Ball and Race Mill

Ramagundam Super Thermal Power Station NTPC Limited

Ball & Race mill

Namendra AnandSr. Engineer (BM/RM)

Ramagundam Super Thermal Power StationNTPC Limited

nanand@ntpc.co.in

Milling system maintenance is a challenge as it requires to use all kinds of

maintenance practices to run the system effectively….

Its an opportunity as it provides vide scope to implement betterment activites….

• To reduce the coal size up to 70 micron size

OBJECTIVE OF PULVERIZER

ARRAMENT OF MILL & PA FAN

1. Raw coal inlet pipe2. Pulverized coal outlet pipe3. Adjustable classifier blades4. Oversize particles return

chute5. Hollow grinding balls6. Rotating lower grinding ring7. Primary air inlet8. Loading cylinders9. Spider guide10.Stationary upper ring11.Air regulating ring12.Gear box13.Reject door

BALL& RACE MILL (E TYPE)

1. Mill gear box to motor coupling2. Reduction Gear Box3. Mill lub oil system4. G/B out put coupling5. Yoke6. Turn plate7. Bottom track/ring8. Coal drop plate9. Grinding ball10. Top track/ring11. Spider/ loading ring12. Loading System13. Classifier Assembly14. Pulverized coal outlet pipe15. Coal inlet pipe16. Pyrites discharge Syste

VARIOUS PARTS OF COAL MILL

REDUCTION GEAR BOX

Gear Box

Gear unit

External

Lub System

InternalFirst reduction one pair of Spiral bevel gears

Second reduction one pair of Single Helical gears

Gear Pump & other

arrangement

Oil distribution system &

Spray nozzles

MILL GEAR BOX

MILL GEAR BOX CROSS SECTION

MILL GEAR BOX INPUT PINION

MILL GEAR BOX I STAGE GEAR

MILL GEAR BOX I STAGE GEAR &II STAGE PINION

MILL GEAR BOX II STAGE GEAR

MILL GEAR BOX EXTERNAL LUB OIL SYSTEM

MILL GEAR BOX INTERNAL LUB OIL LINES

MILL GEAR BOX LABYRINTH SEAL

REDUCTION GEAR BOX

• Speed : 970 / 40.185 rpm• Lub System :

– Roloid gear pump – G/B sump Capacity is 210 liter

HYDRO PNEUMATIC LOADING SYS.

Purpose : exercise a constant load on the ball mills & compensate the wear on same

The system is composed of two parts :1. Pneumatic plant: Determines reqd. load2. Oil system : Lubricate cylinder bushes & keep the gasket in pressure for perfect seal

MILL LOADING SYSTEM

MILL LOADING SYSTEM CABINET

LOADING CYLINDER

TOP & BOTTOM RING

GRINDING BALL

GRINDING BALL (NEW & WORN OUT)

LOADING RING

INLET COAL PIPE WITH INVERTED CONE

CLASSIFIER CONE WITH SKIRT PLATES

CLASSIFIER VANES

RELIEF GATE (CLOSED)

RELIEF GATE (OPENED)

REJECT GATE

CASE STUDY

PROBLEMS FACED&

IMPROVEMENT ACTIONS TAKEN

Improvement actions

1. Highly skilled maintenance practices were developed for mill overhauling

2. Spare mill top housing is being kept ready before hand

3. Readiness of mill bay hoist is ensured

These resulted in reduction of mill overhauling duration from 360 hrs to 72 hrs

Longer mill overhaul period

Problem faced

SPARE MILL TOP BODY ASSEMBLY

Improvement actions

Day 0 : Ensure healthiness of hoistSpare mill top body housing is renovatedAll new spares are available near millErection of necessary approach platforms

Day 1 : Removal of coal pipes

Removal of top mill body assemblyRemoval of loading ringRemoval of top ringRemoval of grinding balls

Day 2 : Cutting & removal of air regulating rings

Removal of bottom ringRemoval of coal drop plateReplacement of middle shell wear platesLoading of new bottom ringReplacement of U clamps, knee assy.

Problem faced

Improvement actions

Day 3 : Replacement of air regulating ringFull welding of middle shell wear plates & air regulating rings with protection bars.Loading of new grinding ballsLoading new top ringLoading of spider ringAssembly of U clamp and knee assy.Replacement of mill top body assy. with spare top body assy.Re-fixing of coal inlet pipeRe-fixing of PF discharge pipe

Problem faced

Improvement actions

1. Rack & pinion arrangement were taken out side of the tramp iron gate assembly

2. Stone pickers are introduced in CHP

This eliminated the problem of choking / jamming

Frequent choking / jamming of reject passages

Problem faced

Improvement actions

1. Highly skilled maintenance practices were developed for mill gear box servicing.

2. Scheduled replacement of gear box was adopted.

3. Stringent QP for procurement4. Photos

These resulted in elimination of mill gear box break down.

Frequent mill gear box break downs

Problem faced

1. Machining of Turn Plate at landing surface

2. Banding of bottom rings3. Photos

These resulted in reduction of premature failure of grinding rings.

Improvement actionsProblem faced

Pre-mature failure of grinding rings

1. Replacement of original carbon steel PF pipes by ceramic pipes

2. Minimizing flanged joint by direct welding, wherever possible

3. Ensuring proper machined mating surface at flanged joint

This resulted in elimination of mill outages due to PF leakages

Improvement actionsProblem faced

Mill outages due to PF leakage

MAINTENANCE PRACTICES

RAMAGUNDAM SUPER THERMAL POWER STATIONNTPC LIMITED

• To maximize availability of generating units with minimum maintenance cost and downtime.

• To carry out maintenance jobs as per laid downprocedures within stipulated time.

• To plan for overhauls, statutory inspection and toobtain statutory certificate from the competentauthority for the safe running of the equipment.

• To ensure safety during isolation, maintenanceand re-commissioning.

OBJECTIVEOF

MAINTENANCE MANAGEMENT SYSTEM

Our philosophy is for Zero breakdown between

overhaul to overhaul by adopting various new

initiatives like – Outage preparedness index.– Activity Based planning and monitoring– Contract rationalization– Rolling plan of equipment.– REAP

MAINTENANCE STRATEGY

Preventive Maintenance. Predictive Maintenance. Proactive Maintenance. Planned maintenance

(Overhaul/Servicing) Break Down Maintenance Life Assessment.

MAINTENANCE PRACTICES

PREVENTIVE MAINTENANCE

PM is time based maintenance. The frequency & scope is based on-

OEM RecommendationSite Experience

After SRCM, this frequency and scope have been revised. This is mainly based on failure modes.

MAINTENANCE PRACTICES :DURING 4 WEEKLY PM CHECKS :

1. Mill Internal inspection & Repair.2. Mill gear box Internal Inspection.3. Loading system rectification.4. Mill coupling Healthiness 5. Checking.6. Measurement of Grinding Rings & Balls.7. Checking of Plough brush assly.

PREDICTIVE MAINTENANCE

Technologies in use : Vibration analysis Infrared thermo graphy Motor current signature analysis Wear debrisanalysis

PdM EXISTING PRACTICE

Vibration monitored at pre determined

frequencies and abnormalities reported

are for corrective action.

Trending done for critical equipments to

keep a track of the equipment behavior.

VIBRATION ANALYSIS

Oil sample is collected from various equipment at pre determined frequencies for wear debris analysis

Being done at site by condition monitoring group If required, sample of the oil is sent to R&D for

detail analysis Trending of the result being done to plan for

inspection Corrective action is taken up after receipt of report

WEAR DEBRIS ANALYSIS

MAINTENANCE PRACTICES :DURING RUNNING :

1. Gear box Oil level and Oil Pressure checking2. Gear box vibration, temperature and abnormal

sound checking3. Monitoring operating parameters4. Checking for PF Leakages5. Checking for hot air leakage6. Checking for any oil leakage7. Any abnormal sound from mill

PROACTIVE MAINTENANCE

Proactive maintenance is in the case of repeated failures. We go in to root cause of failure & do necessary correction by way of change in operating practices / small modification in system / maintenance practices.

PLANNED MAINTENANCE

Practices:• Pre-overhaul• During overhaul • Post overhaul

PLANNED MAINTENANCE

Pre-overhaul:• Preparing Engineering declaration of all the

units as per OPI guidelines.

• Identification of resources ( materials, contracts & T&P).

• Review of the materials & contracts status.

• Distribution of engineering declaration to all concerned before the start of overhaul

PLANNED MAINTENANCE

During overhaul:

Ensuring proper isolation of the equipments

before start of the work.

Making Quality check protocols

Making Re-commissioning check sheets

Maintaining Communication log book

PLANNED MAINTENANCE

During overhaul:• Safety survey & training to agency personnel.

• Constituting Knowledge team

• Daily review meeting

• Review of works planned & actual done.

• Feed back to all concerned.

PLANNED MAINTENANCE

Post-overhaul:

• Preparing executive summary

• Preparation of Unit overhaul completion

report.

• Post overhaul review of improvement

• Pay back calculation

PLANNED MAINTENANCE

BREAKDOWN MAINTENANCE

• Defect registration is normally done by

operation group

• Emergency Nature : Respective Maintenance

to act immediately

• Other Jobs : Scheduled by MTP and then

maintenance to rectify the defect.

BREAKDOWN MAINTENANCE

Management Information System

1. Daily report

2. Weekly report

3. Monthly report

4. Yearly report

5. Unit overhauling report

PULVERIZER TROUBLESHOOTING

• Reasons for Poor fineness

Incorporated slots in air regulating ring to distribute primary air in the mill properly

Classifier cone may be eroded Incorrect adjustment of

classifier bladesExcessive wear out of grinding

elementOverloading of millFuel quality has deterioratedThe Air/Fuel ratio is very high Increased through put

Pulverizer Troubleshooting

• Reason for abnormal sound from mill

Most probable is the entry of foreign matter in the grinding element

Break down of some internal parts, e.g. U clamp, knee assembly

• Reason for High rejects

Change in coal grindability, sulfur & ash

The relief gate is jammed openThe air regulating ring has too

much gap with bottom ring Grinding elements are in

precarious conditions & adding a filler ball is the necessity

• Mill lub oil pressure low

Lub oil pump is not developing pressure

Leakage from lub oil linesAir suction in lub oil lineFilter chockage Improper relief valve adjustment

• Mill coupling failure

Over load in mill due to some foreign material

Mill motor troubleCoupling material defectMill gear box failure Lack of grease in coupling

• Lack of capacity orHigh power consumption

High moisture Low GCV Increased raw coal sizeGrinding too fineExcessive wear out of

• Low coal / air temperature at mill outlet

High moisture in coal Low PA inlet temperaturePassing of cold air damperMill got unloaded

THANK YOURamagundam Super Thermal Power Station

NTPC Limited

Ball and Race Mill

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