Internal Mill Diagnostics: Troubleshooting Guide

The internal mill diagnosis (le diagnostic interne du broyeur) is the clinical examination of the ball mill: the shutdown inspection that opens the mill body and reads the state of the charge, the liners, the diaphragms, the ventilation and the material flow: when the mill output drifts, the residue rises or the energy climbs without the obvious cause, the plant stops the mill, opens it and measures: this guide follows the complete diagnostic procedure: the preparation, the sounding, the lengthwise sampling, the charge and the liner surveys, the interpretation tables and the corrective action plan that comes out of the report.

The Complete Cement Technical Package (931 files including this diagnosis file, the grinding handbooks, the audit forms and the training courses: $249.99 one-time: instant download via the PayPal payment) hosts the full diagnostic method with the forms, the charts and the worked example of a real audit: this article walks the file: the reasons and the timing of the diagnosis, the step-by-step shutdown procedure, the measurement stations, the interpretation of the curves and the actions that follow: the reader becomes able to run a mill diagnosis with his own team.

The diagnosis is the grounding discipline of the grinding department: the energy reports and the daily residues are the symptoms, the internal inspection is the truth: the mill hides its problems in the dark of its shell, and the diagnosis is the lamp: the plants that diagnose their mills on the schedule find the problems early, fix them cheaply, and keep the 90% availability: the plants that guess pay for the guesses in the months of the lost performance: this article teaches the systematic seeing.

1. The Reasons and the Timing of the Diagnosis

The diagnosis is not the emergency tool but the scheduled examination, and the file defines the triggers:

• The scheduled diagnosis: once per year or with the major shutdowns (the liner replacement, the charge audit): the annual rhythm matches the degradation cycle of the internals: the charge decays within the year, the liners within the 2 to 5 years, the diaphragms within the 3 to 6;
• The performance triggers: the output down 5% or more at the equal fineness, the specific energy up 2 kWh/t or more, the residue and the Blaine disconnected (the residue up at the same Blaine), the circulating load beyond the design envelope: any of these justifies the early diagnosis; the drift confirmed over the two weeks, not the two hours;
• The quality triggers: the cement strength falling at the same recipe and the fineness: the PSD change traced to the mill behavior: the customer complaints of the setting or the strength with the unchanged chemistry: the internal state of the mill is a prime suspect;
• The incident triggers: the mill jams, the diaphragm strands break, the liner bolts shear: the diagnosis after the incident documents the damage and the causes before the repair: the incident report of the file includes the diagnosis section;

The timing discipline: the diagnosis needs the shutdown window of 1 to 3 days for the full version, and the file plans the campaigns with the maintenance schedules: the annual mill diagnosis shares the shutdown with the liner survey and the charge audit: the three inspections use the same opening and the same stop: the efficiency of the combined campaign.

2. The Preparation: The Data and the Team Before the Stop

The diagnosis starts on the desk, not in the mill, and the preparation list of the file is the discipline of the campaign:

• The baseline data: the last three months of the daily reports: the feed rate, the fineness, the energy, the circulating load, the mill outlet temperature, the separator settings: the trends tell the doctor when the illness began;
• The design documents: the mill drawings, the charge specification, the liner layout, the previous audit reports: the diagnosis compares the found state against the documented design, never against the memory;
• The operating history: the recharge log, the water injection records, the grinding aid doses, the feed quality changes: the history of the inputs explains the found state of the outputs;
• The team: the process engineer (the team lead), the mill mechanic, the shift operator, the laboratory technician: each member owns his section of the checklist: the roles are assigned before the stop, not improvised in the dust;
• The tools and the forms: the sounding rods, the sampling scoops and bags, the callipers and the measuring tapes, the moisture meter, the templates of the file printed: the complete tool list of the file, checked against the box before the mill stops;

The preparation message: the diagnosis is the evidence-based inspection, and the evidence starts with the written state of the machine: the file’s one-page preparation sheet summarizes the data collection and the team assignment: the plants that complete it run the diagnosis in the planned hours instead of the improvised days.

3. The Sounding of the Charge: The Depth Profile of the Mill

The mill is stopped, cooled and locked, and the first physical measurement is the sounding:

• The method: the sounding rod (the 20 to 30 mm tube, 6 to 8 m long) is lowered through the inspection ports or the open diaphragm access along the mill length: the depth of the charge measured at the stations every 0.5 to 1 m: the depth profile drawn against the mill length;
• The stations: minimum 6 stations in the first compartment and 10 to 12 in the second: the ports and the openings chosen for the repeatability between the audits: the marked stations photographed in the first campaign and reused in the later ones;
• The profile reading: the constant depth along the compartment with the shallow end at the outlet: the hump at the inlet means the first compartment overload (the feed too coarse or the big balls too few): the hollow at the middle means the classifying liner failure: the rising profile toward the outlet means the fine compartment flood:
• The filling computation: the depths converted to the filling degrees per station with the chord-to-area tables: the average filling per compartment against the design: the deviation beyond 1.5 percentage points is the charge correction trigger;

The sounding is the quickest and the most informative single measurement: in the 20 minutes of the rods the engineer reads the flow state of the whole mill: the file’s sounding sheet pre-prints the station columns and the conversion table so the profiles are drawn inside the mill, in the field, before the team forgets the topography.

4. The Lengthwise Sampling: The Material Curve of the Grinding

The material inside the mill tells the grinding story, and the lengthwise sampling is its chapter: the mill is opened at the compartments and the samples taken on the grid:

• The sampling grid: the stations every 0.5 to 1 m along both compartments, the samples at the three vertical levels (the surface, the middle, the bottom of the charge): the composite sample of the station of 0.5 to 1 kg: the total 60 to 120 samples across the full mill;
• The analysis of the samples: the sieve analysis of each station composite (the 200, 90, 45 µm and the retained fractions) and the moisture: the 20 to 30 stations plotted as the residue curves along the length:
• The grinding curve reading: the ideal curve: the steep residue fall in the first compartment (the 25 mm feed to the 2 mm in the first third of the length), the gradual fall in the second, the asymptote before the outlet: the slow fall in the first compartment means the impact insufficient (the balls or the lift), the flat curve in the second means the abrasion zone starved (the small balls missing or the aid problems);
• The retention time: the material hold-up estimated from the samples and the feed rate: the total retention of the cement mill 15 to 30 minutes: the excess retention (the slow grinding) flags the cushioning and the overfine charge;

The lengthwise sampling is the peak of the diagnostic method and the file teaches the interpretation with the worked example: the plant that runs it every year builds the curve library that shows the degradation year over year with the millimeter precision: the sampling campaign of the file is designed to be executed by the plant team in the single shutdown day.

5. The Charge Survey: The Balls Under the Magnifying Glass

The balls are measured, counted and judged at the same stations, and the charge survey completes the mechanics of the grinding tool:

• The ball size measurement: at each station, a sample of 100 to 200 balls measured with the calliper: the mean diameter per station plotted: the design profile (the coarse at the inlet, the fine at the outlet of the second compartment) compared with the found profile:
• The distribution check: the percentage of the balls below the minimum useful size (the <15 mm in the fine compartment), the deformed and the broken balls, the segments: the unusable fraction above 10% of the charge is the sorting trigger:
• The coating inspection: the coating of the compacted fines on the ball surfaces: the coated charge estimated visually per station: the coating above 30% of the surface at the fine end means the moisture, the aid or the ventilation problem: the coated balls grind at the fraction of the efficiency:
• The charge weight verification: the sounding filling and the average ball density give the calculated charge weight per compartment: the comparison with the recharge log: the difference above 5% is the log or the measurement error to resolve:

The charge survey answers the three questions of the tool room: what is the size distribution, what is the damage, what is the weight: the answers feed the recharge and sorting decisions of the same shutdown: the charge file of the course (the charge broyante) and the survey here are the two halves of the same tool management.

6. The Liner Survey: The Armor and Its Wear

The liners protect the shell and shape the grinding, and their wear is measured with the instruments:

• The thickness measurement: the ultrasonic or the mechanical gauges at the marked stations of each row: the residual thickness against the new thickness: the wear rate per year computed: the remaining life = the residual margin divided by the annual rate;
• The profile check: the lifting height of the wave and the step liners with the template: the worn-down profile lifts the charge less, shifting the regime toward the rolling: the grinding rate of the first compartment decays with the profile: the profile loss beyond 30% is the replacement trigger;
• The bolt condition: the bolt heads and the threads: the loose bolts announce themselves in the knocking of the mill: the torque check and the re-tightening in the campaign: the broken bolts counted and replaced with the record:
• The failure patterns: the uneven wear at the inlet rows (the feed impact zone) is normal and the spalling beyond the pattern is the alarm: the wear spikes at the partition walls and the lifters flag the charge segregation: the photographs per station complete the report:

The liner survey is the capital planning instrument: the remaining-life data schedules the liner replacement 12 to 24 months ahead, the purchase order placed in the calm instead of the emergency: the file’s liner survey form matches the liner rows of the standard mills and prints the replacement recommendations with the budget estimates.

7. The Diaphragm and the Grate Inspection

The diaphragms are the gates of the mill, and their state decides the material flow between the zones:

• The slot and the free area: the slot widths measured (the 6 to 8 mm typical) and the free area computed against the design 2.5 to 4%: the free area lost to the debris and the wear raises the hold-up of the first compartment: the blocked slots are the first cause of the first-compartment flooding found in the campaigns;
• The plugging diagnosis: the slots blocked with the moist fine material and the ball fragments: the plugging distribution (the bottom rows first) tells the drying story: the moisture in the feed or the condensation of the mill ventilation: the remedy follows the cause, the cleaning follows both;
• The wear of the plates: the slotted plates wear from the material passage and the ball impact: the worn openings widen and pass the oversize balls into the fine compartment: the diaphragm inspection measures the plate thickness and the opening deviations:
• The lifting and the conveying: the lifters and the conveying plates of the discharge diaphragm checked for the breakage: the broken conveying segments stall the material at the outlet and raise the mill noise and the temperature:

The diaphragm findings often dominate the action plan of the report: the file’s case study found the intermediate diaphragm at 40% free-area loss and the diagnosis explained the 8% output loss of the previous quarter that no separator tuning could recover: the cleaning and the plate section replacement restored the flow within the same shutdown.

8. The Ventilation and the Temperature Profile

The mill is a gas channel as much as a grinding machine, and the diagnosis measures the breathing:

• The pressure profile: the pressure taps along the mill (the inlet, the compartments, the outlet) measured before and after the shutdown: the differentials flag the restricted passages: the delta P up with the constant fan means the internal restrictions (the plugged diaphragm, the clogged outlet):
• The temperature profile: the material and the gas temperatures at the stations: the hot spots of the charge flag the local over-grinding and the coating: the rising temperature toward the outlet above 115 °C confirms the gypsum dehydration risk zone:
• The ventilation audit: the airflow measurement at the mill inlet: the actual flow against the design (the 3.5 to 5 shell volumes per minute): the flow shortfall of 30% explains the mill temperature and the separator feed problems found in the same campaign:
• The moisture map: the material moisture sampled along the length: the moisture above 1% in the second compartment flags the drying shortfall (the raw grinding) or the water injection excess (the finish grinding): the moisture map often explains the coating found in the charge survey:

The combined reading: the pressure, the temperature and the moisture maps drawn along the same length axis become the physiological chart of the mill: the file’s chart template overlays the three maps and the material residue curve in one figure: the multidimensional picture that the report converts into the actions.

9. The Interpretation: The Fault Matrix of the Diagnosis

The measurements are the raw material; the interpretation converts them into the actions, and the file’s fault matrix is the conversion table:

Finding	Likely causes	Treatment
First compartment hump in the sounding	Feed coarse, top ball small, diaphragm restricted	Top size up, feed size control, diaphragm cleaning
Flat residue curve in the first compartment	Charge underfilled, lifters worn, speed low	Top-up + grading, liner profile restoration
Slow tail of the residue curve in the fine compartment	Fine balls starved, coating, aid problems	Graded recharge, ventilation, aid dose review
Coating above 30% on the fine charge	Moisture, poor aids, overventilation imbalance	Drying, aid trial, water injection control
Interior temperature beyond 115 °C	Ventilation low, clinker hot, circulation high	Air flow up, cooler control at the kiln, water injection
Broken diaphragms in the first rows	Impact excess, foreign material, wear	Speed review, replacement, metal detection
Liner wear beyond 30% profile loss	Charge flat, cataracting, soft liner material	Charge correction, liner redesign, steel upgrade

The matrix is the condensed experience of the hundreds of audits: the file presents it with the weights (the priorities) and the expected recovery ranges: the engineer reads the found state, crosses the matrix rows, and builds the action list with the priorities and the owners: the objectivity of the table keeps the team calm in the meeting after the dusty day.

10. The Report and the Action Plan

The diagnosis is completed only when the report is delivered, and the file’s report structure is the professional standard:

• The summary page: the state of the mill in the numbers: the filling degrees, the charge distributions, the liner residuals, the material curves: the executive summary with the verdict: healthy, moderate decay, critical:
• The evidence pages: the sounding profile, the residue curves, the charge charts, the liner maps, the photographs with the station labels: the evidence that the decisions cite:
• The action plan: the findings ranked by the impact and the urgency: the immediate actions (the cleaning, the top-up, the seal work) scheduled in the same shutdown: the short-term (the water injection tuning, the aid trial) in the month: the long-term (the liner replacement, the charge redesign) in the capital plan:
• The expected gains: the quantified recovery per action (the t/h and the kWh/t estimates from the diagnosis curves): the gains traceable in the monthly reports after the implementation:
• The follow-up: the re-measurement date (the 12 months standard) and the KPI watch list for the interim: the report becomes the baseline of the next diagnosis: the archive discipline of the grinding department:

The report is the deliverable that pays the shutdown: the plant that implements the action plan finds the 5 to 15% of the output and the 1 to 3 kWh/t typically identified: the file’s report template and the action sheet are ready to copy into the plant’s document system: the diagnosis converted into the management table.

11. The Special Cases: The Diagnosis of the Raw and the Coal Mills

The diagnostic procedure adapts to the duty, and the file covers the special cases with the adjusted sampling and the priorities:

• The raw mills: the drying dominates: the moisture map at the stations becomes the primary instrument, the hot gas temperature profile the second: the residue curves read against the drying deficit: the grinding faults and the drying faults separated by the correlation of the moisture and the fineness maps: the coating findings interpreted with the gas dew point instead of the cement quality rules:
• The coal mills: the atmosphere and the material differ: the inert gas purging before the inspection is the safety gate: the moisture and the fineness maps read against the explosion risk margins: the coal mill diagnosis focuses the charge wear (the abrasive pyrite) and the drying performance: the report adds the safety section with the inerting and the inspection protocols:
• The wet mills: the slurry lines: the material sampling becomes the pulp sampling with the density measurements: the charge survey underwater with the special procedures: the wear doubled by the corrosion: the corrosion findings feed the media grade review: the wet-mill report carries its own matrix:

The file treats the special cases honestly: the core method is universal, the emphasis shifts with the duty: the plant that masters the dry finish diagnosis transfers the discipline to the other lines with the adjustments of the file’s checklist pages: the three checklists in one document, the right one opened for the right mill.

12. The Training and the Institutionalization of the Diagnosis

The diagnosis is the team skill, and the file closes its method with the training and the institutionalization:

• The training campaign: the process engineers and the mill mechanics trained on the full procedure with the two practice diagnoses on the plant’s own mills: the certification covers the sounding, the sampling, the measurement, the interpretation and the report:
• The knowledge transfer: the alternating team leads per campaign: the senior engineer leads the first, the trained engineer leads the second with the senior in the wings, the third solo with the review: the succession plan of the grinding expertise:
• The standardized forms: the plant’s copies of the file’s forms adopted into the quality system with the form numbers: the filled forms archived with the mill history: the archive is the institutional memory that survives the staff changes:
• The KPI linkage: the diagnosis findings tracked against the mill KPIs in the management reviews: the output, the energy and the availability reviewed with the diagnosis history at the quarterly meetings: the diagnosis becomes the evidence base of the grinding management:

The institutionalization message: the mill diagnosis is not the consultant’s visit but the plant’s own capability: the file provides the complete toolbox and the training plan: the plant that adopts the method owns its mill health: the grinding knowledge stays in the plant, measured and documented, year after year.

13. The Safety of the Mill Entry: The Non-Negotiable Gate

The diagnosis enters the mill, and the entry into the rotating giant is one of the most hazardous jobs of the cement plant: the file puts the safety chapter before the procedures, and every campaign repeats it:

• The isolation: the mill motor locked out, the inching drive de-energized, the breaker tags on the start circuits: the multiple isolation points verified by the operator and the engineer together: the “can the mill move” test: no energy, no gravity, no surprises;
• The atmosphere: the gas measurement of the oxygen, the CO and the combustible gases before the entry and continuously during the work: the inerted coal mills need the positive verification of the breathable atmosphere: the ventilation of the mill with the fresh air before the team enters:
• The temperature: the mill cooled to below 40 °C at the work stations: the hot charge burns and the hot shell radiates: the cooling time after the stop planned in the campaign schedule: the thermometers on the stations confirm, the gloves and the suits protect;
• The loose materials: the charge at the top of the mill and the coating on the liners can slide: the work order requires the charge stabilization (the ratchet or the wedging of the charge in the stopped position) and the entry teams stay in the safe arcs: the harnesses and the lifelines for the high zones:
• The confined space procedure: the permit to work, the attendant outside, the two-person rule, the communication check, the emergency extraction plan: the confined space entry is the standard of the industry and the file’s permit template matches the plant’s own system: no diagnosis is worth an injury, and no campaign skips the gate;

The safety message of the chapter: the diagnostic value is zero if the team does not return whole: the file’s procedure is designed for the practical execution: the safety briefing of 15 minutes before the entry, the permit signed by the authorized person, the checklist completed: the plants that institutionalize the gate run the hundreds of campaigns without the incidents, and the file wants every reader’s plant among them.

14. Frequently Asked Questions

How long does a full internal diagnosis take?

The preparation a week (the data collection and the planning), the shutdown measurements 1 to 3 days depending on the mill size and the depth of the campaign, the report 2 to 3 days: the full cycle 2 to 3 weeks: the combined campaigns with the maintenance cut the incremental time to the single day.

Does the mill need to be emptied for the diagnosis?

No: the standard diagnosis runs with the charge in place, using the sounding rods and the sampling through the ports: the full emptying is needed only for the charge sorting and the major liner work: the in-place diagnosis saves the days of the emptying and the refilling.

What is the cost of the diagnosis versus the value?

The internal team cost is the hours of the shutdown plus the analysis, typically 5,000 to 15,000 USD: the found output gains of 5 to 15% on the 100 t/h mill are worth the 30,000 to 150,000 USD per year: the payback of the diagnosis is the shortest in the grinding expense list: the file’s case economics repeat the ratio in the three plant sizes.

How often should the diagnosis be repeated?

Annually in the continuous plants, with the ad hoc campaigns at the performance triggers (the output down 5%, the energy up 2 kWh/t, the quality disconnection): the annual rhythm matches the charge and the liner decay cycles and feeds the capital planning of the shutdowns.

Can the diagnosis be done on the vertical mill too?

Partially: the VRM has no charge to sound, but the material inspection, the wear measurement of the table and the rollers, and the airflow checks follow the same logic: the VRM has its own diagnostic file in the package: the internal diagnosis of this file is the ball mill tool, and the VRM companion covers its machine.

Who should lead the diagnosis in the plant?

The process engineer of the grinding department, supported by the mill mechanic and the shift operator: the training plan of the file prepares the in-house leads: the external specialists are reserved for the first campaign and the complex cases: the plant owns its expertise: the ownership is the point of the whole institutionalization chapter.

15. Conclusion

The internal mill diagnosis is the discipline that turns the shutdown hours into the knowledge: the sounding, the lengthwise sampling, the charge and the liner surveys, the ventilation maps and the fault matrix: the report with the action plan restores the 5 to 15% of the output and the kWh/t of the energy that drift away with the wear: this guide walked the complete procedure with the forms and the numbers: the plant that adopts the method sees its mills with the new eyes.

The Complete Cement Technical Package includes this diagnosis file with the forms, the charts and the worked example: the one-time $249.99: the instant download: the 931 files of the cement library: the grinding inspection knowledge of the industry, organized: the mill of the plant, opened and understood: the maintenance capital, planned: the engineering career, completed.

This file is part of the Complete Cement Technical Package (931 files) available from cementequipment.org. Respective rights holders; library copy for the licensed single user.