Preventive Maintenance

Preventive maintenance or planned maintenance is the maintenance measures carried out to improve the asset performance and prevent the failures before they happen. 

Different types of Preventive Maintenance include

1.  Time Based Maintenance (TBM) that  recurs periodically and is based on a specific calendar schedule. Weekly, Monthly, Quarterly, Annual, Bi-annual are examples of TBM.  Time based maintenance is generally based on elapsed time and  recurs over fixed intervals, regardless of their usage in that period. 

A variant of TBM is floating, wherein the successive maintenance schedule occurrence will depend on the previous completion.  Eg. Annual Calibration if instrument that’s  due on June 30, was completed on June 15.  Next annual calibration due date is June 14 of next year and not June 30^th. 

2.  Condition Based Maintenance (CBM) is based on the equipment conditions or performance.  Usage based maintenance may recur based on run hours or cumulative production units etc.  Condition based maintenance may recur when the critical parameters of the equipment breach the control levels.

3.  Event Based Maintenance (EBM) occurs upon the incidence of a certain event like annual fire drill, customer visit etc. Turn around or Shut down maintenance is a major event

1. Easy to set up and administer
2. Minimal training of resources required in preparing and executing schedules
3. Does not require complex IT support systems.  Can be easily driven out of excel sheets or paper-based mechanisms, thus requiring no additional investments or budget.
4. Since TBM recurs over set intervels, schedules and checklists are consistent and repeatable.  Standardization and quick roll out is easier
5. Useful and effective for continuously run assets and seasonal maintenance like Boilers, HVACs, furnaces.

1. Optimal period selection is critical.  Too frequent occurrence impacts availability and wastage of resources (spares and labour) and prolonged schedules increases the risk of failures.
2. As the assets age, they require more maintenance.   Time based maintenance occurs over fixed intervals, with out taking into account the equipment ageing.
3. Time based maintenance follows the preset patterns and may overlook other causes of  equipment failures.  Increases the risk as the asset ages.
4. Ineffective for assets not used  regularly
5. Often involves excessive maintenance and increases costs and consequential risks due to excessive maintenance.

1. More reliable as it’s based on hard data of critical parameters
2. Avoids catastrophic failure before they occur saving huge efforts, time and money
3. Factors operating conditions as well
4. Improves the overall reliability of the asset
5. Configuring alerts on occurrence of first symptoms gives enough time to plan maintenance with out causing stoppage.  Avoids costly emergency spares procurement and overtime labour
6. Lays the foundation for predictive maintenance.

1. Need supporting IT infrastructure to monitor critical parameters and reporting
2. Learning curve involved and may require additional training for users
3. Cost of initial procurement and running costs are involved for the supporting IT infrastructure
4. Retrofitting and invasive procedures may be involved.  Inappropriate sensors not suited to operating conditions may result in their frequent failure causing false alarms or frequent outage
5. Maintenance schedules are not predictable impacting negatively on optimal planning and resource levelling.

Shut down maintenance or Turn Around Maintenance is a comprehensive maintenance of all plant machinery, planned with an complete outage of the plant.  Process industries are operated continuously and 24 * 7 and hence their periodic maintenance requires such large scale shut down. This helps in managing all the critical assets and related equipment together and in a coordinated way. 

Shut down is a major activity and hence associated comprehensive planning, budgeting, staffing, organization of resources and spares and continuous monitoring.  Shut down maintenance happens once a few years and includes certain known major repairs earmarked and also comprehensive inspection of all assets to identify any potential failures and corrective measures. 

Project management activities, risk management, critical path, budget management, schedule and cost variance tracking, resource levelling, vendor coordination are some of the critical activities associated with Shut Down maintenance.  Modern CMMS tools like MaintWiz help in managing the Shut Down maintenance planning, execution, budgeting and documentation effectively.

Preventive Maintenance Schedules or PM Schedules are  those trigger conditions,  which when satisfied, will generate a Preventive Maintenance Work Order for a specific asset.  

PM Schedules will follow a set pattern  like weekly on Monday, Wednesday  and Friday, 5^th of every month or last Sunday of every third month etc. or 365 days since completion of last work order generated by the same parent schedule or for every 3,000 equipment run hours.  

In addition to these trigger conditions, PM schedules will also contain the specific asset details, the list of jobs that need to be completed, sequencing of steps, if any, assignment details like individual or group who will execute the job.  PM Schedules may also contain the spares that needs to be changed, maintenance and safety instructions, work permits to be procured, common tools required (eg. Cranes, forklifts etc.), vendor coordinate required, if any and shift in which it needs to be carried out.  PM Schedules are configured as a parent schedule with all the details and when the trigger conditions are met, PM work orders are created by the system automatically with all underlying details and assign it to the relevant person or group.

Preventive Maintenance task lists (PM task lists), sometimes referred as PM Checklists, PM Sheets or Job Plans are the list of tasks that needs to be executed and the inspections to be carried out,  to complete the Preventive Maintenance.   PM Task lists will define who (assignment), what (task name), when (sequence, if any), how (task details and spares to be replaced) etc. 

Preventive Maintenance Compliance (PM Compliance) or Schedule Compliance is the rate of completion of Preventive Maintenance schedules. It is measured as the number of preventive maintenance activities completed to total number scheduled in a given period of time and is expressed as a percentage.

PM Compliance or Schedule Compliance is measured as the number of preventive maintenance activities completed to total number scheduled in a given period of time and is expressed as a percentage.

Effectiveness is the capability of producing a desired result or the ability to produce desired output.  Thus Preventive Maintenance to be effective it has to meet the intended business outcome of preventing breakdowns.   Preventive Maintenance Effectiveness is an indicator on the degree of success or its likelihood in achieving desired result.

PM Compliance does not measure if the PM activities have been completed within their defined timeframe, criticality of equipment, incomplete / overdue tasks, variance, sufficiency of PM schedules and tasks to prevent breakdown etc. and hence these measures are also used in addition to PM Compliance to measure PM effectiveness

1. PMC is a single metric whereas PME is more composite and includes additional vital elements
2. PMC is a standard formula whereas PME is more adaptive and customizable to specific equipment set, operations, conditions and history.
3. PMC is quantitative whereas PM is comprehensive and weights in more qualitative factors as well
4. Besides a measure on completion rate, PMC does not provide additional metrics for improvement. PM Effectiveness is more aligned with the business goals and can be defined from first principles and thus can provide more insights for continuous improvement and best practices