Preventive Maintenanace

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Preventive Maintenance

4.1 Introduction

Preventive maintenance, as its name implies are specific tasks that are designed to prevent the

need for corrective or breakdown maintenance, as well as prolong the useful life of capital assetsand auxiliary equipment. (Definition)

Most preventive maintenance programs are a loose conglomeration of inspections, cleaning,

adjustment, lubrication and similar tasks that do little, if anything, to preserve the reliability ofcritical production assets.

Statistically, between 33 percent and 42 percent of so-called preventive maintenance tasks add

no value, in terms of reliability or maintenance prevention.

Reliability-based preventive maintenance replaces these no-value tasks with specific

maintenance activities that both prevents failures and prolong the useful life of plant assets.

Preventive maintenance (PM) is an important component of a comprehensive maintenance

management plan.

Within a maintenance organization it usually accounts for a major proportion of the total

maintenance effort.

PM may be described as the caring and servicing by individuals involved with maintenance to

keep equipment/facilities in satisfactory operational state by performing systematic inspection,

detection and correction of incipient failures either prior to their occurrence or prior to theirdevelopment into major failure.

Some of the main objectives of PM are to: Enhance capital equipment productive life. Reduce critical equipment breakdowns. Allow better planning and scheduling of needed maintenance work. Minimize production losses due to equipment failures. Promote health and safety of maintenance personnel.

From time to time, PM programs in maintenance organizations end up in failure (i.e. they lose

upper management support) because their cost is either unjustifiable or they take a significanttime to show results.

It is emphasized that all PM must be cost effective.

The most important principle to keep continuous management support is: If it is not going to

save money, then dont do it!


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The preventive maintenance program can be developed using a guided logic approach and is task

oriented rather than maintenance process oriented.

This eliminates the confusion associated with the various interpretations across different

industries.

By using a task-oriented concept, it is possible to see the whole maintenance program reflectedfor any given item.

A decision logic tree can be used to identify applicable maintenance tasks.

Servicing and lubrication are included as part of the logic diagram since this ensures that an

important task category is considered each time an item is analyzed.

The content of any maintenance programme consists of at least two groups of tasks:

Preventive Maintenance tasks. Non-scheduled maintenance tasks.

The preventive maintenance tasks which include failure-finding tasks, are scheduled to be

accomplished at specified intervals or based on condition.

The objective of these tasks is to identify and prevent deterioration below inherent safety and

reliability levels by one or more of the following means:

Lubrication and servicing Operational, visual or automated checking Inspection, functional test or condition monitoring Restoration Discard

This group of tasks is determined by Reliability Centred Analysis, RCM analysis, that is, itconstitutes the RCM-based preventive maintenance program.

The non-scheduled maintenance tasks result from the following:

Findings from the scheduled tasks accomplished at specified intervals of time or usage. Reports of malfunctions or indications of impending failure (including automated

detection).

The objective of the second group of tasks is to maintain or restore the equipment to an

acceptable condition in which it can perform its required function.

An effective program schedules only those tasks necessary to meet the stated objectives.


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It does not schedule additional tasks that increase maintenance costs without a corresponding

increase in the inherent level of reliability.

Experience has clearly demonstrated that reliability decreases when inappropriate or unnecessary

maintenance tasks are performed due to an increased incidence of maintainer-induced faults.

Development of a reliability-based preventive maintenance program follows the logic diagramshown in Fig. 4.1 and the task selection criteria illustrated in Table 4.2 and described in Section4.6 are its principal tools.

The logic diagram are the basis of an evaluation technique applied to each functionallysignificant item (FSI)using all available technical data, as well as the native knowledge ofplant personnel.

This module presents important aspects of PM.


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Fig. 4.1 Development tasks of a Reliability-based Preventive Maintenance Program

4.2 Preventive maintenance elements, plant characteristics in need of a PM program and

a principle for selecting items for PM

There are seven elements of PM as shown in Fig. 4.2.


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Fig. 4.2 Seven Elements of Preventive Maintenance.

The seven elements of a Preventive Maintenance programmed are as follows:

1) Inspection:Periodically inspecting materials/items to determine their serviceability by comparing

their physical, electrical, mechanical, chemical etc., characteristics (as applicable) to

expected standards.2) Servicing:

Cleaning, lubricating, charging, preservation, etc., of items/materials periodically to

prevent the occurrence of incipient failures.

3) Calibration:Periodically determining the value of characteristics of an item by comparison to a

standard.


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It is the process of comparing of two instruments, one of which has a certified standard

with known accuracy, to detect and adjust any discrepancy in the accuracy of thematerial/parameter being used to measure a process or operating parameter.

4)Testing:Periodically testing and/or checking out the integrity of critical items of plant todetermine their serviceability and detect any electrical/mechanical-related degradation.

5) Alignment:Making changes to an items specified variable or adjustable elements for the purpose of

achieving optimum performance.

6) Adjustment:Periodically adjusting specified variable elements of material for the purpose of

achieving the optimum system performance.

7) Installation:Periodic replacement of limited-life items or the items experiencing time cycle or

wear degradation, to maintain the specified system tolerance.

Some characteristics of a plant in need of a good preventive maintenance program are as follows:

Low equipment use due to failures. Large volume of scrap and rejects due to unreliable equipment. Rise in equipment repair costs due to negligence in areas such as regular lubrication,

inspection and replacement of worn items/components.

High idle operator times due to equipment failures. Reduction in capital equipment expected productive life due to unsatisfactory

maintenance.

Table 4.1 presents 17 questions for determining the adequacy of a preventive maintenance

program within an organization.

The answer yes or no to each question is given 5 or 0 points, respectively.

A maybe answer is assigned a score from 1 to 4.

A total score of less than 55 points indicates that the preventive maintenance program requires

further improvements.


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Table 4.1 Preventive Maintenance Evaluation Questions


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4.3 Important Steps for establishing a PM program

To develop an effective PM program, the availability of a number of items is necessary.

Some of those items include the following:

Accurate historical records of equipment. Manufacturers recommendations on spares, maintenance requirements etc. Skilled personnel. Past data from similar equipment. Service manuals.

Unique identification of all equipment.

Appropriate test instruments and tools. Management support and user cooperation. Failure information by problem/cause/action. Consumables and replaceable components/parts. Clearly written instructions with a checklist to be signed off.

There are a number of steps involved in developing a PM program. Figure 4.3 presents six steps

for establishing a highly effective PM program in a short period.


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Fig. 4.3 Six steps for developing a Preventive Maintenance Program

Each step is discussed below.


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1) Identify and choose the areas. Identify and selection of one or two important areas to concentrate the

initial PM effort.

These areas should be crucial to the success of overall plantoperations and may be experiencing a high degree of maintenanceactions.

The main objective of this step is to obtain immediate results in highlyvisible areas, as well as to win concerned management support.

2) Identify the PM needs. Define the PM requirements. Establish a schedule of two types of tasks: daily PM inspections and

periodic PM assignments/tasks.

The daily PM inspections could be conducted by either maintenance orproduction personnel.

An example of a daily PM inspection is to check the waste water solidsconcentration.

Periodic PM assignments/tasks are usually performed by the maintenanceworkers.

Examples of such assignments/tasks are replacing throwaway filters,replacing drive belts and cleaning steam traps and permanent filters etc.

3) Establish frequency of assignment or tasks. Establish the frequency of the assignments/tasks. This involves reviewing the equipment condition and records. Normally, the basis for establishing the frequency is the experience of

those familiar with the equipment and the recommendations of vendors

and relevant engineers.

It must be remembered that vendor recommendations are generally basedon the typical usage of items under consideration.(More details would be provided later in Section 4.7 on establishing the

task frequency/interval)

4) Prepare the PM assignments/tasks. Daily and periodic assignments/tasks are identified and described in detail,

then submitted for approval.

5) Schedule the PM assignments/tasks on an annual basis. The defined PM assignments/tasks are scheduled on the basis of a twelve-

month period.

6) Expand the PM program as necessary.


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After the implementation of all PM daily inspections and periodicassignments in the initially selected areas, the PM can be expanded to

other areas.

Experience gained from the pilot PM projects is instrumental to expandingthe program.

4.4 Reliability-based Preventive Maintenance [Read for general information but can be leftout]

In the development of a reliability-based preventive maintenance program for both new and in-

service equipment, the progressive logic diagram and the task selection criteria are the principaltools.

This progressive logic is the basis of an evaluation technique applied to each functionally

significant item (FSI) using the technical data available.Principally, the evaluations are based on the items functional failures and failure causes.The development of a reliability-based preventive maintenance program is based on the

following:

Identification of FSIs Identification of applicable and effective preventive maintenance tasks using the

decision tree logic.

A functionally significant item is an item whose failure would affect safety or could havesignificant operational or economic impact in a particular operating or maintenance context.

Identification of FSIs is based on the anticipated consequences of failures using an analyticalapproach and good engineering judgment.

The process also use a top-down approach, and is conducted first at the system level, then at thesubsystem level and where appropriate, down to the component level.

An iterative process should be followed in identifying FSIs. Systems and subsystem boundaries

and functions are first identified.

This permits selection of critical systems for further analysis which involves a more

comprehensive and detailed definition of system, system functions, and systems functional

failures.The procedures shown in Fig. 4.1 such as information collection, system analysis and so on,

outline a comprehensive set of tasks in the FSI identification process.

All these tasks should be applied in the case of complex or new equipment.


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However, in the case of well-established or simple equipment, where functions and functional

degradation/failures are well recognized, tasks listed under the heading of 4.4.2 System Analysiscan be covered very quickly.

They should be documented, however, to confirm that they were considered.

The depth and rigor used in the application of these tasks also varies with the complexity andnewness of the equipment.

4.4.1 Information Collection

Equipment information provides the basis for the evaluation and should be assembled prior to the

start of the analysis and supplemented as the need arises.

The following should be included:

Requirements for equipment and its associated systems, including regulatoryrequirements.

Design and maintenance documentation.

Performance feedback, including maintenance and failure data.Also, to guarantee completeness and avoid duplication, the evaluation should be based on an

appropriate and logical breakdown of the equipment.

4.4.2 System Analysis

The tasks just described specify the procedure for the identification of the functionally

significant items and the subsequent maintenance task selection and implementation.

Note that the tasks can be tailored to meet the requirements of particular industries and theemphasis placed on each task depends on the nature of that industry.

4.4.3 Identification of Systems

The objective of this task is to partition the equipment into systems, grouping the componentscontributing to achievement of well-identified functions and identifying the system boundaries.

Sometimes, it is necessary to perform further partitioning into the subsystems that performfunctions critical to system performance.

The system boundaries may not be limited by the physical boundaries of the systems, which may

overlap.

Frequently, the equipment is already partitioned into systems through industry-specific

partitioning schemes.


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This partitioning should be reviewed and adjusted where necessary to ensure that it is

functionally oriented.

The results of equipment partitioning should be documented in a master system index, which

identifies systems, components and boundaries.

4.4.4 Identification of System Functions

The objective of this task is to determine the main and auxiliary functions performed by the

systems and subsystems.

The use of functional block diagrams assist in the identification of system functions.

The function specification describes the actions or requirements the

system or subsystem should accomplish, sometimes in terms of performance capabilities withinthe specified limits.

The functions should be identified for all modes of equipment operation.

Reviewing design specifications, design descriptions and operating procedures, including safety,abnormal operations and emergency instructions may determine the main and auxiliary

functions.

Functions such as testing or preparation for maintenance, if not considered important, may be

omitted.

The reason for omissions must be given. The product of this task is a listing of system functions.

4.4.5 Selection of Systems

The objective of this task is to select and rank systems to be included in the RCM program

because of their significance to equipment safety, availability, or economics.

The methods used to select and rank the systems can be divided into

Qualitative methods based on past history and collective engineering judgment.

Quantitative methods based on quantitative criteria such as criticality rating, safetyfactors, probability of failure, failure rate, or life cycle cost, used to evaluate the

importance of system degradation or failure on equipment safety, performance and costs.

Implementation of this approach is facilitated when appropriate models and data

banks exist.

A combination of qualitative and quantitative methods.


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The product of this task is a listing of systems ranked by criticality.

The systems, together with the methods, the criteria used and the results should be documented.

4.4.6 System Functional Failure and Criticality Ranking

The objective of this task is to identify system functional degradations and failures and rank

them on priority.

The functional degradation or failure of a system for each function should be identified, ranked

by criticality and documented.

Since each system functional failure may have a different impact on safety, availability andmaintenance cost, it is necessary to rank and assign priorities to them.

The ranking takes into account probability of occurrence and consequences of failure.

Qualitative methods based on collective engineering judgment and the analysis of operating

experience can be used.

Quantitative methods such as Simplified Failure Modes and Effects Analysis (SFMEA) orrisk analysis also can be used.

The ranking represents one of the most important tasks in RCM analysis.

Too conservative a ranking may lead to an excessive preventive maintenance program, and

conversely, a lower ranking may result in excessive failures and a potential safety impact.

In both cases, a non-optimized maintenance program results.

The outputs of this task are the following:

List of system functional degradations and failures and their characteristics. Ranked list of system functional degradations and failures.

4.5 Identification of functionally significant items

Based on the identification of system functions, functional degradations and failures and theireffects and collective engineering judgment, it is possible to identify and develop a list of FSIs.

As said before, failure of these items could affect safety, be undetectable during normal

operation and have significant operational or economic impact.

The output of this task is a list of candidate FSIs.


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4.5.1 Functionally Significant Item Failure Analysis

Once an FSI list has been developed, a method such as Failure Modes and Effects Analysis

(FMEA) should be used to identify the following information which is necessary for the logic

tree evaluation of each FSI.

The following examples refer to the failure of a pump providing cooling water flow:

Function. The normal characteristic actions of the item, a cooling water pump (e.g. to providecooling water flow at 100 l/s to 240 l/s to a heat exchanger).

Functional failure. How the item fails to perform its function (e.g., pump fails to providerequired flow).

Failure cause. Why the functional failure occurs (e.g., bearing failure).

Failure effect. What is the immediate effect and the wider consequence of each functionalfailure (e.g. inadequate cooling, leading to overheating and failure of the system).

The FSI failure analysis is intended to identify functional failures and failure causes.

Failures not considered credible such as those resulting solely from undetected manufacturing

faults, unlikely failure mechanisms or unlikely external occurrences should be recorded ashaving been considered and the factors that caused them to be assessed as not credibly stated.

Prior to applying the decision logic tree analysis to each FSI, preliminary worksheets need to be

completed that clearly define the FSI, its functions, functional failures, failure causes, failureeffects and any additional data pertinent to the item (e.g. manufacturers part number, a brief

description of the item, predicted or measured failure rate, hidden functions, redundancy).

These worksheets should be designed to meet the users requirements.

From this analysis, the critical FSIs can be identified (i.e., those that have both significantfunctional effects and a high probability of failure or have a medium probability of failure but are

judged critical or have a significantly poor maintenance record).

4.6 Maintenance Tasks

Explanations of the terms used in the possible tasks as shown in Table 4.2 are as follows:

Lubrication/servicing (all categories).

This involves any act of lubricating or servicing for maintaining inherent design

capabilities.


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Operational/Visual/Automated check (hidden functional failure categories only).

An operational check is to determine that an item is fulfilling its intended purpose. Itdoes not require quantitative checks and is a failure-finding task.

A visual check is an observation to determine that an item is fulfilling its intendedpurpose and does not require quantitative tolerances. This, again, is a failure-finding task.The visual check could also involve interrogating electronic units that store failure data.

Inspection/functional check/condition monitoring (all categories).

An inspection is an examination of an item against a specific standard. A functional

check is a quantitative check to determine if one or more functions of an item performs withinspecified limits.

Condition monitoring is a task, which may be continuous or periodic to monitorthe condition of an item in operation against preset parameters.

Restoration (all categories).

Restoration is the work necessary to return the item to a specific standard.

Since restoration may vary from cleaning or replacement of single parts up to a complete

overhaul, the scope of each assigned restoration task has to be specified.

Discard(all categories).

Discard is the removal from service of an item at a specified life limit.Discard tasks are normally applied to so-called single-cell parts such as cartridges,

canisters, cylinders, turbine discs, safe life

structural members, and the like.

Combination (safety categories).

Since this is a safety category question and a task is required, all possible avenues

should be analyzed.

To do this, a review of the applicable tasks is necessary.From this review, the most effective tasks should be selected.

No task (all categories).

It may be decided that no task is required in some situations, depending on the

effect. Each of the possible tasks just defined is based on its own applicability andeffectiveness criteria. Table


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4.2 summarizes these task selection criteria.

Table 4.2 Task Selection Criteria

4.7 Task Frequencies/Intervals

To set a task frequency or interval, it is necessary to determine the existence of applicableoperational experience data that suggest an effective interval for task accomplishment.

Appropriate information may be obtained from one or more of the following:

Prior knowledge from other similar equipment shows that a scheduled maintenance taskhas offered substantial evidence of being applicable, effective and economically

worthwhile.

Manufacturer or supplier test data indicate that a scheduled maintenance task isapplicable and effective for the item being evaluated.

Reliability data and predictions.Safety and cost considerations need to be addressed in establishing the maintenance intervals.


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Scheduled inspections and replacement intervals should coincide whenever possible and tasks

should be grouped to reduce the operational impact.

The safety replacement interval can be established from the cumulative failure distribution for

the item by choosing a replacement interval that results in an extremely low probability of failureprior to replacement.

Where a failure does not cause a safety hazard but causes loss of availability, the replacement

interval is established in a trade-off process involving the cost of replacement components, thecost of failure and the availability requirement of the equipment.

Mathematical models exist for determining task frequencies and intervals, but these models

depend on the availability of the appropriate data.

These data are specific to particular industries and those industry standards and data sheets

should be consulted as appropriate.

If there is insufficient reliability data or no prior knowledge from other similar equipment, or if

there is insufficient similarity between the previous and current systems, the task interval

frequency can be established initially only by experienced personnel using good judgment andoperating experience in concert with the best available operating data and relevant cost data.

4.7 PM Advantages and Disadvantages

Some advantages of PM are as follows:

Increase in equipment availability. Performed as convenient. i.e. Can be scheduled conveniently. Balanced workload and use of resources. Reduction in overtime costs which are exceptionally high with breakdown maintenance. Increase in production revenue. Consistency in quality. Reduction in need for standby equipment. Increase in preaction (proactive) instead of reaction. Reduction in parts inventory. Improved safety, standardized procedures, times and costs. Scheduled resources on hand. Useful in promoting cost/benefit optimization.

Some disadvantages of PM are as follows: Exposing equipment to possible damage. Using a greater number of parts. Increases in initial costs. Failures in new parts/components.


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Demands more frequent access to equipment/item.

Questions

1)

Discuss at least five important elements of PM.2) What are the symptoms of a plant in need of a good PM program?3) State any five (5) questions that can be asked to determine the adequacy of a PM

program?

4) Comment on the principle or formula proposed to decide whether to go ahead with a PMprogram.

5) List at least ten items whose availability is essential to develop an effective PM program.6) Discuss important steps for developing a PM program.7) What are the benefits and drawbacks of performing PM?8) Using the automobile as an example, list some of the preventive maintenance tasks that

are performed and how they improve the performance and minimize breakdown.

Documents

Preventive Maintenanace