tailieunhanh - Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 43

Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design - Part 43 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. . | Theoretical Overview of Availability and Maintainability in Engineering Design 403 A . ZT AX Operational Time Process Output Equiv. Availability EA ------ x--------A -------- 4 M 7 Time Period MDC _ Y To n MDC T MDC _ 480 x 1 120 x 720 x 1 or 75 where Total time period 720 h Operational time 480 120 600 h MDC maximum dependable capacity MDC 1 x constant representing capacity C Process output 600 720 1 x C Process output Process output 90 of MDC. b Equivalent Maintainability Measures of Downtime and Outage It is necessary to consider mean downtime MDT compared to the mean time to repair MTTR . There is frequently confusion between the two and it is important to understand the difference. Downtime or outage is the period during which equipment is in the failed state. Downtime may commence before repair as indicated in Fig. Smith 1981 . This may be due to a significant time lapse from the onset of the downtime period up till when the actual repair or corrective action commences. Repair time may often involve checks or alignments that may extend beyond the downtime period. From the diagram it can be seen that the combination of downtime plus repair time includes aspects such as realisation time access time diagnosis time spare parts procurement replacement time check time and alignment time. MDT is thus the mean of all the time periods that include realisation access diagnosis spares acquisition and replacement or repair. A comparison of downtime and repair time is given in Fig. . According to the American Military Standard MIL-STD-721B a failure is defined as the inability of an item to function within its specified limits of performance . Furthermore the definition offunction was given as the work that an item is designed to perform and functional failure was defined as the inability of an item to carry-out the work that it is designed to perform within specified limits of performance . From these definitions it is evident that there .