SRC Forum - Message Replies
Forum: Reliability & Maintainability Questions and Answers
Topic: Reliability & Maintainability Questions and Answers
Topic Posted by: Reliability & Maintainability Forum
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998
Posted by: Geoff Hampden-Smith
Date posted: Fri Jun 24 5:58:23 US/Eastern 2005
Subject: Component Confidence levels
The target availability for a 3 component parallel system (valve shutdown system) is 99.95%. The three components have similar failure rates. However, the failures of the components are only revealed on testing.
The question is what confidence limits should be chosen for the individual components to have a 95% chance of shutdown on demand for the parallel system.
Any direction on performing the calculation would be appreciated.
Subject: Component confidence levels
Reply Posted by: V. Narayan
Date Posted: Thu Jul 28 17:14:32 US/Eastern 2005
I am intrigued by your statement that the three elements of the system have similar failure rates. In my experience, the logic or control element is extremely reliable, followed by the detector. In general the reliability of the executive element is poor, and is always the hardest to test. Here I am talking about real tests, not functional tests.
I think you are looking for system availability numbers, not merely the reliability of components. In the end, what matters is whether the system as a whole works on demand. There are many reasons to question numbers that are not fully supported by test results.
Coincidentally, I used to work for Shell UK too, till I retired in 2002.