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Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum (src_forum@alionscience.com )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

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Original Message:

Posted by: Marina Karyagina (marina.karyagina@bigpond.com.au )
Date posted: Thu Jun 15 1:35:56 US/Eastern 2000
Subject: availability modelling
Message:
I have a straightforward 3 pumps running in parallel system. The system is UP if at least 1 pump is working. I need to find Availability of the system. There are formulas, I know that. Here is the complication: the only data I have is that the pumps last at least 500 hrs between repairs (from the vendor). So what we know is more or less useful life=500hrs.What we do not know is the hazard rate during useful life. So we bring a pump down every 500 hrs for an 8 hr (scheduled or un-scheduled) repair. I believe, 3*(1/500) is a good enough estimate of the rate with which the system goes into the '2-pumps UP' state. Now, during the repair we have a chance of the second pump failure. We can devise our maintenance strategies so that while the first pump is repaired the other two are not nearing their useful life. So, using 2*(1/500) as a transition rate from '2-pumps-UP' into 'one-pump-UP', I believe, will result in a gross under-estimation of system's availability. So, there is a question: is there a rule of thumb, estimate, guesstimate or else I can use to estimate the hazard rate during the useful life period from known useful life? MTIA


Reply:

Subject: Pump Failure Rate
Reply Posted by: Jack Farrell (jfarrell@alionscience.com )
Organization: Reliability Analysis Center
Date Posted: Mon Jun 19 14:24:43 US/Eastern 2000
Message:
I know of no technique to estimate the failure rate during useful life given an items wear out life. Here at the RAC we do collect data on field failures for a number of mechanical devices. We publish this data in our Nonelectronic Parts Reliability Data publication. Not knowing your type of pump or its use environment here is failure rate information for several commercial pump types in a ground fixed environment. Fuel Pump 0.1140 Failures Per Million Hours (FPMH) Hydraulic Pump 2.5275 FPMH Pneumatic Pump 23.1562 FPMH Vacuum Pump 21.2202 FPMH Additional information for Nonelectronic Parts Reliability Data can be found at: http://rac.alionscience.com/rac/jsp/webproducts/products.jsp?detail=NPRD-95.


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