Corrective Maintenance 3.1 Introduction
You should realize by now that there are three types of maintenance tasks: (1) breakdown (2) corrective and (3) preventive. The principal difference in these occurs at the point when the repair or maintenance task is implemented. In breakdown maintenance, repairs do not occur until the machine fails to function. Preventive maintenance tasks are implemented before a problem is evident. Corrective tasks are scheduled to correct specific problems that have been identified in plant systems. A comprehensive maintenance program should use a combination of all three. However, many local plants rely almost exclusively on breakdown maintenance to maintain their critical plant production systems. Although every effort is made to make engineering systems as reliable as possible through proper design, application of effective preventive and predictive maintenance programmes etc., from time to time, failures still occur randomly. Consequently, they are repaired to their ope rational state. Thus, repair or corrective maintenance is an important component of maintenance activity. Corrective maintenance may be defined as the remedial action carried out due to failure or deficiencies discovered during normal operations or production, to repair an equipment/item to its operational state.(Definition) Usually, corrective maintenance is an unscheduled maintenance action, basically composed of unpredictable maintenance needs that cannot be preplanned or programmed on the basis of occurrence at a particular time. The action requires urgent attention that must be added, integrated with or substituted in place of previously scheduled work items. This incorporates compliance with “prompt action”, action”, field changes and rectification of deficiencies found during equipment/item operation and performance of repair actions due to incidents or accidents.
A substantial part of overall maintenance effort is devoted to corrective maintenance and over the years many individuals have contributed to the area of corrective maintenance. This module presents some important aspects of corrective maintenance. 3.2 Breakdown Maintenance
In this type of Maintenance programme, less concern is given to the operating condition of critical plant machinery, equipment or systems. Since most of the maintenance tasks are reactive to breakdowns or production interruptions, the only focus of breakdown maintenance is how quickly the machine or system can be returned to service. As long as the machine will function at a minimum acceptable level, this type of maintenance is judged to be effective. This approach to maintenance management is both ineffective and extremely expensive. Breakdown maintenance has two factors that are the primary contributors to high maintenance costs: (1) poor planning and (2) incomplete repair. The first limitation of breakdown maintenance is that most repairs are poorly planned because of the time constraints imposed by production and plant management. As a result, manpower utilization and effective use o f maintenance resources are minimal. Typically, breakdown or reactive maintenance will cost three to four times more for the same repair than when it is well planned. The second limitation of breakdown maintenance is that it concentrates repair on obvious symptoms of the failure, not the root cause. For example, a bearing failure may cause a critical machine to seize and stop production. In breakdown maintenance, the bearing is replaced as quickly as possible and the machine is returned to service. No attempt is made to determine d etermine the root cause of the bearing failure or to prevent a recurrence of the failure. As a result, the reliability of the machine or system is severely reduced. This normal result of breakdown maintenance is an increase in the frequency of repairs and a marked increase in maintenance costs. 3.3 Preventive Maintenance
The concept of preventive maintenance has a multitude of meanings. A literal interpretation of the term is a maintenance program that is committed to the elimination or prevention of corrective and breakdown maintenance tasks. (Definition) A comprehensive preventive maintenance program p rogram will utilize regular evaluation of critical plant equipment, machinery and systems to detect poten tial problems and immediately schedule maintenance tasks that will prevent any degradation in operating condition. In most plants, preventive maintenance is limited to periodic lubrication, adjustments/calibrations and other time-driven maintenance tasks. These programs are not true preventive programs. In fact, most continue to rely on breakdowns b reakdowns as the principal motivation for maintenance activities. A comprehensive preventive maintenance program will include predictive maintenance, timedriven maintenance tasks, and corrective maintena nce to provide comprehensive support for all plant production or manufacturing systems. More details on Preventive Maintenance would be provided in a separate Module.
3.4 Corrective Maintenance
The primary difference between corrective and preventive maintenance is that a problem must exist before corrective actions are taken. Preventive tasks are intended to prevent the occurrence of a problem. Corrective tasks correct existing problems when they occur. Corrective maintenance, unlike breakdown maintenance is focused on regular, planned tasks that will maintain all critical plant machinery and systems in optimum operating conditions. Maintenance effectiveness is judged on the life-cycle costs of critical plant machinery, equipment and systems; not on how fast a broken machine can be returned to service. Corrective maintenance, as a subset of a comprehensive preventive maintenance program, is a proactive approach toward maintenance management.
The fundamental objective of this approach is to eliminate breakdowns, deviations from optimum operating condition and unnecessary repairs and to optimize the effectiveness of all critical plant systems. The principal concept of corrective maintenance is that proper, complete repairs of all incipient problems are made on an as-needed basis. All repairs are well planned, implemented by properly trained craftsmen, and verified before the machine or system is returned to service. Incipient problems are not restricted to electrical or mechanical problems. Instead, all deviations from optimum operating condition, that is, efficiency, production capacity and product quality, are corrected when detected. 3.5 Pre – requisites of Corrective Maintenance
Corrective maintenance cannot exist without specific support efforts. A number of prerequisites must exist before corrective maintenance can be properly implemented. 3.6 Accurate identification of Insipient Problems
Both preventive and corrective maintenance programs must be able to anticipate maintenance requirements before a breakdown can occur. A comprehensive predictive maintenance program that has the ability to accurately identify the root cause of all incipient problems is the first requirement of corrective maintenanc e. Without this ability, corrective actions cannot be planned or scheduled. 3.7 Planning
All corrective repairs or maintenance must be well planned and scheduled to minimize both cost and interruption of the production schedule. Most importantly, adequate time must be allowed to permit complete repair of the root cause and resultant damage caused by each of o f the identified incipient problems. Proper maintenance planning is dependent on well-trained planners, a viable maintenance database and complete repair procedures for fo r each machine train or system within the plant.
Train Traine ed Main Mainttenance Plan Planners
Many plants do not have full-time maintenance planners or their planners lack the knowledge or skills that the job demands. It is therefore imperative that proper training is provided to ensure that each planner has the skills necessary to properly plan repairs and maintenance tasks.
Maintenance History Database The planner must have accurate maintenance history in order to properly plan repairs. As a minimum, he must know the standard mean-time-to-repair for every recurring repair, rebuild procedures and maintenance task required to maintain optimum operating condition of critical plant machinery, equipment, and systems. Without this knowledge, he cannot plan an effective repair. In addition, the planner must know the specific tools, repair parts, auxiliary equipment and craftsmen skills required to complete each maintenance task. This information, in conjunction with proper repair sequence is an absolute requirement of a viable repair plan.
This type of information requires a comprehensive maintenance database that compiles actual mean-time-to-repair, standard repair procedures and the myriad of other information required for proper maintenance planning. 3.8 Proper Repair Procedures
Repairs must be complete and properly implemented. In many cases, poor maintenance or repair practices result in more damage to critical plant machinery than the observed failure mode. A fundamental requirement of corrective maintenance is proper, complete repair of each incipient problem. To meet this requirement, all repairs must be made by craftsmen who have the necessary skills, repair parts and tools required to return the machin e or system to as-new condition.
Craft Skills. A growing number of maintenance craftpersons do not have the minimum skills required to properly maintain or repair plant equipment, machinery or systems. In many cases, they cannot properly install bearings, align machine trains or even balance rotating equipment.
Few have the knowledge and skills required to properly disassemble, repair, and reassemble the complex machinery or systems that comprise the c ritical production systems within many plants. A prerequisite of corrective maintenance is skilled craftsperson. Therefore, plants must implement a continuous training program that will provide the minimum craft skills required to support their production or manufacturing systems. The training program should include the means to verify craft skills by regular evaluations and periodically refresh these skills.
Standard Standard Mainte aintenance Proce Procedures All recurring repairs and maintenance tasks should have a standard procedure that will specifically define the correct method required for completing the tasks. These procedures should include all of the information such as tools, safety concerns and repair parts, required for the task and a step-by-step sequence of tasks required to complete the repair. Each procedure should be complete and and contain all information and steps required to complete the repair or recurring preventive maintenance task. The craftsperson should not be required to find or have supplemental information in order to complete the repair. 3.9 Adequate time to repair
One of the fundamental reasons that most plants rely on or “prefer” breakdown maintenance is that tight production schedules and management constraints limit the time available for maintenance. The only way to reduce the number and frequency of breakdown repairs is to allow sufficient time for proper maintenance. Therefore, Plant Management must permit adequate maintenance time for all critical plant systems before either preventive or corrective maintenance can b e effective. In the long term, the radical change in management philosophy will result in a dramatic reduction in the downtime required to maintain critical production and manufacturing equipment. Machinery that is maintained in as-new condition and not permitted to degrade to a point that breakdown or serious problems can occur occu r will require less maintenance and less downtime than machinery maintained in a breakdown mode. 3.10 Verification of Repair
The final prerequisite of corrective maintenance is that all repairs or rebuilds must be verified by some form of acceptance testing before the machine train or system is returned to service. This verification process will ensure that the repair was properly made and that all incipient problems, deviations from optimum operating conditions or other potential limitations to maximum production capacity and reduced product quality have been corrected.
3.11 Role of Corrective Maintenance
Corrective maintenance will remain a critical part of a comprehensive plant maintenance program. However, the objective of a viable preventive program is to eliminate all breakdown maintenance and severely reduce the number and frequency of unplanned corrective actions. The ultimate objective of any maintenance program should be the elimination of machine, equipment and system problems that require unplan ned corrective actions. 3.12 Corrective Maintenance Types
Corrective maintenance may be classified into five major cate gories as shown in Fig. 3.1. These are: fail-repair, salvage, rebuild, overhaul, and servicing.
Fig. 3.1 Types of Corrective Maintenance
These categories are described below. 1) Fail-repair: The failed item is restored to its operational state. 2) Salvage: This element of corrective maintenance is concerned with disposal of non-repairable material and the use of salvaged material from non- repairable equipment/item undergoing repair, overhaul or rebuild programs. 3) Rebuild: This is concerned with restoring an item to a standard as close as possible to original state in performance, life expectancy and appearance. This is achieved through complete disassembly, ex amination of all components, repair and replacement of worn/unserviceable parts as per specifications and manufacturing tolerances and reassembly and testing to production guidelines.
original original
4) Overhaul: Restoring an item to its total serviceable state as per maintenance serviceability standards, using the “inspect and repair onl y as appropriate” approach. 5) Servicing: Servicing may be needed because of the corrective maintenance action, for example, engine repair can lead to crankcase oil refill, transmission fluid replacement etc.
coolant replacement,
Another example could be that the replacement of an air bottle may require system recharging.
3.13 Corrective maintenance steps, downtime components and time at system level
reduction strategies
Different experts have laid down different sequential steps for performing corrective maintenance. For example, one of these proposes nine steps (as applicable):
Localize. Isolate. Adjust. Disassemble. Repair. Interchange. Reassemble. Align. Checkout.
Another presents seven steps (as applicable): Localization Isolation Disassembly Interchange Reassemble Alignment Checkout. For our purpose, we assume that corrective maintenance is composed of five major sequential steps, as shown in Fig. 3.2. These steps are: Fault recognition Localization Diagnosis Repair Checkout
Fig.3.2 Corrective maintenance sequential steps
The major corrective maintenance downtime components are active repair time, administrative and logistic time and delay time. The active repair time is made up of the following subcomponents:
Preparation time Fault location time Spare item/parts obtainment time Fault correction time Adjustment and calibration time Checkout time
Reduction in corrective maintenance time is useful u seful to improve maintenance effectiveness. Some strategies for reducing the system-level corrective maintenance time are as follows:
Efficiency in fault recognition, location and isolation:
Past experience indicates that in electronic equipment, fault consume the most time within a corrective maintenance activity. In the case of mechanical items, often the largest contributor is
isolation and location
repair time.
Factors such as well designed fault indicators, good maintenance procedures, welltrained maintenance personnel and an unambiguous fault isolation capability are helpful in lowering corrective maintenance time.
E ffective ctiveinter nterchangeabi changeabillity: ty:(Not (Not exact place placem ment) Good physical and functional interchangeability is useful in removing and replacing parts/items, reducing maintenance downtime and creating a positive impact on spares and inventory needs.
Redundancy:(Pumps in parallel to reduce downtime) This is concerned with designing systems or equipment with redundant parts that can be switched in at the moment of need so the equipment/system continues to operate while the faulty part is being repaired. e.g.: Pumps or compressors in parallel with automatic starting on the standby
In this case the overall maintenance workload may not be reduced, but equipment/system downtime could be impacted significantly.
the
Effective accessibility: Often a significant amount of time is spent accessing the failed
part.
Proper attention to accessibility during design can help reduce part accessibility time and, in turn, the corrective maintenance time.
Human factor considerations: Attention is paid to human factors during design in areas such as readability of instructions, size, shape and weight of components, selection and placement of dials and indicators, size and placement of access gates/doors, readability and information processing aids can help reduce corrective maintenance time significantly.
Questions
1. Define corrective maintenance and give an example that you are familiar with. 2. Describe the following types of corrective maintenance: • Overhaul • Rebuild • Servicing 3. Discuss sequential steps associated with corrective maintenance. 4. Define main components of active repair time. 5. Discuss at least four strategies strategies for reducing the system-level system-level corrective maintenance time. 6. For any known piece of equipment of machinery eg. Pump, compressor, automobile etc. list as much corrective maintenance activities as you can. 7. What are the pre-requisites of effective corrective maintenance?
