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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: raja rizwan imtiaz (imtiazrr@petrokemya.sabic.com )
Organization:petrokemya
Date posted: Sun Feb 16 0:57:10 US/Eastern 2003
Subject: functinal failure of safety valves
Message:
Recently, we have faced recurring pressure safety valve popping problem in few of the safety valves installed in high pressure steam service. While conducting safety valve failure investigation, it was observed that the problematic psv's spring has reduced in height about 7.0mm compared to the original (new) spring. original spring height = 207 mm and the used (old) measured spring height = 200mm Currently, we are using only visual inspection criteria for the replacement of safety valve critical parts i.e. spring, spindle etc. However, we are looking for introducing the dimensional criteria as well to avoid such failures. If someone help answer following Are you applying dimensional measurement criteria for the psv's critical parts or only visual inspections are done to adjudge the parts fitness. What is the dimensional measurement criteria for spring replacement. Example could be for any service.


Reply:

Subject: Spring Reliability
Reply Posted by: B.W.Dudley (bdudley@alionscience.com )
Organization: RAC
Date Posted: Tue Feb 18 12:24:56 US/Eastern 2003
Message:
This question on functional failure of safety valves appears to be a spring reliability design problem or a high defect rate on the production units. I would suggest that a design analysis of the spring loading and materials be conducted to see if spring constants meet the operating loads with a safety margin. If this analysis proves that the springs are correctly designed and applied, then you should consider testing a sample to find the defect rate being delivered. Wear-out is the last area that could be the problem, but you do not give any indication that this is the concern. Reliability predictions can be performed using the techniques from the Naval Surface Warfare Center Handbook of Reliability Predictions for Mechanical Equipment. The NSWC Carderock Division is located in Bethesda MD. Thank you for calling the Reliability Analysis Center.


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