Reliability,maintainability and availability
- 1. Reliability, Maintainability and Availability Tesfaye Tamiru Acc. Year: 2021/22
- 2. Lesson Learning Outcomes At the end of this lesson, you should be able to: Understand the implementation of concepts of reliability, maintainability and availability in the industrial set up.
- 3. Reliability Reliability is defined as the probability that a component/system, when operating under given condition, will perform its intended functions adequately for a specified period of time. The four factors required in the determination of reliability are:- 1. Reliability expressed as probability 2. Adequate performance requirements 3. Duration of adequate performance 4. Environmental or operating conditions
- 4. Reliability Cont.. 1. Reliability expressed as probability- A reliability factor can be expressed as probability. A reliability factor=1,means that device performs satisfactorily for the prescribed duration under the given environmental condition. A reliability factor=0, means that all cases the equipment would fail to meet the required performance level. 2. Adequate performance requirements- In reliability
- 5. Reliability Cont.. 3. Duration of adequate performance- The duration of adequate performance is used to state the time up to which the desired performance of system is achieved under the given operating conditions. 4. Environmental or operating conditions- The environmental condition includes the prevailing conditions at which the system is under operation.
- 6. Reliability Model Suppose that products are taken at random from a large group and let of them fail during the time period t, then the probability of failure during the period t is given by = If T= total time Then, = Reliability = - Probability of failure; =
- 7. Reliability Model Cont.. When large number of product are tested then, The exponential probability density function is Where T denotes the required life period If failure rate Then;
- 8. Series reliability model Consider the reliability model in which n components having reliability as ,,……… are connected in series. In this system, the failure of any component puts the complete system in down position. The reliability of the complete system would be
- 9. Parallel reliability model In this model, the system can be partially operative even if some of its components are in the state of failure. Consider the reliability model in which n components having reliabilities as ,,……… are connected in parallel. The overall system reliability is given by
- 10. Types of reliability Inherent reliability- it is associated with the quality of the material and design of machine parts. Achievable reliability- it depends upon other factors such as maintenance and operation of the equipment.
- 11. Maintainability vs Availability Maintainability is defined as the probability that a unit or system will be restored to specified working conditions within a given period. Then maintenance action is taken in accordance with the prescribed procedures and resources. Availability is the ratio of the time at which equipment is available for the designed operation/service to the total time of operation
- 12. Availability Cont.. There are three types of availability are define based on the time element. They are:- Inherent availability Achieved availability Operational availability
- 13. Inherent availability Inherent availability- is the probability that a system or equipment shall operate satisfactorily when used under prescribed condition is an ideal support environment without any scheduled or preventive maintenance at any given time. Inherent availability = Where is mean time between maintenance is mean time to repair =
- 14. Achieved availability Achieved availability- is the probability that a system or equipment shall operate satisfactorily when used under prescribed conditions is an ideal support environment with periodic preventive and corrective maintenance at any given time. Achieved availability = Where M is the mean active maintenance downtime resulting from preventive and corrective maintenance.
- 15. Operational availability Operational availability- is the probability that a system or equipment shall operate satisfactorily when used under prescribed conditions in an actual supply environment without any scheduled or preventive maintenance at any given time. Operational availability = Where MDT is the mean down time is the statistical mean of the downtimes including the supply downtime and administrative downtime.
- 16. Summary Questions Define the following terms: reliability, maintainability, availability, downtime. What do you understand about the use of concepts of reliability, maintainability and availability in the prediction of plant performance.