CRITICALITY | Introduction | Types | Qualitative Analysis | Quantitative Analysis

CRITICALITY is a measure of the frequency of occurrence of an effect.

– May be based on qualitative judgement or

– May be based on failure rate data (most common)

01-static criticality experiment facility-uranium enriched solution

Qualitative analysis:

–Used when specific part or item failure rates are not available.

Quantitative analysis:

–Used when sufficient failure rate data is available to calculate criticality numbers.

01-qualitative vs quantitative analysis-part-item failure rates-calculate criticality number

Qualitative Approach:

  • Because failure rate data is not available, failure mode ratios and failure mode probability are not used.
  • The probability of occurrence of each failure is grouped into discrete levels that establish the qualitative failure probability level for each entry based on the judgment of the analyst.
  • The failure mode probability levels of occurrence are:

–Level A – Frequent

–Level B – Reasonably Probable

–Level C – Occasional

–Level D – Remote

–Level E – Extremely Unlikely

Quantitative Approach

Failure Mode Criticality (CM) is the portion of the criticality number for an item, due to one of its failure modes, which results in a particular severity classification (e.g. results in an end effect with severity I, II, etc…).

  • Category I – Catastrophic: A failure which may cause death or weapon system loss (i.e., aircraft, tank, missile, ship, etc…)
  • Category II – Critical: A failure which may cause severe injury, major property damage, or major system damage which will result in mission loss.
  • Category III – Marginal: A failure which may cause minor injury, minor property damage, or minor system damage which will result in delay or loss of availability or mission degradation.
  • Category IV – Minor: A failure not serious enough to cause injury, property damage or system damage, but which will result in unscheduled maintenance or repair.

The quantitative approach uses the following formula for Failure Mode Criticality:

Cm = βαλpt

Where

Cm = Failure Mode Criticality

β = Conditional probability of occurrence of next higher failure effect

α = Failure mode ratio

λp = Part failure rate

T = Duration of applicable mission phase

01-criticality analysis-example-Plutonium criticality cell pit removal

Reliability | Reliability Analysis | Methods | Failure Rate | Mean Time Between Failures | MTBF | Failure Mode Effective Analysis | FMEA

It is defined as the probability that a given system will perform it’s function adequately for it’s specified period of lifetime under specified operating conditions.

01-reliability-analysis-life time analysis-life time warranty

Common measures are :

1. Failure rate.

2. Mean time between failures(MTBF)

3. Survival percentage.

Failure Rate:

01-the bathtub curve-failure rate vs time - Reliability test-failure analysis-calculate life time of a product

Rate which components of population fail.

R(t)=Ns(t)/Nf(t)

Where,

Ns(t)- No. of components that survived during time ’t’

Nf(t) – No. of failures that occurred during the same time.

01-web- failure analysis-unexpected failure-operational fracture-failure rate

01-preheater-tube-failure-failure rate-circumferential cracking-probability of failure

Mean time between Failures(MTBF):

01-mtbf-mean time between failures-measurement of reliability-repairable items

The reciprocal of the failure rate(1/λ).

Where

λ —-Failure rate.

Failure rate = (No.of failure )/(Time period during all components were exposed to failure)

 

Reliability Analysis:

Failure Mode and Effect Analysis(FMEA)

01-FMEA-failure mode and effect analysis-effective analysis-risk priority number-severity number-probability number-reliability analysis

Purpose :

1. To recommend design changes.

2. To identify design weakness.

3. To help in choosing alternatives.

01-space-shuttle-failure mode-effects-criticality analysis-fmea-reliability test

Four Stages:

Ist Stage – System boundaries and the scope of the analysis is decided.

IInd Stage – Data Collection

Ex: Specification,Operating Procedure,Working Conditions.

IIIrd Stage– Preparing the component or parts list.

IVth Stage – Failure frequency and the functions of the part identified,causes of       failures,Failure detection.

01-Bathtub-life span of product-annual failure-mtbf prediction-mathematical model of reliability-improving quality