MBVH 00418 SÄKERHETSKRITISKA KOMPONENTER - Bilaga 2: FMEA/FMECA
INNEHÅLL
21. FMEA
21.1. Förberedelser
Arbetet förbereds genom att ta fram relevant information, till exempel ritningar, nedbrytning i system, systembeskrivningar, funktionsanalyser och drifterfarenheter (inträffade fel).
21.2. Feltyp
Möjliga orsaker till feltypen anges. En feltyp kan ha flera orsaker. Exempel: felorsaken till läckaget i tätningen kan vara monteringsfel, materialfel eller mindre lyckad konstruktion.
21.3. Felorsak
Möjliga orsaker till feltypen anges. En feltyp kan ha flera orsaker.
21.4. Feleffekt
Feleffekt är konsekvensen av feltypen.
21.5. Felinidikering
Hur upptäcks felet?
21.6. Värdering felintensitet Po
För varje feltyp / orsak görs en uppskattning av med vilken frekvens felen kan förekomma. Projektet har i förväg tagit fram en lista från 1 till 10 med motsvarande felintensitet för beräkning av RPN-talet. Siffran 1 motsvarar då att det är osannolikt att fel kan uppträda, medan siffran 10 motsvarar mycket hög sannolikhet för fel.
21.7. Värdering feleffekt S
Feleffekten redovisas med ett bedömningstal för hur allvarligt felet är (om det uppträder). Minsta konsekvensen (1) kan till exempel vara: Ingen olycksrisk eller inverkan på produktionen, medan högsta konsekvensen (10) kan vara: Allvarlig risk för personskada eller död.
21.8. Värdering upptäcktssannolikhet Pd
Upptäcktssannolikhet kan till exempel betyda med vilken sannolikhet ett fel upptäcks innan det leder till konsekvenser. Minsta risk, det vill säga högsta sannolikhet för upptäckt (1) kan till exempel vara: Felet upptäcks alltid innan det uppträder, medan lägsta sannolikhet för upptäckt (10) kan vara: Osannolikt att felet upptäcks, kan ej provas.
21.9. Risktal RPN
Risktalet RPN = Po * S * Pd. Högsta möjliga risktal i detta fall är 1000, och lägsta risk är 1. Projektet bestämmer vilka risktal som bör utredas för möjlig reduktion eller eliminering med någon åtgärd.
21.10. Åtgärd
Åtgärder för att reducera risken kan vara återkommande provning, omkonstruktion eller annat. Låga risktal är tecken på en god process.
21.11. Att läsa
https://sv.wikipedia.org/wiki/Failure_modes_and_effects_analysis
22. FMECA
Slight differences are found between the various FMECA standards. By RAC CRTA–FMECA, the FMECA analysis procedure typically consists of the following logical steps:
Define the system
Define ground rules and assumptions in order to help drive the design
Construct system block diagrams
Identify failure modes (piece-part level or functional)
Analyze failure effects/causes
Feed results back into design process
Classify the failure effects by severity
Perform criticality calculations
Rank failure mode criticality
Determine critical items
Feed results back into design process
Identify the means of failure detection, isolation and compensation
Perform maintainability analysis
Document the analysis, summarize uncorrectable design areas, identify special controls necessary to reduce failure risk
Make recommendations
Follow up on corrective action implementation/effectiveness
FMECA may be performed at the functional or piece-part level. Functional FMECA considers the effects of failure at the functional block level, such as a power supply or an amplifier. Piece-part FMECA considers the effects of individual component failures, such as resistors, transistors, microcircuits, or valves. A piece-part FMECA requires far more effort, but provides the benefit of better estimates of probabilities of occurrence. However, Functional FMEAs can be performed much earlier, may help to better structure the complete risk assessment and provide other type of insight in mitigation options. The analyses are complementary.
The criticality analysis may be quantitative or qualitative, depending on the availability of supporting part failure data.
System definition
In this step, the major system to be analyzed is defined and partitioned into an indented hierarchy such as systems, subsystems or equipment, units or subassemblies, and piece-parts. Functional descriptions are created for the systems and allocated to the subsystems, covering all operational modes and mission phases.
Ground rules and assumptions
Before detailed analysis takes place, ground rules and assumptions are usually defined and agreed to. This might include, for example:
Standardized mission profile with specific fixed duration mission phases
Sources for failure rate and failure mode data
Fault detection coverage that system built-in test will realize
Whether the analysis will be functional or piece-part
Criteria to be considered (mission abort, safety, maintenance, etc.)
System for uniquely identifying parts or functions
Severity category definitions
Block diagrams
Next, the systems and subsystems are depicted in functional block diagrams. Reliability block diagrams or fault trees are usually constructed at the same time. These diagrams are used to trace information flow at different levels of system hierarchy, identify critical paths and interfaces, and identify the higher level effects of lower level failures.
Failure mode identification
For each piece-part or each function covered by the analysis, a complete list of failure modes is developed. For functional FMECA, typical failure modes include:
Untimely operation
Failure to operate when required
Loss of output
Intermittent output
Erroneous output (given the current condition)
Invalid output (for any condition)
For piece-part FMECA, failure mode data may be obtained from databases such as RAC FMD–91[12] or RAC FMD–97.[13] These databases provide not only the failure modes, but also the failure mode ratios. For example:
Device Failure Modes and Failure Mode Ratios (FMD–91) Device Type Failure Mode Ratio (α)
Relay Fails to trip .55
Spurious trip .26
Short .19
Resistor, Composition Parameter change .66
Open .31
Short .03
Each function or piece-part is then listed in matrix form with one row for each failure mode. Because FMECA usually involves very large data sets, a unique identifier must be assigned to each item (function or piece-part), and to each failure mode of each item.
Failure effects analysis
Failure effects are determined and entered for each row of the FMECA matrix, considering the criteria identified in the ground rules. Effects are separately described for the local, next higher, and end (system) levels. System level effects may include:
System failure
Degraded operation
System status failure
No immediate effect
The failure effect categories used at various hierarchical levels are tailored by the analyst using engineering judgment.
Severity classification
Severity classification is assigned for each failure mode of each unique item and entered on the FMECA matrix, based upon system level consequences. A small set of classifications, usually having 3 to 10 severity levels, is used. For example, When prepared using MIL–STD–1629A, failure or mishap severity classification normally follows MIL–STD–882.[14]
Mishap Severity Categories (MIL–STD–882) Category Description Criteria
I Catastrophic Could result in death, permanent total disability, loss exceeding $1M, or irreversible severe environmental damage that violates law or regulation.
II Critical Could result in permanent partial disability, injuries or occupational illness that may result in hospitalization of at least three personnel, loss exceeding $200K but less than $1M, or reversible environmental damage causing a violation of law or regulation.
III Marginal Could result in injury or occupational illness resulting in one or more lost work day(s), loss exceeding $10K but less than $200K, or mitigable environmental damage without violation of law or regulation where restoration activities can be accomplished.
IV Negligible Could result in injury or illness not resulting in a lost work day, loss exceeding $2K but less than $10K, or minimal environmental damage not violating law or regulation.
Current FMECA severity categories for U.S. Federal Aviation Administration (FAA), NASA and European Space Agency space applications are derived from MIL–STD–882.[15][16][17]
Failure detection methods
For each component and failure mode, the ability of the system to detect and report the failure in question is analyzed. One of the following will be entered on each row of the FMECA matrix:
Normal: the system correctly indicates a safe condition to the crew
Abnormal: the system correctly indicates a malfunction requiring crew action
Incorrect: the system erroneously indicates a safe condition in the event of malfunction, or alerts the crew to a malfunction that does not exist (false alarm)
Criticality ranking
Failure mode criticality assessment may be qualitative or quantitative. For qualitative assessment, a mishap probability code or number is assigned and entered on the matrix. For example, MIL–STD–882 uses five probability levels:
Failure Probability Levels (MIL–STD–882) Description Level Individual Item Fleet
Frequent A Likely to occur often in the life of the item Continuously experienced
Probable B Will occur several times in the life of an item Will occur frequently
Occasional C Likely to occur some time in the life of an item Will occur several times
Remote D Unlikely but possible to occur in the life of an item Unlikely, but can reasonably be expected to occur
Improbable E So unlikely, it can be assumed occurrence may not be experienced Unlikely to occur, but possible
The failure mode may then be charted on a criticality matrix using severity code as one axis and probability level code as the other. For quantitative assessment, modal criticality number C m C_m is calculated for each failure mode of each item, and item criticality number C r C_{r} is calculated for each item. The criticality numbers are computed using the following values:
Basic failure rate λ p \lambda_p
Failure mode ratio α \alpha
Conditional probability β \beta
Mission phase duration t t
The criticality numbers are computed as C m = λ p α β t {\displaystyle C_{m}=\lambda _{p}\alpha \beta t} and C r = ∑ n = 1 N ( C m ) n {\displaystyle C_{r}=\sum _{n=1}^{N}(C_{m})_{n}}. The basic failure rate λ p \lambda_p is usually fed into the FMECA from a failure rate prediction based on MIL–HDBK–217, PRISM, RIAC 217Plus, or a similar model. The failure mode ratio may be taken from a database source such as RAC FMD–97. For functional level FMECA, engineering judgment may be required to assign failure mode ratio. The conditional probability number β \beta represents the conditional probability that the failure effect will result in the identified severity classification, given that the failure mode occurs. It represents the analyst's best judgment as to the likelihood that the loss will occur. For graphical analysis, a criticality matrix may be charted using either C m C_m or C r C_{r} on one axis and severity code on the other.
Critical item/failure mode list
Once the criticality assessment is completed for each failure mode of each item, the FMECA matrix may be sorted by severity and qualitative probability level or quantitative criticality number. This enables the analysis to identify critical items and critical failure modes for which design mitigation is desired.
Recommendations
After performing FMECA, recommendations are made to design to reduce the consequences of critical failures. This may include selecting components with higher reliability, reducing the stress level at which a critical item operates, or adding redundancy or monitoring to the system.
Maintainability analysis
FMECA usually feeds into both Maintainability Analysis and Logistics Support Analysis, which both require data from the FMECA. FMECA is the most popular tool for failure and criticality analysis of systems for performance enhancement. In the present era of Industry 4.0, the industries are implementing a predictive maintenance strategy for their mechanical systems. The FMECA is widely used for the failure mode identification and prioritization of mechanical systems and their subsystems for predictive maintenance.[18]
FMECA report
A FMECA report consists of system description, ground rules and assumptions, conclusions and recommendations, corrective actions to be tracked, and the attached FMECA matrix which may be in spreadsheet, worksheet, or database form.
Fil 1439 datum 2023-04-28