1. Extending Criticality Analysis in order
to Consider Derating and Degradation
Timót Veér, Olav Bolland
Inst. for Energi- og Prosessteknologi, NTNU
Naturgass Seminar, April 2003.
Naturgass Seminar April
29/05/2019

2. Topics of this presentation
Introduction, terms used herein
Failure mode and effects analysis, FMEA
Failure mode, effect and criticality analysis, FMECA
Progressive loss of performance in FMECA
Conclusions
Naturgass Seminar April
29/05/2019

3. What is failure?
Def: failure is the termination of the ability of a system/subsystem to fulfill its designed duties.
Generally, the failure is considered the end effect of a failure mode resulting in outage.
Naturgass Seminar April
29/05/2019

4. Brief regularity nomenclature
Regularity is a term used to describe how a system is capable of meeting demand for deliveries or performance.
Failure rate, the frequency of occurrence of the current failure mode.
Failure mode, describes the way a failure occurs and its impact on equipment operation.
Failure effect, consequence of failure upon operation, function or status of an item.
Criticality is a relative measure of the consequences of a failure mode and its frequency of occurrence.
Naturgass Seminar April
29/05/2019

5. What is FMEA/CA? FMEA – failure mode and effect analysis (EA)
Maps the end effect of different possible failure modes of the subsystems
FMECA – failure mode, effect and criticality analysis (CA)
In addition to EA, it assesses the criticality of individual failure modes.
Naturgass Seminar April
29/05/2019

6. Failure mode and effect analysis - FMEA
Identifies the possible failure modes and the interrelationship between indenture levels – from cause to end effect.
Naturgass Seminar April
29/05/2019

7. Failure mode, effect and criticality analysis – FMECA
Criticality analysis intends to support the ranking of the failure modes.
Qualitative as well as
quantitative approaches are employed.
Basic qualitative approach to criticality assessments
Naturgass Seminar April
29/05/2019

8. Classical quantitative FMECA
Inputs:
lp [f/Mh] : part failure rate, number of failures of the part/subsystem during the considered time span
a [%]: base failure rate, fraction of the part failure rate related to the particular failure mode under consideration.
b [%]: conditional probability is probability that the failure effect will result in the identified criticality classification, given that the failure mode occurs.
Criticality number:
Cm = lp a b t ;
whereas:
Cm [f] : criticality number of the considered failure mode m,
t [h]: time span.
Naturgass Seminar April
29/05/2019

9. FMECA table – antisurge valve failure
Criticality number:
Cm = lp a b t ;
Naturgass Seminar April
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10. What is derating/degradation?
Derating and degradation are main causes of progressive performance loss
(e.g. fouling, erosion, wearing, ambient effects, etc…)
Base line is considered the performance of the regarded system in state definable as ”new and clean”
Naturgass Seminar April
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11. FMEA – fouling, a typical case of derating
The mechanism of fouling of a GT due to airborne particles:
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12. Quantitative CA in case of fouling
Equivalent derated hours (EDH)
Reflects the ratio of the derated energy amount and the current net capacity of the system plus the sum of the outages due to the considered derating cause: = =
Naturgass Seminar April
29/05/2019

13. FMECA table – results
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14. Conclusions
Extension of traditional FMECA to progressive performance loss was proposed
Examples from GT operation are given for CA in both cases
It is shown the relative importance of the progressive performance loss vs. traditional failure
Naturgass Seminar April
29/05/2019