Clinker Cooler Maintenance: Complete Guide

The clinker cooler is the most punishing machine in the cement plant: it receives clinker at 1,300 to 1,450 degrees Celsius from the kiln discharge, quenches it in seconds, recovers the heat for the combustion air and the drying, and delivers a product that the conveying, the grinding and the dispatch systems can handle. The cooler operates at the thermal extremes, in a dusty and abrasive environment, with moving parts — the grates, the drives, the fans, the hydraulic systems and the seals — that are exposed to the hot clinker bed and the process gas at every hour of every campaign. The maintenance of the clinker cooler is therefore not a support activity but a production function: the cooler’s availability, its air distribution, its grate condition and its seal integrity set the kiln production, the clinker quality, the fuel consumption and the emission performance of the entire line. This article is the complete engineering guide to clinker cooler maintenance: the cooler types and the design principles, the maintenance of the grate plates, the hydraulic drives, the cooling air fans and the seals, the refractory care, the instrumentation, the preventive maintenance planning, the troubleshooting, the safety and the performance assessment.

1. The Clinker Cooler and Its Role in the Plant

The clinker cooler sits between the kiln and the clinker storage, and its functions define the performance of the pyro-processing line. The primary function is the rapid quenching: the clinker leaves the kiln with a liquid phase of 20 to 30 percent and a temperature above 1,300 degrees Celsius, and the rapid cooling freezes the alite and the belite in their desirable crystal forms, preserves the clinker’s reactivity and improves its grindability. The secondary function is the heat recovery: the cooling air, heated by the clinker bed, returns to the kiln as the secondary air and to the calciner as the tertiary air, recovering 60 to 70 percent of the clinker’s sensible heat. The third function is the transport: the cooler moves the clinker from the kiln discharge to the downstream conveying, and it must handle the variable rate, the lumps, the snowmen and the dust.

The maintenance implications of these functions are direct. The quenching requires the intimate contact of the air with the clinker, and the grate openings and the air distribution must be kept clean and even. The heat recovery requires the airtight system, and the seals and the refractory must hold the air where it is needed. The transport requires the moving parts — the grates, the drives and the bearings — to operate at the design speed under the load of the hot clinker. Every component is a maintenance commitment, and the plant that treats the cooler as a second-class citizen pays in the kiln availability, the fuel and the refractory life.

2. The Cooler Types and the Design Principles

The maintenance program is defined by the cooler type, and the plant must understand the machine it maintains. The traveling grate cooler moves the clinker on a horizontal or inclined grate that travels continuously, carrying the clinker through the air-quenching zones; its maintenance centers on the grate chains, the sprockets, the side seals and the many rollers and wheels. The reciprocating grate cooler, the dominant type of the modern plants, moves the clinker with a series of stationary and moving grate plates that stroke back and forth, and its maintenance centers on the grate plates, the drive beams, the hydraulic cylinders and the seals. The rotary cooler, used in the older and the smaller plants, is a rotating drum with the internal lifters, and its maintenance centers on the tires, the supports and the internal flights. The modern hybrid and cross-bar coolers combine the grate with the walking floor or the cross bars, adding the hydraulic and the mechanical complexity.

The design principles that govern the maintenance are the air distribution, the grate material and the drive arrangement. The grate plates are cast in the high-chromium alloys and the heat-resistant steels, sized to pass the cooling air while holding the clinker, and their wear is the dominant maintenance cost. The air distribution is engineered through the fixed and the movable air supply zones, and the plugging of the grate and the aeration is the dominant maintenance problem. The drive is hydraulic or mechanical, with the stroke, the speed and the load regulated by the controller, and its oil system and its components are a scheduled maintenance program in themselves. The design determines the criticality: the modern reciprocating coolers have 400 to 1,200 grate plates, each a wear item, and the maintenance plan is built around the plate life, the inspection interval and the replacement campaign.

3. The Grate Plate Maintenance

The grate plates are the working surface of the cooler, and their maintenance is the largest single work package. The plates carry the clinker, distribute the cooling air and wear by the abrasion of the moving clinker, the thermal cycling and the occasional impact of the large lumps. The wear patterns are diagnostic: the wear at the air slots indicates the air velocity and the dust carryover; the wear at the trailing edge indicates the clinker flow direction; the cracking indicates the thermal stress and the restraint; and the melting and the distortion indicate the kiln nose-ring falls and the snowmen. The inspection of the grate surface at the scheduled stops is the core maintenance activity, and the maintenance records track the wear of each plate zone against the production.

The maintenance practice of the grate plates includes the cleaning, the replacement and the aeration care. The cleaning removes the fused clinker and the buildups from the plate surfaces and the air slots, and it restores the air distribution; the plugged slots are a silent production loss because the air redistributes to the open zones and the clinker quenches unevenly. The replacement is planned by the wear measurement: the plate thickness, the slot condition and the distortion are measured at the inspections, and the plates are replaced in zones before the failure, rather than individually at the failure. The replacement campaign is a scaffolded, confined-space operation with the hot clinker above and the drive below, and its planning — the spare plates, the tools, the permits and the sequence — is the difference between a 24-hour and a 72-hour job. The aeration care is the attention to the air slots and the gaps: the modern coolers run the grate with aeration gaps that equalize the pressure, and their condition is part of the inspection.

4. The Hydraulic Drive Systems Maintenance

The reciprocating cooler’s drive moves the grates in the slow, powerful strokes that transport the clinker, and its hydraulic system is the plant’s most demanding hydraulic duty: the high pressures, the continuous operation, the heat and the dust. The hydraulic drive consists of the oil reservoir, the pumps, the control valves, the accumulators, the cylinders and the piping, and the maintenance program covers the oil condition, the filtration, the component wear and the leak integrity. The oil is the system’s lifeblood: the high-temperature environment degrades the oil, the dust and the water contaminate it, and the oil analysis — the viscosity, the acidity, the particle count and the water content — is the scheduled watch on the system’s health.

The common failure modes of the hydraulic drive are the pump wear, the valve sticking, the accumulator failure and the cylinder seal leakage. The pump wear shows as the reduced pressure, the noise and the rising oil temperature; the valve sticking shows as the erratic grate stroke and the uneven clinker transport; the accumulator failure shows as the pressure pulsation and the reduced energy storage; and the cylinder seal leakage shows as the oil consumption, the erratic motion and the contamination of the grate area. The maintenance actions are the scheduled oil changes, the filter replacement, the accumulator charging and testing, the cylinder seal replacement and the pressure and the flow testing of the system. The safety of the hydraulic work is critical: the stored energy in the accumulators and the lines must be relieved before any component is opened, and the hot oil presents the burn and the fire risk.

5. The Cooling Air Fans and the Air Distribution Maintenance

The cooling air is the cooler’s working fluid, and the fans that deliver it are the largest energy consumers of the pyro line. The under-grate fans deliver the air to the grate zones, the overbed and the hood fans shape the flow above the bed, and the vent and the dedusting fans handle the exhaust. The maintenance of the fans covers the impeller, the bearings, the drive and the dampers: the impeller wear by the abrasive dust, the bearing life, the belt or the coupling alignment and the damper operation. The fan performance is measured by the flow, the pressure and the power, and the degradation is diagnosed by the comparison of the operating data with the fan curve.

The air distribution is the maintenance-critical concept: the cooler’s efficiency depends on the uniform air through the clinker bed, and the air takes the path of the least resistance. The uneven bed, the plugged grate, the worn seal and the damaged refractory all redistribute the air, and the symptoms appear in the clinker — the hot spots, the red rivers, the undercooled clinker — and in the process — the rising secondary air temperature, the rising cooler vent temperature and the increased fan power. The maintenance program for the air distribution includes the under-grate pressure monitoring, the zone temperature profiling, the grate cleaning and the seal and the refractory inspection. The diagnostic surveys — the air flow measurement at the zones, the clinker bed sampling across the width and the infrared thermal imaging of the cooler — are the planned activities of the cooler maintenance, and they convert the invisible air distribution into the visible data.

6. The Seal Maintenance

The seals of the clinker cooler are the often-overlooked components whose failure costs the plant its heat recovery and its air control. The seals exist at every interface: the kiln hood seal at the cooler inlet, the stationary-to-moving grate seals between the fixed and the moving frames, the side seals along the grate width, the hydraulic cylinder glands and the duct and the casing joints. The loss of the seal allows the cooling air to bypass the clinker, the hot gas to leak to the environment, the dust to escape and the secondary and the tertiary air to be contaminated. The seal wear is continuous, driven by the abrasion, the thermal cycling and the movement, and the seal inspection is a scheduled stop activity.

The maintenance practice of the seals includes the adjustment, the replacement and the design improvement. The adjustable seals — the side seals with the wear compensation, the hood seals with the spring loading — are adjusted at the scheduled intervals to restore the contact. The worn and the damaged seals are replaced before the leakage distorts the process. The design improvement addresses the chronic failures: the plant that replaces the soft seal with the harder grade, or adds the spring compensation, or installs the wear plates that protect the seal, converts the recurring maintenance into the extended life. The seal performance is measured indirectly: the under-grate pressure, the cooler casing draft, the secondary air temperature and the dust emissions at the leaks are the process indicators of the seal condition.

7. The Refractory and the Lining Care

The cooler’s refractory protects the casing and the grates from the clinker heat and the thermal shock, and its maintenance is the protection of the steel structure. The refractory in the kiln hood, the cooler inlet, the gables, the sidewalls and the dead plates is castable, brick or monolithic, and it degrades by the thermal shock, the abrasion and the chemical attack of the clinker and the alkalis. The failure of the refractory exposes the casing to the hot clinker, distorts the steel and damages the surrounding components, and the repair of the refractory is a scheduled stop activity with the curing requirements that drive the stop duration.

The refractory maintenance includes the inspection, the patching, the replacement and the anchoring care. The inspection at the stops examines the thickness, the cracks, the spalling and the anchoring integrity; the thickness is measured by the drilling and the sampling or the thermographic surveys, and the residual thickness defines the remaining life. The patching repairs the localized damage with the castable or the plastic refractory, and the replacement campaigns renew the zones at the end of their life. The anchoring care — the anchors and the studs that hold the castable — is the hidden condition: the lost anchors precede the lining collapse, and the anchor inspection is part of every refractory check. The refractory records, with the thickness and the condition of each zone at each inspection, are the basis of the lining life prediction and the replacement planning.

8. The Instrumentation and the Monitoring of the Cooler

The modern cooler is instrumented for the control and the diagnosis, and the maintenance of the instrumentation is the maintenance of the cooler’s eyes. The instrument list includes the under-grate pressure transmitters in the zones, the bed temperature probes, the hood temperature, the secondary and the tertiary air temperatures and pressures, the grate speed and the stroke counters, the drive pressure and the hydraulic oil temperature, the fan flow, the pressure and the motor current, and the clinker temperature at the discharge. The signals feed the cooler controller, which regulates the grate speed, the under-grate air flow and the fan dampers to hold the clinker temperature and the bed condition.

The maintenance of the instrumentation covers the calibration, the cleaning and the replacement. The pressure taps and the probes plug with the dust, and their cleaning is a scheduled activity; the plugged tap reads wrong, and the controller responds to the wrong signal. The calibration of the transmitters and the analyzers is scheduled against the standards, and the records of the calibration are part of the instrument care. The replacement of the worn probes — the bed temperature probes burn and erode in the hot clinker — is a stock item, and the spare management of the probes is a maintenance responsibility. The condition monitoring of the mechanical components — the vibration of the fans and the drives, the oil analysis of the hydraulics, the thermal imaging of the casing and the bearings — completes the monitoring picture, and the alarms and the trends are the early warning of the failures that the inspection would find too late.

9. The Preventive Maintenance Planning

The preventive maintenance of the cooler is the schedule of the inspections, the servicing and the replacements that prevents the failures before the stop. The plan is built on the manufacturer’s recommendations, the plant’s history and the condition monitoring, and it is organized by the component, the interval and the stop window. The daily tasks include the visual inspection of the grate area, the lubrication of the accessible bearings, the checking of the hydraulic oil level and the pressure, the review of the cooler trends and the cleaning of the accessible dust accumulations. The weekly tasks include the detailed inspection of the hydraulics, the filter checks and the fan bearing checks. The monthly tasks include the oil sampling, the calibration of the key instruments and the detailed grate inspection.

Component	Typical Inspection Interval	Typical Servicing Action	Failure Indicator
Grate plates	Monthly to quarterly	Cleaning, wear measurement, zone replacement	Hot spots, red rivers, uneven clinker
Hydraulic oil	Weekly to monthly	Sampling, filtration, scheduled change	High temperature, erratic stroke, noise
Hydraulic cylinders	Monthly to quarterly	Seal inspection, gland tightening, seal replacement	Oil consumption, erratic motion
Cooling air fans	Weekly to monthly	Vibration check, bearing greasing, impeller cleaning	Vibration, reduced flow, overheating
Seals	Quarterly	Adjustment, wear plate check, replacement	Under-grate pressure loss, dust leaks
Refractory	Annual or major stop	Thickness check, patching, zone renewal	Casing hot spots, spalling, exposed steel
Instrumentation	Monthly to quarterly	Cleaning of taps, calibration, probe replacement	Drifting signals, plugged taps, bad control

The plan is executed through the work-order system, with the tasks, the procedures, the spares and the crafts defined for each job. The annual major stop is the campaign that renews the cooler: the full grate inspection and the zone replacement, the hydraulic overhaul, the fan inspection and the refractory repair are planned together with the kiln’s refractory campaign, and the sequence and the resources are scheduled to minimize the line downtime. The maintenance plan’s effectiveness is measured by the failure rate, the mean time between failures and the maintenance cost per tonne, and the plan is revised with the history.

10. The Troubleshooting of the Common Cooler Problems

The troubleshooting of the cooler problems is the systematic diagnosis of the symptoms to the causes. The most common problems are the hot clinker at the discharge, the red rivers and the hot spots on the grate, the snowmen and the clinker build-ups, the uneven grate movement, the high vent temperature and the high power consumption. The hot clinker at the discharge is the symptom of the insufficient cooling, and its causes are the low under-grate air, the uneven air distribution, the high clinker rate or the high clinker temperature from the kiln; the diagnosis compares the actual with the design air and the clinker flow, and the correction is the air adjustment or the grate speed change.

The red rivers and the hot spots are the localized hot clinker, the symptom of the air bypass and the uneven bed; the causes are the plugged grate, the worn seal, the clinker channeling and the build-ups that divert the flow, and the correction is the cleaning, the seal repair and the air rebalancing. The snowmen and the build-ups are the sticky clinker and the dust accumulations that block the flow; the causes are the high liquid clinker from the kiln, the low air in the inlet zone, the dead plates and the cold corners, and the prevention is the kiln control, the inlet air management and the periodic cleaning. The uneven grate movement is the symptom of the hydraulic and the mechanical problems, and its diagnosis is the drive pressure, the flow and the mechanical binding. The troubleshooting is structured: the symptom is defined, the process and the machine data are collected, the possible causes are ranked by the evidence and the correction is verified by the result.

11. The Safety in the Cooler Maintenance

The cooler is a confined, hot, moving and dusty environment, and the safety of the maintenance work is the first priority of every task. The major hazards are the heat and the hot material, the moving machinery, the confined space, the dust, the hydraulic energy and the working at height. The heat hazard is managed by the cooling and the inspection: the cooler is purged with the air and the temperature is verified before the entry, and the hot clinker is removed or covered. The moving machinery hazard is managed by the lockout-tagout of the drives, the fans and the hydraulics, and the accidental motion is prevented by the mechanical blocking of the grates and the cylinders. The confined space hazard is managed by the permit system, the gas testing, the ventilation and the attended entry, with the rescue equipment prepared. The dust hazard is managed by the respiratory protection and the dust suppression, and the hydraulic energy hazard is managed by the pressure relief and the blocking of the accumulators and the cylinders.

The safety program is not a separate activity but the framework of the maintenance procedures: every job has its risk assessment, its permit, its isolation and its rescue plan, and the supervision verifies the controls before the work starts. The hot-work permits for the welding and the cutting in the dusty environment, the fire watch during the hot work and the emergency drills for the confined-space rescue are the standard elements of the cooler maintenance safety, and the record of the incidents and the near misses is the input to the improvement of the procedures.

12. The Cooler Performance Assessment

The performance assessment of the cooler is the quantification of its function: the heat recovery, the clinker cooling and the air control. The key performance indicators are the cooler efficiency — the recovered heat as a percentage of the clinker’s sensible heat — the secondary and the tertiary air temperatures, the vent air temperature and the flow, the clinker temperature at the discharge, the specific air consumption (the air per kilogram of clinker) and the specific power consumption. The measurements are taken in the planned surveys: the air flow traverses at the fans and the ducts, the temperature traverses at the vent and the clinker, the clinker sampling at the discharge and the grate, and the under-grate pressure and the bed temperature profiles across the width and the length.

The assessment compares the measured performance with the design and the baseline, and the gaps are the maintenance and the operation opportunities. A high vent air flow with a low secondary air temperature indicates the air that bypasses the heat recovery through the leaks and the poor distribution. A high clinker discharge temperature indicates the low total air or the poor air utilization. A high under-grate pressure with a high fan power indicates the plugged grate or the fine clinker that resists the air. The assessment’s output is the priority list of the maintenance actions, and the re-measurement after the work verifies the improvement. The cooler performance assessment is the maintenance program’s feedback loop, and the plants that measure their coolers manage their heat recovery and their fuel.

13. The Spare Parts and the Stock Management

The cooler’s wear parts are a significant annual expenditure, and their management is a maintenance discipline. The key spares are the grate plates in the grade of the main design, the hydraulic seals and the filters, the fan bearings and the belts, the refractory materials and the instrument probes. The stock levels are set by the failure history and the delivery time: the high-consumption, long-delivery items — the grate plates, the seals — are stocked for the full zone replacement, while the low-consumption items are stocked at the minimum. The inventory review is the annual activity, with the consumption, the usage rate and the delivery time of each item reviewed and the stock adjusted.

The quality control of the spares is the hidden maintenance factor: the grate plates from the wrong casting or the wrong grade fail early, the seals from the wrong compound fail at the wrong temperature, and the wrong grade of the refractory fails in the campaign. The spare parts are specified against the original design, the suppliers are qualified, and the incoming inspection verifies the part and the material certificate. The storage of the spares — the seals out of the light and the ozone, the refractories dry, the bearings clean and greased — is the preservation of the investment. The plants that manage the spares as an engineering inventory, rather than a warehouse, buy the availability of the cooler.

14. The Cooler Upgrades and the Modernization

The older coolers are the modernization opportunities, and the maintenance program is the platform of the upgrade projects. The common upgrades are the grate plate improvements, the air distribution redesign, the hydraulic drive renewal, the seal improvements, the addition of the clinker crushers and the cooler instrumentation and control upgrades. The grate plate improvements — the harder alloys, the optimized air slots, the better geometry — extend the plate life and the air distribution. The air distribution redesign — the zoned air supply, the aeration optimization, the bed fluidization — improves the quenching and the heat recovery. The seal improvements — the spring-loaded side seals, the better hood seals — reduce the air bypass and the dust. The control upgrades — the bed height measurement, the automatic grate speed, the air flow control — stabilize the operation and reduce the operator dependence.

The upgrade projects are executed with the same discipline as the maintenance campaigns: the scope is defined by the performance assessment, the engineering and the risk are reviewed, the stop window is scheduled and the performance is verified after the restart. The upgrade’s economic justification is the improved availability, the reduced fuel and power, the better clinker quality and the reduced maintenance, and the plant that modernizes its cooler systematically converts an old liability into a competitive asset.

15. The Root Cause Analysis of the Cooler Failures

The root cause analysis of the cooler failures is the discipline that converts the recurring problems into the one-time corrections. The method is the structured investigation: the failure is defined, the evidence is collected — the data, the inspection, the metallurgical analysis, the operating history — the causes are analyzed in the layers of the immediate cause, the contributing cause and the root cause, and the corrective actions address the root cause rather than the symptom. The classic example is the recurring grate plate failure: the immediate cause is the plate cracking, the contributing cause is the thermal shock from the snowmen, and the root cause is the kiln operation that produces the snowmen and the inadequate inlet air; the correction is the kiln control and the inlet air management, not the more frequent plate replacement.

The RCA is applied to the high-impact failures — the hydraulic drive failures, the fan failures, the refractory collapses, the grate failures — and its output is the set of the corrective and the preventive actions with the owners and the dates. The RCA register is the plant’s failure memory, and the review of the register at the planning of the stops and the budgets ensures that the corrections are implemented. The plants that analyze their failures, rather than repeat their symptoms, reduce their maintenance costs and their downtime in the compounding effect of the years.

16. The Integration of the Cooler with the Kiln and the Grinding

The cooler is the interface between the kiln and the grinding, and its maintenance cannot be isolated from the two. The kiln side is the clinker condition: the cooler receives the clinker whose temperature, liquid content, lump size and dust content are set by the kiln, and the kiln disturbances — the snowmen, the ring falls, the unstable burning — are the first causes of the cooler problems. The maintenance of the kiln’s discharge end — the nose ring, the kiln hood, the burner pipe — is part of the cooler’s protection, and the coordination of the kiln and the cooler maintenance at the same stop is the standard practice. The grinding side is the clinker quality: the cooled clinker’s temperature, its size distribution and its grindability set the finish mill’s performance, and the clinker storage and the conveying between the cooler and the mill are the shared responsibility.

The integration is operational as well as mechanical: the cooler’s control is coupled to the kiln’s production rate, and the maintenance of the cooler’s control and instrumentation supports the kiln’s stable operation. The cross-functional maintenance planning — the kiln, the cooler and the mill schedules reviewed together — maximizes the use of the stop windows and the crafts. The plant that maintains the cooler as the integral link of the line, rather than a separate machine, achieves the line availability that the component maintenance alone cannot deliver.

17. The Documentation and the Knowledge Management

The cooler’s maintenance documentation is the plant’s memory of the machine: the as-built drawings, the manufacturer’s manuals, the maintenance procedures, the inspection records, the failure analyses and the spare parts lists. The documentation is organized and current, and its use is disciplined: the work orders reference the procedures, the inspection records feed the wear analysis, and the failure analyses feed the planning and the design. The key records are the grate wear history by zone, the hydraulic oil and the component history, the fan vibration history, the refractory thickness history, the seal life history and the cooler performance survey results.

The knowledge management extends the documentation: the experienced maintenance personnel hold the tacit knowledge of the cooler, and the transfer of that knowledge — the training of the new personnel, the procedures that capture the experience, the mentorship and the handover documentation — is a maintenance responsibility. The cooler maintenance training covers the machine, the procedures, the safety and the diagnosis, and the plant that trains its people and documents its practice maintains its cooler with the competence that the documentation alone cannot provide.

Frequently Asked Questions

What is the most critical component of the clinker cooler to maintain?

The grate plates and the air distribution are the most critical: the plates are the largest wear item and the air distribution determines the quenching and the heat recovery. The plugged grates and the worn seals silently degrade the cooler performance, and their maintenance is the core of the cooler program.

How often should the cooler be inspected?

The visual and the operational checks are daily, the detailed component inspections are weekly to monthly, and the full cooler inspection with the grate wear measurement, the hydraulic review and the refractory check is at the scheduled stops, typically the annual major stop aligned with the kiln’s refractory campaign.

Why does the hydraulic drive of the cooler require special attention?

Because it operates continuously under high pressure in a hot, dusty environment. The oil degrades and contaminates, the components wear, and the failure of the drive stops the grate movement and, with it, the kiln operation. The oil analysis, the filtration and the scheduled component checks are essential.

How does the seal condition affect the cooler’s heat recovery?

The worn seals allow the cooling air to bypass the clinker bed and the hot gas to leak. The bypassed air does not recover the heat, the secondary and the tertiary air temperatures fall, and the fuel consumption rises. The seal maintenance is therefore directly linked to the thermal efficiency.

What are the signs that the cooler performance has degraded?

Hot clinker at the discharge, red rivers and hot spots on the grate, rising vent air temperature and flow, rising fan power at the same clinker rate, falling secondary air temperature, and the drifting under-grate pressures are the classic signs, and the scheduled performance surveys quantify them.

Summary

The clinker cooler is the critical interface of the pyro line, and its maintenance is a production discipline: the grate plates, the hydraulic drives, the cooling air fans, the seals and the refractory each require their scheduled care, and the instrumentation and the performance surveys convert the invisible air distribution and the heat recovery into the measurable data. The preventive maintenance plan organizes the daily, the weekly and the annual activities; the troubleshooting diagnoses the common problems to their causes; and the root cause analysis prevents the recurrence. The safety of the cooler work, the management of the spares, the modernization opportunities and the integration with the kiln and the grinding complete the program. The plant that maintains its cooler systematically — measuring its performance, planning its stops, analyzing its failures and training its people — runs its kiln with the availability, the fuel economy and the clinker quality that the cooler makes possible, and it buys the production reliability of the entire line with the maintenance of a single machine.

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