Detailed Analyses Chapter 14

Results of Analyses How final product will operate Problems in operation Malfunctions & failures Hazardous characteristics No one analysis method satisfies all requirements

Failure Modes & Effects Analysis (FMEA) Reliability - probability of successful accomplishment within a specific time & under specified conditions FMEA - evaluates effects (how and how often) of reliability failure

Failure Modes & Effects Analysis (FMEA) Product listed by major assemblies then sub-assemblies & components Each component studied for malfunctions & effects Failure rates determined & listed Establishes overall probability product will operate without failure for specific length of time & operate certain length of time between failures Best & principle means to determine where components & designs need improvement to increase life of product & frequency of service See Figure 14-1, page 152

Failure Modes & Effects Analysis (FMEA) Used for safety purposes Don’t take human error & hazardous conditions into account Do take environmental effects into consideration Don’t consider effects from multiple failures

Failure Modes & Effects Analysis (FMEA) Functions: Systematic review of component failure modes Determine effects of failures Determine parts whose failures critically affect operation Calculate probabilities of failures within multiple component/sub-assembly systems by considering each mode separately Establish test program requirements

Failure Modes & Effects Analysis (FMEA) Functions: Provide input data for trade-off studies Determine how to reduce probabilities of failure Eliminate or minimize adverse effects of failures Deficiencies can be eliminated or minimized through multiple avenues Don’t include effects of human actions on product

Conducting a Failure Modes & Effects Analysis (FMEA) Know & understand: Mission of equipment Constraints within which it is to operate Limits delineating success & failure

Conducting a Failure Modes & Effects Analysis (FMEA) Divide product into assemblies Review diagrams, schematics & drawings to determine inter-relationships Prepare component list & function Establish operational & environmental stresses affecting product Determine significant failure mechanisms that could affect components Identify failure modes of all components

Conducting a Failure Modes & Effects Analysis (FMEA) List each condition which affects a component Indicate hazard category (page 156) List preventive / corrective measures to eliminate or control hazard Enter probabilities of occurrence of each component failure Compute probabilities of failure of sub-assemblies, assemblies & products Determine criticality of components & effects of failure (optional)

Failure Modes & Criticality Analysis (FMECA) Critical components given special attention & analyzed more fully Critical component may be inherently hazardous by nature Effort should be made to safeguard items that could produce injury or damage through single-point failures

Single Point Failure One in which an accident could result from one component loss, human error or other single, untimely & undesirable event Example: One source of power for both critical & non-critical operation (hospitals) Eliminate by separating critical circuits or providing standby power supply

Criticality of Failure Modes Category 1: Failure resulting in potential loss of life Category 2: Failure resulting in potential mission failure Category 3: Failure resulting in delay or loss of operational ability Category 4: Failure resulting in excessive unscheduled maintenance

Criticality Ranking Items to more extensively study Items requiring special attention during production Special requirements included in specifications for suppliers Acceptance standards to be established for components When special procedures, safeguards, protective, warning or monitoring devices are needed When accident prevention efforts could be applied most effectively

Simple Method of Criticality Determination Multiply the probability of failure by the damage that could be generated. Criticality rankings generally expressed as probabilities Ranking does not complete a critical component analysis

Limitations FMEA & FMECA - single units or single failures Fault Tree Analysis - logical diagrams showing how data from FMEA could be interrelated to arrive at a specific event Reverse process now being used - logic analysis establishes events, failures or successful operations contributing to accident. FMEA studies conditions causing those failures, modes & preventive or safety measures to be taken.

Limitations Inadequate attention given to human error problems due to concentration on hardware failures Significant omission Greater number of product failures are result of connector problems, not components Probability of environmental stresses rarely used

Fault Hazard Analysis (FHA) Does not use same logic principles of Fault Tree Analysis Does not use same quantitative aspects of FMEA Originally provided tabulated output Qualitative analysis method Detailed extension of PHA See column headings bottom of page 157

Circuit Logic Analysis Design & evaluation of complex electric & electronic circuitry Determines how a product can be affected by failures of components in a circuit & if circuit can generate damaging output Operation described in terms of interacting electronic components & mechanical devices Circuit elements represented by logic symbols (Figure 14-4, Pages 158-160) Logic equation developed to express condition

Blocking Elements Systems that are designed so they will NOT operate until specific events occur involve this concept Blocking element - device that must be activated or inactivated to change from a safe to an unsafe state Done by a person’s action or result of electrical or mechanical process Example: electrical interlocks

Logic Analysis Determine when a system that has blocking elements that must be removed in order to operate will become unsafe Can show in the progress of an operation when each safety block will be removed

Applications of Logic Analysis Possibilities of inadvertent activation Failure analysis Investigation or interlocks for orderly operation Determination of safeguard & separation needs Evaluations of occurrences that might make single-point failures possible

Interface Analysis Determine incompatibilities between subassemblies & subsystems that could result in accidents Relationships to consider: Physical Functional Flow

Physical Relationships Little clearance that leads to damage upon movement Access to or egress from becomes impossible or restricted due to dimensions or inadequate clearances Inability to join mate parts that should fit closely together Example: Filter in tight spot will not be removed/cleaned

Functional Relationships Zero output - output unit fails completely Degraded output - partial failure occurs Erratic output - intermittent or unstable operations Excessive output - high speed, temperature, voltage Unprogrammed output - inadvertent operation or erroneous output Undesirable side effects - other outputs that could be damaging

Flow Relationships May involve fluid May involve energy May be unconfined Part of interface analysis Most frequent, severe & varied problems involve fluids & energy flowing through confined passages

Potential Flow Problem Causes Faulty connections Full or partial failure of interconnections. Consider: Flammability Toxicity Corrosiveness Loss of pressure Lubricity Loss of material Contamination Moisture & water Odor

Product Subsystems Figure 14-7, Page 166 Products, subsystems, operators & relationship constitute a system Subsystems can affect each other while operating Use of checklists (Figure 7-1, page 57)

Mapping Reveals problems due to location & proximity of units, lines & hazards Distances between fuel lines & ignition sources Locations of tanks in storage farms Mapping of fire zones & fire defense routes Emergency evacuation routes, safety zones & protective structures Noise level contours & scope of effects Accidents involving fires Accidents involving buses / children Determine extent of potential micrometeorological problems

Checklist Reviews Assist designers ensuring no adverse features are incorporated into a product & that appropriate safeguards are provided Lead to good engineering practices Help avoid specifically prohibited or poor practices Ensure mandatory requirements are satisfied

Checklist Reviews Questions based on specific standard (PSM Compliance Officer’s Directive) Source of checklist item included in ( ) at end of question/statement Verification categories: Analysis Examination Demonstration Test

Checklist Reviews Column spaces for remarks regarding compliance Space for entering mark when item is satisfied or other information Standard checklists may not be applicable to product analyzed Arrange questions / statements sequentially Provide designer with checklist before design is begun