Tools

Criticality analysis

The following is an excerpt from The Reliability Engineering Handbook by Bryan Dodson and Dennis Nolan, copyright QA Publishing, LLC

The objective of an FMECA is to identify all failure modes in a system design. Its purpose is to identify all catastrophic and critical failure probabilities so they can be minimized as early as possible. Therefore, the process FMECA should be started as soon as preliminary design information is available and extended as more information becomes available in suspected problem areas.

The effects of all failure modes are not equal with respect to the total impact on the system concerning safety and overall system performance. The designer, faced with this dilemma, needed a tool that would rank the significance of each potential failure for each component in the design alternatives. Because of the need for such justification, the criticality analysis function was added to the Failure Mode and Effects Analysis (FMEA) process, thus creating Failure Mode, Effects, and Criticality Analysis (FMECA).

This tool has been used extensiv>ely by the military in the last three decades. In recent years, more commercial industries have been requiring the FMECA to evaluate new designs and even more recently to improve the reliability of existing equipment. Military Standard 1629 is a good reference for Failure Mode, Effects, and Criticality Analysis.

7.1.1 The qualitative approach to FMECA

This approach should be used when specific failure rate data is not available. Failure modes identified by the FMECA process are assessed by their probability of occurrence. To establish qualitative measures of occurrence, severity, and detection, criteria must be established that subjectively relate to the overall effect on the process. Examples are offered in Tables 7.1, 7.2, and 7.3 to serve as guides in establishing qualitative measures. The product of the measures of occurrence, severity and detection is called the Risk Priority Number (RPN). Tables 7.1, 7.2, and 7.3 are for example only. The numbers or criteria assigned to any particular ranking system are at the discretion of the user. Detailed instructions on how to use this criterion on the FMECA form are explained in Section 7.1.4.

7.1.2 FMECA quantitative approach

Method 102 outlined in MIL-STD-1629 is the quantitative approach used for the FMECA process. Figure 7.1 is the worksheet used for this method.

Table 7.1. Occurrence probabilities.

Rank	Occurrence Criteria	Occurrence Rates (cycles, hrs, etc.)
1	Unlikely. Unreasonable to expect this failure mode to occur.	—
2	Isolated. Based on similar designs having a low number of failures.	1/10,000
3	Sporadic. Based on similar designs that have experienced occasional failures.	1/1,000
4	Conceivable. Based on similar designs that have caused problems.	1/100
5	Recurrent. Certain that failures will ensue.	1/10
NOTE: The ranking criteria selected must be consistent throughout the FMECA.

Table 7.2. Severity Probabilities.

Rank	Severity Criteria
1	Minor. No noticeable effect. Unable to realize that a failure has occurred.
2	Marginal. Annoying. No system degradation.
3	Moderate. Causing dissatisfaction. Some system degradation.
4	Critical. Causing a high degree of dissatisfaction. Loss of system function.
5	Catastrophic. A failure which may cause death or injury. Extended repair outages.
NOTE: The ranking criteria selected must be consistent throughout the FMECA.

The failure mode and criticality number (Cm) is the portion of the criticality number for the item due to a particular failure mode. This criticality number replaces the RPN number used in the qualitative method described in the previous section. The Cm for a failure mode is determined by the expression

Cm = b*a*l_p*t

where:

b = conditional probability of loss of function,

a = failure mode ratio,

l p = part failure rate, and

t = duration or operating time.

Table 7.3. Detection probabilities.

Rank	Detection Criteria	Probability
1	Very high probability of detecting the failure before it occurs. Almost always preceded by a warning.	80% – 100%
2	High probability of detecting the failure before it occurs. Preceded by a warning most of the time.	60% - 80%
3	Moderate probability of detecting the failure before it occurs. About a 50% chance of getting a warning.	40% - 60%
4	Low probability of detecting the failure before it occurs. Always comes with little or no warning.	20% - 40%
5	Remote probability of detecting the failure before it occurs. Always without a warning.	0% - 20%
NOTE: The ranking criteria selected must be consistent throughout the FMECA.

The b values represent the judgment as to the conditional probability that the loss will occur and should be quantified in general accordance with the following:

Failure effect	Probability of loss of function
Actual loss	b =1
Probable loss	0.10<b <1.00
Possible loss	0<b <0.10
No effect	b =0

The failure mode ratio, a , is the probability that the part or item will fail. If all potential failure modes of a particular part or item are listed, the sum of the a values for that part or item will equal one. Individual failure mode multipliers may be derived from failure rate source data or from test and operational data. If failure mode data are not available, the a values should represent the judgment based upon an analysis of the functions.

Part failure rates, l p, are derived from appropriate reliability prediction methods using mean-time-between-failure (MTBF) data or possibly other data obtained from handbooks or reference material. Manufacturers often supply failure data; however, it is important that the environment the item will be subjected to is similar to the environment the manufacturer used when obtaining the failure data.

The operating time, t, is usually expressed in hours or the number of operating cycles of the item being analyzed.

The a and b values are often subjective, thus making the supposed quantitative method somewhat qualitative. All things considered, it is generally understood that the FMECA process is a qualitative method of analysis.

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