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1. degradation Instantaneous Total method Il I Il I creeps months Nic Nic Ne Ne Preventive maintenance 51 1 0 0 52 6 Aleatory observations 2 0 0 0 2 Alarm monitoring 21 3 10 2 36 Others 0 0 0 0 0 Using these data there are obtained for the failure rates the following values Aa 3 01 stops year Apc 1 31 stops year A 0 67 stops year If it is considered that the critical creeps are uniformly distributed on the equipment work periods and an accidental constant failure rate A it may be graphically represented the variation of the failure rate depending on the maintenance interval size at it may be seen in the Figure 2 4 CONCLUSIONS On the grounds of the graphical representations from the figure 2 there may be established an optimal value for the preventive maintenance interval typ taking into account the creeps volume corresponding to the total failure rate with minimal value In the case of the studied equipment as it follows also from the Figure 2 it is recommended to do the specific interventions of preventive maintenance activities in an interval of 12 months according to the graphical representations from the Figure 2 in which the stops rate due to the preventive maintenance actions is the factor that limits the time interval in which it is manifested an acceptable number of critical creeps Marius Gabriel PETRESCU Ion NAE 384 TMP months Figure 2 The influen2. only those instantaneous creeps that are critical the non critical instantaneous creeps are ignored all the calculus are reported at operational time lengths The modelling of the creeps is realized with the help of a state diagram Figure 1 in which the system s state gets the significations presented in table 3 Such a representation is based on Markov processes in order to analyze the system s evolution rendered by the states occupied in different moments of time As it follows from the Figure 1 different possible states of the system are tied between them by so called transfer rate failure rate corresponding to the types of the respective creeps which here have the signification of failure rates The use of the processes type Markov in order to estimate the operational reliability of the system supposes the adoption of some transfer rate with constant value in time This hypothesis allows doing pertinent analysis also in situations in which for the studied system it does not dispose of statistical data enough for the modelling of the creeps distribution in time Marius Gabriel PETRESCU Ion NAE 382 Table 3 Significations of the system states State Signification OK The system is in working condition new system Instantaneous Critical The system is in non working condition due to a critical creep Degradation Non critical The system is in the course of degradation in what it concerns the functions sy
3. S a o Practical experience with a data collection project The OREDA project 2002 11 JORN V OREDA Data Analysis Compressor study SINTEF Report STF 75 F 92034 12 KNUT H PER H HELGE S The analysis of failure data in the presence of critical and degraded failures ESREL 95 Bournemouth 1995 13 OREDA Guidelines for Data Collection 6th edition 1995 14 PER H ANDERS T F The modelling of degraded and critical failures for components with dormant failures London 2000 15 ROGER C The Design of Reliability Data Bases Part I and II Cambridge 2002
4. SISOM 2006 Bucharest 17 19 May PROGRAMMING METHOD OF THE MAINTENANCE ACTIVITIES OF THE PROCESS EQUIPMENT Marius Gabriel PETRESCU Ion NAE Petroleum Gas University of Ploiesti The paper presents the compulsory relation between the volume of the maintenance operations and the accepted risk level though offering a managerial tool for scheduling and observing the operation and maintenance activities Key words maintenance risk Key managerial tool 1 INTRODUCTIVE NOTIONS On the grounds of the superior capitalization of the raw material it was determined the increase of the equipments complexity the intensification of the work regime temperatures pressure speed etc the increase of the installations automation degree the increase of the machines and installations value and also the risks concerning their damage As a result the main objective to whom there are in subordination the designing the execution the installation the exploitation the repairing and the check of the process equipments is the assurance of their functioning in good conditions without damages during the entire period of time foreseen for their use That is why at the same time with the production planning it has to take into consideration as an essential condition the planning of the maintenance and equipments repairing activities A very correct managerial activity has to establish an optimum between the maintenance operations volume and the acc
5. ce of the maintenance interval size on the failure rate for the compression subsystem In order to establish the optimum maintenance interval besides the failure rate it is recommended to take into account also the afferent costs of the equipments maintenance in the interval of two planned stops the results obtained having a better economic justification BRwWNe oO NaN DA 9 REFERENCES BARBET J F Les methods d analyse de la securite des systemes Revue Generale de Preventation 30 pp 42 1984 CATUNEANJU V M MIHALACHE A Bazele teoretice ale fiabilit ii Ed Academiei Bucuresti 1983 GNEDENKO B BELIAEV V SOLOVIEV A Methodes mathematiques en theorie de la fiabilite Moscov 1972 Masschelein C A Ryder D S And Simon J P Immobilized cell technology in beer production Crit Rev Biotechnol 14 pp 155 177 1994 NITU V a Fiabilitatea instala iilor energetice Culegere de probleme Ed Tehnic Bucuresti 1979 ANON G Industry analysts focus on US Asian Latin American markets Oil and Gas Journal Special April 25 pp 45 1994 BATTEAU P MARCIANO J P Probabilite et decision dans l incertain PUF 1976 ANTONESCU N N ULMANU V Fabricarea repararea si ntre inerea utilajului chimic si petrochimic Ed Didactic si Pedagogic Bucure ti 1981 BARON T Calitatea si fiabilitatea produselor Ed Didactic si Pedagogic Bucuresti 1976 10 HELGE
6. epted risk level The classification of the creeps and their production mechanisms is realized on the grounds of the mentions made in the observation papers of the equipment work as it is presented in the tables 1 and 2 Table 1 Classification of the creeps taking into account the severity degree Type of creep Appreciation Critical which causes immediate and complete loss of the system s capacity Incipient which has not an immediate effect on the equipment work With degradation in time which does not produce the immediate and complete fall of the equipment but it compromises its correct work With unknown degree of severity unforeseen events or events that do not endanger the equipment work For simplification it is recommended the classifications of the creeps in two distinct categories critical and non critical It is accepted in the damage non critical category to be included those who are incipient and those with degradation in time Also in the maintenance activities there are taken into consideration two typical creep mechanisms by damage manifested by amplification in time of the wear phenomenon which in the absence of maintenance activities may generate critical creeps instantaneous suddenly or by choc manifested aleatory from the point of view of time without visible degradations before the creep The classification of the creeps critical or non critical is realized mainly by ident
7. failures rates due to the type of damage as it follows For stops due to the non critical damages detected by an aleatory observation a Amp Na Nc NMP NC 3 where na ncrepresents the number of the non critical creeps detected by aleatoy observation in this category are included also the creeps signaled by observation or monitoring nyypnc the number of the stops due to preventive maintenance operations in order to eliminate the non critical damages For stops due to the critical damages which have as basis degradation phenomena Apc dup Npc Dmp NC 4 where np crepresents the number of the critical creeps due to technical state degradation For stops due to some instantaneous damages suddenly appeared considered critical it is used the calculus relation recommended in the papers 1 6 8 9 Ar Ny Ts 5 where n represents the total number of stops due to instantaneous damages Using this algorithm there may be established the failure rates specific to different creep ways corresponding to a pre established maintenance interval t 3 CALCULUS EXAMPLE It is considered the case of an equipment for which there are known Top 3 60 years ts 5 94 years Top 0 50 years and the details of the creeps according to the data from the observation papers table 4 Table 4 Details of the equipment creeps Non critical creeps Critical creeps Interval MP Creep detection By degradation Instantaneous By
8. ification of the methods that were as base of their detection The detection produces in different conditions as it follows during the development of the preventive maintenance activities these are made periodically and consist in 381 Programming method of the maintenance activities of the process equipment the check of the device s working condition the testing of the susceptible damaging elements and replacement of the damaged parts by aleatory observations which have accidental character in detection of the eventual creeps it is based on observation and accidental control of the operator mainly by observation monitoring in this case the creep is observed on the basis of indications signaling of the measuring and control device that supervises the process Table 2 Mechanisms specific to the creep Type of creep Mechanism specific to creep By degradation mechanical wear vibrations oscillations material defect trybological defects contamination of the product wrong operation parameters Instantaneous electrical damage damaged measuring devices lack of process signaling damage alarm The pursuit of the equipments work and the registration with maxim accuracy of the data concerning their damages offers the possibility of optimizing the volume of maintenance operations using statistical methods In the industrial practice it is accepted a classification of the maintenance operat
9. ions in three categories Preventive maintenance MP having as purpose to prevent the creeps due to the wear Inspections made in order to reduce the catastrophic consequences of the creeps Predictive maintenance consisting in the detection of the deviations of some functional parameters from the nominal values and taking the measures needed for the prevention of the some damages that leads from it Specialty literature and practical experience in the field of the functional analysis of the systems recommends the use of the Weibull bi parametrical models in the appreciation of the equipments reliability and maintenance 2 ESTIMATION OF THE MAINTENANCE INTERVALS AT THE PROCESS EQUIPMENTS In the following it is considered that the work of an equipment supposes the following simplifying methods instantaneous critical creeps and the critical ones due to the degradation could not appear in the same time the instantaneous sudden creeps are considered to have a constant appearance rate A no matter if the studied system presents a degradation state due to the wear the repaired components are assimilated with the new components the non critical creeps may be totally detected by actions of preventive maintenance MP aleatory observation OA or alarm A the average of the total time of repairing MTTR are considered having smaller values in comparison with the work duration and may be neglected there will be considered important
10. stem still not functional MP During the preventive maintenance activities MP there was detected a non critical creep Degradation Critical The system is still in non working condition due to a critical degradation Aleatory observation The detection of a non critical creep by the surveillance personnel or their signaling by the installation s alarm systems Instantaneous critical creep Non critical creeps Ap c A Critical creep Aleatory observation Figure 1 State diagram of the considered system The effective preventive maintenance interval is TMPe TMP Tap Us 1 where the lengths typ Top Ts are expressed in time units and represent twp time length between two consecutive stops dedicated to preventive maintenance 1 equipment operation total length T surveillance total length The numerical values of the transfer rate are established starting from the time length elapsed from the finding of a non critical creep till the immediate following preventive maintenance action have an average value equally to t 2 where t represents the time length between two consecutive stops dedicated to preventive maintenance activities In these conditions the failure rate due to the preventive maintenance operation is Amr 2 tope 2 383 Programming method of the maintenance activities of the process equipment Reporting to the relation 2 there may be established the values corresponding to the
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