CGE676 Chapter 3 PDF

MAINTENANCE & RELIABILITY ENGINEERING CGE676 Reliability Analysis Faculty of Chemical Engineering OUTLINE System with series components System with parallel components Complex modular systems - Combined / M-oo-N, Bridge configuration Faculty o...

MAINTENANCE & RELIABILITY ENGINEERING CGE676 Reliability Analysis Faculty of Chemical Engineering OUTLINE System with series components System with parallel components Complex modular systems - Combined / M-oo-N, Bridge configuration Faculty of Chemical Engineering RELIABILITY OF SYSTEM Faculty of Chemical Engineering Faculty of Chemical Engineering Reliability Network Faculty of Chemical Engineering Faculty of Chemical Engineering  Series configuration Most commonly occurring configuration in engineering systems All units must operate successfully for the system success System reliability Faculty of Chemical Engineering Example – Series configuration Faculty of Chemical Engineering  Series configuration MTTFs Example: Assume that a system is composed of five independent and identical subsystems in series. The constant failure rate of each subsystem is 0.0025 failures per hour. Calculate the reliability of the system for a 50-hour mission and the system mean time to failure. Faculty of Chemical Engineering Reliability Network Series configuration Solution: Faculty of Chemical Engineering Reliability Network Reliability Network Parallel configuration Faculty of Chemical Engineering Example – Parallel Configuration Reliability Network Parallel configuration System MTTF Example: A system is composed of three independent and identical subsystems. At least one of the subsystems must operate normally for the system to work successfully. Calculate the system’s reliability if each subsystem’s probability of failure is 0.1. Faculty of Chemical Engineering Reliability Network Parallel configuration Solution: Faculty of Chemical Engineering Reliability Network Faculty of Chemical Engineering Reliability Network Bridge configuration Some units of engineering system may form bridge network System reliability Example five-unit bridge configuration For independently failing units Faculty of Chemical Engineering Reliability Network Bridge configuration System reliability For identical units For constant failure rate MTTF Faculty of Chemical Engineering Reliability Network Bridge configuration Example: A system has five independent and identical units forming a bridge configuration. The unit failure rate is 0.0075 failures per hour. Calculate the network reliability for a 100-hour mission and mean time to failure. Faculty of Chemical Engineering Reliability Network Bridge configuration Solution: Faculty of Chemical Engineering Reliability Network Reliability Network M-out-of-N configuration In this case, the system is composed of a total of n active units, and least m units must operate normally for system success. Faculty of Chemical Engineering Example M-oo-N configuration Reliability Network M-out-of-N configuration System Reliability Failure constant rate for identical units Faculty of Chemical Engineering Reliability Network M-out-of-N configuration MTTF Assume that an engineering system is composed of four independent and identical units in parallel. At least three units must operate normally for system success. Calculate the system mean time to failure if the unit failure rate is 0.0035 failures per hour. Faculty of Chemical Engineering Reliability Network M-out-of-N configuration Solution Faculty of Chemical Engineering Reliability Network Combined configuration There may be larger systems that involve both series and parallel configurations in the overall system. Can be analyzed by calculating the reliabilities for the individual series and parallel sections and then combining them in the appropriate manner. General principle – to reduce sequentially the complicated configuration Faculty of Chemical Engineering Reliability Network Series-parallel configuration Example: Consider a system with three components. Units 1 and 2 are connected in series and Unit 3 is connected in parallel with the first two. What is the reliability of the system if R1 = 99.5%, R2 = 98.7% and R3 = 97.3% at 100 hours? Faculty of Chemical Engineering Reliability Network Series-parallel configuration Solution: Faculty of Chemical Engineering Reliability Allocation Definition: The process of assigning reliability requirements to individual components for achieving the specified system reliability. Faculty of Chemical Engineering Reliability Allocation In the process of developing a new product, the engineer is often faced with the task of designing a system that conforms to a set of reliability specifications. The engineer is given the goal for the system and must then develop a design that will achieve the desired reliability of the system, while performing all of the system's intended functions at a minimum cost. This involves a balancing act of determining how to allocate reliability to the components in the system so the system will meet its reliability goal while at the same time ensuring that the system meets all of the other associated performance specifications. Faculty of Chemical Engineering Reliability Allocation Eg: Distribute the reliabilities uniformly among all components. A system with five components in series has a reliability objective of 90% for a given operating time. The uniform allocation of the objective to all components would require each component to have a reliability of 98% for the specified operating time, since However, it is not the best way to allocate reliability for a system. The optimum method of allocating reliability would take into account the cost or relative difficulty of improving the reliability of different subsystems or components. Faculty of Chemical Engineering Reliability Allocation Benefits To understand and establish the appropriate relationships between reliabilities of components and parts, subsystems, and systems. It forces design engineers to consider reliability equally with other design parameters such as cost, performance, and weight. Reliability optimization - Finding the best combination of component reliability improvements that meet or exceed the performance goals at the lowest cost. Faculty of Chemical Engineering