RAMS Analysis on Maintainability component. Bruno José Azevedo de Lemos

Transcription

1 RAMS Analysis on Maintainability component Bruno José Azevedo de Lemos MSc Thesis on Mechanical Engineering, Instituto Superior Técnico, October 2010 Nowadays we see a constant search and effort to achieve maximum productivity and profit. In that process of optimization, some aspects are left out of the picture and aren t taken into consideration while designing and building plants. This situation occurs because that process only seeks to satisfy the goals and needs related to production, neglecting aspects such as maintainability and safety (DoD, 1995). That explains why suddenly some companies are confronted with costs and problems associated with these factors, that weren t considered in the acquisition or designing of a system, which may lead to costs being greater than profits. The awareness to these situations created the need to create methods that would take into account all the factors with influence to the total cost associated to system (Life Cycle Cost). The RAMS analysis assesses a system s Reliability, Availability, Maintainability and Safety and establishes guidelines and modifications to be applied to those systems. This is a very extensive and dynamic method that implies having access to data regarding all of the system s aspects, before and after the implementation of modifications, to assure a comparison and continuous improvement. This global analysis implies an interconnection between these components, making it necessary to evaluate very carefully the implications in all components when applying any modification to prevent unexpected outcomes. Picture 1 - system RAMS (in: EN 50126)

2 The first step is setting the objectives that have to be achieved. A precisee definition at the beginning of the analysis is crucial to a successful RAMS implementation. The next step is to perform an analysis and characterization of the problems or failures associated with Maintainability that affect Availability and Safety. This analysis is performed by resourcing to a set of methods and techniques with proven value and effectiveness (Smith, 2005), such as: cause and effect diagrams. failure modes, effects and criticality analysis (FMECA). fault tree analysis. total quality management (TQM). These methods and techniques should be applied during the system life cycle phases, providing useful data to be used in the reengineering of design. Some problems can be predicted in analysis carried out before system implementation or production, but there are others that only arise during service phase. This is where one of RAMS main features, the ability to be used as cycle, starts to produce results, by allowing to adjust targets and procedures at the end of each step. Picture 2 - RAMS cycle (in: Smith 2005)

3 After all information is gathered it must be used to evaluate and plan the Maintainability. In this phase, all modifications and procedures must be defined, accordingly to the failures or issues found and their respective causes, and bearing in mind the achievement of objectives defined early. The next step corresponds to validating Maintainability, by conducting analysis, simulations and tests that vouch for the modifications efficiency. Finally, the Maintainability plan must be implemented and submitted to analysis. All data related with the process should be documented and preserved, in order to assure a growth of the company know-how. As is shown in the picture 2, RAMS philosophy consists essentially in doing a loop consisting in: Assess -> Plan -> Apply - > Assess ->. and so on. This is the best approach to guarantee the desired continuous improvement. One other important feature of RAMS is relating his components with each other, and evaluating how the modifications implemented on one component affects the others. This concept can be verified when applying RAMS analysis to Maintainability and we observe the implications it has in Availability and Safety. As an example, providing easy access to a system so it can be repaired is dealt by Maintainability, but it also affects Availability by reducing the mean time down (MTD) and also improves the workers Safety. Appliance of RAMS analysis on Maintainability to the provision of safety valves maintenance to process industry will be used as a case study to demonstrate RAMS effectiveness. This study was conducted in partnership with Instituto de Soldadura e Qualidade (ISQ). The objective of this study is to increase a system s Availability and Safety by improving the Maintainability component. There will exist also the goal to include service users in the process, creating a synergy between them and the service provider, which enables them to achieve an appropriate combination of performance resulting from the application of the RAMS methodology and costs involved. The RAMS analysis has some flexibility, which will be used to adjust it to services provision. The data needed to conduct the analysis was gathered in field expeditions at Integridade and CUF, and by conducting interviews to personnel involved in the safety valves maintenance. All information was sorted and permitted the characterization of problems found with their associated causes. The presentation of problems followed the sequence by which those are felt or affect the workers: 1. Accessing to the valve 2. Removing valve from service 3. Transporting valve to workshop

4 4. Initial testing and maintenance procedures 5. Calibrate and certificate valve 6. Transporting and restoring valve to service This seemed the most efficient approach and provided a good scenery of the needs and issues related to valve maintenance. A different approach was used to present the modifications and solutions to be implemented in the process. These were grouped by items that revealed problems, which made it easier to correlate the problems that were identified. The following measures were defined: Access create conditions to access equipments by including ladders or boardwalks create work platforms in places where maintenance is required ensure that exists enough area to workers operate ensure that is possible removing the valve from location Tools include nut size on valve card (improves logistic) Bolts and nuts use appropriate lubrication/protection on bolt and nuts (Molykote based paste are advised with cost benefits) include bellevilte washers Valve transportation include picture of valve and her location in valve card Maintenance document valve parts condition for future studies chose consumables according to maintenance plan ensure that changes in service conditions are reported create a database to preserve the company know-how acquire portable valve testing equipment include remote monitoring devices (preferably wireless) Safety introduce a tool fall preventing mechanism ensure required protection according to function

5 Valve card introduce a digital valve card create a remote access database Some measures contribute to increase of Availability by diminishing MDT, while others improve Safety by reducing the risk associated with some tasks. In addition to the practical side of Maintainability, there must be also conducted an analysis of the valves maintenance plan. This is a crucial step, as it unravels issues associated with the maintenance actions schedule. The analysis execution must take in account factors such as: legal obligations, components Availability, valve history, consumables and service conditions alterations. All this data must be compiled and computed to produce a more adequate scheduling of preventive actions. To achieve these results it is necessary to utilize the system components Liability data, and resort to probability analysis methods. In this study, the Weibull two parameter distribution (α and β) was adopted, which is described by equation 1 (Assis, 2004). =.. (1) shape parameter: represents the degradation mechanism; scale parameter: characteristic life. This is a very useful and powerful tool to analyze the available Liability data of the valve components (Murthy,2004). Figure 3 contains a method to implement Weibull probability in planning improvement. Picture 3 Weibull implementation diagram

6 The data available to conduct the study was scarce, and this situation implied that some failure dates were estimated in accordance to some reports provided by CUF and from personal experience of maintenance supervisors. The final results, after Weibull implementation, can be seen in figure 4. These were obtained by using Eng. Rui Assis template Intervalos entre inspecções de manutenção preventiva condicionada. Picture 4 Intervals between maintenance inspections The final result was a diminution of the inspection period, so that the desired liability would be achieved. This is a mere example of the possibilities of Weibull, and other probabilistic methods, inclusion on maintenance scheduling. These should always be used whenever they are suitable, because they share the same approach of RAMS analyses: continuous improvement. The final conclusion is that RAMS analysis is a powerful tool that was able to set objectives, identify the system problems, characterize their causes and produce solutions which soften or eliminate the problems. This improved Maintainability and consequently a Safety and Availability increase. Also, the study promoted a healthy communication between the parts involved, which should be encouraged. Therefore, all the study objectives were met.

7 References: Assis, R., Apoio à Decisão em Gestão da Manutenção - Fiabilidade e Manutenibilidade, LIDEL, Lisboa, 2004 Cenelec. EN Railway Applications The Specification and Demonstration of Reliability, Availability, Maintainability and Safety (RAMS). Cenelec, Brussels, 1999 Department of Defense (DoD), Designing and Developing Maintainable Products and Systems MIL-HDBK-470A, Washington, June, 1995 Dhillon, B.S., Engineering Maintainability, Gulf Publishing, Houston, TX, 1999 Leocádio, Miguel., Sobre a Incorporação RAMS no Desenvolvimento de Produtos de Base Tecnológica: Uma Abordagem Holística a Veículos Ferroviários, Dissertação para obtenção do Grau de Mestre em Engenharia e Gestão da Tecnologia, IST, Lisboa, Maio 2004 Murthy, D. N. P.; Xie, M.; Jiang, R., Weibull Models, Wiley-Interscience, Hoboken, 2004 Smith, David J., Reliability, Maintainability and Risk Elsevier, 7º Edition, 2005 Stapelberg, Rudolph F., Handbook ok Reliability, Maintainability and Safety in Engineering Design, Springer, Londres, 2009