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Q1. Briefly explain the maintenance systems that are practiced in production/service units by providing few examples.

A way of reducing the plant breakdowns is to select the best maintenance strategy. Widely adopted maintenance techniques or strategies are:
·         Routine Maintenance.
·         Planned Maintenance.
·         Break down Maintenance or Corrective/ Remedial Maintenance.

·         Preventive Maintenance.
·         Predictive Maintenance.
·         Condition Based Maintenance.
·         Total Productivity Maintenance.

The following sub-sections give a brief explanation of these systems.

1 Routine maintenance
Routine Maintenance (RM) is a procedure followed regularly i.e. a cyclic operation recurring periodically. It includes activities like inspection, cleaning of machines, lubricating systems, small repairs, and so on. For example, checking all compressors first on Mondays, replacing light bulbs, cleaning, repairing any leakages of oil, and lubricating machines daily. RM can be classified as:
Running maintenance: is the work carried out when the equipment or the machine is performing some operations that is, when the machine is working. These include say greasing or lubricating the bearings or systems, outer cleaning, inspection and so on.
Shut down maintenance: certain minor maintenance activities cannot be carried out when the machine is running and hence must be carried out by shutting down the machine. For example, de-scaling furnaces and boilers.

2 Planned or scheduled or productive maintenance
Planned maintenance is the activities carried out according to a predetermined schedule and hence known as scheduled or productive maintenance. It involves inspecting all machineries, overhauling, lubricating, repairing, and carrying out all requisite maintenance before actual break down happens, thus avoiding a situation of emergency maintenance. Planned maintenance reduces the machine downtime, reduces cost of maintenance, and increases productivity as compared to unplanned maintenance.
3 Break down or corrective or remedial maintenance
Breakdown Maintenance is the method of operating the machines to run until they fail and then repair in order to restore them to an acceptable condition. Planned repair or rectifying the problem is carried out when it is more convenient and cost effective. This method is also called as „on-failure maintenance or corrective maintenance. It is carried out when an item has failed or worn out, to bring it back to working order. Corrective maintenance is carried out on all items where, failure or wearing out is not significant and the cost is not greater than preventative maintenance.

4 Preventive maintenance
Preventive Maintenance (PM) is based on the idea “Prevention is better than cure”. PM is a regularly scheduled maintenance activity, with an objective to anticipate problems and correct them before they occur. PM is carried out and normally programmed, to prevent an item failing or wearing out by:
• Providing systematic inspection.
• Using recommended grades of oil.
• Periodical lubrication and cleaning.
• Detecting and preventing incipient failure.

PM is carried out on those items where a failure would result in expensive consequences. For example, lifts, fire alarms, electricity supply, battery back up at sub-station, sub-station transformers, distribution transformers, circuit breakers, isolators, and so on. The figure 1.1 shows the PMs pre-requisites.
Routine and planned maintenance includes PM actions, which could be grouped as:
Fixed-time maintenance: Here maintenance actions are carried out at regular intervals (calendar time). For example, maintenance actions like item replacement, repair and major overhaul.

Condition-based maintenance: This maintenance action is explained below in the further sub heading.

Opportunity maintenance: Here for example: an opportune time to take up the task of repair or reconditioning may arise on those machines, where the work load is not there for that day or product produced on that machine is rejected. The operator has to keep the machine idle since he is waiting for the quality report before producing further quantity.

5 Predictive maintenance
Predictive Maintenance is one of the modern approaches to preventive maintenance. Here sensitive instruments like vibration analyser, amplitude meters, audio gauges, sensors for pressure, temperatures, resistance gauges and so on are used to predict the anticipated failure of machines and equipment. Conditions of the machinery can be checked on line periodically or on continuous basis and maintenance crew can take decision and plan overhaul or repair as warranted. Failure is predicted well in advance using certain techniques like vibration, temperature, misalignments, incorrect installation, rotor imbalance, pump cavitations, oil condition, wear debris analysis, and so on and has proved very effective. Good ability to predict impending failures well in time will result in:
• Maximising the online operations.
• Minimising downtime.
• Increased plant and Personnel safety.
• Optimal maintenance.

6 Condition based maintenance
Condition Based Maintenance (CBM) relies on the fact that the majority of failures do not occur instantaneously, but develops over a period of time. CBM involves recording measurement that gives an indication of the condition. For example, increase in vibration levels, temperature soars, increased leakages, and so on. Hence it is a periodic measurement and interpretation of data to determine the need for maintenance. Condition monitoring is merely a tool that is used by crew through touch, smell, and through their experience to make out the existing condition of the equipment or aggregate.

CBM is also known as dynamic predictive maintenance or diagnostic maintenance. In CBM, the plant is not maintained just after some problem surfaces, but much before the possible breakdown. For example operators who work with equipment every day can listen to equipment and identify changes in noise levels and vibrations, and so on. Temperature changes can be photographed through Infrared (IR) thermograph, which gives warning that something is 'not right'. An investigation can be carried out to identify the exact problem.


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