Trouble Shooting Guidemills97

7/28/2019 Trouble Shooting Guidemills97

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Possible reasons Indications Remedies

Worm shaft bearing

failure

a)Play in worm shaft

b) Shaft seating on

bearing housing.

c) High frequency noise

Replace the bearings.

Improper surface contact

on some teeth on worm

set

Knocking sound Reset the worm gear set

to get proper contact

Interference of pump hub

with bushing

Cyclic rubbing noise Provide proper clearance

Worn out and dry

coupling

Noise from coupling Lubricate /replace

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2/31


Scrapper or scrapper

guard touching

stationary components.

Periodic noise Ensure proper

clearance.

(6-15 mm)

Skirt touching mill bottom Rubbing noise Ensure proper

clearance.

(5-8 mm)

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3/31


Foreign material in mill Noise on no load running

also

Remove foreign material

Roll touching bowl or gap

less

Reset the gap as per

procedure

Bowl assembly touching

stationary part

Periodic noise Minimum gap 3 mm radial

and 8-10 mm vertical

High load vibration Excessive movement of

spring adjusting stud

Proper spring compression

And sufficient ring roll gap

Journal spring assembly

bearing failure

Knocking sound Replace bearings

Loosening of separator

body flange bolts

Dust leakage from flange Tighten the flange bolts

Bull ring segments loose

due to improper assembly

or segment breakage

Visual and physical

inspection

Assemble properly

Cont

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4/31


Excessive wear of bowl

extension ring or bowl.

Proper coal bed

formation not there.

No noise at no load

More noise at low load

Replace extension ring

Spring compression too

high

No noise at no load

More noise at low load

Reduce spring

compression

Roller assembly slipping

in to Bowl due toa)Keeper plate screw

failures.

b)Journal shaft pin or

locknut failure

Visual Correct

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Air entering mill base Oil level goes down if oil

fill pipe cover is opened

Check Mechanical face

seal

Oil cooler puncture.

Cooling water entering

mill base

Over flow stops if cooling

water stopped.

Replace and repair

cooler.

Excess oil in mill base Correct oil level

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6/31


Plugged of vertical shaft

hole

clean

Plugged or broken return

oil line

repair

Improper oil Change oil

Excess clearance

between pump and hub

Replace pump/ hub as

required

Plugged holes in lowerbearing housing clean

Accumulation of debris in

oil collector

clean

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7/31


Improper/damaged

packing gland

Oil over flow from mill

base

Ref 4

Air vent pipe

closed/plugged

Allow venting

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8/31


Breakage/leakage from

return oil line

Repair /replace

Puncture of oil cooler

tubes

Over flow stops if cooling

water stopped

Replace and repair

cooler.

Failure of vertical shaft

oil seal

Replace oil seal

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9/31


Mating surface not

cleaned properly

Seal compound not

applied

Improper gasket used

Bolts not tightened

properly

Venting not adequate

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10/31


Oil cooler pipes choked Clean cooler tubes

Inadequate supply of

cooling water

Maintain proper supply

Oil level high/ low Maintain proper level

Break down of lubricant Lab analysis replace

Air entering mill base Oil level goes down if oil

fill pipe cover is opened

Check Mechanical face

seal

Failure of bearings change

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11/31


Over feeding

( mill capacity depends

on HGI, moisture,fineness etc.)

Limit coal flow to mill

capacity

Insufficient air flow Maintain proper air flow

Separator body cover

plate wear

Air should only flow in

areas identified

Improper internalclearance

Set as per guidelines

Jamming of rolls See

Wear of mill parts like

roll, bullring, liners, bowl

extension ring etc.

Rebuild/ replace wear

parts as required

Cont11


12/31


Improper setting of ring

and roll gap and journal

head and spring seat

gap

Set as per manual

Low spring pressure Excessive jumping Set as per manual

High coal fineness Lab analysis See

Blocking of air inlet vane

/ classifier vane

visual clear

Coal pipe choking Cold pipe see

Sand accumulation in

mill

Purge the mill

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13/31



and roll gap and journal

head and spring seat

gap

Set as per manual

Unequal spring pressure Set as per manual

Unequal roll wear

between journals

Set as per manual

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14/31


Mechanical face seal

failure

Air leakage through

cored holes

Set as per manual

Mill base not properly

cleaned

clean

Running mill with high

rejects

See 10

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15/31


Improper setting of

classifier

visual Reduce number

Excess spring

compression

reduce

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16/31


Improper setting of classifier visual increase number

Low spring compression increase

Improper alignment of

classifier vane

visual correct

Puncture in classifier cone repair

Plugging in classifier cone clear

High mill air flow Maintain proper air flow

Excess wear of mill parts Repair/ replace

Improper setting of ring and

roll gap and journal head

and spring seat gap

Set as per manual

Low mill outlet temperature see

Improper sampling Follow procedure

More gap between cone

and feed pipe

correct

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17/31


High mill outlet

temperature

Keep MOT within limit

High sulfur in coal Keep MOT within limit

High volatile matter incoal

Keep MOT within limit

Mill idle with partially

pulverized coal/ coal

Ensure proper purging/ emptying of

mill during normal shutdown.

Use inert medium if provided.

Putting burning coal inmill avoid

Rapid change in fuel

moisture

Mill outlet temperature should be

kept in auto

Accumulation of rejects

in mill

See 16

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18/31


Mill running with

excessive rejects

See 10

Pyrite hopper choked/

overfilled

clean

Tramp iron spout choked clear

Tramp iron gate closed open

Scrapper failure Repair /replace

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19/31

Failure of vertical shaft is generally due to

fatigue. The shaft is subjected to stress cycles

which may lead to fatigue failure. The shaft has

been designed adequately for keeping the stresslevel beyond endurance limit but due to wrong

operation and maintenance the shaft can be

stresses beyond endurance limit. Under normal

circumstances the bending load of each roll isbalanced on vertical shaft. Cont..

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20/31

Possible reasons Possible reasons Possible reasons

Using rolls with unequal

wear

Using different spring

compressions

Spring breakage

Foreign material entry in

mill

Improper locking of

spring assembly


roll

Jamming of rolls Roller in lifted position

due to jamming of

trunnion bushing

High spring compression

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21/31


Coal HGI reduced Lab analysis

High moisture in coal Low MOT Prevent moisture

addition

High pulverized coal fineness Lab analysis correct

Low spring compression reset

Low air flow Maintain correct air flow

Higher gap between roll & ring

and journal head and spring

assy.

reset

Wear of roll & ring replace

Low mill outlet temperature See

Coal pipe choking see

Wear of bowl extension ring change

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22/31


Instrument air leakage Attend leakage

Solenoid valve damaged replace

Victaulic coupling too

tight

Without coupling MDV

operates

Insert shim between

coupling flanges.

Air cylinder seals

damaged

Change seals

Valve fouling with body Grind and correct

Valve shaft not aligned

properly

Correct the bushes.

Fouling of clevis and

brackets with air cylinder

Grind adjust to suit

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23/31


Classifier vane shaft

bent

Straighten or change

Classifier link lever

length not equal

Use turn back signal to

make free

Bearings / Grease seals

damaged

replace

Classifier vane fouling Remove the fouling

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24/31


O ring oil seal between

upper and lower journal

housing damaged

Oil marks on the flange replace

Upper journal housingseal damaged

Oil leakage from top ofjournal housing

replace

Journal shaft ( seating

area) damaged

By measurement Repair and change shaft

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25/31


Bearings damaged Change bearing

Oil contaminated Replace oil after

cleaning

Oil Inadequate Top up oil

No bearing clearance Follow procedure

Upper journal housing

rubbing with journal head

Ensure clearance

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26/31


Inadequate seal air

supply

Check up air supply

Journal shaft plug open Replace plug after

cleaning journal

assembly.

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27/31


Mill fire

Foreign material

Casting defectHammering

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28/31


Mill fire

Foreign material

Casting defectHammering

Excess wear of bull ring

Improper surface contact

between bull ring and

bowl

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29/31


Oil contamination

Worm shaft bearing.

Housing damagedBearing heated

excessively

Assembly without

clearance

Bearing hammered in toposition

Poor quality bearing

Thrust bearings not

properly assembled

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Scraper rubbing Scoring marks Maintain proper clearance.

Scraper holding bolt failure Change bolt

Lower skirt boss breakage

Scraper blade breakage change

Lower skirt boss thread

damage

Use other boss if available

or use bigger bolt.

Scraper pin excess clearance Change holder

Foreign material

Scraper fouling with cover plate Change warped cover

plate

Excess rejects See 10

Pyrite hopper choked clear

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Increase in raw coal moisture Do not add moisture

Insufficient primary air Maintain proper air flow

Primary air temperature low Try to increase

Excess cold primary air Close cold air damper/gate

Hp heaters not in service Take HP heater in service

Boiler load low Increase boiler load or take

more mills in service

Moisture retained in pulverized

coal low

Eco bypass not used use

SCAPH not charged

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Trouble Shooting Guidemills97

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