until that the outlet temperature and temperature of bag filter reaches 85 centi-degree and continuous heating for one hour.
 
6th, important process parameters
 
Operator of vertical mill needs to set the process parameters, then via comparing to the actual process parameters and changing set point to ensure the reliable operation of equipment.
a. capacity: utilizes hot air furnace to supply heat 120-150t/h, utilizes waste gas at kiln tail to supply heat 190t/h
b. pressure difference: 30-50mber
c. vibration of reducer: 1-2.5mm/s (alarm when it is larger than 3.5mm/s)
d. outlet temperature of mill: 90±5 centi-degree
e. thickness of material bed: 70-85mm
f. pressure of hydraulic tightening force: ( if 120-150t/h) 120-150ber, (190t/h) 150-175ber
g. inlet pressure of mill: <-5mber
h. pressure difference of bag filter: <1700Pa
i. inlet air temperature of mill: <260 centi-degree
j. bearing temperature of reducer: <70 centi-degree
k. oil box temperature of reducer: <60 centi-degree
l: bearing temperature of main motor: <65 centi-degree
m. inlet bag filter temperature: <200 centi-degree
 
7th. pre-loading material into mill
 
In order for successfully start the vertical mill; the process condition of mill should be good.
Too little raw material inside of mill will cause the over-high vibration when starting。 So when 1st starting or starting after maintenance, it should load limestone into the mill or blending scale. This work could be implemented according to the following two methods: start feeding group via live mode, via cancelling grinding and interlock feeding to start while mill is on the stop status. Feeding group will stop when mill has enough raw material. Then distribute the raw material evenly inside of mill by spade on manual.  If the mill is too full, main driving will be overload, now some raw material should be removed out of mill.
 
8th, preparation for starting of vertical mill
 
Starting of vertical mill should comply with the setting order according to chain.
Start raw material conveying group, air channel group and oil supply group of reducer before starting the mill.
Considering safety and successful starting have following check before starting.
 
a.       Check whether all system of vertical mill has been finished, all doors are closed and confirm that there is nobody is on dangerous location. Considering safety, notice field person via phone or interphone.
b.      Check whether the mill is heated correctly or after last operation, it hasn’t been cooled meanwhile it should consider heating time and outlet temperature. Outlet temperature should be no more than 90 degree.
c.       Check the filling condition of mill – too vacant, normal or too full – if necessary take relevant action. Filling degree of mill depends on the last stopping status – loading quantity before stopping, energy stop or stop process.
d.      Check whether there is raw material layer on blending scale.
e.       Check that all necessary equipment are on active mode and no trouble display.
 
ⅰ  all the equipment of feeding group is under preparation, feeding bin has enough limestone
ⅱ  raw meal silo has enough space
ⅲ  oil temperature and oil level of reducer should be proper
ⅳ  check that the operation of water spraying system of vertical mill and sealing motor is normal
ⅴ  check the cooling water and its pipeline and gate valve
ⅵ  compressed air for cleaning bag filter
ⅶ  all groups are on central control mode and are ready
ⅷ  check whether all process parameters has been adjusted to reasonable location and check the setting point
ⅸ  speed of separator
ⅹ  air flow rate
Ⅰ  location of louver valve of fan
Ⅱ  pressure of tensioning hydraulic system
 
9th Starting of vertical mill
 
All preparation for starting are finished and send the starting order to mill group for starting. When main driving of mill is accelerating, operator should monitor the current of main driving motor and condition of pressure difference of mill. After gaining normal value, sends starting order to feeding group. Starting order is as follows:
Start the raw meal feeding group; ash returning group of air conditioning tower; bag filter & discharge conveying group; exhaust blower group at kiln tail; raw meal discharge conveying group; outer circulating group of vertical mill & air-lock discharger; thin oil station of reducer of vertical mill, hydraulic tensioning device, thin oil station of main motor and system fan; separator and sealing motor; system fan; conveying group at bottom of blending silo; water spraying group of vertical mill; carrying roller and main driving motor for starting; feeding & roll down.
Notes: second starting of main motor should be not less than 30 minutes after last stopping time of main motor.
Make the mill stable at the first 5-15 minutes. Operator should carefully monitor process parameters and take the correct steps.
 
The parameters which could indicates stability:
ⅰ vibration of reducer
ⅱ pressure difference of mill
ⅲ current of main driving of mill
ⅳ air flow rate of mill
ⅴ current of circulating motor
ⅵ outlet temperature of mill
ⅶ thickness of material bed
 
Operator should ensure the process request via adjusting following parameters:
ⅰ location of louver valve of circulating fan and other louver valve
ⅱ temperature and volume of hot air
ⅲ feeding quantity
ⅳ rotating speed of separator
ⅴ pressure tensioning hydraulic system
 
10th External system of vertical mill grinding
 
10.1 dust collecting & conveying of finished product
 
Passed finished product of separator enters into cyclone for dust collecting; waste gas continues going and enters into atmosphere via circulating fan, bag filter at kiln tail, exhaust blower at kiln tail. Finished product of cyclone enter into raw meal silo via air-lock dividing, air charging chute and feeding lifter; finished product of bag filter enters into raw meal silo via scraper conveyor, screw conveyor, lifter, air charging chute and feeding lifter into silo.
 
10.2 hot air supply & waste gas exhaustion
 
Primary hot air is supplied by hot air, after normal production, waste gas will enter into vertical mill for drying material, separator, dust collecting cyclone, circulating fan, bag filter at kiln tail and exhaust fan at kiln tail via outlet of air conditioning tower, and then it will enter into atmosphere.
 
10.3 Raw material weighting & feeding
 
Limestone, sandstone, iron powder, etc. raw material will enter into vertical mill via blending silo, apron feeder or unobstructed discharger, belt scale, feeding belt conveyor, electric-hydraulic three way valve and air-lock feeder.
 
10.4 Coarse external-discharge circulating of air ring
 
Iron parts are not allowed to enter into vertical mill
Part of coarse will be fed into mill for re-grinding by belt feeder via external-discharging of air ring, solenoid feeder and lifter.
 
11th Main body system of vertical mill grinding
 
Rated parameters
Supplied vertical roller mill (model MPS 4000B) has the following parameters, mixed grind and dry the raw material of cement: 
Feeding material:  Diwei 1 Mix                         Diwei 2 Mix
Cement raw material mixture                                Cement raw material mixture
Composition:                                                      Composition:
85.07% Limestone                                                88.01% Limestone
14.18% sandrock                                                     10.38% sandrock
0.75% iron powder                                                     1.61% iron powder
Feeding moisture content: Max. 8%                          Max. 8%
Feeding grain size:0-80 mm                                    0-80 mm
Output of finished product:190 t/h                          180 t/h
(wear parts)                                                   (wear parts)
205 t/h(final output of finished product          200 t/h(final output of finished product
if utilizes new wear parts)                             if utilizes new wear parts) 
Fineness of finished product: ≤10% R 0.080 mm        ≤10% R 0.080 mm
Rest moisture content: ≤0.5%                                        ≤0.5%
When feeding moisture is 5.7, hot air (utilizes before mill):
Flow rate:214,505 Nm3/h                                 218,668 Nm3/h
Temperature:229℃                                                 220℃
 
 Mill operation
 
Open ventilation door make the waste air of kiln enter into vertical mill.
Blending belt scale measures the material from feeding bin and then the measured material will be sent into mill via belt conveyor meanwhile on the belt conveyor for mill will install one iron separator and one metal detector. Iron separator will remove the magnetic metal from feeding material and metal detector will start branch chute to discharge non-magnetic metal parts. Branch chute will send the material to the cyclone air-lock valve before mill which could lock air and heat or send the material into middle chamber.
 
Waste gas of kiln will be used for drying material; installed hot air furnace is only used for heating mill during trial-production and stop. Material will be grinded into requested finished and dried inside of mill. Fineness of product will be controlled via adjusting rotor of separator.
 
Finished product will be brought out of separator by air flow via dual line cyclone for separating the air and finished product. Lower bag filter will remove the powder of air flow.
Adjust air volume via adjusting the location of upper louver valve or ventilation door.
Circulating air should be controlled via adjusting control valve.  
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