Condition-Based Maintenance Decision-Making for Equipment

HUANG Jian-xin(黃建新) ，BIAN Ya-qin(邊亞琴)，LI Shu-bin(李樹彬)

Air Force Engineering University，Xi'an 710051，China

Introduction

The modern production equipments are characterized by high technology，complexity and system，and its faults are characterized randomly，which put forwards high requirements on reliability and maintenance support ability.It brings severe test to the personnel quality，maintenance equipment，policy，plan，and cost［1］，and challenges the traditional maintenance theories and methods.Condition-based maintenance， a scientific，advanced，and effective method，is paid more and more attention in academic and enterprises world，and it is becoming the hotspot in the research area of maintenance.Condition-based maintenance for equipment is very important as well as a challenge to the current condition-based maintenance model［2］.Therefore，in this paper，its characteristics，working contents，and decision-making process are presented，which can provide a practical reference for maintenance decision-making for equipment［3］.

At present，researches on condition maintenance are mainly focusing on condition monitoring，fault diagnosis technology，and their application in engineering.Relatively little attention has been paid to condition-based maintenance decision-making.In this paper， the connotation， characteristics， working contents，and process of condition-based maintenance are studied systematically.The influence factors in condition-based maintenance for equipment are analyzed.The condition-based maintenance decision-making policy presented in this paper can provide a practical reference for equipment maintenance，which makes the maintenance decision-making dynamic and scientific.

1 The Connotation of Condition-Based Maintenance

The connotation of condition-based maintenance is put forward early in 1940s.Along with more than 60 years development，it becomes the hotspot of maintenance theory and application research.But，in the related research literature，there isn't unified definition about it.Despite its different definition abroad，there is agreement about the understanding of its concept，that is，condition-based maintenance is a kind of maintenance mode，which confirms the equipment condition by condition monitoring technology， and the maintenance is arranged according to the requirement of condition.In domestic，there are two kinds of understanding about the concept:the narrow and broad condition-based maintenances.The worldwide standpoints about narrow condition-based maintenance are the same，like fault maintenance and hard time maintenance mode，which is specific maintenance method［4-5］.The broad concept is put forward based on the narrow concept.One of the broad concepts is that:the broad condition-based maintenance is an integrated maintenance system combined by different maintenance modes such as fault maintenance，hard time maintenance，narrow condition-based maintenance，and active maintenance.In practice，the maintenance mode is chosen according to the characteristic and importance of the equipment［6］.

Therefore，condition-based maintenance is an advanced method.It obtains information reflecting equipment condition by condition monitoring technology.By using signal analysis，fault diagnosis， reliability evaluation， and life prediction technology，the equipment condition is estimated，and the early signs of defect state are identified;the development trend analysis and forecast are made，and the best maintenance mode is recommended according to the equipment defect condition and the predicted result［7］.

2 Characteristics of Condition-Based Maintenance

Compared with the traditional method，condition-based maintenance has the following distinctive characteristics［8-9］.

2.1 Well targeted maintenance

Condition-based maintenance aims at the operation condition of equipment，and it is the basis of maintenance instead of the equipment usage time.It collects the information of the equipment condition by condition monitoring technology，and the real condition of the tested equipment is evaluated by using all kinds of advanced technologies and methods.According to the evaluation determine maintenance as well as its time，which can avoid blindness，shorten time，and increase equipment utilization rate.It can obviously reduce accidental failures caused by insufficient maintenance，and reduce the waste of resources because of over maintenance.

2.2 Realistic of maintenance plan

Condition-based maintenance has a plan，which is based on the equipment tested condition.It stresses maintenance should be carried on by plan，and the plan is usually made in advance to deal with the possible fault.Since the maintenance measures and time are planned，it is practical.

2.3 More flexible maintenance

Condition-based maintenance monitors the equipment operation by using condition monitoring technology.It predicts the developing trend of the fault through life prediction technology and fault diagnosis;then，maintenance personnel can do preparation work for the maintenance according to the production plan，spare parts，and maintenance ability，which makes the planning and time table more flexible.

3 Condition-Based Maintenance Content

While doing condition-based maintenance，the testing signal of the monitored equipment should be gained firstly，and the characteristic value showing its real condition is picked up and chosen.Then，the present monitored equipment condition is identified to judge whether the equipment is abnormal or not.Once the equipment is tested being abnormal defect state，the defect state is diagnosed and the degree and the trend of the defect is predicted; and the equipment's remain life is evaluated.Finally， maintenance decision-making is done according to the diagnosis and predicting result.The conditionbased maintenance content is shown in Fig.1.It includes data collecting，character value selecting，condition identifying，defect condition diagnosing，and predicting，which is a high technical and complex maintenance process［10］.

Fig.1 Work contents of condition-based maintenance

According to the work contents，the process of conditionbased maintenance can be divided into the following three stages［11-12］.

3.1 Data collecting

Condition-based maintenance needs a great deal of effective data to identify，diagnose，and predict the condition of the tested equipments and support maintenance decision-making.The collected data are mainly four types:

(1)condition monitoring historical data of a certain time，such as，monitoring and analyzing of vibrating，oil analysis data，and pressure data;

(2)historical data of important events related to the tested equipment，which includes the equipment's installing date，failure records (time，mode，and reason)，all the maintenance records，overhaul or replaced records，and maintenance records(fastening，calibration，adjustment，and lubrication);

(3)data about maintenance cost， such as fault maintenance cost including failure cost，material，and labor cost;and preventive maintenance including data of suspending loss，material，and labor cost;

(4)the other related datas including information about production plan，the number of spare parts，and maintenance ability.

3.2 Condition evaluation

Based on the analysis and disposal of the collected data，the equipment can be judged whether it is normal condition or defects operation condition， and the defects condition is diagnosed and predicted.This stage includes character signal pick-up and selection，condition identifying，defects diagnosis，and defects predicting.

3.3 Maintenance decision-making

The optimal maintenance action and plan are determined according to condition evaluation and certain optimizing rule such as suspending time and availability.

As the last stage of the maintenance process，decisionmaking is the important part of the condition-based maintenance system.Enterprises cost huge investment in introducing advanced condition monitoring， diagnosis technology and system to guide and optimize the equipment's maintenance decision-making，which makes the maintenance activity carrying out necessarily.It can ensure the equipment operating safely and reliably，avoid fault，and make a full use of the effective life of the equipment components sufficiently.Therefore，the operating cost of the enterprise is reduced.

Decision-making stage is the essential stage of conditionbased maintenance process，but the attention paid to research about it is not enough.Therefore，this paper mainly researches condition-based maintenance decision-making process.

During the condition-based maintenance working process，the three stages are closely linked.Data collection is the foundation of condition-based maintenance;condition evaluation stage is the key point of realizing condition-based maintenance;the purpose of using condition-based maintenance method is to guide maintenance decision-making.

4 Condition-Based Maintenance Decision-Making Process

4.1 Influencing factors of condition-based maintenance decision-making

The major factors need to be considered while doing condition-based maintenance decision-making.

(1) Remain life predicting of the operating defect equipment at present monitoring moment

It is the practical basis of decision-making，and is the key factor of establishment of condition-based maintenance decisionmaking optimization model.

(2)Economic factors such as maintenance and checking cost

Fault maintenance cost usually includes fault loss，labor cost， maintenance material， and replaced spare parts.

Preventive maintenance cost includes scheduled downtime cost(such as cutting production)，maintenance personal salary，maintenance material，replaced spare parts，and condition checking.

(3)The rationality of fault mode choosing

Fault mode is the description of the fault phenomenon.It is the basis of analysis.Whether the selected model and its description is reasonable or not，it can reflect the fault rule of the equipment.It will directly affect the decision-making.

Moreover，the equipment usually has many fault modes，and different fault modes often have different remain life，and the related maintenance cost may be different greatly.So，in decision-making，all the key fault modes should be analyzed and weighed together to get the optimal decision-making.

(4)Production plan

The implementing of maintenance or replacing plan can be advanced or postponed appropriately according to production situation.

(5)Spare parts

It means the difficulty of ordering the needed spare parts.

(6)Maintenance ability

It includes the number and technical level of maintenance personnel and the rational allocation of the number of maintenance shop，etc.

4.2 Contents of condition-based maintenance decision-making

Since decision-making is the last and most important stage of condition-based maintenance working process.Enterprises cost the huge investment introducing advanced condition monitoring，diagnosis technology and system to guide and optimize the equipment's maintenance decision-making，which makes the maintenance activity carrying out as necessary.

If the equipment is not online real-time monitored，it is only carried out in discrete time.When the equipment is at defect operation stage，according to the current remaining life，two decisions are predicted at each monitoring point:that is maintenance activity decision-making and condition monitoring interval decision-making［13-14］.

(1)Maintenance activity decision-making

It is to determine the best maintenance activity.As a rule，in equipment condition-based maintenance practice，permitted maintenance activities are:

①not to take any maintenance interventions;

②to take preventive maintenance;

③to take preventive replacing.

To simplify it，①and ②two maintenance activities are considered in this paper.Therefore， maintenance activity decision-making is to determine to continue operating or to take preventive replacing.If there is only taking preventive replacing，the best replacing time is to be determined.

In current maintenance practice，in order to prevent serious economic loss caused by fault，as soon as a fault or abnormal is defected，and maintenance or replace are carried out at once，which not only don't make the remain life of the spare parts fully full use，but also lead to great waste of labor，material，and money;and lower the utilization ratio of the equipment，increase downtime loss，and reduce benefit.Therefore，to make decision according to the latest condition information of the monitored equipment is very important，which can make the maintenance activity carrying out necessarily to achieve the true sense of condition-based maintenance.

(2)Condition monitoring interval decision-making

If the equipment needs not any maintenance intervention，it can operate continually and determine the next optimal monitoring interval.

Condition monitoring interval decision-making is very important.Short monitoring interval is favorable to the equipment to define defect early and avoid the defect developing to serious fault，but monitoring frequently brings too much waste such as labor，material，and money，especially when the monitoring cost is high，it is not reasonable economically.On the contrary，long monitoring interval saves expenses，but it may cause miss defection leading to fault even huge economic loss，sometime，it may threat environment and personal safety.Therefore，the monitoring interval should be made scientifically according to latest condition information of the monitored equipment.

4.3 Key link of condition-based maintenance decision-making

In the equipment condition-based maintenance，maintenance activity decision-making is paid the most attention.It is to decide whether the maintenance is needed according to present operating condition of the equipment.If it needs，the best maintenance moment can be decided.If it is expensive offline monitoring， the monitoring interval (monitoring frequency)decision-making is needed，and the best monitoring interval is decided according to the operating condition of equipment.Both the maintenance activity and monitoring interval decision-making are based on the monitored equipment operating condition evaluation.The three key links of the equipment condition-based maintenance decision-making are the prediction of defect condition identifying early-stage，defect condition diagnosis，and defect condition deterioration degree.

(1)Early recognition of the defects condition

The early recognition of the defects condition is one of the key links of decision-making.It can provide maintenance personnel enough time to analyze and diagnose the equipment defect condition，prevent serious effects，and adopt intervention measures to avoid fault happening and serious loss;moreover，it directly affects the prediction of the defect condition deterioration degree，which affects the establishment of remain life prediction model and the related condition-based maintenance decision-making optimizing model.

(2)The diagnosis of defect condition

After the defect is recognized，it needs to be diagnosed to ensure the defect position，analyze the possible reason，and judge fault type to provide scientific basis for the establishment of maintenance plan.

(3)Prediction of defect condition deterioration degree

In the process of condition-based maintenance decisionmaking，as soon as the defect is recognized，the equipment is at a defect operation stage and it is in defect condition.At this stage，it needs maintenance management to pay much attention to the equipment monitoring，and judge the equipment defect condition deterioration trend，then predict it's remain life to get operating and maintenance advice.Therefore，the prediction of defect condition deterioration degree is the key link after the defect recognition.In defect operating condition，as soon as the defect condition deterioration degree predicting model of each monitoring moment is established，according to its optimizing target and maintenance cost，the relevant optimization model of equipment maintenance decision-making can be established easily.

The previous research mainly concentrated on diagnosis of equipment defect condition.It mainly focused on the diagnosis，but was lack of research on early recognition of the defects condition and prediction of defect condition deterioration degree.These two points are very important to the research of equipment maintenance decision-making.

5 Conclusions

Based on available decision-making policy in conditionbased maintenance， the connotation of condition-based maintenance for equipment is defined，and its characteristics and working contents which are divided it into three stages are provided.Finally，the influence factors in condition-based maintenance for equipment are analyzed，and the key links of equipment maintenance contents and decision-making process are proposed.The condition-based maintenance decision-making policy presented in this paper can provide a guide for the equipment condition-based maintenance decision-making practice， which makes the maintenance decision-making dynamic and scientific.

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