Planners will typically utilise a large list of work that can be broken down into two categories:. Some thought needs to be applied to this log and good practice is that you forward schedule these work orders so that they are scheduled to be executed as close as possible to the date that the parts or other scarce resources required to do the job will be available. This, obviously, means that you need to know when these will be available before you schedule the work order. In some cases, we have seen planners push the scheduled start date for all jobs that are awaiting parts to months in the future; this will cause all sorts of cost and labour resource issues within CMMS and ERP systems.
Standardise the layout of these logs wherever possible so abnormal work will stand out visually when you are looking at the work in them. Some possible ways to do this are:. These standardisation techniques will save you time and take away the pain of having to interrogate each task to understand what it means and when it needs to be done.
Parts and Materials are one of the most important aspects of successful planning and scheduling. The end result is a task that cannot be performed and rework will need to be performed. This arrangement can be extremely useful and can complement continuous improvement initiatives where a Root Cause Analysis investigation is performed whenever materials issues have caused work to be deferred. Just like planners, these Materials Coordinators have KPI measures that they will be measured against. For the business it is a win-win situation in reducing delays and ensuring that work is performed safely.
If it is not already in place, we strongly recommend fostering a great working relationship between Maintenance and Supply, as both departments will benefit from this, as will your organisation overall. It is very difficult to visually appreciate how well a process is working unless you put some meaningful Key Performance Indicators in place. Often there will be circumstances that will see a KPI measure go backwards rather than the desired direction but it will alert all concerned that there is an issue that needs to be fixed in order to move forward.
Root Cause Analysis RCA and similar techniques can then be utilised to analyse the problem and pinpoint the issue. This may sometimes require a business case to Management for release of funds and labour to fix it. Without the accompanying measurement data it will be a very hard sell! There is a very real risk of complacency and you can think you are doing well but a deeper dive into the process will often show the opposite.
Keep challenging the status quo of KPI measures and if they are not meaningful or relevant why are we using them? From the forward log of work there will be an expectation from every organisation that you plan ahead to an agreed period. Some very mature organisations with reliable equipment and processes will demand a longer forecast period — in this case up to 12 weeks, or even longer for major shutdown work.
However, this forecast period could be as little as 2 weeks. Regardless of the length of the forecast period the aim is to have fully planned and resourced tasks that can be executed during that period. In order to answer this question you may need to carry out a literature search of FPP maintenance organizations or use your own experience. The thought process that was involved is indicated in Figure 3. It starts with the s a l e s - production reaction to market demand, the resulting change in the plant-operating pattern and the increased plant operation time.
This, in turn, requires amended maintenance life plans and a modified maintenance schedule. Thus, the maintenance workload changes, which brings in the training the need to modify the maintenance organization and systems. Understanding and applying this type of strategic through process is the cornerstone of effective and fruitful maintenance management analysis. Can you explain the effect this had on the following: unit life plans, preventive maintenance schedule, maintenance workload, maintenance organization. Review Ouestions Guidelines R3. For example, a reorganization might influence company profitability through changes in plant availability and maintenance resource costs.
Formulating maintenance strategy: A business-centered approach R3. Sugarcane has a short storage life and has to be processed shortly after cutting. The maintenance strategy of a sugar refinery is based on maintaining the plant over the 6-month sugarcane growing sea-! The maintenance strategy is concerned with maintaining the plant during agreed shutdowns to achieve the longest possible production-operating period. A typical demand for the service number of buses in operation is shown in Case study 6 of Chapter Major maintenance is carried out using the 'spare buses in the fleet'.
Minor maintenance is carried out in the low bus demand periods the maintenance windows. Exercise Guideline Solutions E3. The most likely resource structure see Figure 3. Support whole site! This, in turn, would require a change the administrative structure as shown in Figure 3. I 13 Second-line plant maintenance Second-line workshop maintenance First-line maintenance Figure 3. A proposed improved maintenance organization is shown in Figure 3. The advantage of this when arms are out of repair are evident.
On completion of this chapter you should be able to: model an industrial plant using a process flow diagram; understand how to model the plant services e. In Chapter 3, the various necessary elements of a maintenance system were d e s c r i b e d - and illustrated by reference to the particular case of a food processing plant. This made it clear that the central problem is indeed the formulation of strategy.
While this course will concentrate on industrial plant it will also use case studies to discuss the maintenance management of other physical asset systems, viz. The structure of industrial plant 4. The effectiveness of the production function is usually measured in terms of its rate of output e. In order to discuss an industrial plant in terms of its maintenance strategy it is useful to model the plant in two different but complimentary ways, viz.
A structural m o d e l of an industrial plant can be envisaged as a hierarchy of parts, ranked according to their functional dependencies into units, assemblies, sub-assemblies and components see Figure 4. Each of the units can itself be informatively subdivided into a hierarchy of parts ranked largely according to their replaceability. For example, in Figure 4. This kind of analysis is particularly important when setting up the equipment inventory usually based around the unit , and is especially useful when identifying the maintenance-causing assemblies, sub-assemblies and components of the unit.
The model of Figure 4. The author prefers to use the terms of Figure 4. Exercise E4. Select any one of these examples and draw the following: a A schematic diagram of your selected unit e. I Bearings Seals il Figure 4. Trackerball-driven graphics unit Communications i: port t Figure 4. In this way Figure 4. Models of this kind are an essential aid to understanding the production characteristics of plant and, hence the cost, safety and scheduling considerations which have to be taken into account when determining maintenance strategy.
In the mining industry a conveyor or a coal shearer would be a unit Case study 4 of Chapter A bus would be a unit in a transport fleet Case study 6 of Chapter 12 and a transformer in a power transmission system Case study 10 of Chapter However, different models are required to represent the overall operation of these physical asset systems. Modified process flow diagrams are needed in the mining industry see Figure 1 in Case study 4 and Figure 1 in Case study 5 of Chapter In the case of transport fleets 'status diagrams' are used see Figure 3 in Case study 6 of Chapter In transmission and distribution of electricity simple transmission flow diagrams are used see Figure 1 in Case study 9 of Chapter Although not covered in this book building maintenance could also be approached in a similar way.
A building could be divided into its systems to include the building structure, air conditioning, heating and ventilation, electrical, etc. Within each of these systems units could be identified e. The point that is being made here is that from the 'unit level of plant' downwards the principles and concepts of preventive maintenance can be applied universally across physical asset systems. Review Questions R4. Define a 'unit of plant' and identify some examples from the Case studies 4, 6 and 9 of Chapter When a c o m p o n e n t is unable, according to some predetermined criterion, to perform its designated function it can be said to have failed, and this could be a complete or a partial loss of function.
Such a loss could be contained at unit level The structure of industrial plant temporarily, at least or have consequences at plant level, depending on the design of the plant, e. The loss of function could also have safety or product quality consequences. For technologic and economic reasons, many of the components of a plant will have been designed to have a useful life greater than the longest plant production cycle but less than that of the plant itself. In most cases such maintenance-causing parts, especially the short-life ones, will have been identified at the design stage and made easily replaceable at component level.
Other components will fail for reasons that are not easy to anticipate such as poor design, poor maintenance or malpractice and may be extremely expensive to replace, often requiring a substitution at a higher level of plant, i. In addition, as the plant ages, failure rates and maintenance replacement costs can be expected to increase as the long life, expensive, components - and eventually the assemblies and whole units - reach the limits of their useful lives.
What can be asked at this point is why industrial plant cannot be made maintenance free, or at least designed for minimum maintenance. While this might increase capital cost it would reduce life-time maintenance costs. Perhaps working against this is the fact that the equipment manufacturer knows that companies purchasing equipment tend to use the 'lowest-bid approach' and do not think 'life-cycle costs' see Chapter 2. In addition the equipment manufacturer makes much of his profit from the supply of parts and maintenance services.
Review Question R4. Could a plant be designed to be almost maintenance free? Why is this not done? This division of responsibility is obligatory because the policy for major units and above is influenced by external, often longer term, factors such as obsolescence, sales trends or movements in the cost of capital as well as internal, shorter-term, factors such as operating and maintenance costs.
Decisions involving the replacement of complete units or sections of plant should really be regarded as part of capital replacement policy. Further reading on capital replacement policy is given in Reading 4. A maintenance strategy involves the identification, resourcing and execution of many thousands of repair, replace and inspect decisions.
This should be assembled from the programs of work contained in the unit life plan s but should be dynamic, e. The function is to provide a manufactured product to a market. The objective is to maximize long-term profitability. The best way of modeling such operations is by process flow diagrams down to the unit level of plant supplemented as necessary by systems diagrams for the services. The function of the operation is to provide mined coal to a market. The operation can be modeled using a combination of modified process flow diagrams see Figure 1 in Case study 5 of Chapter 12 and status diagrams for the mobile plant see Figure 2 in Case study 5 of Chapter With a public transport bus fleet the physical assets are mobile and operate over a wide geographic area.
The function is to provide a public transport service. The best way of modeling fleets is via a status diagram see Figure 1 in Case study 6 of Chapter The function is to provide electricity to consumers. With privatized utilities the objective is to maximize long-term profitability.
Each part of the system, e. A unit in an open cast coal mine would be a 'haulage truck'. A unit in a bus fleet would be a 'bus: A unit in a distribution utility would be a 'transformer'. Such parts have to be replacedlrepaired during the life of the unit to ensure the unit remains reliable during production. By selection of the best possible sub-assemblies and component parts a unit could be designed to be maintenance free over its designed life, say 25 years.
This is not done mainly because it would be too expensive but also because i n some cases such long-life parts would not be technologically possible. The maintenance managers advice should be sought but in general he does not take the replacement decision. The greater the number of such considerations that a replacement calculation includes, the much greater is the complexity of the algebra.
In general capital replacement models only take account of a few of the more important variables in any particular case and are, to that extent. The following simplified example is offered as an illustration. It is of deterministic nature in which averaged costs and trends are fairly predictable, as might be the case with a substantial unit of capital equipment.
A fixed-time replacement model for a unit of plant when new, the units-operating cost is 0 Wyear. If the equipment has cost EA to begin with, and were to be sold after n years for ES, the mean annual cost in this respect would be A - S ln Wyear. Second, have the necessary means to achieve your ends; wisdom, money, materials, and methods. Third, adjust all your means to that end. On completion of this chapter you should be able to: identify the main factors that should be included in a statement of objectives; understand the relationship between maintenance resources and plant output factors e.
Therefore, we need to establish what it should consist of, h o w it can be formulated and h o w it can then be used. The main factors that should be taken into account in the formulation of a maintenance objective are shown in Figure 5. The maintenance resources are used to ensure that plant output as specified under production policy , safety standards and plant design life are all achieved, and that energy use and raw material consumption are minimized.
Corporate objective I I Maintenance resources men, spares, tools, information Plant output factors Desired plant-operating pattern Desired output availability; tons of product per period, etc. Desired product quality Plant safety factors Plant life factors longevity Other plant factors Plant energy usage Plant 'shine' cleanliness and tidyness Figure 5.
In practice, however, the formulation of a maintenance objective is more complex than this. Maintenance objectives It usually involves the users, owners and safety department specifying what they want from the plant in negotiation with the maintenance department. Only then can the last of these decide how best to maintain the plant the maintenance strategy in order to achieve the requirements at minimal maintenance resource cost.
This process should provide the basis for maintenance budgeting and cost control. This is relatively easy to measure, using a costing system, and is the maintenance cost the Financial Director is most aware of, it is what he sees maintenance budgeting as being all about. The maintenance manager can change the level of r e s o u r c e s - very quickly in the case of contract arrangements, much more slowly if the resources are in-house.
Failure to ensure this will mean a corresponding loss of capital assets. The maintenance work involved is usually 'protective', e. Although we all recognize that neglect can cause rapid deterioration, determining the relationship between the level of maintenance and the life of a plant is not easy.
The best way of incorporating this into the objective is to establish the standards ofplant condition, which will ensure that the plant will achieve its expected life, and in the light of which the actual plant condition can, periodically, be audited taking into consideration such factors as obsolescence. Clearly, it is important to identify those parts of the plant that will have a major influence on its longevity.
Although it is unlikely t h a t during the life of a p l a n t - the standards of plant condition will change, the level of maintenance needed to ensure compliance with them will probably increase. One of the problems here is that maintenance that is aimed at prolonging plant life is likely not only to be expensive but also to be needed only very infrequently.
Thus, because most costing systems operate on an annual accounting basis, the tendency is often to let such work go until it becomes 'someone else's problem'. The customary procedure is to set safety standards that take account of the estimated probability, consequences and costs of failure. For specific types of plant e. For other items, safety standards will have to be set within the company, although again this will not be easy. Once again the sensible approach is to set safety standards by a process of engineering judgment based on experience and, wherever possible, analyses of plant failures.
The extent to which such standards have been complied with should then be periodically audited. Such a procedure should start with the most senior management. It is their responsibility to understand and comply with the ideas outlined in Figure 5. This is especially true in the case of ageing and hazardous plants. Many major disasters have stemmed from companies' neglect of the relationship between maintenance resources and safety standards. Review Question R5. In the case of the batch chemical plant of Chapter 4 , e.
A possible relationship between output per period and the level of maintenance resources used is suggested in Figure 5. Here, it is assumed that the correct plant maintenance policy is being used, in this case one based on fixed-time maintenance, and that the resources are applied with maximum organizational efficiency. It should be noted that the relationship implies that the cost of achieving increased output rises as the output approaches its maximum.
Maintenance objectives Table 5. It is unlikely that sufficient data will be available to construct such graphs. This would only be w o r t h w h i l e if the income from the additional sales that would accrue from this increase would be greater than the direct maintenance cost involved in achieving it.
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Little effect is felt for the first time period 6 months, say but eventually the output will fall away and money will then have to be spent to bring the plant back to optimal level. If the cost of lost production is known and is as shown in Figure 5. Lost-output cost varies, of course, by orders of magnitude between different industries; in some cases, it can be substantially higher than maintenance resource costs. This pushes the optimum output toward the maximum see Figure 5. In other words, the maintenance objective effectively becomes the maximization of output.
In the case of the batch chemical plant, e. Probably the main thing to emphasize here is that when considering the maintenance strategy for large complex plants the principal focus should initially be not on the various relationships indicated in Figures 5. The relationships shown in Figures 5. F'i cost units compare with Figure 5. Decreasing organizational efficiency causes the curve to move horizontally to the right, i. Think about how such a relationship might be affected by a change in maintenance policy say from fixed-time maintenance to condition-based maintenance.
Explain how the relationship might be affected by organizational efficiency. Figure 5. The relative importance of each of the factors included in this statement varies enormously from one technology to another and from one plant to another. With large, production limited, process plants the cost of lost output may be orders of magnitude greater than resource costs; with large buildings the longevity factor may be important, and so on. The main subdivision is into 'effectiveness' objectives - concerning selection of the best life plan see Figures 5.
Firstly, to determine the right maintenance strategy in order that the plant achieves the required output factors, i. Secondly, to ensure that the maintenance resources are used in the most efficient way organizational efficiency. Use these factors to write a generic statement of a maintenance objective. To illustrate how this might be done let us look again at our example of the batch chemical plant. Clearly, the first thing is to get the various 'user departments' to specify what theywant from each plant stream see Figure 4.
Negotiation to establish these plant objectives should take place at the companies' senior management level see Figure 5. If it is to meet the market demand the production department will need to establish its operating pattern, its plant availability requirements, and the required product mix and quality. Plant longevity and safety requirements will also have to be identified see Figure 5. This then enables the maintenance manager to establish the maintenance objective for the plant stream, viz.
In addition to maintain the plant in such a way so as to ensure a m i n i m u m o f 30 years life. To be meaningful, the above objectives need to be interpreted at plant unit level, e. In the case of the batch chemical plant, it is the responsibility of the production manager with advice from the maintenance manager to establish the production requirements for the reaction stream and hence the individual units. For reaction unit 3 this might be stated as: "to be available for continuous operation on a batch cycle for 50 weeks per year to supply the planned product tonnage and mix at a specified quality" The longevity and safety requirements might be for, say, year life expectancy and zero safety incidents.
Taken together, these requirements form the basis of the user requirement for the reaction unit and effectively define the envelope with which it operates. The reaction units maintenance objective, which is compatible with the plant maintenance objective, can then be stated as being to achieve the user requirement for the reaction unit at least cost. Clear definition of this, and of the user requirement, are a necessary preliminary to establishing the maintenance life plan of the reaction unit see Figure 4.
In addition to t h e s e plant-oriented objectives, organizational objectives of the type illustrated in Figure 5. These, if pursued, will improve organizational efficiency and therefore reduce, for a given workload, the direct cost of maintenance. The implication of Figure 5. In Maintenance objectives practice, the establishment of objectives for each of the principal maintenance r e s o u r c e s - labor and s p a r e s - is needed. The trade-force objective might be: to minimize, for the accomplishment of a given workload, the trade-force cost per period.
This could involve the setting of a target cost, or the setting of sub-objectives expressed in terms of performance targets, utilization levels, flexibility targets, etc. For spare parts the objective could be: to minimize the sum of the stockout and holding costs, which likewise could be used for setting cost targets or sub-objectives expressed via targets for stock turnover, stockout, etc.
Do you think this procedure could be used effectively in your own plant? It is instructive at this early stage of this book to consider the relationship between maintenance objectives, maintenance performance indices MPIs , maintenance control and benchmarks inter-firm comparison indices. They can be derived from the objective hierarchy shown in Figure 5.
As explained in Chapter 3 Figure 3. The monitoring of the MPIs should tell management if their maintenance effort is getting better or worse. The indices of Figure 5. More often than not, a written statement of objectives will not exist. Percent planned work I Overtime index Figure 5. At best there may unit-level data relating output to the level and type of maintenance resources. In the absence of such information production demand, to meet sales requirements, will dominate the setting of the maintenance objective.
This is understandable because production generates the cash flow and has to be responsive to demand, and to variations in raw material availability, which are often unforeseeable. They put pressure on the maintenance department to reduce costs without considering the consequences for plant performance and condition. Under such pressures the maintenance objective more often than not places emphasis on the short as opposed to the long term. In practice, it is often stated along the lines: to achieve the production-demanded output and operating pattern at minimum resource cost, subject to meeting mandatory safety standards.
This might be acceptable in the short term but can eventually result in low availability through neglecting preventive maintenance and a shorter plant life. Sales, production and maintenance must co-operate if a credible maintenance objective is to be established. This will be effective only if the parties concerned appreciate the impact of the relationships modeled in Figures 5.
Reference 1. Exercises E5. Are there company and production objectives? Are there plant maintenance objectives and have they been put in numeric form? Have the plant maintenance objectives been brought down to unit level? Are there maintenance objectives associated with organizational efficiency? Outline the maintenance objectives and user requirements for the unit of plant identified in Exercise E4.
Review Questions Guidelines R5. It is necessary to set standards of 'plant condition' for such parts to prevent them from deteriorating past the 'resource elbow' see Figure 5. These parts and their condition levels should be subjected to periodic plant condition audits. Figures 5. Figure 6. The crucial step in its formulation is establishing the level of preventive work that is needed if the maintenance objective is to be achieved, i.
It indicates that there is a level of Preventive maintenance decision-making - Part 1 Maintenance objective for reaction unit 3 see Figure 4. Production requirements Safety requirements In this case we are employing a policy of fixed-time overhaul on a large chemical plant. The problem is to decide on the optimum shutdown frequency. A shutdown frequency of once per year results in high corrective maintenance costs not enough preventive work.
A frequency of three times per year results in high 87 88 Strategic Maintenance Planning preventive costs. In this case, it would appear that the best policy is to overhaul the plant twice per year every 6 months. Note the shape of the corrective maintenance curve which has a major impact on the optimum frequency. Models of this type have limited applicability, however.
It is not just the frequency or level of preventive work that has to be decided but also its nature time-based, age-based, inspection-based, etc. In addition, the model assumes that the sales, production, or maintenance position is static, which is rarely the case. For example, the cost of lost o u t p u t and hence the optimum level of preventive maintenance - will vary from one time to another depending on whether the plant is sales limited or production limited. Finally, the model does not take into consideration the complexity of the plant. Some units might be more important than others because of the greater impact of their failure on output or safety and should attract more resources.
Indeed, if the model has any applicability at all it is at unit level, e. A procedure for formulating or improving a unit life plan should include the following steps: i Identifying the maintenance-causing assemblies, sub-assemblies and components that make up a unit. This chapter is concerned with identifying the array of maintenance tasks that might be used to maintain the parts of a plant unit.
In addition the principles and the possible applications of each of the identified tasks are discussed. Chapters 7 and 8 use these principles, concepts and ideas to outline procedures for identifying the best maintenance task for each part of the unit. Chapters 7 and 8 will also show how the tasks are assembled into a unit life plan.
Exercise E6. Note: It is usual for the tasks making up the life plan for a particular unit to be embedded within a computerized maintenance schedule. In addition the tasks associated with the major maintenance may not be recorded at all. In this, the complete agitator assembly can be considered as an example of a high-level item because at some point in the life of the reaction unit it may be necessary to decide w h e t h e r to repair or replace it. Conversely, the motor, gearbox, drive belts, coupling, agitator paddle and support frame can also be considered as items.
All of these require an in-situ repair or replace decision to be taken in order to keep the reaction unit operating. As regards the gearbox shaft, however, the decision that is the most likely to have to be made - i. Mechanical plant on the other hand, is not often designed for ease of maintenance, and what modularization there is occurs only because of process function and manufacturing method. The following categories of item are then identifiable: Simple replaceable SRIs, e.
Likely to be maintained by replacement and discard. Likely to be maintained by replacement and repair in workshop. These have an intermittent, particular, function only and are usually safety related. Failure is not observable under normal operation conditions because it has no immediate consequence, but makes Preventive maintenance decision-making - Part 1 the plant's integrity seriously vulnerable to some other failure or process deviation, i. When the item is unable, according to some predetermined criterion, to perform its designated function it can be said to have failed, and this could be a complete or a partial loss of function; i.
The consequences" of the functional failure will depend on whether the loss of function can be contained at unit level temporarily, at least or have consequences at plant level lost production. The functional failure of the item occurs because of one or more failure modes. In the case of the functional failure of SRIs see Figure 6. In the case of a CRI, it is likely that there is more than one failure mode, e.
WHO SHOULD ATTEND?
In the case of CRIs, irrespective of the number of modes of failure the most likely maintenance action will be to 'replace the gearbox' via some form of condition monitoring. It should also be noted that many CRIs may well have a dominant failure mode. The higher-level items, e. Many of these will have been covered by analyzing the agitator system into its lower-level items. The most important reason for identifying failure modes at agitator system level, perhaps using failure modes and effects analysis see Chapter 8 , would be to identify the failure modes that might require complete agitator system replacement e.
The point that is being made is that the definition of an item, and the analysis and categorization of a plant unit into its constituent items simplifies the process of building a unit life plan. Thispragmatic approach concentrates on deciding how to maintain the identified item the hardware rather than on how to deal with the myriad of failure modes.
However, it will be shown later in the course that for some critical items, e. Table 6. These are the building bricks of the maintenance life plan. Alternatively, the need for such work could be designed out. The main alternative first-level tasks for maintaining an item are listed in more detail in Table 6. Thus, each first-level maintenance task consists of: i the maintenance action, ii its timing maintenance policy.
To complete this decision scenario Table 6. In addition third i. The maintenance tasks First-level decisions plant level Some combination of: Action Adjust or calibrate Proof test Always repair Always replace Repair vs replace on condition Timing o f the action Action scheduled before failure on usage hours, miles, etc. Stores decide on inventory policy, e. Lead time of supply of part andlor is it being internally reconditioned? The action is scheduled on usage hours, miles, output , calendar time, or some combination of these. The timing of the action is based o n the condition of the item or performance as indicated via one or more of the following inspection methods: - Operator functional monitoring - Simple inspection look, listen, feel, smell - Condition checking against a limit - Trend monitoring: a At fixed intervals or b At variable intervals or c Via continuous inspection or d Via some combination of these.
Based on a policy of 0TF. The action is carried out after failure. The action can involve considerable pre-planning via spares, quick change , if the item is designated as critical. Based on a policy of 0M. The timing of the action is based o n some other item's maintenance timing. DOM, as indicated by failure-cause investigation after major or recurring failures OM: opportunity maintenance 3 0 D Preventive maintenance decision-making- Part 1 Preventive Maintenance Guideline 2 The best maintenance task for an 'item' or 'failure mode' should be decided on after reviewing all of the maintenance actions, viz.
Can be regarded as largely independent and complimentary to the main actions, and can be considered separately. Also independent of the other actions and can be considered separately. This action is reserved for items or units with a hidden function failure mode s not observable under normal operating conditions. In most practical situations the comparison of these main factors, coupled with engineering judgment, allows the best of options i - iii to be identified. For example see Figure 6. However, with high-level, high-cost items such as the complete agitator assembly of Figure 6.
With such a complex item there are many possible modes of failure, some of which might be cheaper to put right by repair and some of which by agitator assembly replacement e. The point being made is that even when the agitator assembly is held in stores the decision to repair or replace is best left until the failurecausing situation has occurred.
When considering the most likely actions for the items of Figure 6. I Review Question R6. In the case of CRIs e. Item repair is only possible if a component is in stock or can be quickly bought in. Stores policy might be to hold any component that is likely to be required during the life of the plant. The rationale used for assessing the optimum number of components to hold and the optimum time and quantity for reordering is known as the spares inventory policy, which will take account of the rate of demand for the component and therefore of the number of such components in use in the plant.
In some situations it may well be economic to refurbish the component. P r e v e n t i v e m a i n t e n a n c e d e c i s i o n - m a k i n g - Part 1 Preventive Maintenance Guideline 3 As already explained for in-situ repair to be the feasible maintenance action the cost of the repair of the item must be cheaper than the cost of replacement of the item; i. If reconditioning were to be carried out internally the components also would have to be held. Where only item replacement is feasible as a first-level decision the scenario is as shown in Figure 6.
Here it is assumed that the item repair is carried out internally and the workshop decisions involve choosing between repair, recondition or scrap, a decision to scrap having consequences for the stores inventory. Such items are sometimes referred to as rotables. Flow of parts from stores Flow of components Feedback to stores ', I Feedback to stores i. I I Component i V! Such a policy would be adopted partly because of the high cost of the replacement work and partly because of the wide range of possibilities for the type of failure that would o c c u r - each failure mode requiring a different action see Figure 6.
The decision would be influenced by such information as: - probable defective part and in-situ repair methods available; time, labor and material cost of item replacement; unit unavailability cost; running time to next 'window of opportunity'; probable life of item after repair or temporary repair; probable life of item after replacement; condition and probable life of unit.
It is made much easier if some form of inspection procedure has provided prior warning of failure. Spares inventory refurbished. Feedback to- sto res Flow of parts from stores Items scrapped t Feedback to stores Figure 6. Choosing, in a dynamic situation, between the repair or replacement of a complex item is the most difficult, a n d the most commonly occurring, maintenance decision-making problem. It should be noted that the maintenance actions have been looked at separately to their timing policy.
The policies are looked at in Section 6. In practice, the 'action' and 'timing' the maintenance task would be determined together. There are a number of examples and an exercise in Chapter 7 to illustrate the 'maintenance selection procedure'. Identify a CRI that is always or mostly repaired on failure. Identify a CRI that requires a repair vs replace decision to be taken after or shortly before failure. Review Question R6.
How does this differ from the decision scenario of Figure 6. Although this would usually be decided on economic grounds there could well be other influencing factors, viz. Most companies have some combination of internal and contract repair. Controlling this can be one of the most difficult maintenance management problems. Establish the percentage of rotables reconditioned externally and internally.
Machining ,,' rotables! For a maintenance repair technique to be considered as an in situ one, all phases of the repair process must be undertaken at the item's normal location. In-situ techniques have major advantages where the cost of downtime is high. Their adoption can extend running times and improve availability via the reduction of unit outage times, and changes the balance of the tradeoff outlined in Section 6.
Appendix 2 lists the principal in-situ techniques. C Assembly tube The seal is released and the tool withdrawn. The valve is closed and the tool removed. The downstream piping is replaced. Define an in-situ repair process. Can you describe in outline a true in-situ technique. How does the use of in-situ techniques affect the decision scenarios of Figures 6. A maintenance task is made up of an action and its timing the policy. What are the alternative actions? What are the alternative policies? The following is a description of the principles and concepts associated with the selection of alternative maintenance policies - ' t h e timing of the maintenance action'- see Table 6.
They include item replacement, item repair and major strip-down for inspection the author regards CBM as inspection carried out without major strip-down. FTM will improve reliability only if the failure of the item concerned is clearly a result of some form of wear-out and if the useful life of the item is less than that of the unit in which it belongs. For example, consider the case modeled in Figure 6. J Time Figure 6. FTM i. This implies that the vertical axis of Figure 6.
The basics of failure statistics are covered in Appendix 3 to include a description of statistical failure models and their characteristics e. The cost-effectiveness of FTM will depend among other things on the predictability of the time-to-failure of the item concerned, i. The smaller the relative dispersion, the greater the predictability.
For example, the time-to-failure of the motor in Figure 6. In practice, of course, the big difficulty in applying this kind of analysis is that the statistical data is not often agitator available. In general, the lower-level items of Figure 6. In other words, the statistical predictability decreases with complexity and the probable number of failure modes.
It should be noted that in terms of deciding on complete agitator assembly replacement rather than repair the failure distribution should only include those failure modes that require agitator replacement. Gearbox good statistical predictability, i. Thus, the probability of its failure during the life of the system is low and the incidence of such failures random.
FTM is therefore n o t an effective policy for improving reliability. If, in such cases, failure is unacceptable or undesirable then the alternative policies of Table 6. In spite of these limitations FTM, in one of the following forms, is often the most appropriate policy: i Group or phased replacement of large populations of identical or similar items e. For example, consider the maintenance, in a large building, of lamps the failure distribution of which might be as in Figure 6.
Such a policy is usually much c h e a p e r - in terms of the combined cost of labor and materials - than the alternative of replacing each lamp as it fails. Time Figure 6. The idea is indicated in Figure 6. Such a policy might well ensure that an acceptable plant reliability would be achieved at a more affordable cost. All 'simple items' that will last 1 year but are unlikely to last 2 years are changed in annual w i n d o w All 'simple items' that will last 2 years but are unlikely to last 3 years are changed in annual w i n d o w Etc.
O 0 i 1 O 2 i 3 Time from installation of plant years Figure 6. The main difficulty is deciding on the item's replacement period. In most practical situations there is uncertainty caused by the lack of failure data. The best policy is probably to play safe and replace at cautiously short operating periods which can be extended with experience and in the light of information gained via inspection of any items removed for, say, reconditioning see Figure 6.
Inspect online and in yearly Play safe and replace at safewindow. The critical question is operating periods via limited 'will it last another year? Extend periods as assess the situation and replace failure statistics improve. Note that one of the prime uses of CBM is to provide information to enable the shutdown workload to be predicted and planned in advance of the shutdown.
In other words, the timing of the shutdown is fixed in order to resource it but much of the shutdown workload is condition based. Review Questions R6. Explain what this means by drawing a simple failure distribution. Describe two situations where FTM might be used as an effective policy. For example, let us assume that the vibration level of the gearbox of Figure 6. A monthly inspection interval will give adequate notice for planning and scheduling of the maintenance, thus minimizing the effect of lost production. The advantage of this over FTM is that it allows the operation of individual items up to nearly their maximum running time.
CBM is also the prime policy where an item exhibits little failure predictability see Figure 6. It is particularly important for expensive repairable items see Figure 6. Clearly, the inspection interval adopted will be determined by the lead-time-to-failure. The effectiveness of CBM depends on no small measure on how reliably this can be determined from deterioration curves of the type shown in Figure 6. The monitored parameter can provide information about a single component e. The more specific the information provided, the better it is for maintenance decision-making.
Simple inspection: Mainly qualitative checks based on looking, listening and feeling e. The cost of this should be insignificant compared to the cost of replacement or repair. The period between inspections should be sufficiently short so that minor problems can be detected before they develop. Condition checking: Done routinely by measuring some parameter which is not recorded but is used for comparison with a control limit. Such checking only has value where there is extensive experience of identical systems.
Trend monitoring: Measurement and graphic plotting of a performance or condition parameter in order to detect gradual departure from a norm see Figure 6. This application is most effective where little is known about the deterioration characteristics. When enough knowledge of these has been acquired, condition checking can be substituted for trend monitoring. The monitoring of performance and of condition is closely related.
In the former case the parameter monitored would be some measure of a unit or item input or output, e. Changes in this might then be related to deterioration in some condition of concern.
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For example, Figure 6. Where CBM is used extensively the high level of data collection renders mandatory the use of computerized systems of the type shown in Figure 6. These often have trend analysis capability. The extensive range of monitoring techniques available is indicated in Table 6. Image recording and high-resolution analysis is a post-processing possibility.
Can resolve body temperature to within a few degrees and monitoring can be performed f r o m a distance at a glance. Low Thermometers, thermocouples On Manual1 automated Range from stick-on thermometric strips to permanently installed thermocouple sensors. Can give visual temperature readout or an electrical input to a hard-wired monitoring system. Covers a wide range of temperature but acts only o n a small area. Lubricant Vibration Infrared camera 0n Manual As above but can cover a much wider surface area. Can provide a detailed surface temperature picture and can be calibrated to give quantitative measurement.
High Magnetic plugs and filters Onloff Manual Analysis of debris picked up by plugs or filter i n an oil washed system. Mainly large debris picked up, pm. High Ferrography NIA Manual Analytical technique used to separate ferrous debris by size t o enable microscopic examination. Non-ferrous debris can also be separated but not graded. A wide range of debris size can be analyzed from 3 to p m. A contract service is usually available. Generally for small debris size p m A contract service usually available. Crack I I collector Frequency spectrum analysis 0n Manual1 automated Represents the vibration of a rotating or reciprocating machine as a frequency spectrum or signature which reveals the discrete frequency component content of the vibration.
Provides the basis for fault detection, detailed diagnosis and severity assessment. High Structural monitoring Off Manual A variety of vibration-based techniques exists for the detection and location of structural fauIts. The majority of such techniques involve imparting a known vibration into the structure and analyzing the resulting response.
Average Magnetic flux Onloff Manual Detects cracks atlnear the surface of ferrous materials. I I I Continued Table 6. Also useful for detection of inclusions and hardness changes. Expert Ultrasonic Onloff Manual Detects cracks anywhere in a component.
5 tips for more effective Maintenance Planning and Scheduling - Assetivity
Suffers from directional sensitivity, meaning that general searches can be lengthy. Often used to back up other NDT techniques. Involves a radiation hazard. Low Incremental bore holes 0n Manual A series of fine plugged holes of incremental depths which are periodically unplugged and scrutinized for leakage.
thelab.jo/scripts Electrical resistance 0n Manuall automated Electrical element and potentiometer are used to assess resistance change due to material loss. Capable of detecting material thickness reduction of less than 1 nm. Average Polarization resistance 0n A good indicator of corrosion but is unreliable as a means of estimating material loss rate. Explain why this is so? What do you understand by lead-time-to-failure? Exercises E6. Carry out a survey of the condition monitoring techniques used in your company and tabularize them as shown inTable 3 in Case study 1 of Chapter The resulting d e m a n d for corrective work occurs with little or no warning.
This will only be cost-effective if: i the consequences of item failure in terms of lost production, or of danger or damage, can be regarded as negligible or, alternatively, if the cost of letting the item fail is less than that of implementing alternative maintenance policies ; ii the consequences of item failure are serious but do not take effect for some time and it is possible to carry out the necessary repair within this period.
Obviously, such failures have to be identified and planned for in terms of decision guidelines, fault-finding and resources - a k i n d o f p l a n n e d f a i l u r e m a i n t e n a n c e. This is more of a scheduling procedure rather than a maintenance policy. For example, opportunity maintenance OM could be employed in the case of the ammonia plant of course Case study 1 of Chapter If there was a major failure within the normal 4-year operating period the company could 'take the opportunity' to carry out other work during the stoppage.
See Chapter 9 for a fuller discussion of 'opportunity scheduling'. Clearly, this is a design problem but it is often part of the maintenance department's responsibility. In general, DOM only becomes an alternative after some experience of operation; i. Therefore, such a policy can only be implemented effectively if an information system exists which facilitates the choice on cost and safety grounds between the proposed redesign and the best of the various maintenance tasks see Chapter DOM is an area of maintenance management that benefits considerably from the application of failure modes and effects analysis see Chapter 8.
Exercise 6. We now turn to the related question of timing. Special items These have to be treated separately to normal items because they have a hidden function, i. The most appropriate action is to proof test the item at fixed-time intervals. In addition, each special item should be subject to the same decision logic Figure 6. Preventive maintenance decision-making - Part 1 Has the itema normal function or a special function?
Has the item the characteristic of online failure detectability? Has the item the characteristic of offline failure detectability? Has the item statistical failure predictability? Yes lb. The 'best policy' is the most 'cost-effective' policy available within the various constraints e.
See Figure 6.