Preventive Maintenance Process and Program
The preventive maintenance program is 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 of terms such as condition monitoring, on condition, hard time, etc. By using a task oriented concept, it is possible to see the whole maintenance program reflected for a given item. A decision logic tree is used to identify applicable maintenance tasks. Servicing and lubrication are included as part of the logic diagram as this ensures that an important task category is considered each time an item is analyzed. Maintenance Program Content The content of the maintenance program itself consists of two groups of tasks. • A group of preventive maintenance tasks, which include failure-finding tasks, 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: o Lubrication/servicing;
o Operational/visual/automated check;
o Inspection/functional test/condition monitoring;
o Discard. It is this group of tasks, which is determined by RCM analysis, e. it comprises the RCM based preventive maintenance program. • A group of non scheduled maintenance tasks which result from:
• 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 this 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 is one that schedules only those tasks necessary to meet the stated objectives. It does not schedule additional tasks that will increase maintenance costs without a corresponding increase in protection of the inherent level of reliability. Experience has clearly demonstrated that reliability decreases when inappropriate or unnecessary maintenance tasks are performed, due to increased incidence of maintainer-induced faults. Continued...
Published on: Mar 4, 2016
Transcripts - Preventive Maintenance Process and Program
Preventive Maintenance Program
The program is 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 of terms such as condition monitoring,
on-condition, hard time, etc. By using a task-oriented concept, it is possible to see the whole
maintenance program reflected for a given item. A decision logic tree is used to identify
applicable maintenance tasks. Servicing and lubrication are included as part of the logic
diagram as this ensures that an important task category is considered each time an item is
Maintenance Program Content
The content of the maintenance program itself consists of two groups of tasks.
♦ A group of preventive maintenance tasks, which include failure-finding tasks, 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:
o Operational/visual/automated check;
o Inspection/functional test/condition monitoring;
It is this group of tasks, which is determined by RCM analysis, e. it comprises the RCM
based preventive maintenance program.
♦ A group of non-scheduled maintenance tasks which result from:
♦ Findings from the scheduled tasks accomplished at specified intervals of time or usage;
♦ Reports of malfunctions or indications of impending failure (including automated
The objective of this 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 is one that schedules only those tasks necessary to meet the stated
objectives. It does not schedule additional tasks that will increase maintenance costs without
a corresponding increase in protection of the inherent level of reliability. Experience has
clearly demonstrated that reliability decreases when inappropriate or unnecessary
maintenance tasks are performed, due to increased incidence of maintainer-induced faults.
Reliability-Based Preventive Maintenance
This clause describes the tasks in the development of a reliability based preventive
maintenance program for both new and in-service equipment. In the development of a
program the progressive logic diagram and the task selection criteria, illustrated in Table 1,
are the principal tools. 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 functionally significant items (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 have
significant operational or economic impact in a particular operating or maintenance context.
The process of identification of FSIs is based on the anticipated consequences of failures
using an analytical approach and good engineering judgment. FSIs also uses a top down
approach, and is conducted first at the system level, then at the subsystem 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 system functional
The procedures below outline (Figure 3.1) a comprehensive set of tasks in the FSI
identification process. All these tasks should be applied in the case of complex or new
equipment. 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
"system analysis" can be covered very quickly. They should, however, be documented to
confirm that they were considered. The depth and rigor used in the application of these tasks
will also vary with the complexity and newness of the equipment.
Identification of systems
Identification of system
Selection of systems
Identification of system
functional failures and
Identification and analysis
of functionally significant
Maintenance task selection
Technical data feedback
Master system index
List of system functions
Listing of ranked systems
Listing of system functional
failures and ranking
Listing of FSIs
List of maintenance tasks
Initial maintenance program
Figure 3.1 Development tasks of a reliability-based preventive maintenance program.
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
♦ Requirements for equipment and its associated systems, including regulatory
♦ Design and maintenance documentation;
♦ Performance feedback, including maintenance and failure data.
Also, in order to guarantee completeness and avoid duplication, the evaluation should be
based on an appropriate and logical breakdown of the equipment.
The tasks described in the preceding define the procedure for the identification of the
functionally significant items and the subsequent maintenance task selection and
implementation. It should be noted that the tasks can be tailored to meet the requirements of
particular industries and the emphasis placed on each task will depend on the nature of that
Identification of Systems
The objective of this task is to partition the equipment into systems, grouping the
components contributing to achievement of well-identified functions and identifying the
system boundaries. Sometimes it is necessary to perform further partitioning into the
subsystems, which perform functions 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. 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 that identifies systems, components and boundaries.
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 will assist in the identification
of system functions. The function definition describes the actions or requirements which the
system or subsystem should accomplish, sometimes in terms of performance capabilities
within the specified limits. The functions should be identified for all modes of equipment
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 preparations 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.
Selection of Systems
The objective of this task is to select and prioritize systems, which will be included in the
RCM program because of their significance to equipment safety, availability or economics.
The methods used to select and prioritize 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, safety
factors, probability of failure, failure rate, life cycle cost, etc., used to evaluate the
importance of system degradation/failure on equipment safety, performance and
costs. Implementation of this approach is facilitated when appropriate models and
data banks exist;
♦ Combination of qualitative and quantitative methods.
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.
System Functional Failures and Criticality Ranking
The objective of this task is to identify system functional degradation/failures and prioritize
them. The functional degradation/failures of a system for each function should be identified,
ranked by criticality and documented.
Since each system functional failure may have different impacts on safety, availability or
maintenance cost, it is necessary to rank and prioritize them. The ranking takes into account
probability of occurrence and consequences of failure. Qualitative methods based on
collective engineering judgment and based on the analysis of operating experience can be
used. Quantitative methods of Simplified Failure Modes and Effects Analysis (SFMEA) or
risk analysis can also 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 will result. The outputs of this task are the following
♦ Listing of system functional degradation/failures and their characteristics;
♦ Ranking list of system functional degradation/failures.
Identification of Functionally Significant Items (FSIs)
Based on the identification of system functions, functional degradation/failures and effects,
and collective engineering judgment, it is possible to identify and develop a list of candidate
FSIs. As said before, these are items whose failures could affect safety; be undetectable
during normal operation; have significant operational impact; have significant economic
impact. The output of this task is a list of candidate FSIs.
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 that 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 (e.g. to provide cooling water
flow at 100 I/s to 240 I/s to the heat exchanger);
♦ Functional failure: how the item fails to perform its function (e.g. pump fails to
provide required 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
functional failure (e.g. inadequate cooling leading to over-heating and failure of the
The FSI failure analysis is intended to identify functional failures and failure causes. Failures
not considered as credible, such as those resulting solely from undetected manufacturing
faults, unlikely failure mechanisms or unlikely external occurrences, should be recorded as
having been considered and the factors which caused them to be assessed as not credible
should be stated.
Prior to applying the decision logic tree analysis to each FSI, preliminary worksheets need to
be completed which clearly define the FSI, its functions, functional failures, failure causes,
failure effects and any additional data pertinent to the item (e.g. manufacturer's part number,
a brief description of the item, predicted or measured failure rate, hidden functions,
redundancy, etc.). These worksheets should be designed to meet the user's requirements.
(Typical examples of the worksheets are given in annex B).
From this analysis, the critical FSIs can be identified (i.e. those that have both significant
functional 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).
Maintenance Task Selection (Decision Logic Tree Analysis)
The approach used for identifying applicable and effective preventive maintenance tasks is
one that provides a logic path for addressing each FSI functional failure. The decision logic
tree (Figure 5.2.2) uses a group of sequential “YES/NO” questions to classify or characterize
each functional failure. The answers to the “YES/NO” questions determine the direction of
the analysis flow and help to determine the consequences of the FSI functional failure, which
may be different for each failure cause. Further progression of the analysis will ascertain if
there is an applicable and effective maintenance task that will prevent or mitigate it. The
resultant tasks and related intervals will form the initial scheduled maintenance program.
NOTE - Proceeding with the logic tree analysis with inadequate or incomplete FSI failure
information could lead to the occurrence of safety critical failures, due to inappropriate,
omitted or unnecessary maintenance, to increased costs due to unnecessary scheduled
maintenance activity, or both.
Levels of Analysis
Two levels are apparent in the decision logic.
♦ The first level (questions 1, 2, 3 and 4) requires an evaluation of each functional
degradation/failure for determination of the ultimate effect category, i.e. evident
safety, evident operational, evident direct cost, hidden safety, hidden non-safety or
♦ The second level (questions 5, 6, 7, 8 and 9, A to F, as applicable) takes the failure
causes for each functional degradation/failure into account in order to select the
specific type of tasks.
First Level Analysis (Determination of Effects)
Consequence of failure (which could include degradation) is evaluated at the first level using
four basic questions (Figure 3.2).
Figure 3.2.2 Reliability decision logic tree-Level 1-Effects of functional failures
NOTE - The analysis should not proceed through the first level unless there is a full and
complete understanding of the particular functional failure.
Question 1 - Evident or hidden functional failure? The purpose of this question is to
segregate the evident and hidden functional failures and should be asked for each functional
Question 2-- Direct adverse effects on operating safety? To be direct, the functional
failure or resulting secondary damage should achieve its effect by itself, not in combination
with other functional failures. An adverse effect on operating safety implies that damage or
loss of equipment, human injury or death, or some combination of these events is a likely
consequence of the failure or resulting secondary damage.
Question 3 - Hidden functional failure safety effect? This question takes into account
failures in which the loss of a hidden function (whose failure is unknown to the operating
personnel) does not of itself affect safety, but in combination with an additional functional
failure, has an adverse effect on operating safety.
NOTE - the operating personnel consist of all qualified staff who are on duty and who are
directly involved in the use of the equipment.
Question 4 - Direct adverse effect on operating capability? This question asks if the
functional failure could have an adverse effect on operating capability:
♦ Requiring either the imposition of operating restrictions or correction prior to further
♦ Requiring the operating personnel to use abnormal or emergency procedures.
Second Level Analysis (Effects Categories)
Applying the decision logic of the first level questions to each functional failure leads to one
of five effect categories, as follows:
Evident safety effects - Questions 5A to 5E This category should be approached with the
understanding that a task (or tasks) is required to ensure safe operation. All questions in this
category need to be asked. If no applicable and effective task results from this category
analysis, then re-design is mandatory.
Evident operational effects - Questions 6A to 6D A task is desirable if it reduces the risk
of failure to an acceptable level. If all answers are "NO" in the logic process, no preventive
maintenance task is generated. If operational penalties are severe, a redesign is desirable.
Evident direct cost effects - Questions 7A to 7D A task is desirable if the cost of the task is
less than the cost of repair. If all answers are "NO" in the logic process, no preventive
maintenance task is generated. If the cost penalties are severe, a redesign may be desirable.
Hidden function safety effects - Questions 8A to 8F The hidden function safety effect
requires a task to ensure the availability necessary to avoid the safety effect of multiple
failures. All questions should be asked. If no applicable and effective tasks are found, then
redesign is mandatory.
Hidden function non-safety effects - Questions 9A to 9E This category indicates that a
task may be desirable to assure the availability necessary to avoid the direct cost effects of
multiple failures. If all answers are "NO" in the logic process, no preventive maintenance
task is generated. If economic penalties are severe, a redesign may be desirable.
Task determination is handled in a similar manner for each of the five effect categories. For
task determination, it is necessary to apply the failure causes for the functional failure to the
second level of the logic diagram. Seven possible task resultant questions in the effect
categories have been identified, although additional tasks, modified tasks or modified task
definition may be warranted, depending on the needs of particular industries.
Paralleling and Default Logic
Paralleling and default logic play an essential role at level 2. (see Figure 3.3) Regardless of
the answer to the first question regarding "lubrication/servicing", the next task selection
question should be asked in all cases. When following the hidden or evident safety effects
path, all subsequent questions should be asked. In the remaining categories, subsequent to the
first question, a "YES" answer will allow exiting the logic. (At the user's option,
advancement is allowable to subsequent questions after a "YES" answer is derived, but only
if the cost of the task is equal to the cost of the failure prevented).
Figure 5.2.3 Reliability decision logic tree- Level 2- Effects categories and task determination
Default logic: Default logic is reflected in paths outside the safety effects areas by the
arrangement of the task selection logic. In the absence of adequate information to answer
"YES" or "NO" to questions in the second level, default logic dictates that a "NO" answer be
given and the subsequent questions be asked. As "NO" answers are generated, the only
choice available is the next question, which in most cases provides a more conservative,
stringent and/or costly route.
Redesign: Re-design is mandatory for failures that fall into the safety effects category
(evident or hidden) and for which there are no applicable and effective tasks.
Explanations of the terms used in the possible tasks are as follows:
♦ Lubrication/servicing (all categories) This involves any act of lubricating or servicing
for maintaining inherent design capabilities.
♦ Operational/visual/automated check (hidden functional failure categories only) An
operational check is a task to determine that an item is fulfilling its intended purpose.
It does not require quantitative checks and is a failure-finding task. A visual check is
an observation to determine that an item is fulfilling its intended purpose 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 within
specified limits. Condition monitoring is a task, which may be continuous or periodic
to monitor the condition of an item in operation against pre-set 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
♦ 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 disks, safe-life structural members, etc.
♦ 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 tasks,
which are applicable, is necessary. From this review, the most effective tasks should
♦ 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 defined above is based upon its
own applicability and effectiveness criteria. Table 3.4 summarizes these task selection
Table 3.4 Task selection criteria
In order to set a task frequency or interval, it is necessary to determine the existence of
applicable operational experience data that suggest an effective interval for task
accomplishment. Appropriate information may be obtained from one or more of the
♦ Prior knowledge from other similar equipment which shows that a scheduled
maintenance task has offered substantial evidence of being applicable, effective and
♦ Manufacturer/supplier test data which indicate that a scheduled maintenance task will
be applicable and effective for the item being evaluated;
♦ Reliability data and predictions.
Safety and cost considerations need to be addressed in establishing the maintenance intervals.
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
failure prior 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, the cost of failure and the availability requirement of the
Mathematical models exist for determining task frequencies and intervals, but these models
depend on the availability of the appropriate data. This data will be 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 only be established initially by experienced personnel using good judgment
and operating experience in concert with the best available operating data and relevant cost