Corso di Laurea Specialistica in
Ingegneria dell’Automazione
Analysis of the new technologies for
industrial maintenance
Relatore: Chiar.mo Prof. Marco Garetti
Correlatore: Ing. Luca Fumagalli
Tesi di laurea di: Marco Romanò
Matricola: 707300
Abstract (Italian) ... 7 Abstract (English) ... 7 Estratto ... 8 Summary ...11 Cap. 1 Introduction... 14 1.1 Definition ... 14 1.2 Maintenance policies... 15 1.2.1 Corrective maintenance... 15 1.2.2 Preventive maintenance ... 16 Reference... 24 Chap. 2 Surveys ... 26 2.1 Survey ... 26 2.2 Italian situation... 26 2.3 North America... 30 2.4 Sweden ... 31 2.5 Comparison ... 31 Reference... 33
Cap. 3 Condition-based maintenance... 35
3.1 Definition ... 35
3.2 Condition monitoring ... 37
3.3 CBM steps... 37
3.5 Advantages and weak points ... 38
3.5 Prognostic... 39
3.6 Fault prediction methods... 40
3.7 CBM tasks scheduling ... 41
3.8 Common difficulties for the implementation of CBM... 43
Reference... 46 Cap. 4 Standards... 51 4.1 OSA-CBM ... 51 4.2 IEEE 1451 ... 53 4.3 ISO 13373-1 ... 53 4.4 IEEE 1232 ... 54 4.5 MIMOSA ... 54 4.6 ISO 17359 ... 55 4.7 ISO 13379 ... 55 4.8 ISO 13380 ... 56
Cap. 5 Predictive maintenance techniques... 57
5.1 Vibration monitoring ... 57
5.1.1 Vibration monitoring for fans... 58
5.1.2 Vibration monitoring for bearings... 58
5.2 Thermography ... 61 5.3 Oil analysis... 61 5.4 Pressure/temperature/current monitoring... 62 5.5 Visual inspection ... 62 5.6 Noise ... 63 5.7 Weight check ... 63 5.8 Current analysis... 63 5.8.1 NILM ... 64
5.8.2 Electrical signature analysis ... 66
5.8.3 Rotor analysis... 70
5.9 Process parameters check... 71
Reference... 72
Cap. 6 Smart sensors ... 75
6.1 Description ... 75
6.2 Functionality ... 76
6.2.1 Signal processing ... 76
6.2.2 Digital control and manipulation ... 77
6.2.3 Communication and bus interaction ... 78
6.4 Characteristics ... 78
6.5 Sensor communication interface ... 81
6.5.1 Wireless technologies... 82
6.5.1.1 Wireless network Topology... 84
6.5.1.2 Wireless WAN technologies... 87
6.5.2 Wired technologies... 88 6.5.3 Communication protocols ... 89 6.5.4 Initiatives... 91 6.5.4.1 Industrial initiatives... 91 6.5.4.2 Academic initiatives ... 91 6.6 RFID... 92 6.7 Standards ... 92 6.7.1 IEEE 1451 ... 92 6.8 Smart bearings... 97
6.9 Consideration about the smart sensors ... 98
Reference... 100
Cap. 7 Handheld devices... 103
7.1 Description ... 103
7.2 Handheld devices use cases ... 104
7.3 Augmented reality ... 106
Reference... 108
Cap. 8 Analysis of an industrial case ...110
8.1 Manufacturer company profile...110
8.2 Machine 1...114
8.2.1 Machine description ...114
8.2.2 Prescribed maintenance...116
8.2.3 Analysis: possible failure of the machine... 121
8.2.4 Actual maintenance activities on Machine 1... 123
9.2.5 Possible maintenance innovations... 124
8.2.6 Why these solution are not applied ... 126
8.2.7 Possible improvements ... 128
8.3 Machine 2... 130
8.3.1 Machine description ... 130
8.3.2 Prescribed maintenance... 134
8.3.3 Possible failures ... 137
8.3.4 Actual maintenance status ... 139
8.3.5 Possible maintenance innovations... 140
8.3.7 More suitable innovation... 144 Cap. 9 Conclusion ... 146 References ... 149
Table of figures
Figure 1: The bathtub curve ... 17
Figure 2: Reliability and MTBF(W.R.Wessels, 2003) ... 18
Figure 3: Availability comparison (W.R.Wessels, 2003) ... 19
Figure 4: P-F curve ... 36
Figure 5: diagnostics and prognostics are based on multi-source data (VTT, 2006) ... 39
Figure 6: Layers for the identification of a fault ... 40
Figure 7: Example of vibration analisys for bearings ... 59
Figure 8: Examples of mechanical problems of the bearings ... 60
Figure 9: Star network topology ... 85
Figure 10: Mesh network topology ... 86
Figure 11: Hybrid network topology... 87
Figure 12: Wireless sensor data schematic... 93
Figure 13: Smart sensor software architecture... 96
Figure 14: Structure of the smart bearing ... 98
Figure 15: Example of a dispensing system...111
Figure 16: Rotor impregnation...111
Figure 17: Plasma system ...112
Figure 18: Heatstake system ...113
Figure 19: View of the machine 1 ...115
Figure 20: Footprint of the machine 1...116
Figure 21: Top view of machine 2 ... 131
Figure 22: Lateral view of machine 2 ... 132
Figure 23: Side view of machine 2 ... 133
List of table
Table 1: Wireless technologies characteristics ... 84
Table 2: Fieldbuses characteristics... 91
Table 3: TEDS information ... 95
Table 4: Prescribed maintenance for machine 1 ... 120
Table 5: Prescribed maintenance for machine 2 ... 134
Abstract (Italian)
Lo scopo di questo lavoro è di analizzare le nuove tecnologie e servizi relativi alla manutenzione e alle attività connesse ad essa.
Al giorno d'oggi le aziende devono competere nel mercato globale, è quindi necessario ridurre i costi di produzione per poter mantere i prezzi bassi e mantenere la propria quota di mercato.
La manutenzione è un processo aziendale che è in genere considerato solo un costo per l'azienda. Tuttavia è generalmente uno dei processi con la più bassa efficienza. La manutenzione è inoltre generalmente gestita con metodi ormai superati rispetto allo stato dell’arte scientifico e sono presenti numerosi sprechi, c'è quindi ampio margine di miglioramento.
In questa tesi vengono analizzati i più recenti argomenti di ricerca legati alla manutenzione e relativi a nuove soluzioni tecnologie, facendo un'analisi della letteratura, vengono poi presentati due casi di studio reali, viene descritto lo stato attuale della manutenzione di due macchine e viene ipotizzata l'implementazione delle tecnologie descritte, viene infine fatta un'analisi critica delle ragioni per cui non sono implementate attualmente.
Abstract (English)
The aim of this thesis is to analyze the new technologies and methods for the industrial maintenance and to the activities related to it.
Nowadays the companies have to compete in a worldwide market, it is necessary to reduce the production costs to be able to keep a competitive price and hold the market share.
The maintenance is one of the process inside a company that is generally considered only as a cost, because many think it does not generate any return of the investments. Nevertheless maintenance is one process with the lowest efficiency.
The maintenance is generally managed with old methods compared with the present scientific state of the art and there are several wastes, thus there is a great margin for improvements.
In this thesis the latest research topics related to the industrial maintenance are analyzed in a literature analysis, two real case studies are presented, the actual maintenance management of the machine is described, the implementation of the new technologies is hypothesized and in the end a critic analysis is carried out to show the reason why these innovation are not used.
Estratto
Al giorno d'oggi le società dovendo competere in un mercato globale, devono contenere i costi di produzione per poter ridurre i prezzi oppure mantenere o incrementare i propri margini di profitto.
La manutenzione è una delle attività che rappresenta un costo per l'azienda ed è anche una delle attività in cui ci sono ampi margini di miglioramento.
L'obiettivo di questo elaborato è di analizzare le nuove tecnologie e servizi che possono migliorare la gestione della manutenzione e le sue varie attività.
Per fare ciò si è deciso di analizzare gli articoli pubblicati nelle varie riviste scientifiche di settore, un uso reale di queste tecnologie è stato considerato oltre alle opinioni dei ricercatori.
Per ridurre i costi legati alla manutenzione e allo stesso tempo garantire una buona qualità e disponibilità dei macchinari, è necessario tagliare gli sprechi che al momento sono presenti; per ottenere questo risultato è necessario agire su tutti gli aspetti legati alla gestione della manutenzione.
Il primo passo è garantire che tutte le persone che sono coinvolte abbiano accesso a tutte le informazioni di cui necessitano, ciò non è limitato al personale della manutenzione ma è esteso a tutte le persone o dipartimenti che sono legati a questo processo, ad esempio il responsabile degli acquisti, il manager della produzione ma anche l'operatore che lavora sulla macchina possono trarre vantaggio dalla maggiore disponibilità di informazioni.
E' necessario che un sistema informativo sia distribuito ovunque nella fabbrica per poter acquisire tutti i dati che sono prodotti nei vari processi tra cui l'amministrazione, la produzione e la manutenzione. Questi dati vanno poi analizzati e processati e le informazioni estratte possono essere poi fornite alle rispettive persone interessate.
Ogni processo e dipartimento utilizza a differenti tecnologie, queste devono essere capaci di comunicare le une con le altre o con un sistema che sia in grado di gestire la comunicazione tra i vari sistemi, questo però comporta l'aumento della complessità del sistema informativo.
La capacità di avere le informazioni necessarie al momento giusto permette alle persone incaricate di prendere le decisioni di fare le scelte giuste: questo ciò permette di tagliare i costi legati agli errori e all'acquisto di parti di ricambio errate.
Una grossa parte dei costi lagati alla manutenzione possono essere ricondotti alla assenza di produzione durante il periodo in cui la macchina è ferma in seguito a un guasto o alla sostituzione non necessaria di componenti.
Maintenance), il cui scopo è di identificare indizi di un possibile guasto,(.) grazie a queste informazioni i componenti possono essere sostituiti prima del loro guasto, evitando la mancata produzione, inoltre i componenti possono essere usati per quasi tutta la loro vita utile evitando di sostiuirli troppo presto.
La complessità e il grado di automazione dei sistemi CBM è aumentata sempre di più negli ultimi anni e la tendenza è di proseguire in questa direzione.
Al momento nessun metodo è in grado di soddisfare tutti i requisiti di un buon sistema diagnostico, per questo motivo sistemi ibridi con differenti algoritmi per la soluzione del problema possono essere una buona soluzione per gestire gli scenari complessi di un problema diagnostico in un impianto industriale.
Le prossime tecnologie renderanno possibile l'utilizzo del CBM per impianti che al momento hanno una complessità troppo elevata (per differenti motivi) per l'applicazione dei sistemi CBM attuali.
Grazie a questo la gestione della manutenzione sarà resa più semplice e permetterà anche di organizzare le varie attività di manutenzione in modo da minimizzare i tempi di fermo macchina.
Al momento la diffusione del CBM è abbastanza limitata visto che è applicata solo ad alcune macchine, ma in futuro la sua diffusione aumenterà grazie alla possibilità di gestire scenari più complessi, di avere procedure più automatizzate e di rendere la gestione di tutte le attività legate alla manutenzione più semplice. Per supportare il CBM nell'identificazione dei guasti sono necessarie anche le informazioni sullo stato attuale dello stabilimento, la diagnosi deve essere integrata con gli altri processi aziendali.
Metodi qualitativi e quantitativi possono essere combinati per una più accurata identificazione degli indicatori di un guasto.
Per fare questo è necessaria una grossa mole di dati che devono essere il più accurati possibile, specialmente quelli legati agli eventi.
Un altro importante argomento di ricerca è quello legato ai sensore; l'importanza dei sensori è facilmente intuibile poichétutte le informazioni legate allo stato del sistema derivano in modo diretto o indiretto dai valori acquisiti dai sensori.
Sono state sviluppate nuove tecnologie per l'acquisizione affidabile di dati in linea e nuovi algoritmi per il analizzare in modo efficiente e rapido i segnali.
L'analisi e il raggruppamento dei segnali saranno sempre di più svolti dai sensori, in questo modo è possibile ridurre i costi, il consumo energetico e le risorse necessarie e allo stesso tempo incrementare le prestazioni e la precisione.
L'uso di reti wireless per il trasferimento delle informazioni tra i sensori e il sistema di acquisizione permette una maggiore flessibilità nella disposizione dei sensori e riduce is costi dovuti al cablaggio.
possibile creare device intelligenti che possono acquisire il segnali direttamente in digitale e che hanno anche la capacità di monitorare continuamente il loro stato. Per gestire questa accresciuta disponibilità di informazioni il sistema computerizzato di gestione della manutenzione (CMMS computerized maintenance management system) deve essere migliorato. Inoltre sono necessari nuovi e più potenti metodi per estrarre, processare e interpretare le informazioni contenute nei dati acquisiti dai sensori.
Lo scopo di tutto questo è non solo l'identificazione del guasto, ma anche della causa per poter definire un miglior piano di manutenzione ed evitare quando possibile futuri guasti.
L'ultimo interessante ambito di ricerca riguarda l'uso di dispositivi elettronici come ausilio alle operazioni di manutenzione.
Grazie a questi dispositivi l'operatore può accedere alle informazioni di cui necessita come ad esempio lo schema elettrico, i manuali o particolari tecnici; può inoltre collegarsi direttamente ai sensori e fare delle misurazioni, confrontarle con quelle relative alle ispezioni precedenti e identificare segnali di degrado.
Tutto questo in modo quasi istantaneo senza dover perdere tempo per cercare manuali o fogli intorno alla macchina.
L'obiettivo di questo lavoro è di analizzare gli studi legati alla manutenzione e le tecnologie sopra menzionate per descrivere l'evoluzione della gestione della manutenzione dal punto di vista dell'ICT.
Summary
Nowadays, companies have to compete at worldwide level and they have to reduce their production cost, in order to keep the product at low prices, keeping or increasing their profit margin.
Maintenance is one of the activities that generate cost for a company. Nevertheless maintenance practices in the company often reserve margin for improvement. Indeed the objective of this research is to analyse the new technologies and services that can improve maintenance management and maintenance tasks and then link them to a practical industrial case.
The mainly methodology adopted to make this research is literature analysis. Nevertheless, practical implication of use of technology is considered beside the different researchers’ opinion and comments of practical use of the analyzed technologies is considered when addressing the industrial case
To reduce the cost of the maintenance and at the same time assure a good service and availability, the first point is to provide to all the people involved in the process the data that they need, this is not limited only at the maintenance personnel but also the people in the purchasing department, the production manager and even the operator of the production line that can receive benefits from the availability of this information.
For example, it is advised that an ICT system is spread everywhere in the shop floor, in order to collect the data that are produced within different process such as the business, operation and maintenance, analyze and process them and give the information of interest to the correct people.
Each involved process utilizes a number of different technologies, all of these must be able to communicate with each other and this increases the complexity of the ICT environment.
The correct information at the right time will help people to take better decision, this will cut the cost connected to the wrong actions carried out and, for instance, the purchasing of useless spare parts.
The big part of the cost of the maintenance is either due to the lack of production for the down time after a fault or the unnecessary replacement of the components. One of the most interesting research topic seems to be the CBM (Condition Based Maintenance), the goal of this method is to identify the clues of an incoming failure and give the information to the right person so the part can be changed before the failure, avoiding the down-time and, at the same time, without changing the part too early, so almost all the remaining useful life of the component can be used.
The complexity and the grade of automation in CBM systems are increasing and the trend will continue in this direction.
Up to now no method alone is able to meet al. the requirements of a good diagnostic system so hybrid system with parallel ways of reasoning can be an attractive idea to handle a complex and industrial scale diagnostic problem.
According to the literature, in the future the emerging technologies will make possible to use CBM for plant that, at the moment, seem too complex (for many different reasons) for applying CBM. This will make maintenance management of the plant easier, providing all the benefits of many maintenance tasks carried out at the right time.
With the possibility to handle more complex scenarios, to have more automated procedures and the simplification of the management, the use in the industry of CBM will probably increase, while at the moment, CBM is applied in the most of the company solely on few machine or not applied at all.
To support the CBM in the identification of the faults the diagnosis of the plant status must be improved.
The diagnosis should be integrated with other process operations, the advantages of the qualitative and quantitative methods can be combined to be able to identify the early indicators of a fault. To this end, a lot of information is required, this information, especially the event related one, must be accurate.
Another interesting topic in the research is related to the sensors. All the information are provided to the system directly or indirectly by the sensors.
New sensors techniques are envisioned to be developed for robust on-line data acquisition and new algorithms for efficient and fast signal process.
Data processing and sensor fusion will be moved at the sensor node level, in order to reduce costs, power consumption and resources and at the same time increase the performances and the accuracy.
Furthermore, the use of wireless networks should allow more flexibility in the placement of the sensors and avoid the cost of the wired networks.
The CMMS (computerized maintenance management system) must be enhanced to be able to handle all these data and new and more powerful methods are required to extract, process and interpret the information contained in the data acquired by the sensors.
The goal is to identify not only the fault, but also the cause and help to define more efficient maintenance plans.
The last interesting topic is the use of PDAs (Personal Digital Assistant) or handheld devices as a support for the maintenance personnel during their duties. The maintenance operator can access the needed information regarding the machines like electrical schematics and technical drawings, check the status of the
sensors and read all the data regarding them, simply from his handheld device, without losing time to search for the right information.
The objective of this work is to analyze maintenance related studies and technologies above mentioned and to outline the evolution of the maintenance management from the ICT point of view. Two industrial cases are presented to show the problems in the practical use of such innovations.
Cap. 1 Introduction
A foreword about what is maintenance is provided in this chapter to present the general scope of the investigation and provide the reader with the basics of maintenance management in order to introduce the main parts of the dissertation.
1.1 Definition
In OECD’s (Organization for Economic Co-operation and Development) resolution of 1963, maintenance was defined as “a business function entrusted with the constant control of the facilities and all the repair work and services necessary to ensure the smooth running and good state of conservation of the production facilities, services and equipment of the plant”.
The European Standards Committee (CEN) defined maintenance in its standard project WI 319-007 (1997) as “the grouping of all the technical, administrative and management actions taken during the lifecycle of a product in order to maintain or restore it in a state in which it can perform the required task, for which it was designed” (see also EN 13306:2001).
Maintenance commission of UNI (Italian Organization for Standardization) defined maintenance as "a combination of all technical and administrative actions, including supervision actions, intended to maintain or restore an entity in a state where they can perform the required function" (UNI 9910 and UNI 10147) 15 years ago.
In 2003 this norm was replaced by the norm UNI EN 13306, now defining maintenance as "a combination of all the technical, administrative and management activities planned during the life cycle of an entity, to keep it or return it in a state where they can perform the required function".
According to R. Keith Mobley (2002) the major part of the total operating costs for all the manufacturing or production plants in the US is maintenance.
The maintenance costs can bear on the product cost for a percentage that goes from 15% (food or related products) up to 60% (iron, steel, paper and other heavy industries) and can occupy a significant amount of the work force (e.g. up to 30% in the chemical process industry G. Waeyenbergh et al., 2002).
It is worth considering that these percentages include also the expense for the modification or improvement of the machines, this due to the fact that these activities are carried out normally by the maintenance personnel and normally
these activities are not allocated in the correct cost center but are considered as maintenance on the asset.
From the same reference R. Keith Mobley (2002), according to a survey concerning maintenance management effectiveness, one third of the maintenance expenses (33%!) is wasted in improperly or unnecessary actions.
In the actual situation, where a company has to compete in a global market with competitors that have the production in some country where the manwork cost is cheaper or the environmental laws are less strict it is easy to see that cutting the maintenance wastes can reduce the cost of the products without affecting the quality and allow the company to regain the lost competitiveness.
Thanks to the developments in the electronics in the last decades, the machines are now equipped with microprocessor based controller instead of the old electromechanical systems, the computational power and memory available for the programming are continuously increasing, and also the capability to communicate and exchange information between the systems is continuously developing.
Thus, it is now possible to have a better knowledge of the status of the machine or its components, this has brought some advantages for the maintenance.
1.2 Maintenance policies
In this paragraph the different maintenance policies are presented, their advantages and disadvantages are highlighted.
1.2.1 Corrective maintenance
UNI norm defines corrective maintenance as “a maintenance performed following a failure intended to bring an entity in the state where it can perform the required function”
The corrective maintenance is the easiest policy that is possible to apply on a machine, when the machine is working then no actions are performed, when it is broken it will be repaired. It is the first maintenance strategy appeared in industry A plant that is using this run-to-failure management does not spend any money on maintenance until it is necessary. In reality no plant is managed with only corrective maintenance, a basic group of preventive tasks are carried out on the machine like lubrication and small adjustments.
Benefits:
• low direct costs,
• the low need of organizational structure • no planning necessary
Disadvantages:
• lack of failure notice,
• the need of a oversized spare part warehouse • high machine downtime
• low availability
• bad use of the maintenance personnel • low control on costs
• high overtime labor cost
To be able to react rapidly to a failure an extensive spare parts inventories must be maintained, it must include all the major components for all the critical equipment and the maintenance personnel must be available and able to locate and identify the cause of the fault rapidly.
Due to the high downtime of the equipment the cost of this method is usually high, according to an analysis the cost of the a repair performed as a reaction to a fault is in average three times more expensive than the same repair made as a preventive action or scheduled. Being able to schedule a repair minimizes the repair time and the labor cost. Anyway it can be cost-effective in certain cases (Alsyouf, 2007; Kelly, 1997; Pintelon and Gelders, 1992) and when the profit margins are large (Sharma et al., 2005).
Global competition and the reduction of profit margin are forcing maintenance manager to apply more effective maintenance strategies nowadays.
1.2.2 Preventive maintenance
The UNI definition of preventive maintenance is: “maintenance performed at predetermined intervals or according to prescribed criteria and intended to reduce the probability of failure or degradation of operating conditions of an entity“. All the preventive maintenance methods rely over the assumption that statistically the lifetime of a component or the failures on a machine have a standard behavior that can be identified, normally it can be described as a bathtub curve (see figure 1). It can be explained in the following way, a new component or a new machine
has a high probability of failure due to installation problems during the first period of operation, after this initial period the probability of a failure is low for the normal life period. After this period the probability of failures increases rapidly.
Figure 1: The bathtub curve
In preventive maintenance the machine is repaired or the component is replaced based on the MTBF statistic or when the wear out signs start to show.
The scheduled maintenance interval (τ) is the time between two subsequent repair or replacement maintenance actions. The number of intervals expected over the life of the system (k) is an integer value. The time since the last scheduled maintenance interval is the independent variable (t) minus the cumulative time preceding the last scheduled maintenance interval, kτ. The comparison between the reliability function and MTBF for a system without a scheduled maintenance interval and the reliability function and MTBF for a system with a scheduled maintenance interval is shown in Figure 2 Reliability and MTBF.
Figure 2: Reliability and MTBF(W.R.Wessels, 2003)
It is graphically evident that the reliability for a system that implements a scheduled maintenance in which the components are replaced and repaired prior to failure is significantly better than the reliability of a system that is allowed to run to failure. Since the MTBF (θ) is the indefinite integral of the reliability function it is also evident that the MTBF for a system that implements a scheduled maintenance is significantly improved over the MTBF for a system which is allowed to run to failure.
The comparison between the availability function for a system with and without a scheduled maintenance program is shown in the figure 3.
Figure 3: Availability comparison (W.R.Wessels, 2003)
Theoretically the availability of a system with a scheduled maintenance program declines in a small magnitude from one scheduled maintenance activity to the next and as the system is repeatedly restored the availability returns to unity. The magnitude of the decreases in availability over time between scheduled maintenance intervals is justified by the assumption that the decrease for any interval is comparable to the decrease from the condition of the system when new. The availability over time for a system that does not implement a scheduled maintenance interval shows a gradual decline in overall availability of the system. The increases in availability following each maintenance action does not reach unity because the system is not restored by the maintenance action but only the component that has failed is removed and replaced so the other parts that are near to a failure state remain in their place.
The advantages of the preventive maintenance are:
• Management control: preventive maintenance can be planned unlike reactive maintenance, workloads can be scheduled so that equipment is available for preventive activities at reasonable times.
• Overtime: overtime can be reduced or eliminated. Surprises are reduced. Work can be performed when convenient; however, a proper distribution of maintenance tasks is required to ensure that all work is completed without excessive use of overtime.
• Parts inventories: preventive maintenance approach allows the planning of which parts are going to be required and when, those material requirements may be anticipated to be sure that they are on hand for the event. A smaller stock of parts is required.
• Standby equipment: with high demand for production and low equipment availability, reserve, standby equipment is often required in case of breakdowns. Some backup may still be required with preventive maintenance, but the need and investment will certainly be reduced.
• Safety and pollution: if no preventive inspections or built-in detection devices are used, equipment can deteriorate to a point where it is unsafe or may spew forth pollutants. A good detection system catches degrading performance before it reaches too low a level.
• Quality: good preventive maintenance helps ensure costant quality output. Tolerances are maintained within control limits. Naturally, productivity is improved and the investment in preventive maintenance pays off with increased revenues.
And the main disadvantages are:
• Potential damage: every time a person touches a piece of equipment, damage can occur through neglect, ignorance, abuse, or incorrect procedures.
• Infant mortality: new parts and consumables have a higher probability of being defective or failing than exists with the materials that are already in use. Replacement parts are too often not subjected to the same quality assurance and reliability tests as parts that are put into new equipment. • Parts use: replacing parts at preplanned preventive maintenance intervals,
rather than waiting until a failure occurs, will obviously terminate that part’s useful life before failure and therefore require more parts. This is part of the trade-off among parts, labor, and downtime, of which the cost of parts will usually be the smallest component.
• Access to equipment: One of the major challenges when production is at a high rate is for maintenance to gain access to equipment in order to perform preventive maintenance tasks. This access will be required more often than it is with breakdown-driven maintenance. A good program requires the support of production, with immediate notification of any potential problems and willingness to coordinate equipment availability for inspections and necessary tasks.
condition based or predictive.
1.2.2.1 Time based maintenance
Time Based Maintenance (TBM) is the easiest of the preventive maintenance policies, it consist of periodic maintenance tasks carried out according to a defined timeline. When the interval is elapsed since the last maintenance action on the component the component is repaired or exchanged without regard on its actual wear level, this to maintain always the component in the normal operation period on the bathtub curve.
There are two different approaches for this method, in the first the part is exchanged at constant intervals without caring if the component has worked or not, in the second one the exchange is based on the real age of the component counting the effective time that it has worked (Waeyenbergh and Pintelon, 2002; Kumar, 1996)
Benefits:
• easy planning and control
• possibility to schedule maintenance personnel tasks • possibility to schedule the downtime of machine • optimization of the stock of spare parts
Disadvantages:
• necessity of an accurate model or experience to achieve best results • parts can be exchanged also if it is not necessary
This method it is relatively simple to implement, if the wear out of a part is constant or if a time interval between the failures can be identified this method can guarantee good results with low costs.
Unfortunately normally this is not possible and it is difficult to identify a model accurate enough to be sure to carry out the maintenance tasks on the correct time so the result is that the interval is shorter than the optimum and this lead to an high number of interventions (thus higher direct costs) or longer than the optimal so an higher number of failures (thus higher hidden costs).
Normally the manufacturer of the machine prepares a list with the maintenance actions that have to be carried out and their interval and this is included in the documentation of the equipment. This list can be a good point to start for the scheduling, these intervals are usually shorter than necessary because the producer does not know the exact condition of work of the machine and also he want to avoid complains from the customer. With the experience and the knowledge
acquired with the time working on the machine the intervals can be adjusted.
1.2.2.2 Condition based maintenance
UNI 10147 norm defines Condition Based Maintenance (CBM) as “preventive maintenance subordinated to achieve predetermined threshold value”.
The corrective maintenance is basically a set of actions carried out according to the actual condition of the machine or of the component, the idea underneath this method is that indicative prognostic parameters exist, can be detected and used to quantify the possibility of a failure before its occurrence.
The actual status of the equipment is obtained from sensors or measurements taken by the operator, these information are processed to check if the component performances deviates from the acceptable performance level and thus this can be a symptom of an incoming failure.
The common problems of equipments are ageing and deterioration, these trends can be identified through trend analysis of the equipment condition data and this information can be used to recognize when the component is near to the end of its life.
The status of the machine can be evaluated continuously or on time interval Advantages:
• possibility to schedule maintenance personnel tasks • possibility to schedule the downtime of machine • optimization of the stock of spare parts
• exchange of the components only when needed • early identification of the faults
Disadvantages:
• difficult to implement
• requires a deep knowledge of the equipment to identify the parts that need to be observed
This method has a very big potential, the faults are identified early before the failure, thus is possible to avoid the stop of the machine, reduce the spare part warehouse and plan the maintenance actions.
Unfortunately the implementation is not easy, a deep knowledge is required to find which components need to be checked, where the sensor must be placed and especially for complex machines the failure cannot be reconducted straightforwardly to one abnormal trend but to a combination of small changes in
the acquired data.
1.2.2.3 Predictive maintenance
UNI norm defines Predictive Maintenance as “preventive maintenance carried out following the detection and measurement of one or more parameters and extrapolation of remaining time before failure with appropriate models”.
This method is similar to the Condition Based Maintenance but extends its capabilities to predict the future status of the equipment.
The data acquired from the machine are analyzed in order to find a possible temporal trend and so be able to predict when the monitored value will reach or exceed the defined threshold.
Advantages:
• possibility to schedule maintenance personnel tasks • possibility to schedule the downtime of machine • optimization of the stock of spare parts
• exchange of the components only when needed • early identification of the faults
Disadvantages:
• difficult to implement
• requires a deep knowledge of the equipment to identify the parts that need to be observed
This method gives good results in systems where faults are preceded by progressive degradation, the identification and quantification of this trend and the successive analysis will give the possibility to know with a good approximation the remaining life of the component.
As for the Condition Based Maintenance (of which this method can be considered an extension) the implementation is not easy, the amount of data required for the identification of the trend is big and a good knowledge of the machine is required.
Reference
CONTENTS REFERENCES PUBLIC
ATION YEAR WORK CONTRI BUTION An introduction to predictive maintenance Second Edition R. Keith Mobley Butterworth-Heinemann 2002 Specific knowledge A framework for maintenance concept development
Geert Waeyenbergh, Liliane Pintelon
Int. J. Production Economics 77 pp. 299-313 2002 Specific knowledge The role of maintenance in improving companies’ productivity and profitability. Alsyouf I. International Journal of Production Economics, 105, pp. 70–78 2007 Specific knowledge Gestión del mantenimiento industrial
Kelly A. and Harris M.J Publicaciones Fundación Repsol, Madrid. 1997 Specific knowledge Maintenance Management Decision Making Pintelon, L., Gelders, L.F. European Journal of Operational Research 58, 301–317 1992 Specific knowledge Reliability analysis and maintenance scheduling considering operation conditions. Kumar, D. Doctoral Thesis, Lulea University of Technology, Sweeden. 1996 Specific knowledge Maintenance management: literature review and
directions
Amik Garg and S.G. Deshmukh Journal of Quality in Maintenance Engineering Vol. 12 No. 3, 2006pp. 205-238 2006 Specific knowledge
Contracting out maintenance and plan
for future research
H.H. Martin Journal of Quality in Maintenance Engineering, Vol. 3 No. 2, 1997,pp. 81-90. 1997 Specific knowledge
Chap. 2 Surveys
This chapter presents the status of the implementation of the maintenance policies and programs described in the previous chapter in Italy and then the Italian situation is compared with other countries.
2.1 Survey
A survey is a method used to collect in a systematic way, information from a sample of individuals. Although most people are familiar with public opinion surveys that are reported in the press, most surveys are not public opinion polls (such as political polling), but are used for scientific purposes. Surveys provide important information for all kinds of research fields.
Since survey research is always based on a sample of the population, the success of the research is dependent on the representativeness of the population of concern.
2.2 Italian situation
A distinctive feature of the Italian manufacturing companies is the size, almost the 94% of the manufacturing firms have less than 20 employees, 4% have from 20 to 50 employees and only the 2% have more than 50 employees but they provide work for the 25% of the labor force in the manufacturing sector.
A survey on maintenance management in small and medium firms (Cattaneo, 2000) was carried out by AIMAN, the Italian Maintenance Society, in the year 2000. 174 companies with up to 200 employees were involved in this survey; the firms belong to mainly to the mechanical and metal working sectors and to the chemical and pharmaceutical sectors. The survey highlighted that an actual maintenance function exists in about 20 per cent of the micro-firms (with less than 15 employees), in about 50 per cent of small firms (having between 16 and 50 employees) and in about 85 per cent of medium sized firms. The focus of the survey was on the identification of the cost of the maintenance, revealing that it is almost 2 per cent of turnover, that there is no significant difference related to firm size, sector or maintenance policies. It was found that a fire-fighting attitude still prevails in many firms, about the 40% of the maintenance activities are reactive task carried out after a failure, this was also reported by Ferrari et al. (2002). Another survey was carried out in 2002 by a regional section of the AISL, the Italian Society for Work Studies (Ghirardo, 2004), this one was more on local
scale, involving 62 medium firms, and confirmed the percentage of reactive maintenance and the same attitude towards maintenance. It was found out that only the 20 per cent of the examined companies calculate and took into account inefficiency costs (costs of loss of asset availability), caused for example by stops due to reactive or delayed maintenance.
There are also cases of excellence and implementation of the best maintenance policies testified by single case studies (Ferrari et al., 2002) or collections of case studies (Cigolini and Turco, 1997) presented in international literature. It is worth noticing that most surveyed cases regard either large industries or some smaller manufacturing plants that belong to large trusts or multinational groups.
Regarding the structure, we can observe that the internal maintenance structure is usually quite small (about 70 per cent of firms have up to five maintenance operators and about 60 per cent of firms have a spare part inventory value of less than 50,000€). In most cases when needed the internal capacity is assisted by external support. Vertical integration in maintenance apparently is present in only the 6 per cent of firms, who declare to be completely self-sufficient. The majority of firms primarily use the most basic form of maintenance contract, the work package contracts. This contract is task oriented and does not allow the firm to take all the benefits of the maintenance outsourcing, as shown by Tsang (2002) because occasional service supplier normally try to minimize their investments in staff development, equipment and new technologies. The more advantageous and complex performance-contracting mode is selected by only the 15 per cent of firms as main option.
Larger firms are also more capable of adopting more advanced forms of contracting out maintenance (performance contracting is the main option in 31 per cent of large firms against 10 per cent of medium firms and 6 per cent of small ones).
Regarding the technologies, CM is widely adopted, it is present in the 52 per cent of firms, but the diffusion of CMMSs is really limited, it is present in only the 35 per cent of the companies, a result that is comparable to the diffusion in other countries more than 10 years ago, see Ikwan and Burney (1994), Jonsson (1997), Swanson (1997).
The diffusion of CM is not statistically related to the size of the company (it is adopted by 39 per cent of small firms, 60 per cent of medium ones and 55 per cent of large ones) but it grows significantly with operation time (CM is present in about 39 per cent of firms operating on a single shift basis, but in more than 65 per cent of firms operating on a two or three shift basis).
The prevalent usage of CMMSs is for data recording and preventive maintenance planning, while more complex activities involving elaboration of data are seldom
performed (e.g. maintenance budgeting).
The presence of CMMSs is directly related with firm size: CMMSs exist in 29 percent of small firms, in 37 per cent of medium ones and in 41 per cent of large ones.
Concerning CMMS presence, it is worth noticing that the way in which the CMMS is used has an important impact on performance. In particular, a more frequent usage for PM planning is associated to a better safety performance, while a more intense usage for spare parts management and maintenance budgeting is significantly linked to a stronger contribution to lower production costs. Thus, the point is apparently in using a CMMS rather than in having a CMMS.
As to organization, a decentralized maintenance department depending on production functions represents the prevailing structure (54 per cent of firms). In 57 per cent of firms, the head of this department (or of the maintenance function) is a skilled worker and only in the remaining 43 per cent of firms he belongs to middle or senior management.
In small firm is more common to delegate to the operator some maintenance task (77 per cent of small firms, against 47 per cent of medium and 36 per cent of large ones), while a centralized technical department is more common in large enterprises (44 per cent) and in medium enterprises (24 per cent) than in small ones (16 per cent), and a combination of centralized function integrated into production is more common in medium firms (29 per cent) than in small (7 per cent) or large ones (20 per cent).
Regarding the maintenance planning and control the formalization is limited, the maintenance orders are all written only in 35 per cent of firms, a spare parts stock book exists in just 39 per cent of firms and only a minority of companies (11 per cent) has monthly budgeting.
Compared with previous Italian studies (Ghirardo, 2004), a positive sign is the growing awareness about the inefficiency costs, which are taken into account in 48 per cent of firms, even if in a rough way.
Concerning maintenance policies and concepts, there is a limited diffusion of TPM which is present in 16 per cent of the organizations and a reactive maintenance proportion of about 55 per cent, which is well above recommended values of 30-40 per cent (see, for example, Jonsson (1997)). The result is similar to those reported in older surveys concerning Italy (Ferrari et al. (2002) or Ghirardo (2004)) and other countries (Jonsson (1997)). Although widespread, CBM has a limited weight among normal maintenance policies (about 10 per cent of total maintenance).
Among preventive maintenance approaches, condition based maintenance demonstrates to be extremely effective, being positively correlated with better cost, quality and safety performance. This practice appears to be easily available to small and medium firms that can use it to improve their performance.
Also the TPM has proved advantageous, but mostly in terms of quality, and safety without a clear correlation with cost reduction, the same result can be found in other studies, e.g. by McKone et al. (2001), who hypothesize that “TPM allows for effective use of the budgeted maintenance expenses and is able to improve inventory turns, quality, and delivery while maintaining stable production costs”. Also for this maintenance concept applicability and effectiveness do not depend on firm size.
Finally, the performance scores are generally unrelated to the firm size. The only weakly correlation is between yearly turnover and contribution to availability improvement, the smaller the firm, the better the perceived performance.
There are minimal differences in the maintenance strategies in different sectors . Significant ones concern organization (with a prevalence of dependence on production and transfer of maintenance tasks to production in the metal working sector, of centralized maintenance departments in process industry and other industries and of a combination of both structures in the wood working sector) and the diffusion of TPM, which is concentrated in the metal working and machine manufacturing industries (it involves 31 per cent of the firms of this sector but only 7-8 per cent of firms of other sectors).
The general picture evidences some criticalities, such as too much fire-fighting and limited preventive approaches, and, particularly in small firms, low status of maintenance management as to internal capacity, retribution and education of persons in charge and inadequate diffusion and use of planning and control tools, especially CMMSs. Most of these critical issues were nevertheless pointed out by research on manufacturing firms in several western countries
There are also some strong points, including the long experience of most maintenance heads, the growing awareness of inefficiency costs and the increasing diffusion of condition based maintenance and of TPM across all industries, independently of size. The average performances are generally more optimistic than similar measurements presented in literature (see Swanson, 2001), the size of our sample (100 firms) and the numerous and consistent correlations between maintenance best practices and performances highlighted in this study support our trust that these scores describe correctly, if qualitatively, the obtained maintenance results.
As to performance, an interesting finding is that there is no direct correlation between firms’ size and maintenance performance. Good results are equivalently reported by small and large firms.
It is possible to observe that maintenance visions and strategies influence maintenance results significantly, the best performances are achieved thanks to the use of maintenance policies and TPM programs. In particular, a practical implication is the confirmation that CBM can contribute to improve performance and thanks to the fact that these technologies are becoming more widespread and cheaper this practice can be easily adopted even by small firms, leading them to optimize their maintenance results.
Finally, the analysis of data from the examined area confirms the general indication that the usage of preventive maintenance, including CBM, should be extended. In any case this could be done only with the acquisition of opportune maintenance engineering instruments to guarantee that the preventive maintenance programs can harmonize well with production schedules and with the actual state of manufacturing equipment. This requires a more extensive use of the CMMSs and to use these instruments at their full potential it is necessary to train adequately the human resources: maintenance personnel could thus be empowered, waste of resources could be avoided and a synergic positive effect could add up to the extension of proactive maintenance practices.
2.3 North America
The data are taken from the Aberdeen report (2006), there is a big difference in the size of the firms between the italian survey and this one, in fact this survey is based on 43% of respondents from large enterprises (annual revenues above US$1 billion); 27% from mid-sized enterprises (annual revenues between $50 million and $1 billion); and 30% of respondents from small businesses (annual revenues of $50 million or less).
The companies are classified in 3 categories according to performances achieved in the asset management: best in class (those who have mature asset management strategies and operations), industry average (companies that have implemented formalized asset management programs in some areas), and laggard (those companies that are just embarking on asset management and/or are meeting with some resistance).
The use of preventive maintenance is diffused between the 44% of the firms and the RCM is present in the 42% of the companies and the TPM in the 31%.
The CMMS is present in the 72% of the companies but in the 64% it is not completely integrated in real time shop floor system and in the remaining 8% of
the cases is present a fully automated and holistic management system to support the tactical and strategic decisions.
The average of the outsourcing service utilization is the 59% but it has a peak for the best in class companies that utilize the third part services in 85% of the firms. The use of asset performance management is present in the 73% of the best in class firm and in the 47% of the other companies.
2.4 Sweden
The information are taken from the Maintenance practices in Swedish industries: Survey results by Imad Alsyouf (2007). This survey is based on a sample of 185 firms that employ from 37 to 2400 people and a turnover from 100k€ to 1 billion €.
The average number of maintenance employee is 32 and the 66% of them have more than 10 years of experience in the work.
The average budget for the maintenance is the 4% and the 20% of this amount is used for the outsourcing of the services.
The Preventive maintenance either time or use based is the most used maintenance strategy followed by the CBM and the reactive maintenance and then there are TPM and RCM.
2.5 Comparison
The analysis of the survey show that the use of the maintenance policies and programs have a positive effect on the performances of the company but even if the advantages are clear the use of these technologies is limited. The use of these practices is more common in the medium and big firms (41% compared to the 29% of the small companies), this can be correlated to their possibility to spend more money than the small companies because the adoption of these technologies is expensive and the results are visible only years after their adoption.
In the Italian situation with the high number of small or micro firms (94% of the total number of companies has less then 20 employees), the presence of few or no maintenance personnel and a management that is not informed about the best practices the usage of these technologies is less than the other countries.
The main reason of the scarce diffusion of the best maintenance practices is the cost, these technologies are expensive and return of the investment is not guaranteed, if they are correctly applied then this cost will be repaid but it is
difficult to calculate how much time will it take and the risk of not being able to utilize them in the correct way is high.
Reference
CONTENTS REFERENCES PUBLIC
ATION YEAR WORK CONTRI BUTION Maintenance management in italian manufacturing firms
Damiana Chinese, Gianni Ghirardo Journal of Quality in Maintenance Engineering Vol. 16 No. 2, 2010 pp. 156-180 2010 Survey Maintenance in SMEs in Italy Cattaneo, M. AIMAN, Milan 2000 Survey TPM: situation and procedure for a soft introduction in Italian factories
Ferrari, E., Pareschi, A., Persona, A. and Regattieri, A. The TQM Magazine, Vol. 14 No. 6, pp. 350-8. 2002 Survey Maintenance in Pordenone province Ghirardo, G. Manutenzione Tecnica e Management, February, pp. 41-4, available at:
www.manutenzione-online.com (in Italian)
2004 Survey
Total productive maintenance
practices: a survey in Italy
Cigolini, R. and Turco, F. Journal of Quality in Maintenance Engineering, Vol. 3 No. 4, pp. 259-72. 1997 Survey Strategic dimensions of maintenance management Tsang, A.H.C. Journal of Quality in Maintenance Engineering, Vol. 8 No. 1, pp. 7-39 2002 Specific knowledge Strategic dimensions of maintenance management Tsang, A.H.C. Journal of Quality in Maintenance Engineering, Vol. 8 No. 1, pp. 7-39 2002 Survey
Maintenance in Saudi industry
Ikwan, M.A.H. and Burney, F.A.
International Journal of Operations & Production Management, Vol. 14 No. 7, pp. 70-80. 1994 Survey status of maintenance management in Swedish manufacturing firms Jonsson, P. Journal of Quality in Maintenance Engineering, Vol. 3 No. 4, pp. 233-58 1997 Survey An empirical study of the relationship between production technology and maintenance management Swanson, L. International Journal of Production Economics, Vol. 53
1997 Survey
The impact of total productive
maintenance practices on manufacturing performance
McKone, K.E., Schroeder, R.G. and Cua, K.O.
Journal of Operations Management, Vol. 19 No. 1, pp. 39-58. 2001 Specific knowledge Linking maintenance strategies to performance Swanson, L. International Journal of Production Economics, Vol. 70 No. 3, pp. 237-44.No. 1, pp. 39-58. 2001 Specific knowledge The Asset Management Benchmark Report Jane Biddle Aberdeen report 2006 Survey Maintenance practices in Swedish industries: Survey results Imad Alsyouf
Int. J. Production Economics 121 (2009) 212–223
Cap. 3 Condition-based maintenance
This chapter describes one of the most promising maintenance policies that can help companies to improve their performances, reducing the costs and increase the availability. Although there are several advantages this policy it is not widely used as demonstrated by the surveys showed in the previous chapter. This issue is further investigated by the industrial case presented at the end of this work.
3.1 Definition
The Condition Based Maintenance (CBM) is a maintenance policy that lies its basis on the Condition Monitoring (CM), the important parameters of an equipment are acquired and monitored either in an automatic or manual way. The CBM uses the CM to trigger the required maintenance tasks when they are really needed. If the value of a parameter is out of a bounds of a defined threshold then an associated task is triggered.
CBM has been defined as “Maintenance actions based on actual condition (objective evidence of need) obtained from in-situ, non-invasive tests, operating and condition measurement.”
Another commonly agreed definition of CBM is (Jardine et al., 2006): “CBM is a maintenance program that recommends maintenance actions based on the information collected through condition monitoring (CM). CBM attempts to avoid unnecessary maintenance tasks by taking maintenance actions only when there is evidence of abnormal behaviors of a physical asset. A CBM program, if properly established and effectively implemented, can significantly reduce the maintenance cost by reducing the number of unnecessary scheduled preventive maintenance operations”.
CBM is also defined as: preventive maintenance based on performance and/or parameter monitoring and the subsequent actions (EN 13306).
The main point is to assess the condition of the equipment during its normal operation utilizing the data acquired through sensors or the measurement chains and monitor its behavior. If the condition of a component is degrading through the time, like in the P-F curve of the figure 4, it is possible identify its degradation and exchange it before the failure.
Figure 4: P-F curve
The degrading of a component is starting from the normal condition and will end at the failure point F. It is possible to identify a point P that can be used as a threshold to identify the incoming failure. These information are collected and analyzed to recognize whether it is necessary to carry out any maintenance task or not and decide the best time to execute the maintenance to avoid breakdowns or malfunctions. The degree of automation in this process can vary from human visual inspection to a fully automated system based on the sensor reading, data manipulation, condition monitoring, diagnosis and prognosis.
In recent years the CBM has received an increasing attention from the industry due to the improvements in the reliability of the techniques available for the prognosis and diagnosis but also for the developments in the ICT solution to allow the communication between the components of the CBM process chain.
CBM is also one of the most important research topics of maintenance.
CBM aim is to avoid or at least to reduce the failures and the unnecessary maintenance actions on an equipment. Avoiding the breakdowns has a great economic impact thus, in some situations, important savings can be achieved by the use of this policy. According to Mobley (2002) in particular situations and when the technology is properly used to gain maximum benefits, a successful predictive maintenance program should generate a return on investment of 10 and 12 to one, that means that the plant can save 10 or 12€ for each euro invested to deploy the CBM approach.
3.2 Condition monitoring
The Condition Monitoring (CM) can be classified in two ways according to the interval between two subsequent acquisitions of a variable: continuous and periodic.
In the continuous CM the status of the machine is checked continuously, the signals from the sensors are collected and interpreted to individuate the equipment condition. An alarm is triggered whenever the value read from the sensor is different from the normal, this is usually the indicator of degradation or failure. There are two big limitations for the continuous CM, the first one is the cost, the continuous acquisition requires a more powerful hardware and big storage capacity, that is often expensive and the second one is that when the signal acquired is noisy then the results of the diagnostic are not always reliable because of the noise added to the signal that can hide the fault of generate false alarms (Jardine et al., 2006).
Periodic CM is more widely used because it is more cost effective and typically is more accurate because it uses filtered and/or processed data. The risk of a periodic monitoring is that it is possible to miss some failure events if they happen in the interval between two checks or to recognize the failure when it is too late. One of the most important points is the determination of the correct time interval between the checks, this argument has been widely studied to try to find the optimum compromise between the cost and the capability to identify the failures. No preventive maintenance would lead to breakdowns which may affect production, and inflict money losses on the firm, an interval too short would lead to unnecessary prevention costs due to the cost associated to the acquisition of the data, ,on the opposite too long intervals would result in both inconveniences, as they will involve preventive maintenance actions and would lead to uncontrolled breakdowns. The optimal solution is to change the interval dynamically over the equipment life having short intervals on the first phase of the life of the component, longer intervals during the normal operation period and intervals increasingly shorter the more the part shows sign of degradation.
3.3 CBM steps
Lee et al. (2004) has identified three key steps for a CBM program. The main idea of CBM is to utilize the information about the health of an equipment identified to minimize the system downtime and balancing the risk of failure and maintenance costs. The decision making in CBM recommends efficient maintenance policies focusing on prediction, and to do so, many diagnostic tools and methods have been developed with much more success. The three steps are (Lee et al. 2004):
• Data Acquisition: data from the monitored equipments are collected and saved.
• Data Processing: the data collected are cleaned and analyzed, specific analysis tools are used according to the type of data.
• Maintenance Decision Making: after the processing of the data these information are used to decide which maintenance actions have to be taken. Diagnostics and prognostics are two important activities in this stage. Diagnostics is the identification of the nature of a fault; machine fault diagnostics is a procedure to link the measurement or typical feature to the failure, this procedure it is usually done manually, with the support of some auxiliary tools (Jardine et al, 2006) but there are also some automatic diagnostics systems available, most of them exploiting artificial intelligence and neural networks.
3.5 Advantages and weak points
The main advantage of the CBM is the possibility to obtain a significant cost reductions and plant availability improvements; furthermore, it brings to Furlanetto et al. (2006):
• A reduction of component replacing and so a reduction of “early failures” (in cases of bathtub curve validity) ensuring a better maintenance quality; • The possibility to acquire a deeper knowledge of the equipment behavior,
thanks to a more careful analysis of weak signals (A weak signal can be defined as a signal that can be only caught by instruments);
• A better personnel management (as in all preventive maintenance policies); preventive maintenance allows the planning of interventions, and this can be subject to personnel employment optimization: tasks can be organized in order to level the workload and thus obtain a reduction of the number of maintainers needed to manage demand peaks and to better utilization of the personnel.
The main disadvantages of this policy are (Furlanetto et al, 2006):
• The policy is useless if the faults are random and without any identifiable signal of degradation to anticipate them.
• There are some interventions that cannot be postponed because of standards in Health, Safety and Environment (HSE), that define the interval between two maintenance tasks, so not always is possible to use the best possible schedule.
this requires some expenses, whose entity should be compared to the corresponding expected benefits. It is necessary some knowledge to use these instruments so also the cost to teach the user must be considered, this “knowledge cost” can be reduced if these activities are externalized.
• The expenses for the installation of the CBM can be quite large, especially the cost for the instrumentation in particular if the goal is to monitor equipment that is already installed. It is therefore important to decide whether the equipment is important enough to justify the investment. • It is not always easy to achieve the desired accurate maintenance due to
variables such as the complexity of the environment, the inner structure of the equipment, obscure failure mechanisms, etc.
3.5 Prognostic
While the diagnostic gives information about the actual state of a component, like healthy, degraded or faulty, the prognostic goes forward and use the data acquired to try to forecast when it will fail. There are two important quantities that can be estimated: the remaining useful life (RUL), and the risk for one or more failures during a defined period of time (typically, the time to next inspection); in both cases, this information is calculated on the basis of the current machine condition and the past operation profile. So, by a series of instruments and techniques, data coming from the plant are captured and used for decision making, as shown in Figure 5.
.
