III. Maintenance Categories or Types

A. INTRODUCTION

Maintenance programs typically include two basic types of maintenance: corrective maintenance and preventive maintenance (time-based and condition-based). A number of maintenance programs further divide these basic types. For example corrective maintenance may be sub-categorized based on complexity into minor, and planned work. Sub categorizing of preventive maintenance programs is becoming more prevalent. PM programs are typically subdivided into preventive maintenance (time-based) and predictive maintenance (condition-based) as a separate category of maintenance. However, these criteria treat predictive maintenance as a part of preventive maintenance. A proper balance of these types of maintenance provides a high degree of confidence that equipment degradation is identified and corrected, a high level of unit and system availability and reliability is maintained, equipment life is optimized, and the maintenance program resources efficiently used.

This section provides guidelines for establishing the proper relationship of the types of maintenance in the maintenance program. It does not specifically address maintenance activities integrated into operating requirements; however, consider operator surveillance's, ASME inspections and in-service testing requirements when establishing the scope of the preventive maintenance program.

Many factors must be considered when establishing an effective and efficient balance of the different types of maintenance. On important systems and equipment (for example, emission treatment systems or systems important to unit reliability or personnel safety), a thorough technical analysis may be needed to refine this balance. On less important systems, the amount of preventive maintenance may be determined using experience and engineering judgment.

A proper balance of the types of maintenance may include no preventive maintenance for equipment that is allowed to run until failure, provided the failure would not adversely impact operations and is more cost-effective than preventive maintenance. Conversely, for equipment whose failure can adversely impact safety or reliable operation or result in forced outages, or significant derates, extensive preventive maintenance may be required. The purpose of preventive maintenance is to significantly reduce the frequency of equipment failure. Increased preventive maintenance results in improved safety, reliability, and availability, as well as reduced corrective maintenance. However, if performed unnecessarily or excessively, preventive maintenance can consume valuable and limited resources that could otherwise be used to extend the scope of the preventive maintenance program. Additionally, excessive preventive maintenance may unnecessarily increase the potential for operational or maintenance errors, unavailability, rework, and increased costs.

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The elements needed to successfully implement the maintenance program discussed above include the following:

· a comprehensive master equipment list or equivalent to help in selecting and scheduling maintenance activities

· a methodology for determining the correct type and frequency for preventive (time-based) maintenance and predictive (condition-based) maintenance technology for each system and component

· coordination of maintenance actions that allows other related maintenance to be performed concurrently

· review and approval of preventive maintenance actions deferred past a grace period (normally a predetermined percent of the established interval) or postponed until the next scheduled date -- This review should include documented engineering evaluations, as required, to determine the acceptability of the equipment based on a lack of preventive maintenance.

· continuing enhancement of the maintenance program through periodic review of preventive and corrective maintenance effectiveness

B. GUIDELINES

8. Master Equipment List

A detailed master list of equipment, components, and structures to be included in the maintenance program is essential to help in selecting and scheduling preventive maintenance and to evaluate the effectiveness of the maintenance program. The list normally is developed and controlled by or with assistance from the engineering support organization and can be used for other purposes, such as determining the code classification of components, procurement specifications for parts and materials, and creating consistent nomenclature for equipment labeling. Special tools and equipment should also be included in the master list. This list also could be used effectively to help establish the maintenance history program and to determine the necessary parts required to maintain equipment.

2. Types of Maintenance

X. a. Corrective Maintenance

Corrective maintenance is restoration of equipment or components that are degraded or not performing their intended functions. As a rule, if the specific component (for example, packing or bearing) requiring maintenance has degraded or failed, the action required to repair it is classified as corrective maintenance. Corrective maintenance can be minor, or planned (more significant activities that require planning to correct). Non-urgent preparation and planning normally precede much of this work.

Ideally, the majority of these repairs will be to equipment previously selected to run until failure because its loss does not appreciably affect safety and reliability, and it is more cost-effective to run to failure. Only a small fraction of the corrective maintenance effort should be needed to restore equipment that limits safe and reliable operation or results in a forced power reduction.

Corrective maintenance can be performed with the unit in or out of service. In-service maintenance activities are selected based on operational impact and contribution to safety and reliability. Perform activities that can affect plant safety or reliability only after safety and operational assessments, detailed planning and scheduling, job briefings, and coordination with all necessary personnel have been completed. Have contingency plans, resources, tools, materials, and equipment in place in the event of a problem that could result during the corrective maintenance. Schedule work to maximize critical system availability and to ensure maintenance can be performed within committed time limits.

The impact of corrective maintenance activities on the design and configuration of equipment and systems should be evaluated to identify where configuration control processes should be applied. Personnel should be knowledgeable of corrective maintenance work activities that may result in changes in plant configuration, e.g., unauthorized part substitutions, repairs that do not comply with design specifications or code requirements.

XI. b. Preventive Maintenance

Preventive maintenance includes predictive (condition-based) and periodic (time-based) actions taken to maintain a piece of equipment within design operating conditions and to extend its life. It is normally performed before equipment failure in order to reduce the likelihood of equipment failure. The distinction between predictive and periodic maintenance is presented below.

1. Predictive maintenance monitors conditions and the results are used to trend and monitor equipment performance so that needed corrective or preventive maintenance can be performed before equipment failure. Predictive maintenance actions are determined by the data required to monitor equipment condition. Examples are as follows:

· Vibration analysis (includes spectral analysis and bearing temperature monitoring) and lube oil and grease analysis are used to monitor rotating equipment.

· Check valves are tested and analyzed, and a program exists for maintaining a history of test results. Non-intrusive testing techniques, such as acoustic monitoring, may be used for this purpose.

· Infrared surveys (thermography) are performed on heat-producing equipment such as motors, circuit breakers, batteries, load centers, bus ducts, transformers, and insulated areas to monitor for high resistance or insulation breakdown.

· Oil analyses (tribology) are performed on lubrication for rotating equipment to identify degrading equipment and chemical breakdown of lubricants.

· Motor-operated valves are diagnostically tested and analyzed. Tests determine parameters such as run current, valve stem thrust, and torque switch and limit switch actuation points.

· Air-operated valve testing is performed. Test parameters for air-operated valves include operating pressure, stem thrust, and stroke time.

Selected process data is identified; normal values and acceptance criteria for the operating condition of the equipment are established; and readings, such as differential temperatures, pressures, and motor running currents, are monitored and trended for the operating condition of the equipment to detect degradation.

When predictive maintenance requires in-process monitoring of the equipment, such action specifies the proper conditions, system configuration, and operating parameters to establish reliable and trendable data. Equipment-monitoring locations are identified and marked to obtain consistent readings each time the predictive maintenance data is recorded. Knowledgeable individuals gather, review, and analyze this data for the predictive maintenance program. Include an analysis of the equipment history of failed components to help validate the correlation between predictive maintenance data and field conditions and to determine shortcomings in the program.

(2) Periodic maintenance is time-based action taken on equipment to prevent breakdown and involves servicing such as lubrication, filter changes, cleaning, testing, adjustments, calibration, and inspection. Periodicity and required actions should be derived from Original Equipment Manufacturers recommendations, and modified based on actual application and industry experience with the equipment. Periodic maintenance can also be initiated because of the results of predictive maintenance, or experience. Examples are as follows:

· scheduled valve repacking because of anticipated leakage based on previous experience

· replacement of bearings or pump realignment as indicated from vibration analysis and/or lubricating oil analysis

· major or minor overhauls based on experience factors or vendor recommendations

3. Maintenance Action and Frequency Selection

Using the master equipment list, analyze preventive maintenance actions and their frequencies to identify periodic actions to improve equipment performance. Determine the actions selected and their frequencies based on considerations such as the following:

· reliability-centered maintenance analysis techniques, such as problem component or system analysis

· regulatory and code requirements

· vendor recommendations (technical or service information)

· industry and station experience

· maintenance history

· equipment or system availability during operating conditions

· operating history

· engineering judgment

· operator insight/feedback

· cost/benefit evaluations

· function, ease of replacement, and demonstrated reliability of equipment or system

Include in preventive maintenance efforts the analysis of failure modes and frequencies, the determination of failure causes, and the identification of preventive maintenance actions that could improve safety or reliability or reduce operating costs.

Document the justification for the preventive maintenance program. Maintenance and accountable engineering managers approve the program, including new or revised preventive maintenance actions and their frequencies. Effective monitoring and diagnostic methods (predictive maintenance) often are preferred to periodic internal inspection or equipment overhauls.

Although vendor recommendations are an important factor in developing the preventive maintenance program, it is important to establish or validate the basis for the vendor recommendations. Blanket acceptance of vendor recommendations may result in a program that does not reflect the service conditions of the equipment and may cause unnecessary or insufficient preventive maintenance. For example, the environment or operating demands on the equipment may differ from the vendor's assumptions when the preventive maintenance recommendations were established.

When a balance is being established between corrective and preventive maintenance as they pertain to the various systems and equipment, the overriding consideration is safe and reliable operations.

9. Scheduling

Schedule each preventive maintenance action at appropriate intervals. When possible, combine with corrective maintenance activities on the same equipment/system and with other related maintenance based on equipment similarity or proximity. However, when various maintenance activities are combined, activities should be sequenced to obtain accurate as-found data and conditions prior to any repairs or adjustments.

10. Preventive Maintenance Deferral

Preventive maintenance programs should establish not only frequencies or intervals for performance, but should establish a grace period (normally some percent of the established interval) beyond which the PM action is considered overdue. Management controls should be established to manage deferrals of PM activities beyond the grace period, and limit overdue PM tasks. Deferrals should receive review and approval at a level appropriate to the importance of the equipment. Identify the level of review for different types of equipment in the procedures or database used to implement the preventive maintenance program. Reviews for safety or operationally important equipment should have sufficient technical input to ensure any potential consequences are recognized and addressed. Maintenance management should review statistics on overdue and deferred preventive maintenance actions periodically.

11. Preventive Maintenance Program Effectiveness

The preventive maintenance program should be continually reviewed for effectiveness, with subsequent changes based on changes in unit design, operating conditions, regulatory requirements, and as-found conditions. Critical to this continuing assessment is obtaining accurate feedback on preventive maintenance tasks, particularly the as-found conditions and actual work performed. In addition, unexpected equipment failures should trigger a critical self-assessment as to why the previous maintenance activities were insufficient to sustain equipment reliability. The primary objectives of the program are to reduce future component failures, optimize preventive maintenance tasks and use of resources, identify program scope, and satisfy regulatory requirements.