Failure Mode Effect and Criticality Analysis

Adam AdgarSchool of Computing and Technology

FMECA

► Failure Mode Effect and Criticality Analysis Detailed study of a system to determine which

parts are most critical to operation under various failure modes

► Developed in USA in 1950’s► Objective is to prevent failures► Can be used at design stage or process

stage► Different Industries have different standards

but all are very similar in philosophy and procedures

What is FMECA

► A set of systemized group activities intended to:

recognize and evaluate the potential failure of a product or process and its effects

identify and prioritize actions which could eliminate or reduce the chance of the failure occurring

document the process

► It involves: Brainstorming to identify “all” potential failure

causes Assigning numerical priorities to all modes Tracking and documenting corrective actions

Definitions

► Failure Mode Analysis Incorrect behavior of a subsystem or

component due to a physical or procedural malfunction

► Failure Effect Analysis Incorrect behavior of the system caused by

a failure

► Failure Criticality Analysis The combined impact of

The probability that a failure will occur The severity of its effect

FMECA Stages► Stage 1

Identify all components, assemblies etc of the system under consideration

► Stage 2 Make an exhaustive list of

all possible FMs of each component

► Stage 3 Establish the effects that

each FM would have on the overall system or process

► Stage 4 Make a list of all possible

causes of each FM

► Stage 5 Assign a numerical value to

each occurrence for each of the following FM criteria

P = Probability of FM S = Seriousness/Criticality

of FM D = Difficulty of detecting

FM► Stage 6

Determine criticality indexor risk priority number (PSD) for each FM

► Stage 7 Determine corrective action

needed to avoid FM► Stage 8

Rank the RPN for the whole system so action may be taken using resources available

Example CI / RPN► Automobile hydraulic

braking system► Criticality Index / Risk

Priority Number

= 2 x 9 x 5 = 90

► Criteria sometimes ranked on a scale of 1 – 4. (4 = worst)

► Each level may have detailed verbal description

harder to misrepresent a threat

minimizes impact of “judgment” on numerical value

Value 1 2 3 4 5 6 7 8 9 10

P X

S X

D X

Failure M ode & Effects W orksheetP age 1 o f ___P rin ted : D ate and T im e

Area : Identify P lan t A rea or P rocessSystem : S ystem N am eFMEA Number: If U sedTeam Members : W ho prepared th is FM E A

D ate FM E A S tarted :

D ate FM E A C om ple ted:

ControlNum ber

Nam e & Function/Perform ance Requirem ent

Potential Failure M ode Potential Failure Effects M aintenance Approach Rem arks/Continue

Crit

Prob

M ACode

fmea1.vsd

Sample FMECA Data SheetAREA: Central Utilities TEAM: Utilities PAGE 1 0F 1SYSTEM: Chilled Water START:FMEA NO. CW001 END:

DATE PRINTED:3/24/99

CONTROLNUMBER

NAME &FUNCTION

FAILUREMODE

FAILUREEFFECT C

RIT

PR

OB

MA

CO

DE MAINTENANCE

ACTIONCOMMENTS

SEIZEDBEARING

LOSS OFFLOW

8 4 V1NARROWBAND

VIBRATIONQUARTERLY

STATORINSUL.

LOSS OFFLOW

8 3 E1 MEGGER BIENNIAL

LOWFLOW

4 2

CW1CWP

MOTOR101

ROTORBAR LOSS OF

FLOW8 2

E2MOTOR CIRCUIT

EVALUATIONANNUAL

Summary of FMECA

► Is a formalized group-based problem solving method Similar to design review standards in many industries Uses brainstorming to try to identify “all” risks

► Has become an industry-standard approach Is revised and improved based on experience Attempts to define a general “best known method”

► Tries to accomplish the following: Identify risks Prioritize them Define and track corrective actions and their effects Document the entire decision process

Example: Pressure Cooker► OPERATOR:

1. loads cooker2. closes/seals lid3. connects power4. observes pressure5. times cooking at prescribed

pressure6. offloads dinner.

► SYSTEM DESCRIPTION: Electric coil heats cooker. Thermostat controls

temperature Switch opens >120 °C.

Spring-loaded Safety Valve opens on overpressure.

Pressure Gauge red zone indicates overpressure.

High temperature/pressure cooks/sterilizes food — tenderizes and protects against botulin toxin.

► Prepare an FMEA at component level for cooking (after loading/ closing/ sealing).

► Targets are personnel (P), product (R), and the pressure cooker itself (E). Ignore facility/kitchen and energy consumption. Food is for private use.