Planning
This topic covers:
Asset managers need to set the most appropriate maintenance plan for assets and components. The maintenance plan should be developed using the following diagram. The plan should combine planned and unplanned activities, capital and maintenance costs.
Compilation of the maintenance plan is a function of many variables classified into three general categories:
- Item/system maintenance requirements
- Maintenance management requirements
- Operational requirements.
The set of maintenance tasks defined for a site normally include preventative and corrective tasks and condition monitoring with consideration on:
- Grouping of tasks into work packages
- Interval for performance of each work packages.
Decisions are based on the recommendations for each individual maintenance task, considering operational and maintenance requirements.
The planned maintenance workload should be distributed across the maintenance cycle. Using the task groups and intervals, synergies may arise from considering operational requirements, labor resources available and the projected breakdown maintenance workload.
Inadequate maintenance risk planning for assets may result in:
- Detrimental impact on core business
- Inadequate identification of asset demand or maintenance needs
- Poorly targeted maintenance budgets
- Inappropriate asset condition
- Non-compliance with statutory and legal obligations.
Proper risk management and analysis is essential in determining the type of maintenance adopted. The level of risk needs to be determined in order to set priorities.
Stage I Criticality Rating
This stage focuses on purely equipment or asset items. Each item is given a "criticality rating". This indicates optimal maintenance practice for an individual equipment item or asset
Stage II Consequences of Failure
Evaluation is expanded to include all of the Consequences of Failure (CoF). It classifies parts of the system by a simple empirical value (weighted point score), allowing an organization to compare maintenance practices:
- Across like / different asset groups
- Between various regions and areas.
This helps prioritize maintenance practices.
Stage III Cost of Failure (Future)
This stage investigates budget impact on consequences of failure. This allows further prioritizing on “economic” risk basis. The failure of the asset or system has a cost to the organization. The failure can be represented by the cost of the Consequence of Failure. The risk to the organization is only the Risk Cost.
Current risk cost | = | Cost of the Consequences of Failure | X | Probability of failure (within the next 12 months) |
The risk of failure can be increased or decreased with modification to the maintenance regime.
A maintenance philosophy consists of the guiding concepts that allow the formulation of definite policies, in turn leading to specific maintenance plans and procedures.
The maintenance function, while it involves many processes, relates to only four considerations:
- To perform safely
- To perform in an environmentally acceptable manner
- To function as closely as possible to that required
- To meet its role for the least possible long term cost.
Therefore, the objective of maintenance is to:
"Make plant and equipment perform their intended functions, safely, in an environmentally acceptable manner and at least possible long term cost."
This objective is achieved by providing the required reliability and availability using the minimum of resources. It is important to recognize from the outset that the equipment is not intended to be more reliable or more available than necessary. Not that exceeding the targets is bad, but usually to be better than necessary involves using more resources.
This idea may cause many traditional maintenance staff a problem because they developed with the same culture as the airline tradesman who thought more maintenance was better. RCM maintenance philosophy, therefore, addresses maintenance effectiveness by targeting reliability and availability objectives and efficiency by using minimum resources.
The term "reliability" is used to describe the performance of technical equipment, and is defined as "the probability that an item will perform a specified function under specified conditions for a specified length of time"'.
This definition of reliability incorporates three important conditions:
- The measure of reliability relates to the item performing without failure, and depends on the definition of "a failure" for the item
- The conditions under which the item is designed to operate, or has operated must be specified or known
- The measure of reliability must always relate to a finite period of operation, since all items will be perfectly reliable over some very short time interval but will achieve zero reliability if the time interval is sufficiently long i.e. they will all fail.
There is a built-in level of reliability which can be achieved by each like group of items. This inherent level is a direct function of design, and represents the maximum reliability that can be achieved by the item over a nominated time interval when operated under a given set of conditions.
Achieved reliability in actual use can equal but cannot significantly exceed the inherent level for a given set of conditions. Adequate maintenance may serve to maintain achieved reliability close to the inherent level but unnecessary maintenance may either degrade or have no effect on achieved reliability. The inherent reliability can never be increased by maintenance but can only be improved by design changes.
The reliability characteristics of an item are of primary importance, as a means of determining the potential effectiveness of various processes that may be performed on the item. Reliability predictions can only be made by careful consideration of reliability and maintainability factors at the design stage.
Reliability is stated as one or more of the following:
- MTTF - Mean time to failure
- MTBF - Mean time between failures
- MTBMA - Mean time between (or before) maintenance actions
- MTBR - Mean time between (or before) repairs, or failure rate in some specified time period
Determination of Scheduled Maintenance
A range of information must be established about each item or system under consideration to determine maintenance tasks and processes required and timing.
The reliability characteristics of an item are potentially the most significant criterion for determining scheduled maintenance requirements. They form the basis for determining if scheduled maintenance is effective.
The second major criterion for determining scheduled maintenance requirements is the effect of failure of an item on safety, environment, function, cost and/or secondary damage.
Generally, the following items will be analyzed:
- All items for which scheduled maintenance has been defined in existing schedules
- Other items not included, warranting consideration because of its complexity, criticality, failure rate or other reason.
The reliability characteristics of each "item" selected for analysis (i.e. an item, group of items or system) are determined from:
- Failure data and breakdown reports
- Design characteristics
- Maintenance documentation held at the site
- The experience of technicians.
The first step in determining the reliability characteristics establishing the range of item failures over a selected period.
The failure modes should be described briefly in terms of actual fault likely to occur. Modes typical of the item being considered should also be identified for consideration during subsequent task evaluations.
The available data should be examined to determine if failures occur at random, immediately or sporadically.
If likely failure modes do not exist then scheduled maintenance would be ineffective. Scheduled maintenance tasks must be evaluated to determine if tasks are preventing failures, causing the low failure rate.
The effect of every failure must be evaluated to determine its potential effect on safety, environment, operation and damage.
If the failure cannot be detected or prevented by maintenance then the inclusion of a task in the maintenance program cannot be justified. The solution lies in redesign or modification.
The effect on personnel must consider Occupational Health and Safety regulation and equipment. The effect of failure must be established to ensure a valid relationship between reliability of an item and safety identified.
If failure of an item does not have a direct adverse effect on the environment or operation, there is no direct relationship between the reliability of the item and plant reliability.
Items whose failure will cause significant damage should be considered for inclusion in the maintenance program. In this case, the cost implications must also be considered.
This section defines the principles and details the procedures to apply to the principles of maintenance engineering analysis to formulate a Reliability Centered Maintenance plan.
These procedures provide the means to:
- Systematically evaluate the need for scheduled maintenance
- Identify failures that are acceptable, cannot be prevented, predicted or delayed, but that are critical to the objectives of the plant
- Record information and decisions.
Maintenance analysis is necessary to ensure that scheduled maintenance requirements remain effective and efficient. The analysis processes comprise:
- Criticality analysis, determining items required to be operable, and those able to tolerate a breakdown
- Failure Analysis, determining how an item is likely to fail and the effect of that failure.
Maintenance Categories
The maintenance activities determined above will fall into three categories:
- Preventive maintenance, these actions will either prevent or delay failures in equipment.
- Condition monitoring, this process determines when either a preventive or breakdown maintenance action is necessary.
- Breakdown (Corrective) maintenance, this activity is carried out when failure cannot be prevented or delayed or an unlikely mode of failure is experienced.
The assessment of maintenance options and intervals covers the examination of each existing maintenance task. This assesses effectiveness in preventing, delaying or detecting one or more of the identified failure modes.
The process of determining the correct maintenance interval is imprecise because of the differences in equipment reliability performance and a lack of complete knowledge about the item. The assessment will be subjective based on the analysis of the failure data and the characteristics of the item. When very few failures are reported, the possibility the item is inherently reliable should be considered.
The main criterion for this assessment is a comparison of the rate of unscheduled failures arising and the scheduled discoveries. A task is considered ineffective if the number of unscheduled failures exceeds the number of failures discovered during scheduled maintenance.
Alternative Maintenance Tasks
A more efficient means of accomplishing the intent of the task exists, achieving comparable results.
A task may provide an effective means of preventing or detecting likely failure modes identified, categorized as preventive and monitoring tasks.
The consistency tasks/processes expected to produce results must also be considered.
Increasingly, modern equipment is equipped with built-in monitoring systems providing general monitoring of the equipment. For maintenance analysis purposes, this monitoring may be considered as "alternative tasks". When evaluating the potential effectiveness of system monitoring against specific monitoring tasks the following should be considered:
- Scope of Monitoring, should have the same intent and cover the same parameters, items or systems as proposed scheduled tasks.
- The Standard of Monitoring relates to ensuring adequacy, accuracy and reliability of the monitoring system.
- Automatic Condition Monitoring, a built-in condition monitoring system will determine when maintenance is required.
Even though a task may be effective, a final recommendation covering inclusion of the task in the program is dependant on an evaluation of task efficiency. The benefit-derived completion of the task should justify the cost of completion.
Evident advantage should be gained from performing this task. If such benefit cannot be identified, the task should be omitted and the item monitored by general surveillance.
Failure of an item will be evident whenever the equipment is operated. However, some items and systems have a hidden function. A hidden function exists where:
- A function active with no indication performance ceases
- The item has an inactive function with no prior indication.
Secondary damage may result if a failure remains undetected.
Consequently, for items having a hidden function, a maintenance task will normally be included in the scheduled maintenance program.
A preventive task would have been assessed effective or potentially effective when some wear or drift pattern had been identified. This failure pattern (i.e. the failure distribution with respect to time) would also indicate the appropriate interval for the preventive task.
Specific Condition Monitoring
If this monitoring leads to the detection of a fault the existing interval is correct. Alternatively, the following general guidelines are to be followed in making the evaluation:
- If the proportion of unscheduled faults is high, the maintenance interval may be too long
- If few faults are detected, extension of the maintenance interval may be warranted.
Another intention of specific condition monitoring is to predict when failure will occur.
General Condition Monitoring
If there is discovery of a significant number of faults in a particular area or system then the interval should be reduced. Care should be exercised when selecting a period in balancing the requirements for optimum operating time against minimum maintenance downtime.
In some cases the nature of the failure mode is such that the period will be as a result of some event, but only where the condition can be established.
Maintenance costs include:
- Maintenance personnel
- Plant and equipment
- Spares required during emergency (unplanned) breakdown, scheduled corrective/predictive planned maintenance or the intermittent maintenance for major mid-life refurbishment, alterations etc.
An optimal maintenance regime is where the overall maintenance costs would be minimum while providing the required level of service or output, e.g. certain reliability or availability. This requires a combination of capital investment (refurbishment), planned maintenance, and unplanned maintenance.