Reliability Engineering

Reliability Enginnering Presented by: Tarang Jain Rahul Gupta Padmaja Pore

Definition of Reliability The Probability of an item ( component/system) to perform adequately for a specified period of time. under stated conditions

Reliability Engineering • Reliability of a higher level of system – Relationship between different equipment – Reliability of modules which make the equipment – Reliability of system is depend upon reliability of lowest level

Series Model :Automobile with four Wheels

n

System reliability ,Rs=

∏ Ρ i= 1

i

Parallel model: Two Boilers But needed One for operation

m

Rs =

1 − ∏ (1 − Ρi ) i =1

Functionally parallel • The equipment can be in series , but if they act as back up for one another they considered as in parallel

Illustration

Reaction vessel

pump Valve 1 valve 2

Let see one example 0.95

0.70

Cont…. • 1-(1-0.95)*(1-0.70) • 1-0.015=0.985 • The reliability of the parallel system is greater then the reliability of any of its component

Reliability Parameters • Mean Time Between Failures (MTBF) • MTBF is a basic measure of reliability for repairable items. It can be described as the number of hours that pass before a component, assembly, or system fails. It is a commonly-used variable in reliability and maintainability analysis. • MTBF can be calculated as the inverse of the failure rate for constant failure rate systems. For example: If a component has a failure rate of 2 failures per million hours, the MTBF would be the inverse of that failure rate. • MTBF = (1,000,000 hours) / (2 failures) = 500,000 hours

Cont… • Mean Time To Failure (MTTF) • MTTF is a basic measure of reliability for non-repairable systems. It is the mean time expected until the first failure of a piece of equipment. MTTF is a statistical value and is meant to be the mean over a long period of time and large number of units. For constant failure rate systems, MTTF is the inverse of the failure rate. If failure rate is in failures/million hours, MTTF = 1,000,000 / Failure Rate for components with exponential distributions. • Technically MTBF should be used only in reference to repairable items, while MTTF should be used for nonrepairable items. However, MTBF is commonly used for both repairable and non-repairable items.

Cont… • FITS (fail in time) – FITS is a more intuitive way of representing MTBF. FITS is nothing but the total number of failures of the module in a billion hours (i.e. 1000,000,000 hours).

• MTTR – Mean Time To Repair (MTTR), is the time taken to repair a failed module. It should be a goal of system designers to allow for a high MTTR value and still achieve the system reliability goals. – Low MTTR requirement means high operational cost for the system.

Benefits – – – – – –

Product Improvement Planning maintenance Avoiding system failure Reduced Field Failures Reduced Warranty Costs Better Customer Relations

References • Production and operation management :S N CHARY • http://www.i-mtbf.com/ • http://www.ee.iitb.ac.in/

Thank You