Ball Mills & Separators.ppt

BALL MILLS & SEPARATORS

Ankit Agrawal (AAL) Samar Sharma (SSA) Surbhi Bajaj (SBJ) Yash kalra (YK)

OVERVIEW 

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Introduction Circuit of Ball Mill Mechanism Design of Ball Mill Energy Consumption Variants of Ball mill Instrumentation Introduction of Air Separators Principle Forces on a Particle Power Requirement Separator design Types of separators Various makes of separators Maintenance

WHAT IS A BALL MILL?   

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It is a device used for size reduction. It can reduce sizes of 1/4 inches to 20-75 micron Cylindrical in shape Rotates on a horizontal axis Partially filled with the material to be ground and the grinding medium

ADVANTAGES  

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It is a well-established technology. It is a simple technology not requiring many moving parts. Hence, it is a robust construction. It is possible to re-circulate the ground material and achieve finer grinding. It is also possible to regulate the amount of re-circulated product to control final product. Apart from grinding ball mills are also used for mixing. By using different media in the same drum, we can process a wide variety of feed.

COMPONENTS OF BALL MILL

OPEN & CLOSED CIRCUIT MILL

OPEN & CLOSED CIRCUIT MILL 

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In open-circuit grinding the ball-mill discharge passes directly to the next processing step without being screened or classified and no fraction is returned to the ball mill

In closed-circuit grinding the ground material, undersize, in the ball-mill discharge is removed either using a screen or a classifier with the oversize being returned to the mill for additional size reduction

The over size material that is returned to the ball mill is called the circulating load

OPEN & CLOSED CIRCUIT MILL 

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Open-circuit ball-mill grinding requires more power than closed-circuit grinding for products containing similar amounts of top-size material

Example, assuming the desired grind size is 90% passing some specific top size, open-circuit grinding would require 1.40 times the power to achieve similar results as closedcircuit grinding

GRINDING MECHANISM 

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The grinding is primarily done by the grinding media (balls) by a cascading effect. The cylinder is rotated along with the charge and action is similar to tumbling. Due to the centrifugal action of the rotating cylinder, particles are carried towards the top, after which they break contact from the wall and fall down. This angle is called “angle of break”.

GRINDING MECHANISM 

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Grinding occurs primarily due to the impact when falling particles strike the bottom. Grinding also occurs (to a lesser extent) by the slipping and rolling of particles over one another. The speed of rotation and angle of break are important parameters.

CHOICE OF MEDIA 

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Initial feed size – Smaller particles cannot break up larger ones Final particle size – For fine grinding, small sized media are effective Density – Heavier and more dense particles give better results Hardness – Harder media undergo much less wear and contaminate the product less Other considerations include costs, pH resistance, adsorption properties that may affect the charge

CHOICE OF MEDIA 

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Media should also be chosen according to application. Typical examples are flint or pebbles (less hard), stainless steel and metal balls (undergo corrosion), porcelain balls, some specially designed high density media Advantages of using smaller media More grinding contacts per revolution  Provide smaller voids, reducing the chance of agglomeration  More uniform grinding. 

OPTIMAL RUN 

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The voids of the media must be covered by the material to be ground otherwise there will be too much wear of media and contamination will occur. The speed of running typically ranges from 65-80% of the critical speed. Fastest grinding occurs where there is just sufficient material in a batch to fill all voids and slightly cover the grinding media. This is 25% of the total volume with a half ball charge and 18% with a one-third ball charge.

OPTIMAL RUN 

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Batch sizes typically don’t exceed 60% in a conventional ball mill. And, time taken to grind increases faster than the batch size. Only 1-2% of the energy supplied is actually used in the fracturing process. The rest is released as heat. For this reason, a current of air is passed through the mill to keep it cool and sweeping out excessive moisture (dry).

CRITICAL SPEED Speed at which the contents of the mill would simply ride over the roof of the mill due to centrifugal action.  The critical speed (rpm) is given by: nc = 42.29/√d Where “d” is the internal diameter in meters.  Ball mills are normally operated at around 75% of critical speed, so a mill with diameter 5 meters will turn at around 14 rpm. 

DESIGN CRITERIA FOR BALL MILL D = 124.2*(C) + 485.7 L = 85.71*(C) + 185.4

Where “C” = Capacity of Feed (Tons) “L” = Length of mill (mm) “D” = Dia. of mill (mm) 

Volume of Mill is given by {( π /4) * (D2 * L)}

DESIGN CRITERIA FOR BALL MILL 

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The Bulk Volume of balls charge ratio to the volume of mill is known as “Filling Ratio” ( range 30 – 45 % ) F = (bulk volume of the balls / volume of mill) So bulk volume of balls = ( F ) * {(π/4) * (D2 * L)}

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Size of Balls

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D2 { ball } = k * Dp Where k is a constant ranges from 0.9 to 1.4 Mass of Balls Mass of the balls = bulk vol. of balls * bulk density of balls Bulk density of ball = {(1-ε) * (density of ball)

ENERGY CONSUMPTION AND OUTPUT The power (in kW) required to turn a ball mill (cement mill) is approximated by: P = 0.285 d (1.073-j) * m * n where o

“D”

= Internal diameter in meters,

“j”

= Fraction of the mill volume occupied by media

“m”

= Total mass of media in the mill, in tones

“n”

= Mill speed in rpm

Energy Required in a ball mill is directly proportional to • •

Material Hardness Fineness required

Typical mill power consumption for various degrees of fineness. Actual values vary according to mill system efficiency and clinker hardness.

• About 1-2% of the energy supplied is used in fracturing particles, and the remaining energy ends up as heat which has to be dissipated by the mill system

ELECTRICAL ENERGY CONSUMPTION VRM VS BALL MILL (RAW MILL)

30 25 20 15 10 5

19

15

0 VRM

BALL MILL

TYPE OF MILL

ELECTRICAL ENERGY CONSUMPTION VRM VS ROLLER PRESS & BALL MILL (CEMENT GRINDING)

40 35 30 25 20 15

32

27

26

VRM

RP + BM

10 5 0

TYPE OF MILL

BALL MILL

VARIANTS OF BALL MILL 

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Drive System Wet/Dry Ball Mill Multi Chamber Ball Mill

ACCORDING TO DRIVE SYSTEM 

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Drives for tube mills can be of most different design. The following criteria are decisive for the selection: Power to be transmitted Space requirement Type of mill bearing Modern drive systems should require only little maintenance and meet the highest standards as regards noise emission, temperature- resistance and safety.

GIRTH GEAR DRIVE

up to 6,000 kW

CENTRAL DRIVE

from 2,000 to 8,000 kW

CIRCUMFERENTIAL DRIVE

from 3,000 to 10,000 kW and more

MULTI CHAMBER MILL Single Chamber overflow Mill: In the case of this design, the length of the mill cylinder is maximally utilized. Single-chamber tube mill with discharge wall: •Used for wet grinding and dry grinding. •The width of the slots in the discharge diaphragm determines the maximal grain size in the mill discharge material

Two-chamber tube mill This design is used for wet as well as for dry grinding. •The first grinding chamber is normally equipped with a lifter lining and filled with coarse grinding media for the preliminary comminuting of the grinding material. • Via the width of the slots in the intermediate diaphragm, the maximal grain size of the grinding material transferred to the second grinding chamber is limited.

•The second grinding chamber is filled with smaller grinding media. The two-chamber tube mill is the typical mill for grinding systems with separators. Three-chamber tube mill These systems are used for wet as well as for dry grinding. For dry grinding, in an open circuit, for wet grinding also in connection with screens or hydro-cyclones

Two-chamber tube mill with Circumferential discharge:

This design combines two mills to one tube. It is used for dry grinding or combined grinding and drying.

Depending on the plant configuration,one chamber can be used as preliminary grinding chamber and the other one, after the previous separation of the material discharged from the first chamber,as fine grinding chamber. If required, both grinding chambers can be heated.

Two-chamber tube mill with circumferential discharge and drying chamber connected in front: The preliminary grinding chamber is preceded by a drying chamber. To achieve an as uniform distribution of the grinding material over the drying chamber cross section as possible, the drying chamber is provided with lifters.

COMPONENTS OF BALL MILL

SHELL LINING PLATES

These plates are specially designed and incorporate contour features which inject movement into the ball charge during rotation of the mill. Properties required by the plates vary along the length of the mill. For instance, at the inlet region of the mill there is likely to be less protection of the plates by ground products and the balls will be of largest size in use and therefore, toughness is of outmost importance. However, along the length of the mill, in second and third chambers, the ball size is considerably smaller and a protective layer of ground material is available and here wear resistance must be at a maximum.

Effect of Lifters 

Lifters are protrusions on the wall of the mill that help in lifting the material to a greater height and increasing the angle of break.

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The effectiveness depends on the shape and number of lifters.

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Another advantage is that it increases the life of lining because typically lifter undergo most wear and are easy to replace.

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Types of Liners:

1. 2.

Bolted Lines Boltless Liners

Advantages of Boltless Liners: 1. Output Increased 8-10 % 2. Power Consumption Reduced 8-20 % 3. Repair Decreased 98 %

4. Liner Consumption Reduced 8-20 %

DIAPHRAGM

FUNCTIONS OF THE DIAPHRAGM: 

Separation of the grinding media fillings of two grinding chambers or retaining the grinding media in the grinding chamber in the case of discharge diaphragms.

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Restriction of the maximal grain size of the grinding material which is transferred from one grinding chamber into the following one as a function of the slot width.

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Transport from the grinding material from one grinding chamber into the following one or from the grinding chamber into the mill discharge.

Requirement Of Diaphragms: Ensuring a grinding material level in the grinding chambers optimal for the grinding process. As Large free cross section as possible to ensure a sufficient ventilation in the mills. Utmost stability and safety of the frames and the fastening system of the diaphragms. High Wear resistance and security against fracture of the slotted plates and back wall plates.

TYPES OF DIAPHRAGMS

A. Intermediate diaphragm B. Discharge diaphragm

C. Central discharge Diaphragm D. Central discharge diaphragm with separation of the material streams

In Any Grinding Circuit, There Are But A Few Basic Objectives

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Reduce the size of the incoming feed to produce a final product.

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Reduce the size of the feed material to a size where valuable minerals can be liberated and recovered in a subsequent process.

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Reject much of the undesired material to simplify further processing.

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Accomplish the above objectives at minimum cost. The second and third objectives listed, apply to beneficiation of metallic ores while the other two are characteristic of grinding circuits in general.

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Due to the complexity of a grinding circuit, it is difficult if not impossible for an operator to manually achieve the best results from a circuit.

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Variables within a circuit contribute to make measurement and control of certain parameters necessary to achieve optimum performance from the circuit.

These Variables Include •

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Ore type changes media loading liner wear pump dilution classifier feed pulp density reagent additions water pressure changes pump speed and wear sump segregation viscosity product particle size, etc.

Within A Grinding Circuit There Are A Number Of Parameters That Can be Measured Among These Parameters Are The Following:

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Horsepower – can be determined from standard mill amp meter measurements. Horsepower is dependent upon mill speed, media load, ore specific gravity, pulp dilution and type and condition of liners.

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Sound from the mill – Amplitude of mill sound is measured with non relevant frequencies filtered out. Sound can be an indicator of mill loading. Eg: Electronic Ear

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Throughput – can be measured with load cells on the mill feed belt. Throughput is measured as an indication of the quantity of final product produced. Also, it is a means of determining the amount lf makeup water to add to a wet grinding application. Eg: Belt weigher

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Cyclone feed sump level – can be measured using a bubble tube, ultrasonic measurement, nuclear absorptions capacitance or differential pressure measurements. Cyclone sump level is a good indicator of circulating load in a grinding circuit. Eg: Level sensors

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Mass flow – can be determined by a density measurement from a magnetic flow meter. Mass flow can be used to regulate reagent additions and compute mass balances.

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pH – can be measured by standard electrodes. pH can effect the formation of colloidal suspensions from metallic hydroxides. Colloidal suspensions can alter density and have a significant effect upon classification.

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Ball mill head water – can be measured from the pressure drop across an orifice plate. Mill head water controls pulp density and affects grinding efficiency. Eg: Pressure sensors

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Density – often measured by nuclear absorption. Density is significant in classification, flotation, and grinding mills. Eg: Density Gauges

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Particle size of the classifier product – can be determined by screen analysis, direct measurement or inferential technique. Particle size is significant for liberation of valuable minerals and in establishing the proper trade off between throughput and recovery to assure best economic operation and minimizing over grinding to curtail ore beneficiation costs.

Separator: 

Pressure at Inlet and Outlet .

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Temperature Monitoring at: Motor winding Bearing Gear Box Separator Inlet and Outlet.

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Flow Measurement.

1. 2. 3.

Electronic Ear The Fill Level of the Ball Mill is optimized according to the production requirements.

The fill level is measured by an electronic ear. This captures a broad spectrum of frequencies which simplifies the filtrating of external sources such as neighboring mills. The ear measures and memorizes the frequencies of each mill.

Salient Features: 

Enable optimization of mill operation.

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Eliminates excessive dependence on the operator.

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Precautionary measures can be taken before jamming occurs.

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Suitable for double and single chamber mills.

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Helps in close circuiting and automation of the mills.

LATEST DEVELOPMENT

INSTRUMENTED BALL FOR BALL MILLS 

This technology relates to measuring and determining the internal dynamics and kinematics of a ball within an operating ball mill.

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The novel device is an instrumented ball capable of recording data from an industrial mill. A variety of sensors, including accelerometers, gyros, strain gauges, microphones, thermocouples, wear sensors, etc. form the instrument package.

The data collected by the instruments within the ball allow the determination of the force, the moments and the rotational kinetic energy as a function of time.

This technology will allow for Verification and Tuning of the models. Thus , with better models , the operation parameters will improve leading to Steel and Energy savings for ball mill operations.

This technology will help the mining industry optimize its milling performance through: 1. 2.

3. 4.

Increase energy efficiency. Reduced ball consumption through optimized ball to rock ratio. Improved equipment life. Reduction of operation costs, and better design of balls and liners.

AIR SEPARATORS

AIR SEPARATORS  

Definition It is the division of a given material stream into two separate streams using air as a carrying medium.

PRINCIPLE Action of an air current of a certain velocity upon a mass particle is proportional to the surface presented by this particle to the air current. Thus to the square of mean dimension of the particle.  Action of force of gravity upon a mass particle is proportional to the volume. Thus to the cube of mean dimension of particle. 

PRINCIPLE 

If these two forces are concurrent, the force of gravity will prevail over the effect of air current as particle dimension increases.

FORCES ON A PARTICLE 1. 2. 3.

Centrifugal force, Fc The force of ascending air current, Fd. The force of gravity, Fg.

Fd Fc Fg

The distribution plate must impart sufficient amount of centrifugal force to through the particle in separating zone before the new feed is received.  As heavier and large particles are thrown outwards their Fc is decreased therefore they settle by the action of gravity. 

DIAGRAM

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Fine particles are lifted by the ascending air currents and pass between the blades of both auxiliary and main fan into the outer separator cone, which is also called fines cone. Underneath the separating zone there are air return vanes. The main fan moves the air from fines cone into the separating zone through grating of vanes.

The separation of fines in the outer separator is due to decrease in their velocity as well as change in direction of air current.

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The auxiliary fan acts against the intake air current caused by the main fan. This counter action can be controlled by number of blades of auxiliary fan. Horizontal control valves make it possible to change the cross section of ascending air current for fine adjustment. The separator finish product is judged by the amount of coarse particles in the fine.

The finer the particle size of finish product, the lower the separator’s production capacity.

POWER REQUIREMENT Quality of separator feed.  The circulating load.  Desired fineness of the finished products. 

Figures in magnitude vary from 2Kwh/t-6Kwh/t.

TYPES OF SEPARATOR DESIGN 

1st generation

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2nd generation

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3rd generation

HEYD or Sturtevant separators Cyclone separator QDK or High Performance cage rotor separator

TYPES OF SEPARATOR DESIGN 

1ST GENERATION SEPARATOR Circulating fan Separation Chamber Auxiliary fan Precipitating Chamber

Distribution plate Circulating Fan Tailing Cone

TYPES OF SEPARATOR DESIGN 

2nd Generation Separators

TYPES OF SEPARATOR DESIGN 

3rd Generation Separator

COMPARISON

TYPES OF SEPARATORS 

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Grit Separator Mechanical separators High efficiency cyclone separators Dynamic classifiers

TYPES OF SEPARATORS 

Grit Separators

TYPES OF SEPARATORS 

Grit Separator  Used to de-dust mill air sweep.  Have no moving parts.  Effect separation by cyclonic air flow induced by guide vanes.  Dust separation: 

Radial settings of vanes gives minimum. Tangential settings of vanes gives maximum.

TYPES OF SEPARATORS 

Mechanical Separators

TYPES OF SEPARATORS 

Mechanical separators  The traditional classifier of mill product.  Has a rotating dispersion plate.  Coarse particles either fall directly from the dispersion plate or are rejected between auxiliary fan blades and the control valve.  Operating adjustments   

Number of auxiliary fan blades. Clearance between blades and control valve. Position of main fan blades.

TYPES OF SEPARATORS 

High efficiency cyclone separator

TYPES OF SEPARATORS 

High efficiency cyclone separator  Introduced to improve on the mechanical separator’s low efficiency in fines recovery.  Material is fed onto a rotating dispersion plate, whence it is dispersed into classifying air stream & then sucked by tangential inlet ducts.  Loading is up to about 2.5 kg feed/m3 air flow.  Horizontal vortex formed classifies particles between centrifugal force and inward air flow.  Increasing rotor speed increases fineness.

TYPES OF SEPARATORS 

Dynamic Classifiers

TYPES OF SEPARATORS 

Dynamic Classifiers  Used integrally with roller mill.  Involves upward flow of dust entrained air into a segregating area above grinding table.  With decreasing air velocity coarse particles fall back while fines leave with exhaust.

VARIOUS MAKES 

Turbo Separator Turbopol (Polysius)

VARIOUS MAKES 

Turbo Separator  

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It is furnished by an auxiliary fan driven by separate drive arrangement. A Special feeding arrangement to improve the separation process by uniform distribution of separation feed from distribution plate in circular separating space. Sizes= 3200-8500mm dia. Maximum capacities= 160t/h of cement with fineness of 2800 cm2/gm Blaine and for raw mix 200t/h with a fineness 12% residue on 88 micron.

VARIOUS MAKES 

Cyclone air separator (KHD Humboldt Wedag AG)

VARIOUS MAKES 

Cyclone air separator  Possible to achieve fine product capacities upto 500 metric t/h.  The cross current separation process with distribution plate and auxiliary fan.  According to manufacturers: 

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Specific load of separating zone can be increased. Variation in the separation feed (circulating load) do not affect fineness of separator product. This circulating load is reduced to the capacity of centrifugal system, the separation runs without vibration. The blower located outside works better than the fan located inside separator.

VARIOUS MAKES 

Separator with external blower : SKET/ZAB

VARIOUS MAKES 

Separator with external blower  Combination of a speed controlled air separator with principle of instrumental splitting of separation & precipitation process.  Similar features as cyclone separator.

VARIOUS MAKES 

Cyclone air separator (Krupp polysius)

VARIOUS MAKES 

Cyclone air separator 

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The material is introduced laterally into the separator by an air slide & is uniformly distributed in the separating chamber. Air generated by external mounted blower separates the fines & coarse by various forces. The separating zone has a higher loading capacity than that of turbopol separator. Specific power consumption is higher than that of Turbopol separator due to the separation in external cyclone.

VARIOUS MAKES 

O-Sepa air separator

VARIOUS MAKES 

O-Sepa air separator  Developed by Onoda Cement Co. Ltd., Tokyo Japan.  Provides sharp classification & mill system energy saving.  Material is fed by chutes directly on the distribution plate over the rotor.  The primary & secondary air create a vortex in the separating zone of separator.  Adjustments can be made for desired product size by adjusting the rotor speed.

MAINTENANCE 

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Lubrication of separator bearings. Lubrication and tightness of gear box. Guide vanes Timely maintenance of separator cone. Proper functioning of fans. Continuous Monitoring o o o o o o

Temperature Power Current Speed Position Pressure

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