The Use of Condition Monitoring Information for Maintenance ...

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Nordisk kemesikkerhedsforskning Norrænar kjamoryggisrannsoknir Pohjoismainen ydinturvallisuustutkimus Nordisk kjemesikkerhetsforskning Nordisk kåmsåkerhetsforskning Nordic nuclear safety research

DK0300012

NKS-80 ISBN 87-7893-136-3

The Use of Condition Monitoring Information for Maintenance Planning and Decision-Making Kari Laakso and Tony Rosqvist VTT Industrial Systems, Finland Jette L. Paulsen Risø National Laboratory, Denmark

. .34/15

December 2002

Abstract A survey is presented outlining the use of condition monitoring information in three Nordic nuclear power plants. The questions of the survey relate to the role of condition monitoring in strategic, as well as operative, maintenance planning and decision-making. The survey indicates that condition monitoring is increasingly implemented at nuclear power plants, but very selectively and in a rather slow pace for predictive maintenance. A combined strategy of condition based maintenance and predetermined preventive maintenance is applied for important equipment such as main circulation pumps and steam turbines. A realistic aim is to reduce the number of costly or error prone maintenance and disassembling inspection activities by condition monitoring given that the approach enables a good diagnosis and prediction. Systematic follow-up and analysis of such condition monitoring information followed by a case-specific planning and decision making of timely and rightly directed maintenance actions can justify an extension of the intervals of a number of predetermined inspection, maintenance or periodic testing tasks.

Key words Condition monitoring, maintenance planning, decision making

NKS-80 ISBN 87-7893-136-3 Pitney Bowes Management Services Denmark A/S, 2003 The report can be obtained from NKS Secretariat P.O. Box 30 DK - 4000 Roskilde, Denmark Phone +4546774045 Fax +45 4677 4046

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The use of condition monitoring information for maintenance planning and decision-making Customer: NKS/SOS-2.2 project Kari Laakso & Tony Rosqvist VTT Industrial Systems Jette L. Paulsen, Risø National Laboratory

Technical Report 27.12.2002

The use of condition monitoring information for maintenance planning and decision-making Kari Laakso, Tony Rosqvist VTT Industrial Systems, Finland Jette L. Paulsen, Risø National Laboratory, Denmark

Abstract In a modern operating and maintenance organisation for nuclear and advanced power plants, condition monitoring plays an important role in managing the maintenance and operability activities. Basically, condition monitoring helps to control the safety and economy of the plant operations. In order to establish a framework for understanding problems and solutions related to the use of condition monitoring information in strategic and operational maintenance decision-making, the respective decision contexts are modelled as process models. The report includes a survey of the utilisation of condition monitoring information for predictive maintenance in three Nordic nuclear power plants. Based on the survey the following general observations can be stated; •

A combined strategy of condition based maintenance and predetermined preventive maintenance is applied for very important equipment such as main circulation pumps, steam turbines and turbine condensers;

•

For less important equipment, the maintenance management at a nuclear power plant tends to wait for experiences of condition based maintenance in other type of power plants;

•

The reluctance to change manners of proceedings in nuclear power plants is relatively high and risk is not taken in implementing new condition monitoring for predictive maintenance unless a high fault coverage of the monitored equipment and cost benefits are demonstrated (in many cases, existing condition monitoring systems at plants are used for condition monitoring but at a rather slow pace for condition based maintenance);

•

A realistic aim of condition monitoring is to reduce the number of costly or error prone predetermined maintenance and disassembling inspection activities. Condition monitoring, together with an increase of proper tailoring of maintenance actions (timing, localisation, work), based on the analysis of condition monitoring information, can justify an extension of the intervals of comparable predetermined inspections and preventive maintenance.

•

Display systems for online condition monitoring of equipment by using existing process monitoring and control data have also a great potential in increasing the condition based maintenance.

The current report is a working report that will be completed at the HRP MTO research period 2003 - 2005. The feedback from the plants is very crucial in developing process and decision models for condition monitoring that meet the generic and specific needs at the plants.

LIST OF CONTENTS ABSTRACT

2

1.

5

INTRODUCTION

2. THE DIAGNOSTIC AND PROGNOSTIC CAPABILITY OF CONDITION MONITORING APPROACHES

6

3.

CONDITION MONITORING IN MAINTENANCE DECISION-MAKING

9

3.1 OBJECTIVES FOR STRATEGIC AND OPERATIVE MAINTENANCE DECISION-MAKING 3.2 STRATEGIC MAINTENANCE PLANNING UTILISING CONDITION MONITORING 3.3 OPERATIVE MAINTENANCE STEERING BASED ON CONDITION MONITORING

9 10 14

4.

SURVEY OF THE USE OF CONDITION MONITORING IN THREE NORDIC NPPS

4.1 THE QUESTIONNAIRE 4.2 ANSWERS OBTAINED FROM THE MAINTENANCE MANAGERS AND PERSONNEL 5.

CONCLUSIONS

17

17 18 22

REFERENCES

24

APPENDIX 1 TO THE NKS/SOS-2.2 REPORT ON THE USE OF CONDITION MONITORING INFORMATION FOR MAINTENANCE PLANNING AND DECISION MAKING

26

ADVANCED DISPLAY SYSTEMS FOR CONDITION MONITORING

26

INTRODUCTION BACKGROUND WHAT IS CONDITION MONITORING? METHODS PUMPS HEAT EXCHANGERS

2.6 2.6 2,7 2,8 29 31

LIST OF FIGURES FIGURE 1. COST EFFECTIVENESS WITH RESPECT TO DEPENDABILITY ADDED VALUE (VALUE THAT IS SEIZED DUE TO IMPROVED DEPENDABILITY) FOR DISTINCT MAINTENANCE STRATEGIES. 5 FIGURE 2. INFORMATION FLOW FROM CONDITION MONITORING INTO CONDITION BASED MAINTENANCE, [ADAPTED FROM LUUKKANEN, 2002]. 7 FIGURE 3 . ACCURACY, PRECISION AND DETECTION TIMES OF TWO CONDITION MONITORING APPROACHES. 9 FIGURE 4. IDEFO-MODEL OF THE DECISION AND ACTIVITY CONTEXT RELATED TO STRATEGIC MAINTENANCE DECISION-MAKING. 10 FIGURE 5. A LOGIC TREE FOR THE IDENTIFICATION OF EFFECTIVE MAINTENANCE ACTIONS [MSG-3 1988]. 11 FIGURE 6. A SIMPLE COST MODEL FOR MAINTENANCE STRATEGY EFFICIENCY EVALUATION. 12 FIGURE 7. A FUNDAMENTAL OBJECTIVES HIERARCHY RELATED TO THE EVALUATION OF MAINTENANCE OPTIONS. 13

FIGURE 8. A MEANS-ENDS OBJECTIVES NETWORK RELATED TO THE EVALUATION OF MAINTENANCE OPTIONS. 13 FIGURE 9. IDEFO-DIAGRAM OF THE DECISION CONTEXT RELATED TO OPERATIVE MAINTENANCE DECISION-MAKING. 15 FIGURE 10. TIME POINTS RELATED TO THE OPERATIONAL MAINTENANCE DECISION OPTIONS AND REALISATIONS OF THE FAILURE TIME (T,, T2) MEASURED FROM THE TIME OF OBSERVATION OF AN INCIPIENT FAILURE (S0). 16 FIGURE 11. A CONDITION MONITORING DISPLAY FOR A CENTRIFUGAL PUMP. 29 FIGURE 12. A CONDITION MONITORING DISPLAY FOR A DISPLACEMENT PUMP. 30 FIGURE 13. A CONDITION DISPLAY OF A TUBE HEAT EXCHANGER. 31 FIGURE 14. A CONDITION MONITORING DISPLAY ON THE TURBINE CONDENSER. 32

LIST OF TABLES TABLE 1. END OBJECTIVES AND CORRESPONDING EXAMPLE ATTRIBUTES AND MEASURES. 13 TABLE 2. OPERATIONAL MAINTENANCE DECISION OUTCOMES RELATED TO PRIME COSTS GIVEN INCIPIENT FAULT INDICATION 16 TABLE 3. THE EXPERTISE AVAILABLE DURING THE INTERVIEWS 17 TABLE 4. QUESTIONS PRESENTED TO THE MAINTENANCE PERSONNEL AT THE THREE NPPS. 18 TABLE 5. THE STRATEGY USED ON A CENTRIFUGAL PUMP FOR CONDITION MONITORING DISPLAY DESIGN 29 TABLE 6. THE STRATEGY USED FOR DESIGN OF A CONDITION MONITORING DISPLAY FOR A POSITIVE DISPLACEMENT PUMP 30 TABLE 7. STRATEGY FOR DESIGN OF A CONDITION MONITORING DISPLAY OF TURBINE CONDENSER 32

1. Introduction In a modern operating and maintenance organisation for nuclear and advanced power plants, condition monitoring plays an important role in optimising the maintenance activities. Basically, condition monitoring helps to control the safety and economy of the plant operations. The objective of condition based maintenance is twofold: Firstly, to avoid extraneous outages or damages due to unexpected failures in equipment, secondly, to decrease costs related to maintenance (e.g. periodic replacements, repairs or fault finding tasks). Thus, condition-based maintenance is a means to achieve a better control of risks but also a better return on investments on maintenance. The development of condition monitoring approaches and techniques, and information technology, together with nowadays more stringent economic and safety requirements of power plants has led to the development of systematic maintenance planning towards condition based maintenance. Condition based maintenance means preventive maintenance consisting of parameter and performance monitoring and the subsequent actions. Predictive maintenance is condition based maintenance carried out following a forecast derived from analysis and evaluation of significant parameters of degradation of the item [CEN - prEN 13306, 1999]. Fig. 1 illustrates the change of cost effectiveness with respect to added value due to improved dependability for specific maintenance strategies.

Reliability Centered Maintenance Degraded equipment -> Functional breakdown. The main purpose of condition monitoring is to detect a fault, or a degradation process, that has reached a certain symptomatic level and to provide an indication of the abnormality in time before the functional breakdown occurs.

Condition monitoring in power plants may be based on several condition monitoring approaches, for instance [Hytonen & Savikoski, 1998; Kokko 2000, Luukkanen 2002]. In the following figure 2 different condition monitoring methods and an ideal information flow from the monitoring into condition based maintenance is given. Msmtenance history Operating history Design information

Operators logbooks IMPLEMENTATION OF

CONVERSION INTO ^ontftion montonng • via^l check • ttMMiml efficiency monitoring • calibration monitoring - water chemstry and

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INFORMATION Identification of COLLECTION

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WORK FLANMNG

ACTIONS - timing and localizdion of prevsntiw msntanancB actions - timing and localiz^ion of repairs - timing of actions into or operation period

corrosion fTDntoring • etectrotBchnical

ffiGditonny -dlandysis • rjKxnu^uiJJiy -NOT inspections - fl'idoscupy inspection - thermal and pressus loadng montonnc; Process monitoring • temperature -pressus •flow • control settings

Figure 2. Information flow from condition monitoring into condition based maintenance, [adaptedfrom Luukkanen, 2002]. The above figure on information and activity flow shows how condition based maintenance consists of parameter and performance monitoring and subsequent actions. In the words of Hunt of condition monitoring techniques are given [Hunt, 2000]: 'Machine faults produce certain signals which certain monitors can monitor', i.e. there has to be the aligning of Fault - Signal (symptom) - Monitor. This is also reflected in the causal chain of degradation as indicated above. It is important to realise that the 'Signal' can be related both to equipment condition and state of the process [Paulsen, 2000]. With an example related to advanced condition monitoring of process instrumentation for nuclear power plants, the benefits of condition monitoring, given that the process and condition monitoring signals are reliable, can be grouped according to [Fantoni, 1999]: • • •

Technical Specifications compliance Efficiency optimisation Incipient fault detection

In the first case generally, periodic testing and condition monitoring are used in verifying the operability of safety related equipment, whereas in the second case, process monitoring data is used to verify that the operating system or equipment is functioning at the specified or optimal operating point [Sunde et al., 2001]. In the third case, the condition of the equipment is monitored to ensure economic and safe operation of the equipment. Generally in this case, prediction of time to failure is crucial for operative maintenance decision-making. In the

following we will not distinguish condition monitoring - signals from process monitoring signals unless otherwise noted. For strategic maintenance planning, the diagnostic and prognostic capability of a condition monitoring approach has to be known, before any decision regarding the potential usefulness of the approach can be evaluated. For instance, periodic walk-around checks are useful for identification of a large range of fault signals and symptoms and, combined with good personnel experience and expertise, they can provide a pertinent diagnosis and forecast of the degradation of e.g. some rotating equipment and valves. But visual and walk around checks alone cannot reveal hidden faults or generally the underlying failure mechanism and location (the degraded item) or facilitate a good prediction of the time to functional breakdown of different important equipment. The condition monitoring - approaches thus vary with respect to capabilities for diagnosis and capabilities for prognosis. The diagnostic and prognostic capability is here defined as the capability of condition monitoring approaches to produce an early indication (diagnosis) and to yield an accurate and precise prediction [Pulkkinen 1991] of a functional breakdown, given that a degradation process is under way.. Accuracy, precision and detection sensitivity, are defined in Fig. 3. Both condition monitoring approaches in Fig. 3 have the same accuracy (both are unbiased in the sense that on average they produce right predictions) and precision (both have the same spread depicted by the 'arrow'). Approach 'A' has, however, a lower detection threshold and gives more time to steer operative maintenance activities in the case of on-line condition monitoring. In the case of diagnostic testing for a hidden fault at a predetermined time point in the interval [IA, te], method A would yield an indication and a prediction, whereas method B would leave the fault undetected. The reason for a lower detection level may be a better sensitivity of the condition monitoring - equipment for the detection of a symptom, or the measurement of another symptom that is more apt for fault detection, that is, fault resolution and fault coverage, respectively. Concepts related to diagnostic testing are found in IEC / 706-5 (1994).

Level of degradation predictions by two monitoring methods given incipient fault detection

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Level of functional breakdown

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Levels of detection

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degradation di ie to one or several failure mechanisms

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time of detection

Figure 3. Accuracy, precision and detection times of two condition monitoring approaches.

3. Condition monitoring in maintenance decision-making 3.1 Objectives f or strategic and operative maintenance decision-making We define two objectives of the strategic and operative decision contexts related to condition monitoring. In the rest of section, these definitions will guide us in detailing the role of condition monitoring information in the respective decision contexts, and put decision models used to evaluate and select the best course of action into a broader perspective.

Itefil; 'Strategic condition mentoring objptive' ij^ffbiecttve of strategic njc^ntenance d^cision^atiin£ re ^monttofmg^in^mation^^to^ improve the cost »*c—*""*"***"» use ojF&mditibn monitoring information. '

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