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Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 5

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Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 5 studies the combination of various methods of designing for reliability, availability, maintainability and safety, as well as the latest techniques in probability and possibility modelling, mathematical algorithmic modelling, evolutionary algorithmic modelling, symbolic logic modelling, artificial intelligence modelling, and object-oriented computer modelling, in a logically structured approach to determining the integrity of engineering design. . | 20 1 Design Integrity Methodology Designing for maintainability as it is applied to an item of equipment includes the aspects of testability repairability and inter-changeability of an assembly s inherent components. In general the concept of designing for maintainability is concerned with the restoration of equipment that has failed to perform over a period of time. The performance variable used in the determination of maintainability that is concerned with the measure of time subject to equipment failure is the mean time to repair MTTR . Thus besides providing for visibility accessibility testability repairability and inter-changeability designing for maintainability also incorporates an assessment of expected performance in terms of the measure of MTTR in relation to the performance capabilities of the equipment. Designing for maintainability during the preliminary design phase would be to minimise the MTTR of a system by ensuring that failure of an inherent assembly to perform a specific duty can be restored to its expected performance over a period of time. Similarly designing for maintainability during the detail design phase would be to minimise the MTTR of an assembly by ensuring that failure of an inherent component to perform a specific function can be restored to its expected initial state over a period of time. d Designing for Safety Traditionally assessments of the risk of failure are made on the basis of allowable factors of safety obtained from previous failure experiences or from empirical knowledge of similar systems operating in similar anticipated environments. Conventionally the factor of safety has been calculated as the ratio of what are assumed to be nominal values of demand and capacity. In this context demand is the resultant of many uncertain variables of the system under consideration such as loading stress pressures and temperatures. Similarly capacity depends on the properties of materials strength physical dimensions constructability .